intro to fire sim-027

An introduction toFire Simulationwith FDS and Smokeview

Emanuele Gissi

Updated to FDS 5.4.1

Development

Download a revised version of this manual from:

• http://www.corbezzolo.org/

You can participate in the development of this manual. Please, contact me at:

• [email protected]

This manual was produced using LYX and OpenOffice.org on Ubuntu Linux 9.04.The LATEX file intro_to_fire_sim-027.tex was compiled on September 21,2009.

Copyright notice

This work is licensed under the Creative Commons Attribution-Share Alike 3.0License. To view a copy of this license, visit:

• http://creativecommons.org/licenses/by-sa/3.0/

You are free to use, share, adapt this work. Remember to cite the sources andto use the same open license for derivative work.

http://www.corbezzolo.org/

mailto:[email protected]

http://creativecommons.org/licenses/by-sa/3.0/

About the Author

Emanuele Gissi ([email protected]) is a fire officer since 2002, servingat the Comando provinciale dei Vigili del Fuoco, Genova (Italy), a branch ofthe governmental fire safety national organization (http://www.vigilfuoco.it/).He is a mechanical engineer and obtained a doctorate in technical physicsin 2001. His work is focused on incident command, review of performancebased fire safety design, and fire investigation. He provides basic and ad-vanced training on fire simulation to fire officers and professionals.

iii

mailto:[email protected]

http://www.vigilfuoco.it/

Preface

This manual was born as a small tutorial for students of fire safety engineeringcourses. Then it grew to the current state.The main goal of this manual is to introduce the student to the complex world offire simulation with Fire Dynamics Simulator and Smokeview, and complementsthe official documentation. The official documentation remains an invaluablesource of reference for advanced users and this manual is heavily based on it.Some large parts are even copy-pasted and adapted.In this manual, topics are organized in a strict logical order and the basics arethoroughly explained to improve the learning curve. Some advanced topics arecompletely omitted for the sake of simplicity.According to teaching experience, students understand the logic behind FireDynamics Simulator and become autonomous learners after 16 hours of course:they learn to work independently and are able to develop reasonable engineeringlevel applications by themselves.

v

Acknowledgments

People

The National Institute of Standards and Technology (NIST), a federal agencywithin the Department of Commerce of the United States, is the major drivingforce behind Fire Dynamics Simulator development.The Fire Dynamics Simulator has been publicly released on 2000. Since its firstrelease, continued improvements have been made to the software based largelyon feedback from its users and on the hard work of some NIST employees.Fire Dynamics Simulator documentation is written and maintained by KevinMcGrattan, Bryan Klein, Simo Hostikka, and Jason Floyd fromvarious organizations. Their user support through the discussion group is alwayscomplete, fast, and precious. In a simple word: friendly.I owe to them large parts of this manual and most of my knowledge on firesimulation. Thus, I thank them once more.

Ideas

This manual is open content and free, because it’s published in a format thatexplicitly allows copying and modifying of its information by anyone.Fire Dynamics Simulator itself is free and in the public domain. Linux1 andOpenoffice.Org2 are free and open source.I thank the open source movement as a whole, as the best things in life are free.

1Linux is an open source operating system. See http://www.ubuntu.com/ for an userfriendly Linux distribution.

2Openoffice.org is an open source office and productivity program for Linux, MS Windows,and MacOS X. See http://www.openoffice.org/ for further details.

vii

http://www.ubuntu.com/

http://www.openoffice.org/

Contents

1 Introduction 11.1 Complexity of fire phenomena . . . . . . . . . . . . . . . . . . . 11.2 Approaches to fire simulation . . . . . . . . . . . . . . . . . . . 21.3 Simplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Inside FDS5 72.1 What are FDS5 ans Smokeview? . . . . . . . . . . . . . . . . . 72.2 Engineering applications . . . . . . . . . . . . . . . . . . . . . . 82.3 Who develops FDS5? . . . . . . . . . . . . . . . . . . . . . . . 82.4 Who uses FDS5? . . . . . . . . . . . . . . . . . . . . . . . . . . 92.5 How much does it cost? . . . . . . . . . . . . . . . . . . . . . . 92.6 How does FDS5 work? . . . . . . . . . . . . . . . . . . . . . . . 9

2.6.1 Hydrodynamic model . . . . . . . . . . . . . . . . . . . 92.6.2 Combustion model . . . . . . . . . . . . . . . . . . . . . 92.6.3 Radiation transport . . . . . . . . . . . . . . . . . . . . 102.6.4 Geometry and multiple meshes . . . . . . . . . . . . . . 102.6.5 Parallel processing . . . . . . . . . . . . . . . . . . . . . 112.6.6 Boundary conditions . . . . . . . . . . . . . . . . . . . . 11

2.7 Limitations of FDS5 . . . . . . . . . . . . . . . . . . . . . . . . 112.7.1 Low speed flow assumption . . . . . . . . . . . . . . . . 112.7.2 Rectilinear geometry . . . . . . . . . . . . . . . . . . . . 112.7.3 Fire growth and spread . . . . . . . . . . . . . . . . . . 122.7.4 Combustion . . . . . . . . . . . . . . . . . . . . . . . . 122.7.5 Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . 13

ix

x CONTENTS

3 Running FDS5 153.1 Online resources and user support . . . . . . . . . . . . . . . . . 153.2 Version numbers . . . . . . . . . . . . . . . . . . . . . . . . . . 163.3 Hardware requirements . . . . . . . . . . . . . . . . . . . . . . . 163.4 Serial and parallel calculations . . . . . . . . . . . . . . . . . . . 173.5 Installing on Windows XP . . . . . . . . . . . . . . . . . . . . . 183.6 Installing on Ubuntu Linux . . . . . . . . . . . . . . . . . . . . . 18

3.6.1 First install . . . . . . . . . . . . . . . . . . . . . . . . . 183.6.2 Installing a new version . . . . . . . . . . . . . . . . . . 19

3.7 Compiling an optimized binary . . . . . . . . . . . . . . . . . . . 203.8 Performing a calculation . . . . . . . . . . . . . . . . . . . . . . 20

3.8.1 Running serial FDS5 on Windows XP . . . . . . . . . . . 203.8.2 Running serial FDS5 on Ubuntu Linux . . . . . . . . . . 213.8.3 Running parallel FDS5 on Ubuntu Linux . . . . . . . . . 21

3.9 Monitoring progress . . . . . . . . . . . . . . . . . . . . . . . . 233.10 Stop a calculation . . . . . . . . . . . . . . . . . . . . . . . . . 233.11 Visualizing results . . . . . . . . . . . . . . . . . . . . . . . . . 233.12 Output files . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Input file basics 254.1 Syntax of the input file . . . . . . . . . . . . . . . . . . . . . . 254.2 Writing an input file . . . . . . . . . . . . . . . . . . . . . . . . 274.3 The logic behind most FDS5 input files . . . . . . . . . . . . . . 294.4 Keep it simple . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.5 Each model, its input data . . . . . . . . . . . . . . . . . . . . . 334.6 Units of measurement . . . . . . . . . . . . . . . . . . . . . . . 344.7 Reference coordinate system . . . . . . . . . . . . . . . . . . . . 344.8 Prescribing geometric entities . . . . . . . . . . . . . . . . . . . 354.9 Prescribing orientations . . . . . . . . . . . . . . . . . . . . . . 364.10 Prescribing colors and aspect . . . . . . . . . . . . . . . . . . . 37

CONTENTS xi

5 General configuration 395.1 Naming the job, HEAD . . . . . . . . . . . . . . . . . . . . . . . 395.2 Simulation time, TIME . . . . . . . . . . . . . . . . . . . . . . . 405.3 Miscellaneous, MISC . . . . . . . . . . . . . . . . . . . . . . . . 40

6 Combustion and radiation 436.1 Combustion is not pyrolysis . . . . . . . . . . . . . . . . . . . . 436.2 Prescribing a fire . . . . . . . . . . . . . . . . . . . . . . . . . . 436.3 Modeling gas phase combustion, REAC . . . . . . . . . . . . . . 44

6.3.1 Ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.3.2 Burning . . . . . . . . . . . . . . . . . . . . . . . . . . 45

6.4 CO production in under-ventilated fires . . . . . . . . . . . . . . 496.5 Flame extinction . . . . . . . . . . . . . . . . . . . . . . . . . . 496.6 Radiation transport, RADI . . . . . . . . . . . . . . . . . . . . . 50

7 Computational domain 537.1 Defining a mesh, MESH . . . . . . . . . . . . . . . . . . . . . . . 537.2 Multiple meshes . . . . . . . . . . . . . . . . . . . . . . . . . . 557.3 Conformity to the mesh . . . . . . . . . . . . . . . . . . . . . . 567.4 Choosing the right mesh dimension:

a sensitivity study . . . . . . . . . . . . . . . . . . . . . . . . . 577.5 Initial conditions of the computational domain, INIT . . . . . . . 58

8 Materials 618.1 Defining a material, MATL . . . . . . . . . . . . . . . . . . . . . 618.2 Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . 628.3 Burning properties . . . . . . . . . . . . . . . . . . . . . . . . . 63

8.3.1 Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638.3.2 Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 648.3.3 HEAT_OF_COMBUSTION in a MATL line? . . . . . . . . . . 65

8.4 Properties hell . . . . . . . . . . . . . . . . . . . . . . . . . . . 658.5 Resources for material property data . . . . . . . . . . . . . . . 66

xii CONTENTS

9 Extra gas species 67

9.1 Defining extra gas species, SPEC . . . . . . . . . . . . . . . . . . 67

9.2 CARBON DIOXIDE and carbon dioxide . . . . . . . . . . . . . 68

10 Lagrangian particles 71

10.1 Defining Lagrangian particles, PART . . . . . . . . . . . . . . . . 71

10.2 Massless particles . . . . . . . . . . . . . . . . . . . . . . . . . 72

10.3 Water droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

11 Boundary conditions 75

11.1 Defining boundary conditions, SURF . . . . . . . . . . . . . . . . 75

11.2 Predefined boundary conditions . . . . . . . . . . . . . . . . . . 77

11.3 Coloring boundary conditions . . . . . . . . . . . . . . . . . . . 78

11.4 Examples of boundary conditions . . . . . . . . . . . . . . . . . 79

11.4.1 Adiabatic surface . . . . . . . . . . . . . . . . . . . . . . 79

11.4.2 Fixed temperature and heat flux . . . . . . . . . . . . . . 79

11.4.3 Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

11.4.4 Fans injecting extra gas species . . . . . . . . . . . . . . 81

11.4.5 Dynamic pressure at an open boundary . . . . . . . . . . 82

11.4.6 Prescribing an heat release rate . . . . . . . . . . . . . . 82

11.5 Geometric conformity and rates . . . . . . . . . . . . . . . . . . 83

11.6 Boundary conditions for solids . . . . . . . . . . . . . . . . . . . 83

11.6.1 Backing . . . . . . . . . . . . . . . . . . . . . . . . . . 84

11.6.2 Setting an initial temperature . . . . . . . . . . . . . . . 86

11.7 Time dependent boundary conditions . . . . . . . . . . . . . . . 86

11.7.1 Simplified ramps . . . . . . . . . . . . . . . . . . . . . . 86

11.7.2 User defined ramps . . . . . . . . . . . . . . . . . . . . 87

11.8 Injecting Lagrangian particles . . . . . . . . . . . . . . . . . . . 89

CONTENTS xiii

12 Solid geometry 9112.1 Defining solid obstructions, OBST . . . . . . . . . . . . . . . . . 9112.2 Creating voids inside obstructions, HOLE . . . . . . . . . . . . . . 9412.3 Prescribing a different boundary condition, VENT . . . . . . . . . 9512.4 Default boundary condition . . . . . . . . . . . . . . . . . . . . 9712.5 How thick is a wall? . . . . . . . . . . . . . . . . . . . . . . . . 9812.6 Thin sheet obstructions . . . . . . . . . . . . . . . . . . . . . . 9912.7 Activating and deactivating objects . . . . . . . . . . . . . . . . 10012.8 Stair stepping complex geometries . . . . . . . . . . . . . . . . . 10012.9 Coloring individual objects . . . . . . . . . . . . . . . . . . . . . 10112.10Making burning solids disappear . . . . . . . . . . . . . . . . . . 102

13 Devices and control logic 10313.1 Devices, DEVC and PROP . . . . . . . . . . . . . . . . . . . . . . 10313.2 Examples of devices . . . . . . . . . . . . . . . . . . . . . . . . 105

13.2.1 Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . 10513.2.2 Thermometer . . . . . . . . . . . . . . . . . . . . . . . 10513.2.3 Smoke detector . . . . . . . . . . . . . . . . . . . . . . 10513.2.4 Beam smoke detector . . . . . . . . . . . . . . . . . . . 10613.2.5 Sprinkler and heat detector . . . . . . . . . . . . . . . . 106

13.3 Basic control logic . . . . . . . . . . . . . . . . . . . . . . . . . 10713.4 Advanced control logic . . . . . . . . . . . . . . . . . . . . . . . 109

14 Output 11114.1 Prescribing output . . . . . . . . . . . . . . . . . . . . . . . . . 11114.2 Output control parameters, DUMP . . . . . . . . . . . . . . . . . 11314.3 Point measurement devices, DEVC . . . . . . . . . . . . . . . . . 113

14.3.1 Record a gas phase quantity . . . . . . . . . . . . . . . . 11414.3.2 Record a solid phase quantity . . . . . . . . . . . . . . . 11414.3.3 Record integrated quantities . . . . . . . . . . . . . . . . 115

xiv CONTENTS

14.4 Animated planar slices, SLCF . . . . . . . . . . . . . . . . . . . 11514.5 Animated boundary quantities, BNDF . . . . . . . . . . . . . . . 11614.6 Animated isosurfaces, ISOF . . . . . . . . . . . . . . . . . . . . 11814.7 Realistic smoke and fire . . . . . . . . . . . . . . . . . . . . . . 11914.8 Heat release rate . . . . . . . . . . . . . . . . . . . . . . . . . . 12014.9 Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12014.10Upper and lower layers . . . . . . . . . . . . . . . . . . . . . . . 12114.11Heat fluxes and thermal radiation . . . . . . . . . . . . . . . . . 12214.12Interfacing with structural models . . . . . . . . . . . . . . . . . 12214.13Visualizing Lagrangian particles . . . . . . . . . . . . . . . . . . 12314.14Frequently used output quantities . . . . . . . . . . . . . . . . . 123

15 Real world examples 12715.1 Building a ventilator . . . . . . . . . . . . . . . . . . . . . . . . 12715.2 Prescribing a simplified burning material . . . . . . . . . . . . . 12715.3 Simulation and revelation of smoke of a smoldering fire . . . . . 12815.4 A pan filled of ethanol . . . . . . . . . . . . . . . . . . . . . . . 12915.5 A simple car parking . . . . . . . . . . . . . . . . . . . . . . . . 129

15.5.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . 12915.5.2 Input file and results . . . . . . . . . . . . . . . . . . . . 131

16 Using a GUI 141

List of Figures

3.1 A cluster of Linux computers . . . . . . . . . . . . . . . . . . . 173.2 Starting a serial calculation on Windows XP and on Linux Ubuntu 213.3 Running a parallel calculation and monitoring the system on Ubuntu

Linux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.4 Running Smokeview on Windows XP and on Ubuntu Linux . . . . 24

4.1 The structure of an FDS5 namelist group . . . . . . . . . . . . . 254.2 Modeling reality in FDS5 . . . . . . . . . . . . . . . . . . . . . 334.3 The reference system, a volume, a face, a segment, a point, and

a plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.1 Combustion is not pyrolysis . . . . . . . . . . . . . . . . . . . . 446.2 Combustion and pyrolysis in a flaming match . . . . . . . . . . . 456.3 Flame extinction criteria . . . . . . . . . . . . . . . . . . . . . . 50

7.1 The computational domain composed by four meshes . . . . . . 557.2 Mesh connections: (a) ideal, (b) allowed, and (c) forbidden . . . 567.3 Geometric object: before and after automatic shifting . . . . . . 56

11.1 Extending the computational domain beyond the vent . . . . . . 7811.2 brick wall: multiple layers of different materials . . . . . . . . 8411.3 HRRPUA as function of time after SURF activation . . . . . . . . . 8711.4 VEL and TMP_FRONT as function of time after SURF activation . . 88

12.1 Boundary conditions prescribed with SURF_ID, SURF_IDS, andSURF_ID6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

xv

xvi LIST OF FIGURES

12.2 OBST, HOLE and VENT . . . . . . . . . . . . . . . . . . . . . . . 9512.3 Setting boundary conditions to exterior boundaries of the com-

putational domain . . . . . . . . . . . . . . . . . . . . . . . . . 9612.4 Flow velocity on two sides of an oblique wall. Lower side has

SAWTOOTH=.FALSE. set. . . . . . . . . . . . . . . . . . . . . . . 101

14.1 Output of animated planar slices SLCF as viewed in Smokeview . 11514.2 Output of animated boundary quantities, BNDF as viewed in Smoke-

view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11714.3 Output of animated isosurfaces, ISOF as viewed in Smokeview . . 11814.4 Output of soot MASS FRACTION and HRRPUV as viewed in Smoke-

view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

15.1 Car parking plan . . . . . . . . . . . . . . . . . . . . . . . . . . 13015.2 The entered geometry . . . . . . . . . . . . . . . . . . . . . . . 13515.3 Heat release rate curve . . . . . . . . . . . . . . . . . . . . . . . 13515.4 Thermocouples at the center of the car parking. . . . . . . . . . 13615.5 Gas temperatures in front of window panes vs time. . . . . . . . 13615.6 FED vs time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13715.7 Layer height vs time. . . . . . . . . . . . . . . . . . . . . . . . . 13715.8 AST (Adiabatic Surface Temperature) boundary file at 1200 s. . . 13815.9 Visibility slice file at 120 s. . . . . . . . . . . . . . . . . . . . . . 13815.10Visibility (10 m) isosurface at 300 s. . . . . . . . . . . . . . . . . 13915.11Net heat flux on boundary surfaces at 1200 s. . . . . . . . . . . 139

16.1 A Blender session . . . . . . . . . . . . . . . . . . . . . . . . . 141

List of Tables

4.1 Systematic organisation of the input file . . . . . . . . . . . . . . 314.2 COLOR values . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.1 HEAD parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 405.2 TIME parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 405.3 MISC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.1 Mixture fraction species . . . . . . . . . . . . . . . . . . . . . . 476.2 REAC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 476.3 RADI parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 51

7.1 IJK values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547.2 MESH parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 547.3 INIT parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 59

8.1 MATL parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 618.2 RAMP (temperature) parameters . . . . . . . . . . . . . . . . . . 63

9.1 Predefined extra species . . . . . . . . . . . . . . . . . . . . . . 679.2 SPEC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 68

10.1 PART parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 72

11.1 SURF parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 7611.2 RAMP (time) parameters . . . . . . . . . . . . . . . . . . . . . . 88

xvii

xviii LIST OF TABLES

12.1 OBST parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 9312.2 HOLE parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 9412.3 VENT parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 97

13.1 DEVC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 104

14.1 DUMP parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 11314.2 Output of devices in mycase_devc.csv file . . . . . . . . . . . . 11314.3 SLCF parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 11614.4 BNDF parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 11714.5 ISOF parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 11914.6 Output of HRR in mycase_hrr.csv file . . . . . . . . . . . . . 12014.7 Frequently used output quantities . . . . . . . . . . . . . . . . . 123

Chapter 1

Introduction

This chapter presents the problems embedded in fire simulation. Thetext has been adapted from [FDS5 user’s guide], Introduction byHoward Baum, NIST Fellow Emeritus.

1.1 Complexity of fire phenomena

The idea that the dynamics of a fire might be studied numerically dates backto the beginning of the computer age. Indeed, the fundamental conservationequations governing fluid dynamics, heat transfer, and combustion were firstwritten down over a century ago. Despite this, practical mathematical modelsof fire, as distinct from controlled combustion, are relatively recent due to theinherent complexity of the problem.Indeed, in his brief history of the early days of fire research, Hoyt Hottelnoted:

“A case can be made for fire being, next to the life processes, themost complex of phenomena to understand” [Hottel 1984].

The difficulties revolve about three issues:

• First, there are an enormous number of possible fire scenarios to considerdue to their accidental nature.

• Second, the physical insight and computing power required to perform allthe necessary calculations for most fire scenarios are limited. Any funda-mentally based study of fires must consider at least some aspects of bluff

1

2 CHAPTER 1. INTRODUCTION

body aerodynamics, multi-phase flow, turbulent mixing and combustion,radiative transport, and conjugate heat transfer; all of which are activeresearch areas in their own right.

• Finally, the fuel in most fires was never intended as such.

Thus, the mathematical models and the data needed to characterize the degrada-tion of the condensed phase materials that supply the fuel may not be available.Indeed, the mathematical modeling of the physical and chemical transformationsof real materials as they burn is still in its infancy.In order to make progress, the questions that are asked have to be greatly sim-plified.To begin with, instead of seeking a methodology that can be applied to all fireproblems, we begin by looking at a few scenarios that seem to be most amenableto analysis. Hopefully, the methods developed to study these simple problemscan be generalized over time so that more complex scenarios can be analyzed.Second, we must learn to live with idealized descriptions of fires and approximatesolutions to our idealized equations.Finally, the methods should be capable of systematic improvement. As our phys-ical insight and computing power grow more powerful, the methods of analysiscan grow with them.

1.2 Approaches to fire simulation

To date, four distinct approaches to the simulation of fires have emerged. Eachof these treats the fire as an inherently three dimensional process evolving intime.The first to reach maturity, the zone models, describe compartment fires. EachZone modelscompartment is divided into two spatially homogeneous volumes, a hot upperlayer and a cooler lower layer. Mass and energy balances are enforced for eachlayer, with additional models describing other physical processes appended asdifferential or algebraic equations as appropriate. Examples of such phenomenainclude fire plumes, flows through doors, windows and other vents, radiativeand convective heat transfer, and solid fuel pyrolysis. Model development hasprogressed to the point where documented and supported software implementingthese models are widely available, such as CFAST1.

1The Consolidated Model of Fire and Smoke Transport (CFAST) is a two-zone fire modelused to calculate the evolving distribution of smoke, fire gases and temperature throughoutcompartments of a building during a fire. Visit http://cfast.nist.gov/ for further information.

http://cfast.nist.gov/

1.2. APPROACHES TO FIRE SIMULATION 3

The relative physical and computational simplicity of the zone models has led totheir widespread use in the analysis of fire scenarios. So long as detailed spatialdistributions of physical properties are not required, and the two layer descriptionreasonably approximates reality, these models are quite reliable. However, by theirvery nature, there is no way to systematically improve them.The rapid growth of computing power and the corresponding maturing of com-putational fluid dynamics (CFD), has led to the development of CFD based fieldmodels applied to fire research problems. Virtually all this work is based on theconceptual framework provided by the Reynolds-averaged form of the Navier-Stokes equations (RANS). The use of CFD models has allowed the description RANSof fires in complex geometries, and the incorporation of a wide variety of physicalphenomena.However, these models have a fundamental limitation for fire applications – theaveraging procedure at the root of the model equations.RANS models were developed as a time-averaged approximation to the conser-vation equations of fluid dynamics. While the precise nature of the averagingtime is not specified, it is clearly long enough to require the introduction of largeeddy transport coefficients to describe the unresolved fluxes of mass, momentumand energy. This is the root cause of the smoothed appearance of the resultsof even the most highly resolved fire simulations. The smallest resolvable lengthscales are determined by the product of the local velocity and the averaging timerather than the spatial resolution of the underlying computational grid.Unfortunately, the evolution of large eddy structures characteristic of most fireplumes is lost with such an approach, as is the prediction of local transient events.It is sometimes argued that the averaging process used to define the equations isan ensemble average over many replicates of the same experiment or postulatedscenario. However, this is a moot point in fire research since neither experimentsnor real scenarios are replicated in the sense required by that interpretation ofthe equations.The application of Large Eddy Simulation (LES) techniques to fire is aimed at LESextracting greater temporal and spatial fidelity from simulations of fire performedon the more finely meshed grids allowed by ever faster computers.The phrase LES refers to the description of turbulent mixing of the gaseousfuel and combustion products with the local atmosphere surrounding the fire.This process, which determines the burning rate in most fires and controls thespread of smoke and hot gases, is extremely difficult to predict accurately. Thisis true not only in fire research but in almost all phenomena involving turbulentfluid motion. The basic idea behind the LES technique is that the eddies thataccount for most of the mixing are large enough to be calculated with reasonable


accuracy from the equations of fluid dynamics. The hope (which must ultimatelybe justified by comparison to experiments) is that small-scale eddy motion caneither be crudely accounted for or ignored.The fourth approach is Direct Numerical Simulation (DNS). DNS is a simula-DNStion in computational fluid dynamics in which the Navier-Stokes equations arenumerically solved without any turbulence model. This means that the wholerange of spatial and temporal scales of the turbulence must be resolved in thecomputational mesh.The computational cost of DNS is very high, even at low Reynolds numbers.For the Reynolds numbers encountered in most industrial applications, the com-putational resources required by a DNS would exceed the capacity of the mostpowerful computers currently available.

1.3 Simplification

The equations describing the transport of mass, momentum, and energy by thefire-induced flows must be simplified so that they can be efficiently solved for thefire scenarios of interest. The general equations of fluid dynamics describe a richvariety of physical processes, many of which have nothing to do with fires.Retaining this generality would lead to an enormously complex computationaltask that would shed very little additional insight on fire dynamics. The simplifiedequations, developed by Rehm and Baum [Rehm 1978], have been widely adoptedby the larger combustion research community, where they are referred to as thelow Mach number combustion equations. They describe the low speed motionof a gas driven by chemical heat release and buoyancy forces.The low Mach number equations are solved numerically by dividing the physicalspace where the fire is to be simulated into a large number of rectangular cells.Within each cell the gas velocity, temperature, etc., are assumed to be uniform;changing only with time. The accuracy with which the fire dynamics can besimulated depends on the number of cells that can be incorporated into thesimulation. This number is ultimately limited by the computing power available.Present day, single processor desktop computers limit the number of such cellsto at most a few million. This means that the ratio of largest to smallest eddylength scales that can be resolved by the computation (the dynamic range of thesimulation) is on the order of 100. Parallel processing can be used to extend thisrange to some extent, but the range of length scales that need to be accountedfor if all relevant fire processes are to be simulated is roughly 104 to 105 because

1.3. SIMPLIFICATION 5

combustion processes take place at length scales of 1mm or less, while the lengthscales associated with building fires are of the order of tens of meters.

Chapter 2

Inside FDS5

This chapter gives a concise insight on how Fire Dynamics Simula-tor works and its limitations. Possible engineering applications arepresented.

2.1 What are FDS5 ans Smokeview?

Fire Dynamics Simulator, version 5 (FDS5) is a computational fluid dynamics FDS5(CFD) model of fire-driven fluid flow. The model solves numerically a form ofthe Navier-Stokes equations appropriate for low-speed, thermally-driven flow withan emphasis on smoke and heat transport from fires. The partial derivatives ofthe conservation equations of mass, momentum and energy are approximatedas finite differences, and the solution is updated in time on a three-dimensional,rectilinear grid. Thermal radiation is computed using a finite volume techniqueon the same grid as the flow solver. Lagrangian particles are used to simulatesmoke movement, sprinkler discharge, and fuel sprays.Smokeview is a companion program to FDS5 that produces images and anima- Smokeviewtions of the results. Smokeview is able to visualize fire and smoke in a fairlyrealistic way. Via its three-dimensional realistic renderings, Smokeview is an in-tegral part of the physical model, as it allows one to assess the visibility within afire compartment in ways that ordinary scientific visualization software cannot.Although not part of the FDS5/Smokeview suite maintained at NIST, there areseveral third-party and proprietary add-ons to FDS5 either available commerciallyor privately maintained by individual users.The first version of FDS5 was publicly released in February 2000.

7

8 CHAPTER 2. INSIDE FDS5

2.2 Engineering applications

Throughout its development, FDS5 has been aimed at solving practical fire prob-lems in fire protection engineering, while at the same time providing a tool tostudy fundamental fire dynamics and combustion.It is generally recognized that FDS5 can be effectively used in engineering appli-Basic usecations to model the following phenomenas:

• Low speed transport of heat and combustion products from fire;

• Radiative and convective heat transfer between the gas and solid surfaces;

• Sprinkler, heat detector, and smoke detector activation.

FDS5 can be used to model the following problems, too:Advanced use

• Pyrolysis;

• Flame spread and fire growth;

• Sprinkler sprays and suppression by water.

Currently, the users interested in engineering applications should probably avoidusing FDS5 to model the latter problems, as they are still subject to intenseresearch study in academic environments.To date, about half of the applications of the model have been for design ofsmoke control systems and sprinkler/detector activation studies. The other halfconsist of residential and industrial fire reconstructions.

2.3 Who develops FDS5?

Currently, FDS5 is maintained by the Building and Fire Research Laboratory(BFRL) of National Institute of Standards and Technology. The developers atNIST have formed a loose collaboration of interested stakeholders, including:

• VTT Technical Research Centre of Finland, a research and testing labora-tory similar to NIST;

• The Society of Fire Protection Engineers (SFPE);

• Fire protection engineering firms that use the software;

• Engineering departments at various universities with a particular emphasison fire.

2.4. WHO USES FDS5? 9

2.4 Who uses FDS5?

The use of fire models currently extends beyond the fire research laboratoriesinto the engineering, fire service and legal communities.FDS5 is intended for use only by those competent in the fields of fluid dynamics,thermodynamics, heat transfer, combustion, and fire science, and is intendedonly to supplement the informed judgment of the qualified user. The softwarepackage is a computer model that may or may not have predictive capability whenapplied to a specific set of factual circumstances. Lack of accurate predictionsby the model could lead to erroneous conclusions with regard to fire safety.Sufficient evaluation of any model is necessary to ensure that users can judge theadequacy of its technical basis, appropriateness of its use, and confidence levelof its predictions.

2.5 How much does it cost?

FDS5 is free and its source code is in the public domain.

2.6 How does FDS5 work?

A brief description of the major features of FDS5 follows.

2.6.1 Hydrodynamic model

FDS5 solves numerically a form of the Navier-Stokes equations appropriate forlow- speed, thermally-driven flow with an emphasis on smoke and heat trans-port from fires. The core algorithm is an explicit predictor-corrector scheme,second order accurate in space and time. Turbulence is treated by means ofthe Smagorinsky form of Large Eddy Simulation (LES). It is possible to performa Direct Numerical Simulation (DNS) if the underlying numerical mesh is fineenough. LES is the default mode of operation.

2.6.2 Combustion model

For most applications, FDS5 uses a single step chemical reaction whose productsare tracked via a two-parameter mixture fraction model. The mixture fraction


is a conserved scalar quantity that represents the mass fraction of one or morecomponents of the gas at a given point in the flow field. By default, two com-ponents of the mixture fraction are explicitly computed. The first is the massfraction of unburned fuel and the second is the mass fraction of burned fuel, asfor example the mass of the combustion products that originated as fuel.A two-step chemical reaction with a three parameter mixture fraction decomposi-tion can also be used with the first step being oxidation of fuel to carbon monoxideand the second step the oxidation of carbon monoxide to carbon dioxide. Thethree mixture fraction components for the two step reaction are unburned fuel,mass of fuel that has completed the first reaction step, and the mass of fuel thathas completed the second reaction step. The mass fractions of all of the majorreactants and products can be derived from the mixture fraction parameters bymeans of state relations.Lastly, a multiple-step finite rate model is also available.

2.6.3 Radiation transport

Radiative heat transfer is included in the model via the solution of the radiationtransport equation for a gray gas, and in some limited cases using a wide bandmodel.The equation is solved using a technique similar to finite volume methods forconvective transport, thus the name given to it is the Finite Volume Method(FVM). Using approximately 100 discrete angles, the finite volume solver requiresabout 20% of the total CPU time of a calculation, a modest cost given thecomplexity of radiation heat transfer.The absorption coefficients of the gas-soot mixtures are computed using [Grosshandler 1993]narrow-band model. Liquid droplets can absorb and scatter thermal radiation.This is important in cases involving mist sprinklers, but also plays a role in allsprinkler cases.

2.6.4 Geometry and multiple meshes

FDS5 approximates the governing equations on a rectilinear mesh. Rectangularobstructions are forced to conform with the underlying mesh.It is possible to prescribe more than one rectangular mesh to handle cases wherethe computational domain is not easily embedded within a single mesh.

2.7. LIMITATIONS OF FDS5 11

2.6.5 Parallel processing

It is possible to run an FDS5 calculation on more than one computer using theMessage Passing Interface (MPI).

2.6.6 Boundary conditions

All solid surfaces are assigned thermal boundary conditions, plus informationabout the burning behavior of the material. Heat and mass transfer to andfrom solid surfaces is usually handled with empirical correlations, although it ispossible to compute directly the heat and mass transfer when performing a DirectNumerical Simulation (DNS).

2.7 Limitations of FDS5

Although FDS5 can address most fire scenarios, there are limitations in all of itsvarious algorithms. Some of the more prominent limitations of the model arelisted here.

2.7.1 Low speed flow assumption

The use of FDS5 is limited to low-speed1 flow with an emphasis on smoke andheat transport from fires. This assumption rules out using the model for anyscenario involving flow speeds approaching the speed of sound, such as explosions,choke flow at nozzles, and detonations.

2.7.2 Rectilinear geometry

The efficiency of FDS5 is due to the simplicity of its rectilinear numerical gridand the use of a fast, direct solver for the pressure field. This can be a limita-tion in some situations where certain geometric features do not conform to therectangular grid, although most building components do. There are techniquesin FDS5 to lessen the effect of “sawtooth” obstructions used to represent nonrectangular objects, but these cannot be expected to produce good results if,for example, the intent of the calculation is to study boundary layer effects. For

1Mach numbers less than about 0.3


most practical large-scale simulations, the increased grid resolution afforded bythe fast pressure solver offsets the approximation of a curved boundary by smallrectangular grid cells.

2.7.3 Fire growth and spread

Because the model was originally designed to analyze industrial-scale fires, it canbe used reliably when the Heat Release Rate (HRR) of the fire is specified and thetransport of heat and exhaust products is the principal aim of the simulation. Inthese cases, the model predicts flow velocities and temperatures to an accuracywithin 10% to 20% of experimental measurements, depending on the resolutionof the numerical grid2.However, for fire scenarios where the heat release rate is predicted rather thanspecified, the uncertainty of the model is higher. There are several reasons forthis:

1. Properties of real materials and real fuels are often unknown or difficult toobtain;

2. The physical processes of combustion, radiation and solid phase heat trans-fer are more complicated than their mathematical representations in FDS5;

3. The results of calculations are sensitive to both the numerical and physicalparameters. Current research is aimed at improving this situation, but itis safe to say that modeling fire growth and spread will always require ahigher level of user skill and judgment than that required for modeling thetransport of smoke and heat from specified fires.

2.7.4 Combustion

For most applications, FDS5 uses a mixture fraction-based combustion model.The mixture fraction is a conserved scalar quantity that is defined as the fractionof gas at a given point in the flow field that originated as fuel. In its simplestform, the model assumes that combustion is mixing-controlled, and that thereaction of fuel and oxygen is infinitely fast, regardless of the temperature.

2It is extremely rare to find measurements of local velocities and temperatures from fireexperiments that have reported error estimates that are less than 10%. Thus, the most accuratecalculations using FDS5 do not introduce significantly greater errors in these quantities thanthe vast majority of fire experiments.

2.7. LIMITATIONS OF FDS5 13

For large-scale, well-ventilated fires, this is a good assumption. However, if a fireis in an under-ventilated compartment, or if a suppression agent like water mistor CO2 is introduced, fuel and oxygen are allowed to mix and not burn, accordingto a few empirically-based criteria.The physical mechanisms underlying these phenomena are complex, and are tiedclosely to the flame temperature and local strain rate, neither of which are readily-available in a large scale fire simulation.Subgrid-scale modeling of gas phase suppression and extinction is still an area ofactive research in the combustion community.Until reliable models can be developed for building-scale fire simulations, simpleempirical rules are used by FDS5 that prevent burning from taking place whenthe atmosphere immediately surrounding the fire cannot sustain the combustion.

2.7.5 Radiation

Radiative heat transfer is included in the model via the solution of the radiationtransport equation (RTE) for a gray gas, and in some limited cases using awide band model. The RTE is solved using a technique similar to finite volumemethods for convective transport, thus the name given to it is the Finite VolumeMethod (FVM).There are several limitations of the model:

• First, the absorption coefficient for the smoke-laden gas is a complex func-tion of its composition and temperature. Because of the simplified com-bustion model, the chemical composition of the smokey gases, especiallythe soot content, can effect both the absorption and emission of thermalradiation.

• Second, the radiation transport is discretized via approximately 100 solidangles, although the user may choose to use more angles. For targets faraway from a localized source of radiation, like a growing fire, the discretiza-tion can lead to a non-uniform distribution of the radiant energy. This erroris called “ray effect” and can be seen in the visualization of surface tem-peratures, where “hot spots” show the effect of the finite number of solidangles. The problem can be lessened by the inclusion of more solid angles,but at a price of longer computing times. In most cases, the radiative fluxto far-field targets is not as important as those in the near-field, wherecoverage by the default number of angles is much better.

Chapter 3

Running FDS5

This chapter explains how to obtain, install and run FDS5 andSmokeview. The explanation covers Ubuntu Linux and WindowsXP.

3.1 Online resources and user support

The primary resource for detailed instructions on how to download executables,manuals, source code and related utilities, is the FDS5-SMV website:

• http://fire.nist.gov/fds

FDS5 has two separate manuals:

• [FDS5 technical reference];

• [FDS5 user’s guide].

The [FDS5 technical reference] guide is broken into three volumes:

• Mathematical model;

• Verification;

• Experimental validation.

Smokeview has its own manual:

15

http://fire.nist.gov/fds

16 CHAPTER 3. RUNNING FDS5

• [Smokeview user’s guide].

The FDS5 and Smokeview user’s guides only describe the mechanics of usingthe computer programs. The technical reference guides provides the theory,algorithm details, and verification and validation work.Along with the FDS5 manuals, there are resources available on the Internet.These include an issue tracker, that allows you to report bugs, feature requestsand ask specific clarifying questions, and group discussions, which support moregeneral topics than just specific problems.Before using these on-line resources, it is important to first try to solve your ownproblems by performing simple test calculations, or debugging your input file.

3.2 Version numbers

Each release of FDS5 and Smokeview is identified by a version number suchas FDS_5.0.1 or SMV_5.0.1, where the first number is the major release (5),the second number (0) is the minor release, and the third (1) indicates themaintenance release number.As a general pattern, major releases will occur every year or so. As the nameimplies, they dramatically change the functionality or capabilities of the model.Minor releases occur every few months, and may cause minor changes in func-tionality. Maintenance releases are more frequent and are just bug fixes or smallrefinements, and should not affect code functionality. The release notes can helpyou decide whether the changes should effect the type of applications that youtypically do.

3.3 Hardware requirements

FDS requires one or more fast CPUs and a substantial amount of random-accessmemory (RAM) to run efficiently. For minimum specifications, the system shouldhave at least a 1 GHz CPU, and 1 GB RAM. The CPU speed will determine howlong the computation will take to finish, while the amount of RAM will determinehow many mesh cells can be held in memory.1 GB RAM can hold around 106 cells. To understand the physical meaning, a20m x 10m x 5m computational domain contains 106 cells, when discretizedwith cubic cells of 10 cm side.

3.4. SERIAL AND PARALLEL CALCULATIONS 17

Figure 3.1: A cluster of Linux computers

A large hard drive is required to store the output of the calculations. It is notunusual for the output of a single calculation to consume many gigabytes ofstorage space.Smokeview needs an OpenGL graphic card. Look for graphics cards that specif-ically list OpenGL support for the operating system you intend to use.

3.4 Serial and parallel calculations

FDS can perform serial and parallel calculations:

Serial calculations are performed in a single process that uses one only core ofcurrent multi-core CPUs.

Parallel calculation splits the computational burden on many processes thatcan be assigned to many different cores or CPUs. These CPUs can resideon a single workstation or in a cluster of networked computers.

Setting up a cluster of computers is a complex task and it is out of the scope ofthis manual.


3.5 Installing on Windows XP

To install or reinstall FDS5 and Smokeview on a Windows XP system, downloadWindows installation package from official website.The Windows file is a self-extracting compressed archive that will install FDS5,Smokeview and all associated files in the Program Files/FDS folder.Launch the installation by double-clicking on the downloaded file.At the end of the install process, your Windows XP system is ready to performserial calculations.

3.6 Installing on Ubuntu Linux

These instructions require a basic knowledge of an Ubuntu Linux computer.Ubuntu Linux operating system comes in two basic flavors:

Ubuntu Linux 32 bit, that can be installed on any type of computer.

Ubuntu Linux 64 bit, that can be installed on all AMD 64 bit CPUs withAMD64 extension and all Intel CPUs with EM64T extension.

The 64 bit version has the ability to address more RAM memory than 32 bitversion (over 4 GB). The 32 bit version is limited to 4 GB of RAM memory.Be aware that Smokeview works much better on a good dedicated graphic card.Some cheap graphic cards can prevent you from using it on Linux.

3.6.1 First install

To install FDS5 and Smokeview on an Ubuntu Linux system, first, download thelatest version of the precompiled FDS5 and Smokeview for Linux from downloadpage on the official web site. Depending on your Ubuntu flavor, download the32 bit or the 64 bit compressed archive of the FDS distribution.After downloading to your computer, extract the archive by right clicking on itsicon and selecting Extract here in the context menu.Then, move the extracted FDS folder to your preferred location, for exampleyour home directory. In my case the resulting path to FDS folder would be/home/egissi/FDS, as /home/egissi is my home folder.

3.6. INSTALLING ON UBUNTU LINUX 19

After that, look at hidden files of your home folder by selecting View . Showhidden files menu in the Nautilus file browser. Locate the .bashrc file inyour home directory, and open it for editing by double-clicking its icon.Append the following text to the .bashrc file in the editor window:

### FDS5 and Smokeview environment# Actual path to FDS folderFDS=/home/egissi ← set your actual path hereecho "FDS5 setup ($FDS)"# Setting limitsulimit -s unlimitedulimit -v unlimited# Setting executable and library pathsexport PATH=$PATH:$FDS/FDS/FDS5/binexport LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$FDS/FDS/FDS5/bin/lib32export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$FDS/FDS/FDS5/bin/lib64

Edit the emphasized line and set your actual path to the FDS folder. After that,save the .bashrc file and close the text editor.Open Synaptic package manager by selecting System . Administration . Synapticpackage manager menu and install:

On Ubuntu Linux 32 bit: lam-runtime software package;

On Ubuntu Linux 64 bit: lam-runtime and ia32-libs software packages.

Ubuntu Linux takes care for you of all the needed software dependencies.Close your session and log-in again to put into effect the environment modifi-cations. Your Ubuntu Linux system is now ready to perform serial and parallelsimulations on a multi-core or multi-CPU workstation.

3.6.2 Installing a new version

When installing a new version of FDS5 and Smokeview, just delete the old FDSfolder. After downloading the new version, extract the new FDS folder and dragit to the same position as before.Your Ubuntu Linux system is now ready to perform serial and parallel simulationswith the new version.


3.7 Compiling an optimized binary

As FDS5 is open source software, users can always download the latest versionof FDS5 source code and compile it by themselves using a Fortran 90 and Ccompiler. Direct compilation is often applied to obtain an FDS5 binary that isoptimized for the specific hardware and platform and to fully take advantage ofits speed.Compilation is a complex task and is out of the scope of the present manual.

3.8 Performing a calculation

The typical procedure for using FDS5 and Smokeview is to:

1. Set up an FDS5 input file, as mycase.fds, and put it in a folder, asmycase folder. See following chapters to learn how to do it.

2. Run FDS5 on the input files. FDS5 starts and creates many output filesin the mycase folder.

3. While FDS5 is running, monitor the development of the calculation.

4. Analyze the generated output files with Smokeview.

3.8.1 Running serial FDS5 on Windows XP

After having set up an input file, open up a command prompt window: selectStart . Run menu, then type cmd. Move into the mycase folder, where the inputfile for the case is located, with the cd command.Then run the code by typing the following command:

fds5 mycase.fds

where fds5 is the name of FDS5 binary and mycase.fds is the input file name.A serial calculation starts and its progress is indicated by diagnostic output thatis written out onto the screen.

3.8. PERFORMING A CALCULATION 21

Figure 3.2: Starting a serial calculation on Windows XP and on Linux Ubuntu

3.8.2 Running serial FDS5 on Ubuntu Linux

On an Ubuntu Linux computer select Applications . Accessories . Terminalmenu to open the command prompt. Move into the mycase folder, where theinput file for the case is located, with the cd command.Then execute one of the following commands:

On Ubuntu Linux 32 bit: fds5_intel_linux_32 mycase.fds

On Ubuntu Linux 64 bit: fds5_intel_linux_64 mycase.fds

If unsure, just look inside FDS/FDS5/bin folder to discover the right name forFDS5 binaries.Then the simulation starts and its progress is indicated by diagnostic output thatis written out onto the screen.

3.8.3 Running parallel FDS5 on Ubuntu Linux

FDS5 uses the Message Passing Library (MPI) for parallel computing.MPI is a language-independent communications protocol used to program paral-lel computers. Both point-to-point and collective communication are supported.MPI’s goals are high performance, scalability, and portability. MPI is the domi-nant model used in high-performance computing today.The input file for both single and parallel versions of FDS5 are the same. In fact,it is recommended that before embarking on parallel processing, you should runyour input file in serial mode to ensure that it is properly set up.


Figure 3.3: Running a parallel calculation and monitoring the system on UbuntuLinux

To run FDS5 in parallel, you must break up the computational domain into mul-tiple meshes so that the workload can be divided among the available processors.For the parallel version to work well, there has to be a comparable number ofcells in each mesh, or otherwise most of the computers will sit idle waiting forthe one with the largest mesh to finish processing each time step.On an Ubuntu Linux computer as configured before, type the following commandsto perform a parallel calculation:

On Ubuntu Linux 32 bit:

lamboot -vmpirun -np 4 fds5_mpi_intel_linux_32 mycase.fdslamhalt -v

On Ubuntu Linux 64 bit:

lamboot -vmpirun -np 4 fds5_mpi_intel_linux_64 mycase.fdslamhalt -v

The lamboot command starts the MPI environment.The mpirun command starts an MPI application, in our case fds5_mpi_intel_linux_32or fds5_mpi_intel_linux_64, where -np 4 is the number of started processes.The number of processes must match the number of meshes that span the com-putational domain of the input case.At the end of the calculation, the MPI server is safely stopped with lamhaltcommand.

3.9. MONITORING PROGRESS 23

3.9 Monitoring progress

Diagnostics for a given calculation are written into a text file called mycase.out,contained in the case folder mycase. The CPU usage and simulation time arewritten here, so you can monitor it to see how far along the program has pro-gressed.The application System . Administration . Gnome System Monitor is a pro-cess viewer that provides a dynamic real-time view of a running system.

3.10 Stop a calculation

To stop a calculation before its scheduled time, create an empty file in themycase folder called mycase.stop. The mere existence of this file stops theprogram gracefully, causing it to dump out the latest flow variables for viewingin Smokeview.Since calculations can be hours or days long, FDS5 has a restart feature. See[FDS5 user’s guide] for broader details.

3.11 Visualizing results

Smokeview is used before, during and after model runs:

• before, to check the input data;

• during a calculation, to monitor a simulation’s progress;

• in a post-processing step, to visualize FDS5 data after a calculation hasbeen completed.

On Windows XP, Smokeview may be started by double-clicking on the file namedmycase.smv, contained in the case folder.On Ubuntu Linux, Smokeview is run from the command prompt by typing:

On Ubuntu Linux 32 bit: smv5_linux_32 mycase.smv

On Ubuntu Linux 64 bit: smv5_linux_64 mycase.smv


Figure 3.4: Running Smokeview on Windows XP and on Ubuntu Linux

If unsure, just look inside FDS/FDS5/bin folder to discover the right name forSmokeview binary.Inside Smokeview, menus are accessed by clicking on the graphical window withthe right mouse button. The Load/Unload menu may be used to read in thedata files to be visualized. The Show/Hide menu may be used to change howthe visualizations are presented.For the most part, the menu choices are self explanatory. In case of need, helpon using Smokeview can be found on [Smokeview user’s guide].

3.12 Output files

FDS5 writes out many output files in the mycase folder:

Diagnostic output: the file mycase.out consists of a list of input parameters,and an accounting of various important quantities, including CPU usage.

Heat release rate and related quantities: the HRR of the fire, plus other globalenergy-related quantities, are automatically written into a text file calledmycase_hrr.csv

Device output data: data associated with particular devices (link tempera-tures, smoke obscuration, thermocouples, etc.) is output in comma de-limited format in a file called mycase_devc.csv

and many other types of files, used by Smokeview for visualisation.

The comma delimited format files can easily be imported into Openoffice.orgCalc or Microsoft Excel for further analysis.

Chapter 4

Input file basics

This chapter teaches the logic of FDS5 input file, its organizationand its grammar. Then the standard reference system is explainedand some tips and tricks are exposed.

4.1 Syntax of the input file

All the necessary information to perform an FDS5 simulation has to be containedin a single text file. The input file is saved with a name such as mycase.fds.There should be no blank spaces in the job name.Data is specified within the input file by using namelist groups. Each namelist Namelist groupsgroup record occupies a line of text and begins with the & character followedimmediately by the name of the namelist group. Then a comma-delimited listof the input parameters is inserted, and finally a forward slash / character closesthe namelist group, as shown in Figure 4.1.

Figure 4.1: The structure of an FDS5 namelist group

Spaces and new lines can be freely inserted to visually format the namelist group.Comments and notes should be written outside the & / delimiters. For example: Comments

25

26 CHAPTER 4. INPUT FILE BASICS

&OBST XB=0.5,1.1,0.5,1.1,0.0,0.1 / A commentAnother comment

It is recommended that each namelist group be clearly commented to justify thechoice of its parameters and to link it to literature references or direct experi-mentation. For example, comments like these:

&REAC ID=’polyurethane’, SOOT_YIELD=0.1875, CO_YIELD=0.02775,C=1.0, H=1.75, O=0.25, N=0.065,HEAT_OF_COMBUSTION=25300., IDEAL=.TRUE. /Gas phase reaction: polyurethane flexible foam (means)from Tewarson SFPE Handbook 3rd ed,SFPE handbook table 3-4.14, p. 3-112.

help the reviewer keeping track of the sources of information employed by theuser.By deeply commenting the code, the input file becomes the complete and onlysource of information about the simulated case.The parameter values can be of the following types:Parameter values

Integers, as in T_END=5400

Real numbers, as in CO_YIELD=0.008

Groups of real numbers, as in XYZ=6.04,0.28,3.65

Groups of integers, as in IJK=90,36,38

Character strings, as in CHID=’this_is_a_string’

Groups of character strings, as in SURF_IDS=’burner’,’steel’

Logical parameters, as in POROUS_FLOOR=.FALSE. or POROUS_FLOOR=.TRUE.The periods must be included.

Sometimes the parameters are multidimensional arrays. For example:Parameter arrays

MATL_ID(2,3)=’brick’

4.2. WRITING AN INPUT FILE 27

indicates that the third material component of the second layer is brick.To speed up data input, you can use this notation:

MATL_ID(1:3,1)=’plastic’,’insulation’,’steel’

which means that the surface is composed by three different layers made re-spectively of plastic, insulation and steel. The notation 1:3 means arrayelement 1 through 3, inclusive.A simplified notation is accepted, too:

MATL_ID=’plastic’,’steel’

is equivalent to:

MATL_ID(1:2,1)=’plastic’,’steel’

These last surfaces are composed by two different layers made respectively ofplastic and steel.The code is case sensitive: my_burner is not the same as MY_BURNER. Case sensitivity

To ensure that FDS5 reads the entire input file, add &TAIL / or a comment as TAILthe last line at the end of the input file.

4.2 Writing an input file

When looking at a new scenario, first select a pre-written input file that resem-bles the case, make the necessary changes, then run the case at fairly low gridresolution to determine if the geometry is set up correctly.The following file is a slightly modified and simplified version of pplume5.fds,generally included in FDS5 software distribution:

### General configuration

&HEAD CHID=’pplume5’, TITLE=’Plume case’ /name of the case and a brief explanation

&TIME T_END=10.0 /the simulation will end at 10 seconds


&MISC SURF_DEFAULT=’wall’, TMPA=25. /all bounding surfaces havea ’wall’ boundary conditionunless otherwise specified,the ambient temperature is set to 25°C.

&REAC ID=’polyurethane’, SOOT_YIELD=0.10,N=1.0, C=6.3, H=7.1, O=2.1 /predominant fuel gas for the mixture fraction modelof gas phase combustion

### Computational domain

&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,0.0,0.8 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,0.8,1.6 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,1.6,2.4 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,2.4,3.2 /

four connected calculation meshesand their cell numbers

### Properties

&MATL ID=’gypsum_plaster’, CONDUCTIVITY=0.48,SPECIFIC_HEAT=0.84, DENSITY=1440. /thermophysical properties of ’gypsum plaster’ material

&PART ID=’tracers’, MASSLESS=.TRUE., SAMPLING_FACTOR=1 /a type of Lagrangian particles

&SURF ID=’burner’, HRRPUA=600.,PART_ID=’tracers’, COLOR=’RASPBERRY’ /a type of boundary conditions named ’burner’

&SURF ID=’wall’, RGB=200,200,200, MATL_ID=’gypsum_plaster’,THICKNESS=0.012 /a type of boundary conditions named ’wall’

### Solid geometry

&VENT XB=0.5,1.1,0.5,1.1,0.1,0.1, SURF_ID=’burner’ /the ’burner’ boundary conditionis imposed to a plane face

&OBST XB=0.5,1.1,0.5,1.1,0.0,0.1, SURF_ID=’wall’ /a solid is created, ’wall’ boundary conditionis imposed to all its faces

4.3. THE LOGIC BEHIND MOST FDS5 INPUT FILES 29

&VENT XB=0.0,0.0,0.0,1.6,0.0,3.2, SURF_ID=’OPEN’/&VENT XB=1.6,1.6,0.0,1.6,0.0,3.2, SURF_ID=’OPEN’/&VENT XB=0.0,1.6,0.0,0.0,0.0,3.2, SURF_ID=’OPEN’/&VENT XB=0.0,1.6,1.6,1.6,0.0,3.2, SURF_ID=’OPEN’/&VENT XB=0.0,1.6,0.0,1.6,3.2,3.2, SURF_ID=’OPEN’/

the ’OPEN’ boundary condition is imposed tothe exterior boundaries of the computational domain

### Output

&DEVC XYZ=1.2,1.2,2.9, QUANTITY=’THERMOCOUPLE’, ID=’tc1’ /send to output: the data collected by a thermocouple

&ISOF QUANTITY=’TEMPERATURE’, VALUE(1)=100.0 /3D contours of temperature at 100°C

&SLCF PBX=0.8, QUANTITY=’TEMPERATURE’, VECTOR=.TRUE. /vector slices colored by temperature

&BNDF QUANTITY=’WALL TEMPERATURE’ /surface ’WALL_TEMPERATURE’ at all solid obstructions

&TAIL / end of file

Not all kinds of FDS5 namelist groups are listed in this input files. In fact,another general rule of thumb when writing input files is to only add to the fileparameters that are to change from their default value. That way, you can moreeasily distinguish between what you impose and FDS5 defaults.In general, the namelist records can be entered in any order in the input file, butit is a good idea to organize them in some systematic way.Be aware that the order of identical namelist groups can be significant. When Order conventionproperties overlap the general rule is first-come, first-served.For the sake of clarity, users often group similar namelists in homogeneous sec-tions identified by heading comments, as shown in the former example inputfile.

4.3 The logic behind most FDS5 input files

This section presents the logic behind most FDS5 input files; this same logic isused for the organization of this manual:

• First, general configuration is performed.


– The case receives a name via the HEAD namelist group, the simula-tion time is set via the TIME namelist group. Other miscellaneousparameters are prescribed via the MISC namelist group.

– Then gas phase combustion reaction is set up via the REAC namelistgroup, the radiation model is configured with RADI.

• Second, the computational domain is defined via the MESH namelist group.All FDS5 calculations must be performed within a domain that is made upof rectilinear volumes called meshes. Each mesh is divided into rectangularcells, the number of which depends on the desired resolution of the flowdynamics.Some initial conditions are prescribed for the flow domain via the INITnamelist group.

• Third, some properties are set up:

– the properties of each material (MATL),– the properties of extra gas species (SPEC),– the properties of Lagrangian particles (PART),– and the types of boundary conditions (SURF).

This is the most challenging part of setting up the simulation: first, for bothreal and simulated fires, the growth of the fire is very sensitive to the thermalproperties of the surrounding materials. Second, even if all the material proper-ties are known to some degree, the physical phenomena of interest may not besimulated properly due to limitations in the model algorithms or resolution of thenumerical mesh.It is your responsibility to supply the thermal properties of the materials, and thenassess the performance of the model to ensure that the phenomena of interestare being captured.

• Fourth, the solid geometry is entered via OBST, VENT, HOLE namelistgroups.

– A considerable amount of work in setting up a calculation lies inspecifying the geometry of the space to be modeled and applyingboundary conditions to these objects. The geometry is described interms of obstructions to the gas phase flow.

4.3. THE LOGIC BEHIND MOST FDS5 INPUT FILES 31

– A boundary condition needs to be assigned to each bounding surfaceof the gas phase domain describing its thermal properties. Both solidobstruction faces and the exterior boundaries of the computationaldomain need a boundary condition assigned. A fire is just one typeof boundary condition.

• Fifth, some control logic and automation is introduced via PROP, DEVC,CTRL namelist groups: devices can be used to control various actions, likecreating and removing obstructions, or activating and deactivating fansand vents.

• Finally, the user prescribes the output quantities (DEVC, SLCF, BNDF, ISOF).All output quantities must be specified at the start of the calculation. Inmost cases, there is no way to retrieve information after the calculationends if it was not specified from the start. Much like in an actual experi-ment, the user must decide before the calculation begins what informationto save.

The table summarizes this logic and shows a proposed systematic organizationof an input file in sections:

Table 4.1: Systematic organisation of the input file

Section Content Namelist groups

General configuration General information required toperform a simulation, as its name,duration and other miscellaneousparameters.

HEAD, TIME, MISC

Main gas phase combustion reactionand radiation model.

REAC, RADI

Computational domain Computational domain:dimensions and grid.

MESH

Initial conditions of thecomputational domain.

INIT

Properties Materials, temperature dependentthermophysical properties.

MATL, RAMP(temperature)

Extra gas species properties. SPEC

continued on next page


from previous pageLagrangian particles properties. PART

Boundary conditions, time dependentboundary conditions.

SURF, RAMP (time)

Solid geometry Description of solid geometry,assignment of boundary conditionsto bounding surfaces.

OBST, HOLE, VENT

Control logic General properties of devices, devicesand control functions used to controlvarious actions, like creating andremoving solid obstructions oractivating and deactivating boundaryconditions.

PROP, DEVC, CTRL

Output List of calculated quantities tooutput.

DUMP, DEVC, SLCF,BNDF, ISOF

4.4 Keep it simple

Novice users tend to forget that FDS5 is not a Computer Aided Design (CAD)tool, but a CFD code.First, not all geometrical details, all physical and chemical properties of all in-volved objects need to be entered in the input file.Looking at the example proposed in Figure 4.2 on the next page, chair and tableframe effect on the fluid flow can be considered negligible. On the contrary, theinfluences to fluid flow of the separating wall, the table top and seats can becomeimportant, depending on the objective of the analysis.So, the first step of the analysis process is to formulate the problem by seekinganswers to the following questions:

• What is the objective of the analysis?

• What is the easiest way to obtain that objective?

• What input data needs to be included?

Approximations of the geometry and simplifications of the properties are alwaysrequired to allow an analysis with reasonable effort.

4.5. EACH MODEL, ITS INPUT DATA 33

Figure 4.2: Modeling reality in FDS5

It is better to start off with a relatively simple file that captures the main featuresof the problem without getting tied down with too much detail that might maska fundamental flaw in the calculation.Initial calculations ought to be meshed coarsely so that the run times are lessthan an hour and corrections can easily be made without wasting too much time.As you learn how to write input files, you will continually run and re-run yourcase as you add in complexity.

4.5 Each model, its input data

When entering data into the input file, it is suggested to always consider howmodels inside FDS5 will use that data.For example:

• The hydrodynamic model needs to know which cells of the computationaldomain are open to fluid flow and which are instead occupied by solidobstructions. The geometry is discretized and the maximum resolution isthe grid cell size.


• The heat transfer model needs the characteristics and the thicknesses of thebounding surfaces of the flow domain to perform heat transfer calculation.

Imagine that the wall separating room 1 and room 2 of Figure 4.2 on the pre-ceding page is 0.19m thick. Thus, during the calculation:

• Taken the cell size equal to 0.30m, the hydrodynamic model considers thatwall as if it was 0.30m thick, because the geometry must conform to theunderlying grid. That information is used to obstacle the fluid flow.

• The heat transfer model performs a one-dimensional heat transfer calcula-tion of the wall using the real 0.19m thickness and the material properties.

It may sound strange to novice users, but that wall is. . . both 0.30m and 0.19mthick for FDS5.

4.6 Units of measurement

FDS5 employs the units of measurement from the International System (SI).

Lengths are expressed in m, time in s, mass in kg, temperature in °C, pressurein Pa, heat in kJ, power in kW, conductivity in W/m/K, heat flux in kW/m2,molecular weight in g/mol. . .

This manual contains a comprehensive list of frequent namelist parameters andtheir units. For a complete list consult the [FDS5 user’s guide].

4.7 Reference coordinate system

FDS5 coordinate system conforms to the right hand rule. By default, the z axisis considered the vertical.

For computational reasons, it is always preferable for the longest horizontal di-mension of the model be aligned with the x axis. This often shortens the calcu-lation time.

4.8. PRESCRIBING GEOMETRIC ENTITIES 35

Figure 4.3: The reference system, a volume, a face, a segment, a point, and aplane

4.8 Prescribing geometric entities

Many namelist groups extend their action to volumes, faces, segments, points orplanes. As shown in Figure 4.3, FDS5 geometrical entities are always describedusing some conventional rules.A volume is always represented by a single right parallelepiped with edges parallel Volumesto the axis. Its position and dimensions are described by the coordinates of twoopposite vertexes: if point A = (xA, yA, zA) and point B = (xB, yB, zB) are theopposite vertexes, its coordinates are entered as xA, xB, yA, yB, zA, zB. Forexample,

&OBST XB=0.5,1.5,2.0,3.5,-2.0,0., SURF_ID=’wall’ /

uses the parameter XB to define a solid obstacle that spans the volume startingat the origin (0.5, 2.0,−2.0) and extending 1m in the positive x direction, 1.5min the positive y direction, and 2m in the positive z direction.A face is represented by a right plane face with edges parallel to the axis. Its Facesposition and dimensions are described by the coordinates of two opposite vertexes,that must lie on the same plane. For example:

&VENT XB=0.5,1.1,2.0,3.1,-2.0,-2.0, SURF_ID=’fire’ /

uses the parameter XB to define a flat face perpendicular to the z axis imposinga particular boundary condition over a solid. Two of the six coordinates are thesame, denoting a flat face as opposed to a solid.A segment is bounded by two end points. If point A = (xA, yA, zA) and point SegmentsB = (xB, yB, zB) are the end points, its coordinates are entered following thesame convection valid for volumes. For example,


&DEVC XB=0.5,1.5,2.0,3.5,-2.0,0., QUANTITY=’PATH OBSCURATION’,ID=’beam1’, SETPOINT=0.33 /

is a beam smoke detector between (0.5, 2.0,−2.0) and (1.5, 3.5, 0.) end points.A point is simply identified by its 3 coordinates. For example, the line:Points

&DEVC XYZ=2.,3.,4., QUANTITY=’THERMOCOUPLE’, ID=’termo1’ /

uses the parameter XYZ to insert a thermocouple at the point of coordinates(2., 3., 4.).A plane is represented by a right plane perpendicular to one of the reference axis.PlanesFor example, these lines:

&SLCF PBX=0.5, QUANTITY=’TEMPERATURE’ /

is a plane perpendicular to the x axis and intersecting its point (.5, 0., 0.).

&SLCF PBY=1.5, QUANTITY=’TEMPERATURE’ /

is a plane perpendicular to the y axis and intersecting its point (0., 1.5, 0.).

&SLCF PBZ=-.5, QUANTITY=’TEMPERATURE’ /

is a plane perpendicular to the z axis and intersecting its point (0., 0.,−.5).All use the parameters PBX, PBY, PBZ to specify the coordinate in the directionof the perpendicular axis.

4.9 Prescribing orientations

Some FDS5 entities need the prescription of a particular orientation. This isdone with one of the following parameters: IOR or ORIENTATION.The parameter IOR, index of orientation, is used to prescribe one of the sixIORpossible orientations parallel to axis: if the orientation is in the positive x directionset IOR=1, negative x direction IOR=-1, positive y IOR=2, negative y IOR=-2,positive z IOR=3, negative z IOR=-3.For example, the line:

4.10. PRESCRIBING COLORS AND ASPECT 37

&DEVC XYZ=0.7,0.9,2.1, QUANTITY=’WALL TEMPERATURE’,IOR=-2, ID=’ST-1’ /

designates the surface temperature of a wall facing the negative y direction.The parameter ORIENTATION is used for entities that require a free directional ORIENTATIONspecification, like a sprinkler. ORIENTATION is specified with a triplet of realnumber values that indicate the components of the direction vector. The defaultvalue of ORIENTATION is (0, 0,−1).For example, the line:

&DEVC XYZ=23.91,21.28,0.50, PROP_ID=’nozzle’,ORIENTATION=1.,1.,0., ID=’noz_1’ /

designates a nozzle oriented towards the (1, 1, 0) vector.

4.10 Prescribing colors and aspect

Colors of objects can be prescribed with two parameters: RGB and COLOR.

The RGB parameter is followed by a triplet of integer numbers in the range from RGB0 to 255, indicating the amount of red, green and blue that make up the color.The COLOR parameter calls the name of a predefined color that must be entered COLORexactly as it is listed in the color table:

Table 4.2: COLOR values

ACQUAMARINE, BANANA, BEIGE, BLACK, BLUE, BRICK, BROWN, CADMIUMORANGE, CARROT, COBALT, CORAL, CRIMSON, CYAN, FIREBRICK, FLESH,GOLD, GRAY, GREEN, INDIGO, MAGENTA, MAROON, MELON, MINT, NAVY,OLIVE, ORANGE, ORCHID, PINK, PURPLE, RASPBERRY, RED, SALMON,SEPIA, SIENNA, SILVER, TAN, TEAL, TOMATO, TURQUOISE, VIOLET,WHITE, YELLOW, ...

You can rapidly find the whole color table of more than 500 colors by googlingfor FDS COLOR TABLE on the Internet.For example, both the parameter RGB=0,0,255 and the parameter COLOR=’BLUE’can be used to obtain a blue object.


Objects can be made semi-transparent by assigning a TRANSPARENCY parameter.TRANSPARENCYThe parameter value is a real ranging from 0 to 1, with 0 being fully transparent.The parameter should always be set along with RGB or COLOR.Using COLOR=’INVISIBLE’ causes the object not to be drawn in Smokeview.The parameter OUTLINE=.TRUE. causes the object to be drawn as an outline.

Chapter 5

General configuration

First, general configuration is performed.The case receives a name via the HEAD namelist group, the simula-tion time is set via the TIME namelist group. Other miscellaneousparameters are prescribed via the MISC namelist group.

5.1 Naming the job, HEAD

The first thing to do when setting up an input file is to give the job a name. Thename of the job is important because often a project involves numerous simula-tions in which case the names of the individual simulations can help organize theeffort. The namelist group HEAD contains two parameters, as in this example:

&HEAD CHID=’mycase’, TITLE=’This is a short description’ /

CHID is a string of 30 characters or less used to name the output files created byFDS5. No periods or spaces are allowed. TITLE is a string of 60 characters orless that describes the simulation. It is simply a descriptive text that is passedto various output files.It is always convenient to exactly use the same string for the name of the inputfile and the CHID. For example, if you name mycase.fds the input file, then setCHID=’mycase’ in the HEAD namelist group.Only one HEAD line can be entered in the input file. The following table summa-rizes some HEAD parameters:

39

40 CHAPTER 5. GENERAL CONFIGURATION

Table 5.1: HEAD parameters

Parameter Type Description Unit Default

CHID String Job identifier ’output’

TITLE String Short description of the job

5.2 Simulation time, TIME

TIME is the namelist group that define the time duration of the simulation.Usually, only the duration of the simulation is required on this line, via theparameter T_END. The default is 1 s.For example, the following line will instruct FDS5 to run the simulation for 5400 s:

&TIME T_END=5400. /

If T_END is set to zero, only the set-up work is performed, allowing you to quicklycheck the geometry in Smokeview.Only one TIME line can be entered in the input file. The following table summa-rizes some TIME parameters:

Table 5.2: TIME parameters


T_BEGIN Real Starting time for calculation s 0.

T_END Real Ending time for calculation s 1.

5.3 Miscellaneous, MISC

MISC is the namelist group of global miscellaneous input parameters. Manyparameters for MISC exist, some of them are explained later in this manual.For example:

&MISC SURF_DEFAULT=’steel’ /

5.3. MISCELLANEOUS, MISC 41

establishes that all bounding surfaces are to be made of steel unless otherwisespecified.Only one MISC line can be entered in the input file. The following table summa-rizes some MISC parameters:

Table 5.3: MISC parameters


SURF_DEFAULT String Default boundary condition SURF forsurfaces

’INERT’

TMPA Real Ambient temperature °C 20.

U0,V0,W0 Real Initial prevailing velocity field m/s 0.

GVEC(3) Real Gravity vector m/s2 0,0,-9.81

HUMIDITY Real Relative Humidity % 40.

CO_PRODUCTION Logical Start three-parameters mixturefraction model

.FALSE.

RESTART Logical Restart previous calculation .FALSE.

42 CHAPTER 5. GENERAL CONFIGURATION

Chapter 6

Combustion and radiation

. . . then gas phase combustion reaction is set up via the REAC namelistgroup, the radiation model is configured with RADI.

6.1 Combustion is not pyrolysis

A common source of confusion in FDS5 is the distinction between gas phasecombustion and solid phase pyrolysis.

Pyrolysis is the decomposition or transformation of a compound caused by heatthat produce the gaseous fuel. It is the first chemical reaction that occursin the burning of many solid fuels, like wood, cloth, paper, and plastic.

Gas phase combustion refers to the exothermic chemical reactions betweenthe gaseous fuel and oxygen accompanied by the production of heat andlight in the form of flames.

So solid phase pyrolysis refers to the generation of fuel vapor at a solid or liquidsurface, while the visible flames are not due to combustion of the solid fuel itself,but rather of the gases released by its pyrolysis.

6.2 Prescribing a fire

In FDS5, a fire is a particular boundary condition applied to a surface boundingthe flow field. There are two ways of designating a fire:

43

44 CHAPTER 6. COMBUSTION AND RADIATION

Figure 6.1: Combustion is not pyrolysis

• The first is to specify an heat release rate on a surface; this is the same asprescribing a well defined burner. How to do this is described in detail inSection 11.4.6 on page 82.

• The other is to specify thermophysical properties of fuel materials andto let them pyrolyze. In this case the burning rate of the fuel dependson the net heat feedback to the surface. This approach is explained inSection 8.3.1 on page 63 for solid fuels and in Section 8.3.2 on page 64for liquids.

Both burners and pyrolyzing materials inject the calculated quantities of gaseousfuels in the flow field. In a realistic fire scenario, there may be various types ofgaseous fuels originating from the various burning objects in the building andinjected into the flow field.

6.3 Modeling gas phase combustion, REAC

6.3.1 Ignition

Once injected into the flow field, the gaseous fuels mix with air and burn. Thereis no need to prescribe an ignition source: the combustion model assumes that

6.3. MODELING GAS PHASE COMBUSTION, REAC 45

Figure 6.2: Combustion and pyrolysis in a flaming match

fuel gas and oxygen burn on contact. We can imagine that every grid cell hostsa virtual spark plug, that initiates combustion when temperature and local ratioof fuel gas and oxygen are appropriate (See Figure 6.3 on page 50).

6.3.2 Burning

The burning process releases heat and smoke.Whereas there can be many types of combustibles in an FDS5 fire simulation,one only gaseous fuel can be simulated by FDS5. In general, you should setthe chemistry of the modeled burning gaseous fuel to coincide with the actualpredominant burning gaseous fuel.This model simplification is due to computational cost: it is expensive to solvetransport equations for multiple gaseous fuels.FDS5 adjusts automatically the burning rates of solids and liquids to accountfor the difference in the heats of combustion of the various combustibles. If thestoichiometry of the burning material differs from the global reaction, the heat ofcombustion of each burning material is used to ensure that an equivalent amountof fuel is injected into the flow domain from the burning object.FDS5 can describe the gas phase reaction in two ways.By default, a so-called mixture fraction model is used to account for the evolution Mixture fraction

modelof the fuel gas from its surface of origin through the combustion process.


The alternative is what is referred to as the finite-rate approach , where all of the Finite-rate approachindividual gas species involved in the combustion process are defined and trackedindividually. This is a costlier and more complicated approach than the mixturefraction model.This manual covers the mixture fraction model only, as it is simpler and commonlyemployed for engineering level problems.When the mixture fraction model is applied, a set of scalar variables, Zi, representthe state of the combustion process from pure fuel (∑Zi = 1) to pure air(∑Zi = 0).FDS5 provides two types of mixture fraction model:

Two-parameter mixture fraction model: the first parameter (Z1) is the massfraction of unburned fuel and the second (Z2) is the mass fraction of burnedfuel, as for example the mass of the combustion products that originatedas fuel. FDS5 uses the two-parameter model by default.

Three-parameter mixture fraction model: this combustion model simulatesa two-step chemical reaction with three parameters. The first step of thereaction is the oxidation of fuel to carbon monoxide and the second stepthe oxidation of carbon monoxide to carbon dioxide. The three mixturefraction components for the two step reaction are unburned fuel (Z1), massof fuel that has completed the first reaction step (Z2), and the mass offuel that has completed the second reaction step (Z3). See Section 6.4 onpage 49 to understand why and how to use the three-parameter model.

The mass fractions of all of the major reactants and products of combustion – asfuel, O2, CO2, H2O, N2, CO and soot – can be derived from the mixture fractionparameters by means of state relations: a set of pre-tabulated functions of themixture fraction parameters, Zi. In other words, the values of Zi in any givenmesh cell determines the mass fraction of all the gases listed.The stoichiometry of the predominant gas phase combustion reaction is pre-scribed in the input file by one only REAC namelist group: the specified parame-ters are used to generate the table associating the mass fractions with Zi. FDS5defaults to propane combustion if no REAC line is entered.In the mixture fraction model, each reaction is assumed to be of the form:

Cx Hy Oz Nv Otherw + νO2O2 →→ νCO2CO2 + νH2OH2O + νCOCO + νsootSoot + νN2N2 + νH2H2 + νotherOther

6.3. MODELING GAS PHASE COMBUSTION, REAC 47

You need only specify the chemical formula of the fuel along with the yields of CO,soot, and H2, and the amount of hydrogen in the soot, Hfrac. For completenessyou can specify the N2 content of the fuel and the presence of other species.FDS5 will use that information internally to determine the amount of combustionproducts that are formed.The species implicitly defined by FDS5 when doing a mixture fraction calculationfor gas phase combustion are as follows:

Table 6.1: Mixture fraction species

fuel, oxygen, nitrogen, water vapor, carbon dioxide, carbonmonoxide, hydrogen, soot, other

Note that these species are identified by a lowercase name, and are not to beconfused with the species identified by uppercase names defined by the SPECnamelist groups. See Section 9.2 on page 68 for further discussion.The Table 6.2 lists some of the parameters that may be prescribed on the REACline. Note that the various *YIELD are for well-ventilated, post-flame conditions.There are options to predict various species yields in under-ventilated fire scenar-ios, but these special models still require the post-flame yields for CO, soot andany other species listed in the table.

Table 6.2: REAC parameters


ID String Identifier

C Real Number of carbon atoms in the fuel 3

H Real Number of hydrogen atoms in thefuel

8

O Real Number of oxygen atoms in the fuel 0

N Real Number of nitrogen atoms in the fuel 0

OTHER Real Number of other atoms in the fuel 0

MW_OTHER Real Average molecular weight of OTHER,defaults to N2

g/mol 28

CO_YIELD Real The fraction of fuel mass convertedinto carbon monoxide.

kg/kg 0



from previous page

H2_YIELD Real The fraction of fuel mass convertedinto hydrogen.

kg/kg 0

SOOT_YIELD Real Fraction of soot from the fuel. Thefraction of fuel mass converted intosmoke particulate.

kg/kg 0.01

SOOT_H_FRACTION Real Atom fraction of hydrogen in soot 0.1

HEAT_OF_COMBUSTION Real The amount of energy released perunit mass of fuel consumed

kJ/kg

EPUMO2 Real Energy per unit mass oxygen. If theheat of combustion is not explicitlyspecified, it is calculated as:consumed O2×EPUMO2

kJ/kg 13100

IDEAL Logical Adjust for minor product yields .FALSE.

VISIBILITY_FACTOR Real Visibility parameter (see Section 14.9on page 120)

3

MASS_EXTINCTION_COEFFICIENT Real Visibility parameter (see Section 14.9on page 120)

m2/kg 8700

IDEAL is a logical value indicating whether or not the EPUMO2 or HEAT_OF_COMBUSTIONvalues represent values for complete combustion (.TRUE.) or for incomplete com-bustion (.FALSE.). If IDEAL=.TRUE., then FDS5 internally adjusts the resultingheat of combustion to account for products of incomplete combustion specifiedin CO_YIELD, H2_YIELD, and SOOT_YIELD.A few sample REAC lines are given here, the values are for demonstration only:

&REAC ID=’methane’, C=1., H=4. /&REAC ID=’ethylene’, C=2., H=4., SOOT_YIELD=0.05 /&REAC ID=’propane’, SOOT_YIELD=0.01, C=3., H=8.,

HEAT_OF_COMBUSTION=46460., IDEAL=.TRUE. /&REAC ID=’propane’, SOOT_YIELD=0.01, C=3., H=8.,

HEAT_OF_COMBUSTION=46124., IDEAL=.FALSE. /&REAC ID=’wood’, SOOT_YIELD=0.02, O=2.5, C=3.4, H=6.2,

HEAT_OF_COMBUSTION=17700 /Ritchie, et al., 5th IAFSS

&REAC ID=’polyurethane’, SOOT_YIELD=0.1875, CO_YIELD=0.02775,

6.4. CO PRODUCTION IN UNDER-VENTILATED FIRES 49

C=1.0, H=1.75, O=0.25, N=0.065, OTHER=0.002427, MW=27.,HEAT_OF_COMBUSTION=25300., IDEAL=.TRUE. /Polyurethane flexible foam (means) fromTewarson SFPE Handbook 3rd ed,SFPE handbook table 3-4.14, p. 3-112.

6.4 CO production in under-ventilated fires

An algorithm has been implemented that computes the gas phase combustion asa two step reaction and that predicts the formation and destruction of CO. Thisalgorithm is used when the parameter CO_PRODUCTION is set to .TRUE. on theMISC line:

&MISC CO_PRODUCTION=.TRUE. /

Even though the algorithm predicts CO formation and its eventual oxidation atelevated temperature, it cannot predict the post-flame yield of CO. For example,within a flashed over compartment, the algorithm predicts the elevated CO levels,but it cannot predict the CO concentration of the exhaust gases that exit theflaming region. Thus, even if using this model, you must specify the CO_YIELDthat is expected of a well-ventilated fire.Note that when active, this algorithm requires the use of three parameters forthe mixture fraction instead of the two parameters used when it is disabled andwill therefore increase run times and memory usage accordingly. If the simulationyou are performing will not result in an under-ventilated fire, then there will belittle if any benefit to enabling the CO production algorithm.

6.5 Flame extinction

Modeling suppression of a fire due to the introduction of a suppression agent likeCO2 or water mist, or due to the exhaustion of oxygen within a compartmentis challenging because the relevant physical mechanisms occur at length scalessmaller than a single mesh cell.Flames are extinguished due to lowered temperatures and dilution of the oxygensupply. A simple suppression algorithm has been implemented in FDS5 thatattempts to gauge whether or not a flame is viable at the fuel-oxygen interface.


Figure 6.3: Flame extinction criteria

The default values for the the limiting oxygen index and the critical flame temper-ature are 15% (volume fraction) and 1427°C, respectively as shown in Figure 6.3on the next page.

6.6 Radiation transport, RADI

For most FDS5 simulations, thermal radiation transport is computed by defaultand you need not set any parameters to make this happen. However, there aresituations where it is important to be aware of issues related to the radiativetransport solver.The most important issue involves the fraction of energy released from the fireas thermal radiation, commonly referred to as the radiative fraction. It is a func-tion of both the flame temperature (T4 dependence) and chemical composition,neither of which are reliably calculated in a large scale fire calculation. In fact,because of the size of the mesh cells, the flame sheet is not well-resolved.To compensate the underestimation of the fire radiation, the RADIATIVE_FRACTIONis not calculated and is set to 35% by default: every mesh cell cut by the flameradiates that fraction of the chemical energy being released into it. Some ofthat energy may be reabsorbed elsewhere, yielding a net radiative loss that is lessthan RADIATIVE_FRACTION, depending mainly on the size of the fire and thesoot loading.For example:

&RADI RADIATIVE_FRACTION=0.45 /

sets the fraction of energy released from the fire as thermal radiation to 45%.The following table summarizes some RADI parameters:

6.6. RADIATION TRANSPORT, RADI 51

Table 6.3: RADI parameters


NUMBER_RADIATION_ANGLES Integer Number of solid angles 104

RADIATIVE_FRACTION Real Radiative Loss Fraction 0.35

Chapter 7

Computational domain

Second, the computational domain is defined via the MESH namelistgroup.All FDS5 calculations must be performed within a domain that ismade up of rectilinear volumes called meshes. Each mesh is dividedinto rectangular cells, the number of which depends on the desiredresolution of the flow dynamics. Some initial conditions are pre-scribed for the flow domain via the INIT namelist group.

7.1 Defining a mesh, MESH

MESH is the namelist group that defines the volume of the computational domain.For example,

&MESH IJK=10,20,30, XB=0.0,1.0,0.0,2.0,0.0,3.0 /

defines a mesh that spans the volume starting at the origin (0., 0., 0.) andextending 1m in the positive x direction, 2m in the positive y direction, and 3min the positive z direction.The mesh is subdivided into uniform cells via the parameter IJK. In this example,the mesh is divided into 10 cm cubes: 10 cubes in x direction, 20 cubes in ydirection, and 30 cubes in z direction.Any obstructions or vents that extend beyond the boundary of the mesh are cutoff at the boundary. There is no penalty for defining objects outside of the mesh,and these objects will not appear in Smokeview either.

53

54 CHAPTER 7. COMPUTATIONAL DOMAIN

Note that it is best if the mesh cells resemble cubes, that is, the length, widthand height of the cells ought to be roughly the same.Keep in mind that the Large Eddy Simulation technique (LES) is based on theassumption that the numerical mesh should be fine enough to allow the formationof eddies that are responsible for the mixing. In general, eddy formation is limitedby the largest dimension of a mesh cell, thus shrinking the mesh resolution inone or two directions may not necessarily lead to a better simulation if the thirddimension is large.Because an important part of the calculation uses a Poisson solver based onFast Fourier Transforms (FFTs) in the y and z directions, the second and thirddimensions of the mesh should each be of the form 2k × 3m × 5n, where k, mand n are integers. For example, 64 = 26, 72 = 23 · 32 and 108 = 22 · 33 aregood mesh cell divisions, but 37, 99 and 109 are not.The first number of mesh cell divisions (the I in IJK) does not use FFTs andneed not be given as a product of small numbers.Here is a list of numbers between 1 and 1024 that can be factored down to 2’s,3’s and 5’s:

Table 7.1: IJK values

2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 25, 27, 30, 32, 36, 40, 45,48, 50, 54, 60, 64, 72, 75, 80, 81, 90, 96, 100, 108, 120, 125, 128, 135,144, 150, 160, 162, 180, 192, 200, 216, 225, 240, 243, 250, 256, 270,288, 300, 320, 324, 360, 375, 384, 400, 405, 432, 450, 480, 486, 500,512, 540, 576, 600, 625, 640, 648, 675, 720, 729, 750, 768, 800, 810,864, 900, 960, 972, 1000, 1024. . .

The following table summarizes some MESH parameters:

Table 7.2: MESH parameters



IJK(3) Integer Number of cells in x, y, and zdirections

10

XB(6) Real Volume m

7.2. MULTIPLE MESHES 55

Figure 7.1: The computational domain composed by four meshes

7.2 Multiple meshes

The computational domain can consist of many connected mesh. Each meshmust have its MESH namelist group.For example,

&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,0.0,0.8 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,0.8,1.6 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,1.6,2.4 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,2.4,3.2 /

describes a domain composed of four connected meshes, as in Figure 7.1.The connections must always follow a simple rule of mesh alignment depicted in Mesh alignmentFigure 7.2 on the next page: an integer (1, 2, 3. . . ) number of fine cells exactlyabuts each coarse cell.The following rules of thumb should also be followed when setting up a multiplemesh calculation:

• Avoid putting mesh boundaries where critical action is expected, especiallyfire. Sometimes fire spread from mesh to mesh cannot be avoided, but


Figure 7.2: Mesh connections: (a) ideal, (b) allowed, and (c) forbidden

Figure 7.3: Geometric object: before and after automatic shifting

if at all possible try to keep mesh interfaces relatively free of complicatedphenomena since the exchange of information across mesh boundaries isnot yet as accurate as cell to cell exchanges within one mesh.

• If a planar obstruction is close to where two meshes abut, make sure thateach mesh sees the obstruction. If the obstruction is even a millimeteroutside of one of the meshes, that mesh does not account for it, in whichcase information is not transferred properly between meshes.

• Experiment with different mesh configurations using relatively coarse meshcells to ensure that information is being transferred properly from meshto mesh. There are two issues of concern. First, does it appear that theflow is being badly affected by the mesh boundary? If so, try to move themesh boundaries away from areas of activity. Second, is there too muchof a jump in cell size from one mesh to another? If so, consider whetherthe loss of information moving from a fine to a coarse mesh is tolerable.

7.3 Conformity to the mesh

All geometric objects must conform to the rectangular mesh. If you creategeometrical objects that do not precisely conform to the underlying mesh, FDS5shifts them to the closest mesh cell as shown in Figure 7.3.

7.4. CHOOSING THE RIGHT MESH DIMENSION:A SENSITIVITY STUDY57

7.4 Choosing the right mesh dimension:a sensitivity study

The most important numerical parameter in FDS5 is the grid cell size. CFD mod-els solve an approximate form of the conservation equations of mass, momentum,and energy on a numerical grid. The error associated with the discretization ofthe partial derivatives is a function of the size of the grid cells and the type ofdifferencing used. FDS5 uses second-order accurate approximations of both thetemporal and spatial derivatives of the Navier-Stokes equations, meaning thatthe discretization error is proportional to the square of the time step or cell size.In theory, reducing the grid cell size by a factor of 2 reduces the discretizationerror by a factor of 4. However, it also increases the computing time by a factorof 16 (a factor of 2 for the temporal and each spatial dimension). Clearly, thereis a point of diminishing returns as one refines the numerical mesh. Determiningwhat size grid cell to use in any given calculation is known as a grid sensitivitystudy.In general, you should build an FDS5 input file using a relatively coarse mesh,and then gradually refine the mesh until you do not see appreciable differencesin your results.A point of diminishing returns is reached when the improvement in the qualityof the results is outweighed by the cost of the computation. When this point isreached depends on the application. It also depends on the quantities that areof interest. Some quantities, like hot gas layer temperature or height, do nottypically require as fine a numerical grid as quantities such as the heat flux totargets near the fire.For simulations involving buoyant plumes, a measure of how well the flow fieldis resolved is given by the non-dimensional expression D∗/δx, where D∗ is acharacteristic fire diameter and δx is the nominal size of a mesh cell. D∗ isdefined as:

D∗ =(

Q̇

ρ∞ cp T∞√g

) 25

(7.1)

where Q̇ is the heat release rate of the fire in kW, ρ∞ air density (∼1.2 kgm3 ),

cp air thermal capacity (∼1 kJkg K), T∞ ambient air temperature (∼293 K), g

gravitational acceleration (∼9.81 ms2 ).

The quantity D∗/δx can be thought of as the number of computational cellsspanning the characteristic (not necessarily the physical) diameter of the fire.


The more cells spanning the fire, the better the resolution of the calculation.It is better to assess the quality of the mesh in terms of this non-dimensionalparameter, rather than an absolute mesh cell size. For example, a cell size of10 cm may be adequate, in some sense, for evaluating the spread of smoke andheat through a building from a sizable fire, but may not be appropriate to studya very small, smoldering source.As an example, in the mesh sensitivity study for [NUREG 1824], the D∗/δxvalues ranged from 4 to 16. These values were used to adequately resolve plumedynamics, along with other geometrical characteristics of the models as well.This range does not indicate what values to use for all models, only what valuesworked well for that particular set of models.

7.5 Initial conditions of the computational do-main, INIT

At the start of any calculation, the temperature is ambient everywhere, the flowvelocity is zero everywhere, nothing is burning, and the mass fractions of allspecies are uniform.To change the starting ambient conditions within some volumetric region of theflow domain add lines of the form:

&INIT XB=0.5,0.8,2.1,3.4,2.5,3.6, TEMPERATURE=30. /

the initial temperature of the gas phase shall be 30°C instead of the ambientwithin the prescribed volume. This construct can also be used for DENSITY orMASS_FRACTION(n).The INIT construct may be useful in examining the influence of stack effect ina building, where the temperature is different inside and out.For setting initial temperature of a solid obstruction see Subsection 11.6.2.Also the MISC namelist group can be used to set a variety of initial conditions.An initial velocity on the domain can be prescribed via U0, V0, and W0 parameters.Normally, the initial values of the gas velocity in each of the coordinate directionsare all 0m/s, but there are a few applications where it is convenient to start theflow immediately, like in an outdoor simulation involving wind.A different ambient temperature of the domain can be prescribed via the TMPAparameter.

7.5. INITIAL CONDITIONS OF THE COMPUTATIONAL DOMAIN, INIT 59

To model a sloping roof or tunnel you can change the direction of the gravityvector. The GVEC parameter contains the 3 components of gravity, in m/s2. Thedefault is GVEC=0,0,-9.81

For example,

&MISC U0=2., TMPA=25., GVEC=-0.114377,0.,-9.809333 /

generates an initial wind speed to 2 m/s in +x direction, sets ambient tempera-ture to 25°C, and bends the gravity vector in −x direction.The following table summarizes some INIT parameters:

Table 7.3: INIT parameters


DENSITY Real Initial value of density kg/m3 Ambient

MASS_FRACTION(n) Real Initial value of specie n kg/kg Ambient

TEMPERATURE Real Initial value of temperature ◦C TMPA

XB(6) Real Volume m

Chapter 8

Materials

Third, some properties are set up, as the properties of each material(MATL). This chapter covers RAMP (temperature dependent) namelistgroup, too.

8.1 Defining a material, MATL

The properties of each material used in the model are designated via the MATLnamelist group. These properties indicate how rapidly the materials heat up, andhow they burn. Each MATL entry in the input file must have an ID that can bereferred by other namelist groups.The following table summarizes some MATL parameters:

Table 8.1: MATL parameters



DENSITY Real Solid mass per unit volume kg/m3 0.

EMISSIVITY Real Emissivity 0.9

CONDUCTIVITY Real Thermal conductivity W/m/K 0.

CONDUCTIVITY_RAMP String Ramp ID for conductivity

SPECIFIC_HEAT Real Specific heat kJ/kg/K 0.continued on next page

61

62 CHAPTER 8. MATERIALS

from previous pageSPECIFIC_HEAT_RAMP String Ramp ID for specific heat

HEAT_OF_COMBUSTION Real Heat of combustion kJ/kg 0.

HEAT_OF_REACTION Real Heat of reaction kJ/kg 0.

ABSORPTION_COEFFICIENT Real Absorption Coefficient 1/m 50 000.

BOILING_TEMPERATURE Real Boiling temperature ◦C 5000.

8.2 Thermal properties

The MATL namelist group can be used to specify thermal CONDUCTIVITY ( Wm K),

DENSITY ( kgm3 ), SPECIFIC_HEAT ( kJ

kg K), and EMISSIVITY (0.9 by default) ofmaterials, for example:

&MATL ID=’steel’, EMISSIVITY=.95, DENSITY=7850.,CONDUCTIVITY=45.8, SPECIFIC_HEAT=0.46, /

&MATL ID=’concrete’, DENSITY=2200.,CONDUCTIVITY=1.2, SPECIFIC_HEAT=0.88, /

&MATL ID=’copper’, SPECIFIC_HEAT=0.38,CONDUCTIVITY=387., DENSITY=8940. /

&MATL ID=’gypsum plaster’, CONDUCTIVITY=0.48,SPECIFIC_HEAT=0.84, DENSITY=1440. /

Thermal properties like conductivity and specific heat can vary significantly withtemperature. In such cases, use the RAMP function like this:

&MATL ID=’steel’, SPECIFIC_HEAT_RAMP=’c_steel’,CONDUCTIVITY_RAMP=’k_steel’, DENSITY=7850. /

&RAMP ID=’c_steel’, T=20., F=0.45 /&RAMP ID=’c_steel’, T=377., F=0.60 /&RAMP ID=’c_steel’, T=677., F=0.85 /&RAMP ID=’k_steel’, T=20., F=48. /&RAMP ID=’k_steel’, T=677., F=30. /

&MATL ID=’calcium silicate’,CONDUCTIVITY_RAMP=’k_casi’, DENSITY=770.,SPECIFIC_HEAT=0.96 /

8.3. BURNING PROPERTIES 63

&RAMP ID=’k_casi’, T= 25., F=0.18 /&RAMP ID=’k_casi’, T=200., F=0.19 /&RAMP ID=’k_casi’, T=500., F=0.20 /

For this kind of ramps the parameter F is the value of the actual physical quantity.If CONDUCTIVITY_RAMP is used, there should be no value of CONDUCTIVITYgiven. Note also that for values of temperature, T, below and above the givenrange, FDS5 will assume a constant value equal to the first or last F specified.Each set of RAMP lines must be listed with monotonically increasing T. Thefollowing table summarizes some RAMP (temperature) parameters:

Table 8.2: RAMP (temperature) parameters



T Real Temperature °C

F Real Function value

8.3 Burning properties

8.3.1 Solids

The MATL namelist group can be used to specify the parameters employed inthe solid phase pyrolysis process. As already explained in Chapter 6 on page 43,pyrolysis is the decomposition or transformation of a compound caused by heatthat produce the gaseous fuel, that is burned during gas phase combustion.

FDS5 contains a fairly general description of multi-layered, multi-component,multi-reaction solid: while burning, each material can undergo several reactionsthat may occur at different temperatures and consume different amounts of heat.Each individual reaction can produce a single solid residue, water vapor, or fuelgas.

Here is an example of a material that burns in the neighborhood of 350°C,converting all its mass to fuel gases with NU_FUEL(1)=1.:


&MATL ID=’my fuel’, SPECIFIC_HEAT=1.0, CONDUCTIVITY=0.1,DENSITY=100.0, HEAT_OF_COMBUSTION=15000., N_REACTIONS=1,NU_FUEL(1)=1., REFERENCE_TEMPERATURE(1)=350.,HEAT_OF_REACTION(1)=3000. /

See next Sections and [FDS5 user’s guide] for broader description of the problemand its complexity.

8.3.2 Liquids

The MATL namelist group is also used to specify the parameters for burningliquids.For a liquid fuel, the thermal properties are similar to those of a solid material,with a few exceptions. The evaporation rate of the fuel is governed by theClausius-Clapeyron equation. The only drawback of this approach is that the fuelgases burn regardless of any ignition source. Thus, if a liquid fuel is specified,the fuel begins burning at once.As an example:

&MATL ID=’ethanol’, EMISSIVITY=1.0, NU_FUEL=0.97,HEAT_OF_REACTION=880.,CONDUCTIVITY=0.17, SPECIFIC_HEAT=2.45, DENSITY=787.,ABSORPTION_COEFFICIENT=40., BOILING_TEMPERATURE=76. /

The inclusion of BOILING_TEMPERATURE on the MATL line tells FDS5 to use itsliquid pyrolysis model.It also automatically sets N_REACTIONS=1: the only reaction is the phase changefrom liquid to gaseous fuel. Thus, HEAT_OF_REACTION in this case is the latentheat of vaporization. The gaseous fuel yield, NU_FUEL, is 0.97 instead of 1 toaccount for impurities in the liquid that do not take part in the combustionprocess.The thermal conductivity, density and specific heat are used to compute theloss of heat into the liquid via conduction using the same one-dimensional heattransfer equation that is used for solids. Obviously, the convection of the liquidis important, but is not considered in the model.Note also the ABSORPTION_COEFFICIENT for the liquid. This denotes the ab-sorption in depth of thermal radiation. Liquids do not just absorb radiation atthe surface, but rather over a thin layer near the surface. Its effect on the burningrate is significant.

8.4. PROPERTIES HELL 65

8.3.3 HEAT_OF_COMBUSTION in a MATL line?

The HEAT_OF_COMBUSTION is the energy released per unit mass of fuel gas thatmixes with oxygen and burns. This has nothing to do with the pyrolysis process.This parameter would better be used in gas phase combustion!

If you remember what was said in Chapter 6 on page 43, whereas there can bemany types of combustibles in an FDS5 fire simulation, only one gaseous fuelcan be simulated by FDS5. The stoichiometry of the predominant reaction isspecified via the REAC namelist group.In fact, the HEAT_OF_COMBUSTION specified on the REAC line pertains to the onlygaseous fuel modeled in gas phase combustion.The HEAT_OF_COMBUSTION specified on the MATL line is that specific to gaseousfuel produced by pyrolysis.If the HEAT_OF_COMBUSTION is specified on the MATL line, FDS5 automaticallyadjust the mass loss rate of the gaseous fuel injected by the pyrolyzing material,so that the corrected mass loss rate multiplied by the single, global, gas phaseheat of combustion produces the expected heat release rate.If, for example, the HEAT_OF_COMBUSTION specified on the REAC line is twicethat specified on the MATL line, the mass of pyrolyzing material contained withinwall cell will be decremented by that determined by the pyrolysis model, but themass of fuel gas added to gas phase would be reduced by 50%.

8.4 Properties hell

The scientific community agrees that there is no standardized way of obtaining allof the parameters needed to run FDS5. This is especially true of materials thatburn. There are various devices used to measure various properties, but there isno consensus on the exact physical and mathematical description of these, andthus, no standard way of taking bench-scale data and converting it into an FDS5input file.Recently Nick Dempsey of WPI, Marc Janssens of Southwest Research,and Morgan Hurley of the SFPE were awarded a three year grant to developan engineering guide that will document the standard test methods used toobtain material properties, and more importantly the physical and mathematicalinterpretation of these methods that will enable us all to understand what to dowith measurements made in the various bench scale devices.


A prediction is called blind, if the results are not compared to experimental mea-sures. Grid sensitivity and uncertain material properties make blind predictionsof fire growth on real materials beyond the reach of the current version of themodel.However, the model can still be used for a qualitative assessment of fire behavioras long as the uncertainty in the flame spread rate is recognized.For engineering level applications, it’s strongly advised to recur to simplified firemodeling, directly prescribing the HRR of the fire scenario taken from literatureor direct experimentation, as shown in Section 11.4.6 on page 82.

8.5 Resources for material property data

Here are some web resources for material property data; a broader list of links ismaintained on FDS5 web site:

• NIST Chemistry Webbook:http://webbook.nist.gov/chemistry/

• ChemFinder:http://chemfinder.cambridgesoft.com/

• Parital INSC Material Properties Database:http://www.insc.anl.gov/matprop/thermo.php

• Cone calorimeter data from Worcester Polytechnic Institute:http://www.wpi.edu/Academics/Depts/Fire/Lab/Cone/Data

• MatWeb:http://matweb.com

• Engineering Toolbox:http://engineeringtoolbox.com

http://webbook.nist.gov/chemistry/

http://chemfinder.cambridgesoft.com/

http://www.insc.anl.gov/matprop/thermo.php

http://www.wpi.edu/Academics/Depts/Fire/Lab/Cone/Data

http://matweb.com

http://engineeringtoolbox.com

Chapter 9

Extra gas species

Third, some properties are set up, as the properties of extra gasspecies (SPEC).

9.1 Defining extra gas species, SPEC

Gases that are introduced into the domain that are neither reactants nor productsof combustion, like carbon dioxide from an extinguisher, are tracked separatelyfrom the mixture fraction model for gas phase combustion via an additional scalartransport equation. In fact, there does not need to be any fire at all, as FDS5can be used to transport a mixture of non-reacting ideal gases.The namelist group SPEC is used to specify each additional species. Each SPECline should include at the very least the name of the species via a character stringcalled ID.The following gases are predefined in FDS5 and do not need any property to beset up:

Table 9.1: Predefined extra species

AIR, ARGON, CARBON DIOXIDE, CARBON MONOXIDE, HELIUM, HYDROGEN,METHANE, NITROGEN, OXYGEN, PROPANE, WATER VAPOR.

For example:

&SPEC ID=’HELIUM’ /

67

68 CHAPTER 9. EXTRA GAS SPECIES

adds the predefined HELIUM gas as an additional specie that can be injected inthe domain.To specify a gas not included in the list, the user should input several chemicalproperties. See [FDS5 user’s guide] for broader description.If the ambient initial mass fraction of an extra gas specie is something other than0, then the parameter MASS_FRACTION_0 is used to specify it. For example, theline:

&SPEC ID=’ARGON’, MASS_FRACTION_0=0.1 /

specifies that 10% in mass of ARGON is to be included in the calculation, inaddition to the 90% unlisted default ambient specie named AIR.The following table summarizes some SPEC parameters:

Table 9.2: SPEC parameters



MASS_FRACTION_0 Real Initial mass fraction 0

9.2 CARBON DIOXIDE and carbon dioxide

These extra gas species are identified by an uppercase name, and are not to beconfused with the lowercase species implicitly defined by FDS5 when doing a mix-ture fraction calculation for gas phase combustion, as fuel, oxygen, nitrogen,water vapor, carbon dioxide. . . See Section 6.3 on page 44 for reference.If the user introduces an extra gas in the calculation that is the same as a productof combustion, as in:

&SPEC ID=’CARBON DIOXIDE’ /

FDS5 will take into account two different gases: the implicitly defined carbondioxide and the extra gas species CARBON DIOXIDE, injected for example tosimulate a CO2 extinguisher.

9.2. CARBON DIOXIDE AND CARBON DIOXIDE 69

The first is a product of combustion, while the second is just another gas: itdoes not participate to combustion, but it can dilute oxygen and contribute tofire suppression.The two gases are tracked separately: carbon dioxide is tracked via the mix-ture fraction variable and CARBON DIOXIDE is tracked via its own transportequation.

70 CHAPTER 9. EXTRA GAS SPECIES

Chapter 10

Lagrangian particles

Third, some properties are set up, as the properties of Lagrangianparticles (PART).

10.1 Defining Lagrangian particles, PART

Lagrangian particles are used in FDS5 as water or liquid fuel droplets, flow tracers,and various other objects that are not defined or confined by the numerical mesh.Sometimes the particles have mass, sometimes they do not. Some evaporate,absorb radiation, etc. PART is the namelist group that is used to prescribeparameters associated with Lagrangian particles.All Lagrangian particles must be explicitly defined via the PART namelist group.Once a particular type of particle or droplet has been described using a PARTline, then the name of that particle or droplet type is invoked elsewhere in theinput file via the parameter PART_ID.There are no reserved PART_ID, all must be defined. For example, an inputfile may have several PART lines that include the properties of different types ofLagrangian particles:

&PART ID=’my tracer’, MASSLESS=.TRUE. /

Then these Lagrangian particles can be introduced in the fluid flow from a solidsurface via a boundary condition as explained in Section 11.8 on page 89.The following table summarizes some PART parameters:

71

72 CHAPTER 10. LAGRANGIAN PARTICLES

Table 10.1: PART parameters



MASSLESS Logical Massless particles .FALSE.

WATER Logical Water droplets .FALSE.

AGE Real Droplet lifetime s 100000.

COLOR String Color

RGB(3) Integer Color

DT_INSERT Real Time between insertions s 0.01

XB(6) Real Volume, initial particle location m

10.2 Massless particles

The simplest use of Lagrangian particles is for visualization, in which case theparticles are considered massless tracers. In this case, the particles are definedvia the line:


10.3 Water droplets

WATER=.TRUE. declares that the liquid droplets evaporate into WATER VAPOR,a separate gas phase specie that is automatically added to the calculation bythis command. By default, WATER=.FALSE., even though the default propertiesof droplets are that of water. Setting WATER=.TRUE. instructs FDS5 to addWATER VAPOR as an explicitly defined specie, and it also invokes appropriateconstants related to the absorption of thermal radiation by the water droplets.It also causes the droplets to be colored blue in Smokeview.For example:

&PART ID=’droplets’, WATER=.TRUE. /

10.3. WATER DROPLETS 73

When a droplet strikes a solid surface, it sticks and is reassigned a new speedand direction. If the surface is horizontal, the direction is randomly chosen. Ifvertical, the direction is downwards.

74 CHAPTER 10. LAGRANGIAN PARTICLES

Chapter 11

Boundary conditions

Third, some properties are set up, as the types of boundary conditions(SURF).This is the most challenging part of setting up the simulation: first,for both real and simulated fires, the growth of the fire is very sensi-tive to the thermal properties of the surrounding materials. Second,even if all the material properties are known to some degree, thephysical phenomena of interest may not be simulated properly dueto limitations in the model algorithms or resolution of the numericalmesh.It is your responsibility to supply the thermal properties of the ma-terials, and then assess the performance of the model to ensure thatthe phenomena of interest are being captured.This chapter covers RAMP (time dependent) namelist group, too.

11.1 Defining boundary conditions, SURF

This chapter describes how to specify the properties of the bounding surfacesof the flow domain. The namelist group that defines the types of boundaryconditions is SURF. For example,

&SURF ID=’warm_surface’, TMP_FRONT=25. /

defines a surface named warm_surface. Its temperature is fixed to 25°C.

75

76 CHAPTER 11. BOUNDARY CONDITIONS

While building the solid geometry, the types of boundary conditions will be appliedto each of the bounding surfaces of the flow domain: the faces of the solidobstructions and the exterior boundaries of the computational domain.The following table summarizes some SURF parameters:

Table 11.1: SURF parameters



ADIABATIC Logical Adiabatic thermal boundarycondition

.FALSE.

EMISSIVITY Real Emissivity 0.9

HRRPUA Real HRR per unit area kW/m2 0.

MLRPUA Real Mass loss rate per unit area kg/m2/s 0.

HEAT_OF_VAPORIZATION Real Heat of vaporisation forspecified HRR only

kJ/kg 0.

IGNITION_TEMPERATURE Real Ignition temperature ◦C 5000.

MATL_ID(i,j) String Material name (Layer,Component)

MATL_MASS_FRACTION(i,j) Real Mass fraction ofcomponents (Layer,Component)

THICKNESS(i) Real Thickness of layers (Layer) m 0.

BACKING String Back boundary condition ’VOID’

TMP_BACK Real Back surface temperature ◦C 20.

TMP_FRONT Real Front surface temperature ◦C 20.

TMP_INNER Real Initial solid temperature ◦C 20.

BURN_AWAY Logical Burn away solid .FALSE.

NET_HEAT_FLUX Real Net heat flux at surface kW/m2 0.

CONVECTIVE_HEAT_FLUX Real Convective heat flux atsurface

kW/m2 0.


11.2. PREDEFINED BOUNDARY CONDITIONS 77

from previous pageEXTERNAL_FLUX Real External heat flux to

surfacekW/m2 0.

MASS_FLUX_TOTAL Real Total mass flux kg/m2/s

MASS_FLUX(n) Real Mass flux for specie n kg/m2/s 0.

MASS_FRACTION(n) Real Mass fraction for specie n

VEL Real Normal velocity m/s 0.

VEL_T(2) Real Tangential velocitycomponents

m/s 0.,0.

VOLUME_FLUX Real Normal velocity × area m3/s 0.

POROUS Logical Porous boundary condition .FALSE.

TAU_MF(n) Real Ramp time for specie n s 1.

TAU_Q Real Ramp time for HRR s 1.

TAU_T Real Ramp time for temperature s 1.

TAU_V Real Ramp time for velocity s 1.

RAMP_MF(n) String Ramp ID for specie n

RAMP_Q String Ramp ID for HRR

RAMP_T String Ramp ID for temperature

RAMP_V String Ramp ID for velocity

COLOR String Color

RGB(3) Integer Color 255,204,102

TRANSPARENCY Real Transparency 1

PART_ID String Lagrangian particle ID

11.2 Predefined boundary conditions

FDS5 contains some predefined boundary conditions that do not need to be setwithin a SURF namelist group: INERT, OPEN, and MIRROR.An INERT boundary condition represents an isothermal wall with the temperature INERTfixed at ambient temperature. INERT allows for heat loss and is not the same


Figure 11.1: Extending the computational domain beyond the vent

as an adiabatic surface. An INERT solid is something that never heats up, like apiece of steel that has cold water constantly flowing across its back side.In general, this boundary condition should not be used, as it is better to assignactual material properties to everything.An OPEN boundary condition assumes that ambient conditions exist beyond thatOPENVENT. OPEN can only be prescribed at an exterior boundary of the computationaldomain.If you are concerned about the flow through a particular vent, do not use an OPENboundary because the constant pressure assumption is just an approximation.You should extend your computational domain beyond the vent and build it outof obstructions, as shown in Figure 11.1. The flow in and out will then be treatednaturally as part of the solution of the governing equations.A MIRROR boundary condition denotes a symmetry plane. A MIRROR should spanMIRRORan entire face of the computational domain, essentially doubling the size of thedomain. The flow on the opposite side of the MIRROR is exactly reversed. Froma numerical point of view, a MIRROR is a no-flux, free-slip boundary.MIRROR can only be prescribed at an exterior boundary of the computationaldomain.

11.3 Coloring boundary conditions

As explained in Section 4.10 on page 37:

&SURF ID=’upholstery’, RGB=0,255,0 /&SURF ID=’carpet’, COLOR=’VIOLET RED’ /

will cause objects with a boundary condition of type upholstery to be coloredgreen and the objects of type carpet to be violet red.

11.4. EXAMPLES OF BOUNDARY CONDITIONS 79

It is highly recommended that colors be assigned to solid obstructions via theSURF line because, as the geometries of FDS5 simulations become more complex,it is very useful to use color as a spot check to determine if the desired surfaceproperties have been assigned throughout the room or building under study.Another example:

&SURF ID=’glass’, RGB=0,255,0, TRANSPARENCY=.3 /

will cause objects with a boundary condition of type glass to be colored greenand to be partially transparent.

11.4 Examples of boundary conditions

In the following Sections a list of simple boundary conditions are presented. Morecomplex examples can be found in Chapter 15 on page 127.

11.4.1 Adiabatic surface

For some special applications, it is often desired that a solid surface be adiabatic,that is, there is no net heat transfer (radiative and convective) from the gas tothe solid. The line:

&SURF ID=’adiabatic_surface’, ADIABATIC=.TRUE. /

defines an adiabatic surface named adiabatic_surface. FDS5 will compute awall temperature so that the sum of the convective and radiative fluxes to thewall is zero.

11.4.2 Fixed temperature and heat flux

The line:

&SURF ID=’warm_surface’, TMP_FRONT=25. /

fixes the surface temperature to 25°C.The following line specifies a NET_HEAT_FLUX in units of kW/m2:


&SURF ID=’warm_surface’, NET_HEAT_FLUX=25. /

FDS5 will compute the surface temperature required to ensure that the combinedradiative and convective heat flux from the surface is equal to the prescribed flux.The following line specifies separately the CONVECTIVE_HEAT_FLUX, in units ofkW/m2 and the radiative heat flux using TMP_FRONT temperature in °C andEMISSIVITY:

&SURF ID=’warm_surface’, CONVECTIVE_HEAT_FLUX=25.,TMP_FRONT=150., EMISSIVITY=.9 /

The sign convention is that positive heat flux from a surface heats up the gas.Sign convention

11.4.3 Fans

For most applications, the ventilation system of a building is described in FDS5using velocity boundary conditions. For example, fresh air can be blown into, andsmoke can be drawn from a compartment by specifying a velocity in the normaldirection to a solid surface. However, there are various other facets of velocityboundary conditions that are described below.For example, the line:

&SURF ID=’supply’, VEL=-1.2, TMP_FRONT=50. /

defines a surface supplying hot air to the domain at a velocity of 1.2m/s andtemperature of 50°C. The volume flux depends on the prescribed area and itsalignment with the computational mesh.The line:

&SURF ID=’supply’, VOLUME_FLUX=1.2 /

defines a surface extracting air from the domain at a volume flow of 1.2m3/s. Thevelocity depends on the prescribed area and its alignment with the computationalmesh.This line:

&SURF ID=’supply’, MASS_FLUX_TOTAL=-1.2 /

11.4. EXAMPLES OF BOUNDARY CONDITIONS 81

supplies air to the domain at a mass flow rate of 1.2 kg/s. The MASS_FLUX_TOTALis converted internally into a velocity boundary condition whose value for anoutflow is adjusted based on the local density.The line:

&SURF ID=’louver’, VEL=-1.2, VEL_T=0.5,-0.3 /

represents a boundary condition for a louvered vent that pushes air into the spacewith a normal velocity of 1.2m/s and a tangential velocity of 0.5m/s in eitherthe x or y direction and -0.3m/s in either the y or z direction, depending onwhat the normal direction is.In cases of limited mesh resolution, it may not be possible to describe a louveredvent or slot diffuser using VEL_T because there may not be enough mesh cellsspanning the opening. In these cases, you might consider simply specifying a flatplate obstruction in front of the VENT with an offset of one mesh cell. The platewill simply redirect the air flow in all lateral directions.Note that either VEL, VOLUME_FLUX, or MASS_FLUX_TOTAL should be prescribed,not both. The choice depends on whether an exact velocity is desired at a givenvent, or whether the given volume flux or mass flux is desired.The sign convention is that positive volume or mass flux is drawn out of the Sign conventiondomain.

11.4.4 Fans injecting extra gas species

There are two species boundary conditions that can be specified: MASS_FLUX(n)and MASS_FRACTION(n) where n refers to a given specie SPEC via its place inthe input file.If the mass fraction of the n specie is to be some value at a forced flow boundary(VEL or MASS_FLUX_TOTAL) set MASS_FRACTION(n) equal to the desired massfraction on the appropriate SURF line.If the mass flux of the n specie is desired, set MASS_FLUX(n) instead of MASS_FRACTION(n).If MASS_FLUX(n) is set, no VEL should be set. It is automatically calculated basedon the mass flux.The inputs MASS_FLUX(n) and MASS_FRACTION(n) should only be used forinflow boundary conditions. MASS_FLUX(n) should always be positive with unitsof kg/m2/s.For example, the lines:


&SPEC ID=’ARGON’, MASS_FRACTION_0=0.1 /&SPEC ID=’HELIUM’ /&SURF ID=’inlet’, MASS_FRACTION(2)=0.2, VEL=-0.3 /

specify that ARGON and HELIUM are to be included in the calculation in additionto the unlisted default AIR. At the inlet, a mixture of helium (20% by mass),argon (10% by mass because nothing different is specified), and air (70% by massmaking up the rest) flows out at a velocity of 0.3m/s into the flow domain.

11.4.5 Dynamic pressure at an open boundary

In some situations, it is more convenient to specify a dynamic pressure, ratherthan a velocity, at a boundary.Suppose that you are modeling the interior of a tunnel, and a wind is blowingat one of the portals that affects the overall flow within the tunnel. If (and onlyif) the portal is defined using an OPEN vent, then the dynamic pressure at theboundary can be specified like this:

&VENT XB=0.,0.,0.,4.,0.,3., SURF_ID=’OPEN’,DYNAMIC_PRESSURE=2.4 /

A dynamic pressure of 2.4 Pa is applied to the specified face. See Section 12.3on page 95 for a description of the VENT namelist group.

11.4.6 Prescribing an heat release rate

Solids and liquid fuels can be modeled by specifying their relevant properties viathe MATL namelist group. However, if you simply want to specify a fire of agiven Heat Release Rate (HRR), you need not specify any material properties.A specified fire is basically modeled as the ejection of gaseous fuel from a solidsurface or vent. This is essentially a burner, with a specified heat release rateper unit area, HRRPUA, in units of kW/m2. For example, the line:

&SURF ID=’burner’, HRRPUA=500. /

defines a surface that injects a flow of fuel gas that, when properly mixed withambient air, burns and produces 500 kW per m2 of emitting surface.

11.5. GEOMETRIC CONFORMITY AND RATES 83

An alternative to HRRPUA with the exact same functionality is MLRPUA, exceptthis parameter specifies the mass loss rate of fuel gas per unit area in kg/m2/s.Do not specify both HRRPUA and MLRPUA on the same SURF line.For example:

&SURF ID=’burner’, MRLPUA=5. /

specifies the a mass loss rate of fuel gas per unit area of 5 kg/m2/s.By specifying HRRPUA or MRLPUA, you are controlling the burning rate ratherthan letting the material pyrolyze based on the conditions of the surroundingenvironment.

11.5 Geometric conformity and rates

Be aware that, whenever geometric objects are transformed to become conformto the underlying mesh, their face areas can change. FDS adjusts the value ofHRRPUA, MRLPUA, and of other mass fluxes to guarantee the user prescribed rates.

11.6 Boundary conditions for solids

The thermal and burning properties of each material are specified via the MATLnamelist group. Then materials are invoked by the SURF namelist group to defineboundary conditions for solids.FDS5 performs a one-dimensional heat transfer calculation at each surface ofthe solid to provide a reasonable bounding surface temperature for the gas phasecalculation.A solid boundary can consist of one or multiple layers of different materials, andeach layer can consist of multiple material components.These combinations of layers and material components are specified on the SURFline via the array parameter called MATL_ID(i,j). The argument i is an integerindicating the layer index, starting at 1, the layer at the exterior boundary. The ar-gument j is an integer indicating the component index. MATL_ID(2,3)=’brick’indicates that the third material component of the second layer is brick.The components of the solid mixtures are treated as pure substances with novoids.The following is an example of a multi-layer, multi-component surface:


Figure 11.2: brick wall: multiple layers of different materials

&MATL ID=’water’, CONDUCTIVITY=0.60, SPECIFIC_HEAT=4.19,DENSITY=1000. / material

&MATL ID=’brick’, CONDUCTIVITY=0.69, SPECIFIC_HEAT=0.84,DENSITY=1600. / material

&MATL ID=’insulator’, CONDUCTIVITY=0.041, SPECIFIC_HEAT=2.09,DENSITY=229. / material

&SURF ID=’brick wall’, MATL_ID(1,1:2)=’brick’,’water’,MATL_MASS_FRACTION(1,1:2) = 0.95,0.05,MATL_ID(2,1)=’insulator’,THICKNESS(1:2)=0.1,0.2 / boundary condition

First, materials are defined, then a boundary condition brick wall is prescribed.In brick wall surface (see Figure 11.2), the first layer is composed of a mix-ture of brick and water. This is given by the MATL_ID array which spec-ifies component 1 of layer 1 to be of brick material, and component 2 oflayer 1 to be of water material. The mass fraction of each is specified viaMATL_MASS_FRACTION: brick is 95% by mass and water is 5%. The first layeris 0.1m thick.The innermost layer is made of one only component, insulator, and is 0.2mthick.

11.6.1 Backing

The heat transfer condition of the innermost layer of a wall is set using theBACKING parameter. This parameter can be set to VOID, INSULATED, or EXPOSED.

11.6. BOUNDARY CONDITIONS FOR SOLIDS 85

For example: VOID

&SURF ID=’double_layer’, MATL_ID(1:2,1)=’plastic’,’steel’,THICKNESS(1:2)=0.1,0.2, BACKING=’VOID’, TMP_BACK=30. /

defines a two layers surface. The external layer is made of one only component,plastic, and is 0.1m thick. The innermost layer is made of one only component,steel, and is 0.2 m thick. The innermost layer backs up to an air gap. The airgap is at a TMP_BACK temperature of 30°C. If TMP_BACK is not set, the air gapdefaults to ambient temperature. BACKING=’VOID’ can be safely omitted as itis the default value.A second example: INSULATED

&SURF ID=’double_layer’, MATL_ID(1:2,1)=’plastic’,’steel’,THICKNESS(1:2)=0.1,0.2, BACKING=’INSULATED’ /

defines the same two layers surface. In this second case, the innermost layerbacks up to an insulating (adiabatic) material, so that no heat is lost to thebacking material.As a last example: EXPOSED

&SURF ID=’double_layer’, MATL_ID(1:2,1)=’plastic’,’steel’,THICKNESS(1:2)=0.1,0.2, BACKING=’EXPOSED’ /

defines the same two layers surface. In this third case, the innermost layer backsup to the room on the other side of the wall. EXPOSED only works if the wall isless than or equal to one mesh cell thick, and if there is a non-zero volume ofcomputational domain on the other side of the wall. FDS5 calculates the heatconduction through the entire THICKNESS and uses the gas phase temperatureand heat flux on the front and back sides for boundary conditions.A redundant calculation is performed on the opposite side of the obstruction, sobe careful how you specify multiple layers: if the layering is symmetric, the sameSURF line can be applied to both sides; however, if the layering is not symmetric,you must create two separate SURF lines and apply one to each side.For example, a asymmetric layered hollow box column that is made of steel andcovered on the outside by a layer of insulation material and a layer of plastic ontop of the insulation material, would have to be described with two SURF lineslike the following:


&SURF ID=’column exterior’, BACKING=’EXPOSED’,MATL_ID(1:3,1)=’plastic’,’insulation’,’steel’,THICKNESS(1:3)=0.002,0.036,0.0063 /

&SURF ID=’column interior’, BACKING=’EXPOSED’,MATL_ID(1:3,1)=’steel’,’insulation’,’plastic’,THICKNESS(1:3)=0.0063,0.036,0.002 /

11.6.2 Setting an initial temperature

A solid obstruction can be given an initial temperature via the parameter TMP_INNERon the SURF line:

&SURF ID=’stuff’, MATL_ID=’steel’, THICKNESS=.1, TMP_INNER=30. /

the initial temperature shall be 30°C within the concerned face, instead of theambient.

11.7 Time dependent boundary conditions

When solid obstacles are activated (See Section 12.7 on page 100), the prescribedboundary conditions for their faces begin to come into effect.After activation, temperatures, velocities, burning rates, etc., are ramped-upfrom their initial values to their prescribed values in roughly 1 s, because nothingcan happen instantaneously.This default 1 s ramp can be modified by the user: many SURF parameters canbecome time dependent and follow a different trend after the activation instant.

11.7.1 Simplified ramps

The parameters TAU_Q, TAU_T, TAU_V, TAU_MF(n) indicate that the heat re-lease rate (HRRPUA), surface temperature (TMP_FRONT), normal velocity (VEL,VOLUME_FLUX) or MASS_FLUX_TOTAL, mass fraction or mass flux of specie n areto ramp up to their prescribed values in TAU_* seconds after SURF activation andremain there.If TAU_* is positive, then the related quantity ramps up like tanh(t/τ). IfSign convention

11.7. TIME DEPENDENT BOUNDARY CONDITIONS 87

Figure 11.3: HRRPUA as function of time after SURF activation

negative, then it ramps up like (t/τ)2. As stated before, the default value for allTAU_* is 1 s.For example, this line:

&SURF ID=’burner’, HRRPUA=4000., TAU_Q=-120 /

specifies a boundary condition for a burner that activates when the calculationstarts at t=0 s, ramps up like HRRPUA=4000. (t/120)2 while t<120 s and remainsat 4000 kW/m2 after 120 s, as shown in Figure 11.3.

11.7.2 User defined ramps

If something other than a tanh or t2 ramp up is desired, then a user-definedfunction must be input. To do this, set RAMP_Q, RAMP_T, RAMP_V or RAMP_MF(n)equal to a character string designating the ramp function to use for that particularsurface type, then somewhere in the input file generate lines of the form:

&RAMP ID=’rampname1’, T= 0.0, F=0.0 /&RAMP ID=’rampname1’, T= 5.0, F=0.5 /&RAMP ID=’rampname1’, T=10.0, F=0.7 /

For this kind of ramp, T is the time passed from activation, and F indicates thefraction of the heat release rate, wall temperature, velocity, mass fraction, etc.,to apply. Linear interpolation is used to fill in intermediate time points.Note that each set of RAMP lines must be listed with monotonically increasing T.For example, a simple blowing fan can be controlled via the lines:


Figure 11.4: VEL and TMP_FRONT as function of time after SURF activation

&SURF ID=’blower’, VEL=-2., TMP_FRONT=50.,RAMP_V=’blower_v’, RAMP_T=’blower_t’ /

&RAMP ID=’blower_v’, T=20.0, F=1.0 /&RAMP ID=’blower_v’, T=30.0, F=0.5 /&RAMP ID=’blower_v’, T=60.0, F=0. /&RAMP ID=’blower_t’, T=40.0, F=1.0 /&RAMP ID=’blower_t’, T=50.0, F=1.5 /&RAMP ID=’blower_t’, T=60.0, F=0.5 /

that produce the time trend shown in Figure 11.4 for SURF parameters.The following table summarizes some RAMP (time) parameters:

Table 11.2: RAMP (time) parameters



T Real Time s

F Real Function value

11.8. INJECTING LAGRANGIAN PARTICLES 89

11.8 Injecting Lagrangian particles

Lagrangian particles can be introduced into the fluid flow from a solid surfacevia boundary conditions.For example, the following line defines a type of Lagrangian particles:


These Lagrangian particles are then introduced into the fluid flow from a solidsurface that has the following boundary condition prescribed:

&SURF PART_ID=’my tracer’ /

Note that a surface on which particles are specified must have a non-zero normalvelocity directed into the computational domain. This happens automatically ifthe surface is burning, hence injecting fuel gas into the flow domain, but mustbe specified if it is not.

Chapter 12

Solid geometry

Fourth, the solid geometry is entered via OBST, VENT, HOLE namelistgroups.A considerable amount of work in setting up a calculation lies inspecifying the geometry of the space to be modeled and applyingboundary conditions to these objects. The geometry is described interms of obstructions to the gas phase flow. A boundary conditionneeds to be assigned to each bounding surface of the gas phase do-main describing its thermal properties. Both solid obstruction facesand the exterior boundaries of the computational domain need aboundary condition assigned. A fire is just one type of boundarycondition.

12.1 Defining solid obstructions, OBST

The namelist group OBST contains parameters used to define obstructions. EachOBST line define a solid volume within the computational domain.The boundary condition for whole faces of the obstruction can be easily speci-fied prescribing one of the following three parameters: SURF_ID, SURF_IDS, orSURF_ID6.The parameter SURF_ID designates one SURF boundary condition to apply to allthe faces of the obstruction. For example:

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2, SURF_ID=’brick wall’ /

91

92 CHAPTER 12. SOLID GEOMETRY

builds a solid obstruction and apply the brick wall surface type to all its sixfaces.If the obstruction has different properties for its top, sides and bottom, use in-stead the parameter SURF_IDS, an array of three character strings specifying theboundary condition for the top, sides and bottom of the obstruction, respectively.For example:

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2,SURF_IDS=’burner’,’brick wall’,’INERT’ /

builds a solid obstruction and applies the burner surface type to the top face(+z direction), brick wall surface type to sides, and INERT surface type tothe bottom face (−z direction).If the obstruction has different properties for all its faces, use instead the parame-ter SURF_ID6, an array of six character strings specifying the boundary conditionfor each face. For example:

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2,SURF_ID6=’bc-x’,’bc+x’,’bc-y’,’bc+y’,’bc-z’,’bc+z’ /

builds a solid obstruction and applies:

• the bc-x surface type to the x = 2.3 face (face in −x direction),

• the bc+x surface type to the x = 4.5 face (face in +x direction),

• the bc-y surface type to the y = 1.3 face (face in −y direction),

• the bc+y surface type to the y = 4.8 face (face in +y direction),

• the bc-z surface type to the z = 0.0 face (face in −z direction),

• the bc+z surface type to the z = 9.2 face (face in +z direction).

Note that SURF_ID6 complies to the same convention as the XB parameter.The following table summarizes some OBST parameters:

12.1. DEFINING SOLID OBSTRUCTIONS, OBST 93

Figure 12.1: Boundary conditions prescribed with SURF_ID, SURF_IDS, andSURF_ID6

Table 12.1: OBST parameters


XB(6) Real Volume m

SAWTOOTH Logical Sawtooth .TRUE.

THICKEN Logical Force at least one cell thick .FALSE.

SURF_ID String Set boundary condition (all faces) ’INERT’

SURF_IDS(3) String Set boundary conditions (top, side,bottom faces)

’INERT’

SURF_ID6(6) String Set boundary conditions (each of sixfaces)

’INERT’

ALLOW_VENT Logical Allow VENT on OBST .TRUE.

PERMIT_HOLE Logical Allow OBST to be carved by a HOLE .TRUE.

COLOR String Color

RGB(3) Integer Color 255,204,102

TRANSPARENCY Real Transparency 1

OUTLINE Logical Draw as outline in Smokeview .TRUE.

DEVC_ID String ID of DEVC that controls OBST’sexistence

CTRL_ID String ID of CTRL that controls OBST’sexistence


12.2 Creating voids inside obstructions, HOLE

The HOLE namelist group is used to carve a hole out of an existing obstructionor set of obstructions. To do this, add lines of the form:

&HOLE XB=2.0,4.5,1.9,4.8,0.0,9.2 /

Any solid mesh cells within the volume 2.0 < x < 4.5, 1.9 < y < 4.8, 0.0 <z < 9.2 are removed. Obstructions intersecting the volume are broken up intosmaller blocks.If the hole represents a door or window, a good rule of thumb is to punch morethan enough to create the hole. This ensures that the hole is created throughthe entire obstruction. For example:

&OBST XB=1.0,1.1,0.0,5.0,0.0,3.0 /&HOLE XB=0.99,1.11,2.0,3.0,0.0,2.0 /

the OBST line denotes a wall 0.1m thick; the HOLE line creates a door. The extracentimeter added to the x coordinates of the hole make it clear that the hole isto punch through the entire obstruction.If an obstruction is not to be punctured by a HOLE, add the parameter PERMIT_HOLE=.FALSE.to the OBST line.Note that a HOLE has no effect on a VENT or a mesh boundary. It only appliesto obstructions.The following table summarizes some HOLE parameters:

Table 12.2: HOLE parameters


XB(6) Real Volume, cutout m

COLOR String Color for resulting obstruction

RGB(3) Integer Color for resulting obstruction

TRANSPARENCY Real Transparency of resulting obstruction

DEVC_ID String ID of DEVC that controls HOLE’sexistence

CTRL_ID String ID of CTRL that controls HOLE’sexistence

12.3. PRESCRIBING A DIFFERENT BOUNDARY CONDITION, VENT 95

Figure 12.2: OBST, HOLE and VENT

12.3 Prescribing a different boundary condition,VENT

OBST namelist group can easily prescribe boundary conditions of entire faces ofobstacles. But very often you will need to apply a particular boundary conditionto a rectangular patch of an entire face or to the exterior boundaries of thecomputational domain.The VENT namelist group is used to prescribe boundary conditions:

• on flat faces adjacent to obstructions,

• or exterior boundaries of the computational domain.

For example, the lines:

&VENT XB=1.0,2.0,2.0,2.0,1.0,3.0, SURF_ID=’burner’ /&OBST XB=0.0,5.0,2.0,3.0,0.0,4.0, SURF_ID=’brick wall’ /

build a solid obstacle made of brick wall and apply a burner boundary conditionto a rectangular patch on the solid face in −y direction.To set boundary conditions to exterior boundaries of the computational domainproceed as in the following example:


Figure 12.3: Setting boundary conditions to exterior boundaries of the compu-tational domain

### Computational domain&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,0.0,0.8 /&MESH IJK=32,32,16, XB=0.0,1.6,0.0,1.6,0.8,1.6 /### Properties&SURF ID=’brick wall’, COLOR=’BROWN’ /&SURF ID=’floor’, COLOR=’SILVER’ /&SURF ID=’ceiling’, COLOR=’SLATE GRAY’ /### Solid geometry&VENT XB=0.0,0.0,0.0,1.6,0.0,1.4, SURF_ID=’brick wall’ /

lower part of -x exterior boundary&VENT XB=0.0,0.0,0.0,1.6,1.4,1.6, SURF_ID=’OPEN’ /

upper part of -x exterior boundary&VENT XB=1.6,1.6,0.0,1.6,0.0,1.6, SURF_ID=’OPEN’ /

+x exterior boundary&VENT XB=0.0,1.6,0.0,0.0,0.0,1.6, SURF_ID=’brick wall’ /

-y exterior boundary&VENT XB=0.0,1.6,1.6,1.6,0.0,1.6, SURF_ID=’OPEN’ /

+y exterior boundary&VENT XB=0.0,1.6,0.0,1.6,0.0,0.0, SURF_ID=’floor’ /

-z exterior boundary&VENT XB=0.0,1.6,0.0,1.6,1.6,1.6, SURF_ID=’ceiling’ /

+z exterior boundary

The result is shown in Figure 12.3.A shortcut exists to select mesh boundaries: the MB parameter. This manual willnot cover it because it leads to error and confusion when employed with multiplemeshes.

12.4. DEFAULT BOUNDARY CONDITION 97

Note that:

• Only one VENT may be specified for any given wall cell. If additional VENTlines are specified for a given wall cell, FDS5 will output a warning messageand ignore the subsequent lines. The first defined VENT will be applied:first-come, first-served.

• VENT overrides the boundary condition defined by the underlying obstruc-tion: VENT boundary condition wins on OBST boundary condition.

• A VENT must always be attached to a solid obstruction or exterior bound-aries of the computational domain: a flying VENT is not allowed.

• If FDS5 outputs an error message requesting that the orientation of a VENTbe specified, check to make sure that the VENT is a plane and that it isnot buried within a solid obstruction.

The following table summarizes some VENT parameters:

Table 12.3: VENT parameters


XB(6) Real Face m

PBX, PBY, PBZ Real Plane m

IOR Integer Index of orientation

SURF_ID String Set boundary condition ’INERT’

DEVC_ID String ID of DEVC that controls VENT’sexistence

CTRL_ID String ID of CTRL that controls VENT’sexistence

12.4 Default boundary condition

INERT is the default SURF boundary condition for all solid surfaces and theexterior boundary of the computational domain, if not otherwise specified.If you want to change the default boundary condition, set SURF_DEFAULT pa-


rameter on the MISC line. For example:

&MISC SURF_DEFAULT=’steel’ /

If the default boundary condition is desired for the faces of an obstruction, thenSURF_ID* need not be set. For example:

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2 /

builds a solid obstruction and apply the default surface type to all its faces.The same is true for a VENT:

&VENT XB=2.3,4.5,1.3,4.8,0.0,0,0 /

12.5 How thick is a wall?

The lines:

&MATL ID=’brick’, CONDUCTIVITY=0.69, SPECIFIC_HEAT=0.84,DENSITY=1600. / material

&SURF ID=’brick wall’, MATL_ID(1,1)= ’brick’, THICKNESS= 0.1 /boundary condition

&OBST XB=0.,10.,0.,.2,0.0,2.7, SURF_ID=’brick wall’ /solid obstruction

first define a brick material and describe a brick wall boundary condition,then build a solid obstruction applying the brick wall surface type to all itsfaces.If you carefully examine the prescriptions:

• The OBST object is 10m long, 2.7m high, and 0.2m thick in the y direction.

• The brick wall boundary condition (SURF line) prescribe a 0.1 m thick-nesses for the same wall.

12.6. THIN SHEET OBSTRUCTIONS 99

At first sight, these two parameters seem in contradiction.But, if you remember what was anticipated in Section 4.5 on page 33, theTHICKNESS parameter indicated by the SURF line, does not need to match theXB dimensions of the solid obstruction prescribed by OBST.In fact, the two parameters are independent from each other:

• The OBST line describes the overall geometric structure of a solid thatobstacles the gas phase flow: it provides data to the hydrodynamic model.

• The SURF line describes the characteristics of the surfaces of the solid, usedto provide a reasonable bounding surface temperature for the gas phasecalculation: it provides data to the solid heat transfer model.

When a SURF boundary condition is applied to the faces of a solid obstruction,FDS5 performs a separate one-dimensional heat transfer calculation at each faceof the solid, using the THICKNESS parameter. There is no communication amongthe faces.Obviously, this is not an ideal way to perform solid phase heat transfer, but that’scurrently beyond FDS5 scope!

12.6 Thin sheet obstructions

Obstructions can be flat. Often, thin sheets, like a window, form a barrier,but if the numerical mesh is coarse relative to the thickness of the barrier, theobstruction might be unnecessarily large if it is assumed to be one layer of meshcells thick.All faces of an obstruction are shifted to the closest mesh cell. If the obstructionis very thin, the two faces may be approximated on the same cell face.FDS5 and Smokeview render this obstruction as a thin sheet, but it is allowed tohave thermally thick boundary conditions, in other words a not null THICKNESSof the applied SURF.A thin sheet obstruction can only have one velocity vector on its face, thus a gascannot be injected reliably from a thin obstruction because whatever is pushedfrom one side is necessarily pulled from the other. For full functionality, theobstruction should be specified to be at least one mesh cell thick.Thin sheet obstructions work fine as flow barriers, but other features are fragileand should be used with caution.


To prevent FDS5 from allowing thin sheet obstructions, set THICKEN_OBSTRUCTIONS=.TRUE.on the MISC line, or THICKEN=.TRUE. on each OBST line for which the thin sheetassumption is not allowed.

12.7 Activating and deactivating objects

By default the objects and their prescribed boundary conditions are activatedand come to existence when the calculation starts, then they are deactivated anddisappear when the calculation ends.If needed, activation and deactivation times of single OBST, VENT and HOLE canbe prescribed by a control logic, as explained in Chapter 13 on page 103.

12.8 Stair stepping complex geometries

The efficiency of FDS5 is due to the simplicity of its numerical mesh. However,there are situations in which certain geometric features do not conform to therectangular mesh, such as a sloped ceiling or roof. In these cases, construct thecurved geometry using rectangular obstructions, a process sometimes called stairstepping.A concern is that the stair stepping changes the flow pattern near the wall.To lessen the impact of stair stepping on the flow field near the wall, prescribethe parameter SAWTOOTH=.FALSE. on each OBST line that makes up the stairstepped obstruction. The effect of this parameter is to prevent vorticity frombeing generated at sharp corners, in effect smoothing out the jagged steps thatmake up the obstruction.For example, the lines:

&OBST XB=0.00, 0.05,-0.01, 0.01, 0.00, 0.05,SAWTOOTH=.FALSE., COLOR=’GREEN’ /

&OBST XB=0.05, 0.10,-0.01, 0.01, 0.00, 0.10,SAWTOOTH=.FALSE., COLOR=’GREEN’ /

&OBST XB= 0.10, 0.15,-0.01, 0.01, 0.05, 0.15,SAWTOOTH=.FALSE., COLOR=’GREEN’ /

...&OBST XB=0.00, 0.05,-0.01, 0.01, 0.05, 0.10,

SAWTOOTH=.TRUE., COLOR=’TEAL’ /

12.9. COLORING INDIVIDUAL OBJECTS 101

Figure 12.4: Flow velocity on two sides of an oblique wall. Lower side hasSAWTOOTH=.FALSE. set.

&OBST XB=0.05, 0.10,-0.01, 0.01, 0.10, 0.15,SAWTOOTH=.TRUE., COLOR=’TEAL’ /

&OBST XB=0.10, 0.15,-0.01, 0.01, 0.15, 0.20,SAWTOOTH=.TRUE., COLOR=’TEAL’ /

...

create a two sided oblique wall. The resulting flow is shown in Figure 12.4.This is not a complete solution of the problem, but it does provide a simpleway of ensuring that the flow field around a non-rectangular obstruction is notinhibited by extra drag created at sharp corners.

12.9 Coloring individual objects

Objects may be colored individually, overriding SURF prescription, by specifyinga COLOR or RGB value on the respective OBST or VENT line:

&OBST XB=..., SURF_ID=’carpet’, COLOR=’INDIGO’ /

This is not recommended. As explained in Section 11.3 on page 78, it is muchbetter to assign a color to each boundary condition.


12.10 Making burning solids disappear

If a burning object is to disappear from the calculation once it is consumed, setBURN_AWAY=.TRUE. on the corresponding SURF line. The solid object disappearsfrom the calculation cell by cell, as the mass contained by each mesh cells isconsumed either by the pyrolysis reactions or by the prescribed HRR. The massof each mesh cell is the cell face area multiplied by the surface density of theSURF type.An example:

&SURF ID=’stuff’, MATL_ID(1:2,1)=’fabric’,’foam’,THICKNESS(1:2)=0.01,0.1, BURN_AWAY=.TRUE. /

Keep in mind the following issues:

• For reacting surfaces, the surface density is computed as a sum of the layerdensities multiplied by the layer thicknesses. This value can be overriddenby setting SURFACE_DENSITY on the SURF line.

• For surfaces with prescribed HRRPUA, add SURFACE_DENSITY parameter asthis is the only way of defining the mass of the object.

• Use BURN_AWAY parameter cautiously. If an object has the potential ofburning away, a significant amount of extra memory has to be set aside tostore additional surface information as the rectangular block is eaten away.

• If BURN_AWAY is prescribed, the SURF should be applied to the entire object,not just to a face of the object, because it would be unclear how to handleedges of solid obstructions that have different SURF_IDs on different faces.

See [FDS5 user’s guide] for broader discussion on the topic.

Chapter 13

Devices and control logic

Fifth, some control logic and automation is introduced via PROP,DEVC, CTRL namelist groups.Devices can be used to control various actions, like creating andremoving obstructions, or activating and deactivating fans and vents.

13.1 Devices, DEVC and PROP

From the point of view of FDS5, sprinklers, smoke detectors, heat flux gauges,timers, thermocouples, etc., are devices that operate in specific ways dependingon the properties assigned to them.All devices are designated via the DEVC namelist group. The various parametersassociated to devices can be grouped and entered in a PROP namelist group.Each PROP line is identified by a unique ID, and it is invoked by the DEVC linesthat share the same properties. For example, these lines:

&PROP ID=’acme heat detector’, QUANTITY=’LINK TEMPERATURE’,ACTIVATION_TEMPERATURE=74. /

&DEVC ID=’heat detector 1’, XYZ=22.88,19.76,7.46,PROP_ID=’acme heat detector’ /



103

104 CHAPTER 13. DEVICES AND CONTROL LOGIC

define a group of heat detectors of the same model.Every device has an associated QUANTITY. When the QUANTITY value passesabove, or below, a prescribed threshold the device is activated or deactivated,and changes its state from .FALSE. to .TRUE. or the other way back.The threshold can be as simple as a numeric SETPOINT or can be the result of anactivation algorithm. For example, an activation algorithm is used for sprinklers,heat and smoke detectors. . .The following table summarizes some DEVC parameters:

Table 13.1: DEVC parameters



QUANTITY String Name of quantity to output. SeeTable 14.7 on page 123.

XB(6) Real Volume, face or segment ofmeasurement

m

XYZ(3) Real Point of measurement m

IOR Integer Index of orientation

ORIENTATION(3) Real Direction vector 0,0,-1

PROP_ID String Associated PROP line

SETPOINT Real Value at which device changes state

TRIP_DIRECTION Integer Sign of derivative at first statechange

1

INITIAL_STATE Logical Initial state of device .FALSE.

LATCH Logical Device cannot change state multipletimes

.TRUE.

See [FDS5 user’s guide] for a detailed description of PROP parameters.

13.2. EXAMPLES OF DEVICES 105

13.2 Examples of devices

In the following paragraphs some sample devices are listed.

13.2.1 Timer

The simplest example of device is just a timer:

&DEVC XYZ=1.2,3.4,5.6, ID=’my timer’,QUANTITY=’TIME’, SETPOINT=30. /

It changes its state from .FALSE. to .TRUE. when the simulation time increasespast 30 s.

13.2.2 Thermometer

The following is a thermometer:

&DEVC XYZ=1.2,3.4,5.6, ID=’my thermometer’,QUANTITY=’TEMPERATURE’, SETPOINT=60. /

It changes its state when the temperature at point (1.2, 3.4, 5.6) increases past60°C.

13.2.3 Smoke detector

A smoke detector is defined in the input file with an entry similar to:

&PROP ID=’acme smoke detector’, QUANTITY=’CHAMBER OBSCURATION’,LENGTH=1.8, ACTIVATION_OBSCURATION=3.28 /

&DEVC ID=’SD_29’, PROP_ID=’acme smoke detector’, XYZ=2.3,4.6,3.4 /

The SD_29 smoke detector has the following properties:

• LENGHT is the characteristic length for the single parameter Heskestadmodel and defaults to 1.8m.

• ACTIVATION_OBSCURATION threshold can be set according to the valueprovided by the manufacturer. The default setting is 3.28%/m.

The smoke detector changes its state following the specific activation algorithm.


13.2.4 Beam smoke detector

A beam detector is defined by:

&PROP ID=’acme beam’, QUANTITY=’PATH OBSCURATION’,SETPOINT=0.33 /

&DEVC ID=’B_4’, PROP_ID=’acme beam’,XB=1.0,1.1,0.0,5.0,0.0,3.0 /

The B_4 beam detector has the following properties:

• SETPOINT is the total % obscuration at which the detector activates.

• XB specifies the segment covered by the beam. The two endpoints mustlie in the same mesh.

The beam detector changes its state following the specific activation algorithm.

13.2.5 Sprinkler and heat detector

Here is a very simple example of a sprinkler:

&PART ID=’water drops’, WATER=.TRUE. /&PROP ID=’acme spk’, QUANTITY=’SPRINKLER LINK TEMPERATURE’,

RTI=148., ACTIVATION_TEMPERATURE=74.,PART_ID=’water drops’, FLOW_RATE=189.3 /

&DEVC ID=’S_60’, XYZ=22.88,19.76,7.46, PROP_ID=’acme spk’ /

A sprinkler, known as S_60, is located at a point in space given by XYZ. It is anacme spk type sprinkler, whose properties are given on the PROP line:

• RTI is the Response Time Index in units of √m · s.

• ACTIVATION_TEMPERATURE is the link activation temperature in °C.

• FLOW_RATE is the flow rate of released water expressed in units of l/min.

• PART_ID refers the properties of the droplets water drops, defined by aPART namelist group.

13.3. BASIC CONTROL LOGIC 107

This example defines a heat detector, which uses essentially the same activationalgorithm as a sprinkler, without the water spray:

&PROP ID=’acme heat’, QUANTITY=’LINK TEMPERATURE’, RTI=132.,ACTIVATION_TEMPERATURE=74. /

&DEVC ID=’HD_66’, PROP_ID=’acme heat’, XYZ=2.3,4.6,3.4 /

13.3 Basic control logic

In many fire scenarios, the opening or closing of a door or window can lead todramatic changes in the course of the fire. Sometimes these actions are takenintentionally, sometimes as a result of the fire. Within the framework of anFDS5 calculation, these actions are represented by the creation or removal ofsolid obstacles, or the opening or closing of vents.The change of state of a device can be used to trigger an action to occur, likeactivating and deactivating obstructions, boundary conditions and holes. Forexample, devices change of state can create or remove obstructions, switch onor off burners, ventilators and fans.For example, in the lines:

&DEVC XYZ=1.2,3.4,5.6, ID=’my timer’, QUANTITY=’TIME’,SETPOINT=30. /

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2, SURF_ID=’brick wall’,DEVC_ID=’my timer’ /

the DEVC creates a timer named my timer. The OBST existence depends onthe DEVC, thanks to the DEVC_ID=’my timer’ parameter. When the simulationstarts the timer has a .FALSE. initial state, thus the OBST is not activated anddoes not obstacle the flow domain. Then at 30 seconds the timer changes itsstate to .TRUE., the OBST is activated and comes to life.The following parameters dictate how a device will control something:

• SETPOINT is the value of the device at which its state changes. For adetection type of device (heat or smoke detectors) this value is calculatedfrom the device’s activation algorithm and need not be specified on theDEVC line.


• TRIP_DIRECTION, a positive integer means the device will change statewhen its value increases past the SETPOINT and a negative integer meansthe device will change state when its value decreases past the SETPOINT.

• LATCH, if this logical value is set to .TRUE. the device will only changestate once. Otherwise it can change its state several times.

• INITIAL_STATE, this logical value is the initial state of the device.

To remove and then re-create an obstruction in the same place, use two linessince the code simply sees this as two different obstructions:

&DEVC XYZ=1.2,3.4,5.6, ID=’timer1’, QUANTITY=’TIME’,SETPOINT=30., INITIAL_STATE=.TRUE. /

&DEVC XYZ=1.2,3.4,5.6, ID=’timer2’, QUANTITY=’TIME’,SETPOINT=60., INITIAL_STATE=.FALSE. /

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2,SURF_ID=’brick wall’, DEVC_ID=’timer1’ /

&OBST XB=2.3,4.5,1.3,4.8,0.0,9.2,SURF_ID=’brick wall’, DEVC_ID=’timer2’ /

A last example:

&PROP ID=’acme heat’, QUANTITY=’LINK TEMPERATURE’,RTI=132., ACTIVATION_TEMPERATURE=74. /

&DEVC ID=’HD_66’, PROP_ID=’acme heat’, XYZ=2.3,4.6,3.4 /&HOLE XB=2.3,4.5,1.3,4.8,0.0,9.2, DEVC_ID=’HD_66’ /

the hole is created when the activation algorithm of the heat detector gives theconsensus. Note that a DEVC that is being used for a HOLE should not be usedfor anything else other than additional HOLE lines.When a HOLE is deactivated, the obstruction to be cut out can have a differentcolor and transparency than the original obstruction: just set the COLOR and aTRANSPARENCY value on the HOLE line. When the HOLE becomes activated, thevoid is created and the prescribed color disappears.Also VENT activation can be controlled by DEVC state. For example, to control aburner with the device my timer, do the following:

13.4. ADVANCED CONTROL LOGIC 109

&DEVC XYZ=1.2,3.4,5.6, ID=’my timer’,QUANTITY=’TIME’, SETPOINT=30. /

&SURF ID=’burner’, HRRPUA=4000., TAU_Q=-120 /&VENT XB=0.6,1.0,0.3,0.7,0.0,0.0,

SURF_ID=’burner’, DEVC_ID=’my timer’ /

Note that at 30 s, the VENT is activated and the burner boundary conditionturns on. The prescribed t2 ramp begins at 30 s and ends at 150 s, when HRRPUAreaches its maximum of 4000 kW/m2.Note that a MIRROR or OPEN VENT should not be activated or deactivated. Thereason for this restriction is that abrupt changes in pressure can cause numericalinstabilities.

13.4 Advanced control logic

If you desire to control FDS5 objects using more complex logic than can beprovided by the use of a single device and its SETPOINT, control functions canbe specified using the CTRL namelist group.A control function take as input the outputs of one or more devices and controlfunctions. In this manner, complicated behaviors can be simulated by makingfunctions of other functions. For most of the control function types, the logicalvalue output of the devices and control functions and the time they last changedstate are used as the inputs.For any object that a DEVC_ID can be specified for (such as OBST or VENT), aCTRL_ID can be specified instead.See [FDS5 user’s guide] for broader discussion of the topic.

Chapter 14

Output

Finally, the user prescribes the output quantities (DEVC, SLCF, BNDF,ISOF).All output quantities must be specified at the start of the calcula-tion. In most cases, there is no way to retrieve information after thecalculation ends if it was not specified from the start.

14.1 Prescribing output

FDS5 computes the temperature, density, pressure, velocity and chemical com-position within each numerical grid cell at each discrete time step. There aretypically hundreds of thousands to millions of grid cells and thousands to hun-dreds of thousands of time steps. In addition, FDS5 computes at solid surfacesthe temperature, heat flux, mass loss rate, and various other quantities.So the user must carefully select what data to save, much like one would doin designing an actual experiment. Even though only a small fraction of thecomputed information can be saved, the output typically consists of fairly largedata files (it can be many GBytes!).Remember that all output quantities must be specified at the start of the calcu-lation. Before a calculation is started, carefully consider what information shouldbe saved.Typical output quantities for the gas phase include:

• Gas temperature;

• Gas velocity;

111

112 CHAPTER 14. OUTPUT

• Gas species concentration (water vapor, CO2, CO, N2);

• Smoke concentration and visibility estimates;

• Pressure;

• Heat release rate per unit volume;

• Mixture fraction (or air/fuel ratio);

• Gas density;

• Water droplet mass per unit volume.

On solid surfaces, FDS5 predicts additional quantities associated with the energybalance between gas and solid phase, including:

• Surface and interior temperature;

• Heat flux, both radiative and convective;

• Burning rate;

• Water droplet mass per unit area.

Global quantities recorded by the program include:

• Total Heat Release Rate (HRR);

• Sprinkler and detector activation times;

• Mass and energy fluxes through openings or solids.

Time histories of various quantities at a single point in space or global quantitieslike the fire’s heat release rate (HRR) are saved in simple, comma-delimited textfiles that can be plotted using a spreadsheet program, like Openoffice.org Calcor Microsoft Excel.Most field or surface data are visualized with Smokeview. FDS5 and Smokevieware used in concert to model and visualize fire phenomena. Smokeview performsthis visualization by presenting animated tracer particle flow, animated contourslices of computed gas variables and animated surface data. Smokeview alsopresents contours and vector plots of static data anywhere within a scene at afixed time.

14.3. POINT MEASUREMENT DEVICES, DEVC 113

14.2 Output control parameters, DUMP

The namelist group DUMP contains parameters that control the rate at whichoutput files are written, and various other global parameters associated withoutput files.For example, NFRAMES parameter is the number of output dumps per calculation.The default is 1000. Device data, slice data, particle data, isosurface data, 3Dsmoke data, boundary data, solid phase profile data, and control function data aresaved every (T_END - T_BEGIN) / NFRAMES seconds unless otherwise specified.The following table summarizes some DUMP parameters:

Table 14.1: DUMP parameters


NFRAMES Integer Number of output dumps percalculation

1000

14.3 Point measurement devices, DEVC

The same devices DEVC used for control logic (see Chapter 13 on page 103)are employed to save a given quantity at a single point in space so that thisquantity can be plotted as a function of time, like for example a thermocoupletemperature measurement.The prescribed QUANTITY is recorded as a column in an output file namedmycase_devc.csv. This type of file can be imported into any spreadsheet pro-gram.An example:

Table 14.2: Output of devices in mycase_devc.csv file

s °C °C

FDS Time TC_A01 TC_A02

0.00E+000 2.00E+001 2.00E+001continued on next page


from previous page5.00E+000 2.98E+001 2.77E+001

1.00E+001 2.35E+001 2.26E+001

14.3.1 Record a gas phase quantity

For example, if you just want to record the time history of the temperature at agiven point, add:

&DEVC XYZ=6.7,2.9,2.1, QUANTITY=’TEMPERATURE’, ID=’T-1’ /

and a column will be added to the output file mycase_devc.csv under the labelT-1.

FDS5 reports the value of the QUANTITY calculated in the cell where the pointXYZ is located.

14.3.2 Record a solid phase quantity

When prescribing a solid phase quantity, be sure to position the probe at a solidsurface. It is not always obvious where the solid surface is since the mesh doesnot always align with the input obstruction locations.

To help FDS5 locate the appropriate surface, suggest the right orientation of theface using the parameter IOR (index of orientation).

There are still instances where FDS5 cannot determine which solid surface isbeing designated, in which case an error message appears in the diagnostic outputfile. Re-position the probe and try again. For example, the line

&DEVC XYZ=0.7,0.9,2.1, QUANTITY=’WALL TEMPERATURE’,IOR=-2, ID=’wt1’ /

designates the surface temperature of a wall facing the negative y direction.

14.4. ANIMATED PLANAR SLICES, SLCF 115

Figure 14.1: Output of animated planar slices SLCF as viewed in Smokeview

14.3.3 Record integrated quantities

In addition to point measurements, the DEVC group can be used to report inte-grated quantities. For example, you may want to know the mass flow out of adoor or window. To report this, add the line:

&DEVC XB=0.3,0.5,2.1,2.5,3.0,3.0,QUANTITY=’MASS FLOW’, ID=’mf1’ /

Note that in this case, a face is specified rather than a point.QUANTITY=’HRR’ can be used to compute the total heat release rate within asubset of the domain. In this case, the sextuplet XB ought to define a volumerather than a face.Remember to avoid faces or volumes that cross multiple mesh boundaries: FDS5has to decide which mesh to use in the integration.

14.4 Animated planar slices, SLCF

The SLCF namelist group parameters allows you to record various gas phasequantities at more than a single point. A slice refers to a subset of the wholedomain. It can be a line, face, or volume, depending on the values of XB.Animated vectors can be created in Smokeview if a given SLCF line has theattribute VECTOR=.TRUE. If two SLCF entries are in the same plane, then onlyone of the lines needs to have VECTOR=.TRUE. Otherwise, a redundant set ofvelocity component slices will be created.For example, the line:


&SLCF PBX=0.8, QUANTITY=’TEMPERATURE’, VECTOR=.TRUE. /

records the temperature and animated velocity vectors on the plane x = 0.8The following table summarizes some SLCF parameters:

Table 14.3: SLCF parameters



VECTOR Logical Include flow vectors .FALSE.

XB(6) Real Face m

PBX, PBY, PBZ Real Plane m

14.5 Animated boundary quantities, BNDF

The BNDF namelist group parameters allows you to record surface quantities atall solid obstructions. As with the SLCF group, each quantity is prescribed witha separate BNDF line. No physical coordinates need be specified, however, justQUANTITY.For example, the line:

&BNDF QUANTITY=’WALL_TEMPERATURE’ /

records WALL_TEMPERATURE for all solid obstructions faces.For certain output quantities, additional parameters need to be specified via thePROP namelist group. In such cases, add the character string, PROP_ID, to theBNDF line to tell FDS5 where to find the necessary extra information.Note that BNDF files can become very large, so be careful in prescribing the timeinterval.One way to reduce the size of the output file is to turn off the recordingof boundary information on desired obstructions. On any given OBST line,

14.5. ANIMATED BOUNDARY QUANTITIES, BNDF 117

Figure 14.2: Output of animated boundary quantities, BNDF as viewed in Smoke-view

if the string BNDF_OBST=.FALSE. is included, the obstruction is not consid-ered for boundary quantities output. To turn off all boundary recording, setBNDF_DEFAULT=.FALSE. on the MISC line. Then individual obstructions can beturned back on with BNDF_OBST=.TRUE. on the appropriate OBST line. Individ-ual faces of a given obstruction can be controlled via BNDF_FACE(IOR)=.TRUE.,where IOR is the index of orientation.The following table summarizes some BNDF parameters:

Table 14.4: BNDF parameters




Figure 14.3: Output of animated isosurfaces, ISOF as viewed in Smokeview

14.6 Animated isosurfaces, ISOF

The ISOF namelist group is used to specify the output of gas phase scalar quanti-ties, as three dimensional animated contours. For example, a 300°C temperatureisosurface shows where the gas temperature is 300°C. Three different values ofthe temperature can be saved via the line:

&ISOF QUANTITY=’TEMPERATURE’,VALUE(1)=50., VALUE(2)=200., VALUE(3)=500. /

where the values are in °C.Any gas phase quantity can animated via isosurfaces, but use caution. To renderan isosurface, the desired quantity must be computed in every mesh cell at everyoutput time step. For quantities like TEMPERATURE, this is not a problem, asFDS5 computes it and saves it anyway. However, soot density or oxygen demandsubstantial amounts of time to compute at each mesh cell.The following table summarizes some ISOF parameters:

14.7. REALISTIC SMOKE AND FIRE 119

Table 14.5: ISOF parameters



VALUE(3) Real Contour values

14.7 Realistic smoke and fire

When you do a fire simulation using the default mixture fraction combustionmodel, FDS5 automatically creates two output files that are rendered by Smoke-view as realistic looking smoke and fire.

Figure 14.4: Output of soot MASS FRACTION and HRRPUV as viewed in Smoke-view

By default, the output quantities MASS FRACTION of soot and HRRPUV (heatrelease rate per unit volume) are used in the visualization.


14.8 Heat release rate

Quantities associated with the overall energy budget are reported in the commadelimited file mycase_hrr.csv. This file is automatically generated.The file consists of six columns. The first column contains the time in seconds.The second through fifth columns contain integrated energy gains and losses, allin units of kW. The second column contains the total heat release rate, the thirdcontains the radiative heat loss to all the boundaries (solid and open), the fourthcontains the convective and radiative heat loss to the boundaries (i.e. the energyflowing out of or into the domain), and the fifth contains the energy conductedinto the solid surfaces. The sixth column contains the total burning rate of fuel,in units of kg/s. It is included merely as a check of the total heat release rate.An example:

Table 14.6: Output of HRR in mycase_hrr.csv file

s kW

FDS_HRR_Time HRR

0.00E+000 0.00E+000

5.00E+000 2.04E+002

1.00E+001 2.16E+002

14.9 Visibility

If you are performing a fire calculation using the mixture fraction approach, thesmoke is tracked along with all other major products of combustion.The intensity of monochromatic light I passing a distance L through smoke isattenuated according to:

I/I0 = e−KL (14.1)

The light extinction coefficient K, is a product of the density of smoke particulate,ρYS, and a mass specific extinction coefficient Km that is fuel dependent:

K = KmρYS (14.2)

14.10. UPPER AND LOWER LAYERS 121

As showed by [Mulholland 2002], an estimate of visibility through smoke can bemade by using the equation:

S = C/K (14.3)

where C is a non-dimensional constant characteristic of the type of object beingviewed through the smoke.Since K varies from point to point in the domain, as it depends on smoke density,the visibility S does as well.Keep in mind that FDS can only track smoke whose production rate and com-position are specified. Predicting either is beyond the capability of the presentversion of the model. Three parameter control smoke production and visibility;each parameter is input on the REAC line:

• SOOT_YIELD is the fraction of fuel mass that is converted to soot.

• MASS_EXTINCTION_COEFFICIENT, the Km of Equation 14.2 on the facingpage. The default value is 8700m2/kg, a value suggested for flamingcombustion of wood and plastics.

• VISIBILITY_FACTOR, the C of Equation 14.3. C = 8 for a light-emittingsign and C = 3 for a light-reflecting sign. C is 3 by default.

The visibility S is output via the QUANTITY keyword VISIBILITY.

14.10 Upper and lower layers

Fire protection engineers often need to estimate the location of the interfacebetween the hot, smoke-laden upper layer and the cooler lower layer in a burn-ing compartment. Relatively simple fire models, often referred to as two-zonemodels, compute this quantity directly, along with the average temperature ofthe upper and lower layers. In a computational fluid dynamics (CFD) modellike FDS5, there are not two distinct zones, but rather a continuous profile oftemperature. Nevertheless, there are methods that have been developed to es-timate layer height and average temperatures from a continuous vertical profileof temperature.The quantities LAYER HEIGHT, UPPER TEMPERATURE and LOWER TEMPERATUREcan be designated via DEVC lines in the input file. For example, the entry:


&DEVC XB=2.0,2.0,3.0,3.0,0.0,3.0,QUANTITY=’LAYER HEIGHT’, ID=’whatever’ /

produces a time history of the smoke layer height at x = 2 and y = 3 betweenz = 0 and z = 3. If multiple meshes are being used, the vertical path cannotcross mesh boundaries.

14.11 Heat fluxes and thermal radiation

There are various ways of recording the heat flux at a solid boundary. If youwant to record the net heat flux to the surface, qconvective+ qradiative, use theQUANTITY=’HEAT FLUX’. The individual components, the net convective andradiative fluxes, are CONVECTIVE HEAT FLUX and RADIATIVE HEAT FLUX, re-spectively.If you want to compare predicted heat flux with a measurement, you often needto use GAUGE HEAT FLUX. The difference between NET HEAT FLUX and GAUGEHEAT FLUX is that the former is the rate at which energy is absorbed by the solidsurface; the latter is the amount of energy that would be absorbed if the surfacewere cold or some temperature specified with GAUGE_TEMPERATURE.All of the above heat flux output quantities are defined at a solid surface.

14.12 Interfacing with structural models

FDS5 solves a one-dimensional heat conduction equation for each boundary cellmarking the interface between gas and solid, assuming that material propertiesfor the material layers are provided. The results can be transferred (via eitherDEVC or BNDF output) to other models that predict the mechanical response ofthe walls or structure. For many applications, the one-dimensional solution ofthe heat conduction equation is adequate, but in situations where lateral heatconduction within the solid is significant, another approach can be followed.FDS5 includes a calculation of the Adiabatic Surface Temperature (AST), aquantity that is representative of the heat flux to a solid surface, following theidea proposed by [Wickstrom 2007].Obviously, the objective in passing information to a more detailed model is toget a better prediction of the solid temperature (and ultimately its mechanicalresponse) than FDS5 can provide.

14.13. VISUALIZING LAGRANGIAN PARTICLES 123

14.13 Visualizing Lagrangian particles

Tracer particles can be injected into the flow field from vents or obstacles, asshown in Section 11.8 on page 89, and then observed in Smokeview. Streak linescan be drawn, too.

14.14 Frequently used output quantities

The following table shows an organized list of frequently used output quantities.The column “Namelist” details where the concerned QUANTITY can be specified:B boundary file (BNDF), D device (DEVC), I isosurface file (ISOF), S slice file(SLCF).

Table 14.7: Frequently used output quantities

QUANTITY Description Unit Namelist

NET HEAT FLUX Sum of radiative heat flux andconvective heat flux at a solidboundary

kW/m2 B,D

RADIATIVE HEAT FLUX Radiative heat flux at a solidboundary

kW/m2 B,D

CONVECTIVE HEAT FLUX Convective heat flux at a solidboundary

kW/m2 B,D

MASS FLUX∗ Mass flux at solid surface of thespecified specie

kg/m2/s B,D

MASS FRACTION∗ Mass fraction of the specified specie kg/kg D,I,S

VOLUME FRACTION∗ Volume fraction for the specifiedspecie

mol/mol D,I,S

LAYER HEIGHT Estimate height of the interfacebetween the hot, smoke-laden upperlayer and the cooler lower layer in aburning compartment

m D

UPPER TEMPERATURE Upper layer temperature °C D

LOWER TEMPERATURE Lower layer temperature °C Dcontinued on next page


from previous pageMASS FLOW Calculate integrated mass flow

through a given planar area.kg/s D

HEAT FLOW Calculate integrated heat flowthrough a given planar area.

kJ/s D

VOLUME FLOW Calculate integrated volume flowthrough a given planar area.

m3/s D

VISIBILITY Visibility length through smoke m D,I,S

SOOT VOLUME FRACTION Soot volume fraction mol/mol D,I,S

FED Fractional effective dose based onCO, CO2, and O2 as proposed by[Purser 2002]

D

TEMPERATURE Gas temperature. °C D,I,S

THERMOCOUPLE Temperature of a simulatedthermocouple, usually close to thegas temperature.

°C D

WALL TEMPERATURE Solid interface temperature. °C B,D

ADIABATIC SURFACE TEMPERATURE See Section 14.12 on page 122. °C B,D

VELOCITY Gas phase velocity. m/s D,I,S

NORMAL VELOCITY Gas phase wall normal velocity. m/s D,B

WALL CLOCK TIME Elapsed wall clock time s D∗These quantities require the specification of a gas specie using SPEC_ID.

Here are some examples of output quantities:

&DEVC XYZ=0.1,0,2.39, QUANTITY=’THERMOCOUPLE’,ID=’2.4’ /

&DEVC XB=0.1,0.1,0,0,0.0,2.4,QUANTITY=’LAYER HEIGHT’, ID=’layer_h’ /

&ISOF QUANTITY=’TEMPERATURE’, VALUE(1)=60.0 /&SLCF PBY= -3., QUANTITY=’TEMPERATURE’,

VECTOR=.TRUE. /&BNDF QUANTITY=’ADIABATIC SURFACE TEMPERATURE’ /

There are some output quantities that require a specie name via SPEC_ID, asfor example the MASS FRACTION quantity. The SPEC_ID can select one of

14.14. FREQUENTLY USED OUTPUT QUANTITIES 125

the species implicitly defined when doing a mixture fraction calculation (fuel,oxygen, nitrogen, water vapor, carbon dioxide, carbon monoxide, hydrogen,soot, other), or an extra gas specie from those defined via a SPEC line:

&SLCF PBX= 0.1, QUANTITY=’VOLUME FRACTION’,SPEC_ID=’carbon dioxide’ /volume fraction of carbon dioxideissued from mixture fraction modelgas phase combustion model

&SLCF PBX= 0.1, QUANTITY=’VOLUME FRACTION’,SPEC_ID=’CARBON DIOXIDE’ /volume fraction of carbon dioxidedefined in a SPEC namelist groupand injected separately in the flow domain

Chapter 15

Real world examples

This chapter shows some real world examples.

15.1 Building a ventilator

If you just want to blow gases in a particular direction, create a thin (zerocells thick) OBST and apply to it, via SURF_ID only, a SURF line that has theparameter POROUS=.TRUE. along with the other velocity parameters describedabove. This allows hot, smokey gases to pass through the obstruction, much likea free-standing fan. These obstructions are merely flat plates, by necessity. Forexample:

&SURF ID=’blow’, POROUS=.TRUE., VEL=5.0 /pushes towards +x direction

&OBST XB=4.0,5.0,-0.2,-0.2,1.4,1.8 /

The velocity VEL associated with a POROUS surface represents the velocity in thepositive or negative coordinate direction, as indicated by its sign. Sign convention

15.2 Prescribing a simplified burning material

Real objects are often difficult to describe via the SURF and MATL parameters. Sosimplified descriptions of burning solid fuels are possible. If ignition temperatureand burning rate as a function of time from ignition are known, add lines similarto the following:

127

128 CHAPTER 15. REAL WORLD EXAMPLES

&MATL ID=’stuff’, CONDUCTIVITY=0.1,SPECIFIC_HEAT=1.0, DENSITY=900.0 /

&SURF ID=’my_surf’, MATL_ID=’stuff’, HRRPUA=1000.,IGNITION_TEMPERATURE=500., HEAT_OF_VAPORIZATION=1000.,RAMP_Q=’fire_ramp’, THICKNESS=0.01 /

&VENT XB=0.6,1.0,0.3,0.7,0.0,0.0, SURF_ID=’my_surf’ /&RAMP ID=’fire_ramp’, T=0.0, F=0.0 /&RAMP ID=’fire_ramp’, T=10.0, F=1.0 /

• IGNITION_TEMPERATURE delays the emission of fuel gas until the specifiedtemperature is reached: when the surface temperature reaches 500°C, theobject starts emitting fuel gas.

• HEAT_OF_VAPORIZATION tells FDS5 to account for the energy used tovaporize the fuel; in the example, the net heat flux at the material surfaceis reduced by a factor 1000 kJ/kg times the instantaneous burning rate.

If a MATL line is invoked by a SURF line containing a specified HRRPUA, then thatMATL ought to have only thermal properties and no reaction parameters, productyields. . .By specifying HRRPUA, you are controlling the burning rate rather than lettingthe material pyrolyze based on the conditions of the surrounding environment.

15.3 Simulation and revelation of smoke of asmoldering fire

By default, FDS5 assumes that the smoke from a fire is generated in directproportion to the heat release rate. A value of SOOT_YIELD=0.01 on the REACline means that the smoke generation rate is 1% of the fuel burning rate. Thesmoke in this case is not explicitly tracked by FDS5, but rather is assumed to bea function of the mixture fraction variables that are explicitly tracked.Suppose, however, that you want to define your own smoke and that you wantto specify its production rate independently of the HRR (or even instead of anactual fire, like a smoldering source). You also want to define a mass extinctioncoefficient for the smoke and an assumed molecular weight (as it will be trackedlike a gas specie). Finally, you also want to visualize the smoke using the SMOKE3Dfeature in Smokeview.Use the following lines:

15.4. A PAN FILLED OF ETHANOL 129

&SPEC ID=’my smoke’, MW=29.,MASS_EXTINCTION_COEFFICIENT=8700. /

&SURF ID=’smolder’, TMP_FRONT=1000., MASS_FLUX(1)=0.0001,COLOR=’RED’ /

&VENT XB=0.6,1.0,0.3,0.7,0.0,0.0, SURF_ID=’smolder’ /&PROP ID=’Acme Smoke’, QUANTITY=’CHAMBER OBSCURATION’,

SPEC_ID=’my smoke’ /&DEVC XYZ=1.00,0.50,0.95, PROP_ID=’Acme Smoke’, ID=’smoke_1’ /&DUMP SMOKE3D_QUANTITY=’my smoke’, DT_PL3D=30. /

15.4 A pan filled of ethanol

Here is an example of a steel pan filled with a thin layer of ethanol:

&MATL ID=’ethanol’, EMISSIVITY=1.0, NU_FUEL=0.97,HEAT_OF_REACTION=880.,CONDUCTIVITY=0.17, SPECIFIC_HEAT=2.45, DENSITY=787.,ABSORPTION_COEFFICIENT=40., BOILING_TEMPERATURE=76. /

&MATL ID=’steel’, EMISSIVITY=.95, DENSITY=7850.,CONDUCTIVITY=45.8, SPECIFIC_HEAT=0.46, /

&MATL ID=’concrete’, DENSITY=2200.,CONDUCTIVITY=1.2, SPECIFIC_HEAT=0.88, /

&SURF ID=’pool’, MATL_ID=’ethanol’,’steel’,’concrete’,THICKNESS=0.01,0.001,0.05 /

&VENT XB=0.,1.,0.,1.,0.,0., SURF_ID=’pool’ /

15.5 A simple car parking

15.5.1 Description

In Figure 15.1 on the following page a simple case of a fire in a car parking isproposed. The data are for demonstration only, and the proposed solution is onlyone of the many possible solutions.The objectives of the analysis are:

• Study the critical times for evacuation and rescue operations from firebrigade of the car parking.


Figure 15.1: Car parking plan

15.5. A SIMPLE CAR PARKING 131

• Obtain adiabatic surface temperature to predict the mechanical responseof the structure.

The following input file is called car_parking.fds, and it is deeply commented.

15.5.2 Input file and results### General configuration

&HEAD CHID=’car_parking’,TITLE=’Covered car parking (10 cars)’ /Name of the case and a brief explanation.

&TIME T_END=3600.0 /The simulation ends at 3600 seconds.

&MISC SURF_DEFAULT=’wall’, CO_PRODUCTION=.TRUE. /All bounding surfaces have a ’wall’ boundary conditionunless otherwise specified.Calculation of formation and destruction of CO at elevatedtemperatures activated.

&REAC ID=’polyurethane’, SOOT_YIELD=0.1875, CO_YIELD=0.02775,C=1.0, H=1.75, O=0.25, N=0.065,HEAT_OF_COMBUSTION=25300., IDEAL=.TRUE. /Gas phase reaction: polyurethane flexible foam (means) fromTewarson SFPE Handbook 3rd ed,SFPE handbook table 3-4.14, p. 3-112.

### Computational domain

&MESH IJK=32,30,10, XB= 0.0, 8.5, 0.5, 8.5, 0.0,2.4 /&MESH IJK=32,30,10, XB=-8.5, 0.0, 0.5, 8.5, 0.0,2.4 /&MESH IJK=32,30,10, XB= 0.0, 8.5,-7.5, 0.5, 0.0,2.4 /&MESH IJK=32,30,10, XB=-8.5, 0.0,-7.5, 0.5, 0.0,2.4 /

Four connected calculation meshes and their cells numbers,total 38400 cells.

### Properties

# Walls&MATL ID=’gypsum plaster’, CONDUCTIVITY=0.48,

SPECIFIC_HEAT=0.84, DENSITY=1440. /Thermo-physical properties of gypsum plaster.

&SURF ID=’wall’, COLOR=’BRICK’, MATL_ID=’gypsum plaster’,


THICKNESS=0.03 /Type of boundary condition for walls.

# CarsBurning surface of the car: (4 + 2 + 4 + 2) * 1.3 + 4 * 2 = 23.6 m2HRR max for a car: 5000 kW, HRR ramps up in 600 sHRRPUA: 5000 / 23.6 = 211.86 kW/m2 (approx: whole car burns)&SURF ID=’first_car’, HRRPUA=211.86, TAU_Q=-600, COLOR=’FLESH’ /

Type of boundary conditions for the first burning car, a burner.&MATL ID=’car_mat’, CONDUCTIVITY=54.0,

SPECIFIC_HEAT=0.465, DENSITY=7850.0 /Properties for steel taken from NUREG-1805 pg. 2-11

&SURF ID=’car’, MATL_ID=’car_mat’, HRRPUA=211.86, TAU_Q=-600,IGNITION_TEMPERATURE=250., THICKNESS=0.005,BACKING=’INSULATED’, COLOR=’DARK OLIVE GREEN 1’ /Type of boundary conditions for other cars.

### Solid geometry

&OBST XB= 7.5 , 7.75,-7.5, 7.5 ,0.0, 2.4 / E wall&OBST XB= -7.5 ,-7.75,-7.5, 7.5 ,0.0, 2.4 / W wall&OBST XB= -7.75, 7.75, 7.5, 7.75,0.0, 2.4 / N wall&HOLE XB= -1.5 , 1.5 , 7.0, 8.0 ,0.0, 2.0 / N entrance

The entrance is open since the beginning.&HOLE XB= 7.0 , 8.0 , 2.0, 7.0 ,2.0, 2.2, COLOR=’PALE GREEN’,

DEVC_ID=’NE_broke’, TRANSPARENCY=.6 / NE window&HOLE XB= -7.0 ,-8.0 , 2.0, 7.0 ,2.0, 2.2, COLOR=’PALE GREEN’,

DEVC_ID=’NW_broke’, TRANSPARENCY=.6 / NW window&HOLE XB= 7.0 , 8.0 ,-2.0,-7.0 ,2.0, 2.2, COLOR=’PALE GREEN’,

DEVC_ID=’SE_broke’, TRANSPARENCY=.6 / SE window&HOLE XB= -7.0 ,-8.0 ,-2.0,-7.0 ,2.0, 2.2, COLOR=’PALE GREEN’,

DEVC_ID=’SW_broke’, TRANSPARENCY=.6 / SW windowWindow panes are broken by temperature.

&VENT XB= -7.5 , 7.5 ,-7.5, 7.5 ,2.4, 2.4, SURF_ID=’wall’ / soffit&VENT PBX= 8.5 , SURF_ID=’OPEN’ / E opening&VENT PBX=-8.5 , SURF_ID=’OPEN’ / W opening&VENT PBY= 8.5 , SURF_ID=’OPEN’ / N opening&VENT PBZ= 2.4 , SURF_ID=’OPEN’ / top opening

Domain borders are open.&OBST XB= 3.0 , 7.0 , 5.0, 7.0 ,0.2, 1.5, SURF_ID=’car’ / +E2 car&OBST XB= 3.0 , 7.0 , 2.0, 4.0 ,0.2, 1.5, SURF_ID=’car’ / +E1 car&OBST XB= 3.0 , 7.0 , -1.6, 0.4 ,0.2, 1.5, SURF_ID=’car’ / E0 car,

Asymmetric parking


&OBST XB= 3.0 , 7.0 , -2.0,-4.0 ,0.2, 1.5,SURF_IDS=’first_car’,’first_car’,’car’ / -E1 car

&OBST XB= 3.0 , 7.0 , -5.0,-7.0 ,0.2, 1.5, SURF_ID=’car’ / -E2 car&OBST XB=-3.0 ,-7.0 , 5.0, 7.0 ,0.2, 1.5, SURF_ID=’car’ / +W2 car&OBST XB=-3.0 ,-7.0 , 2.0, 4.0 ,0.2, 1.5, SURF_ID=’car’ / +W1 car&OBST XB=-3.0 ,-7.0 , -1.0, 1.0 ,0.2, 1.5, SURF_ID=’car’ / W0 car&OBST XB=-3.0 ,-7.0 , -2.0,-4.0 ,0.2, 1.5, SURF_ID=’car’ / -W1 car&OBST XB=-3.0 ,-7.0 , -5.0,-7.0 ,0.2, 1.5, SURF_ID=’car’ / -W2 car

### Control logic

&DEVC ID=’NE_broke’, XYZ= 7.0, 4.5, 2.1,QUANTITY=’TEMPERATURE’, SETPOINT=300. /

&DEVC ID=’NW_broke’, XYZ=-7.0, 4.5, 2.1,QUANTITY=’TEMPERATURE’, SETPOINT=300. /

&DEVC ID=’SE_broke’, XYZ= 7.0,-4.5, 2.1,QUANTITY=’TEMPERATURE’, SETPOINT=300. /

&DEVC ID=’SW_broke’, XYZ=-7.0,-4.5, 2.1,QUANTITY=’TEMPERATURE’, SETPOINT=300. /These devices effectively break window panes at 300°C.

### Output

&DEVC XYZ=0.1,0,2.39, QUANTITY=’THERMOCOUPLE’, ID=’2.4’ /&DEVC XYZ=0.1,0,2.0 , QUANTITY=’THERMOCOUPLE’, ID=’2.0’ /&DEVC XYZ=0.1,0,1.6 , QUANTITY=’THERMOCOUPLE’, ID=’1.6’ /&DEVC XYZ=0.1,0,1.2 , QUANTITY=’THERMOCOUPLE’, ID=’1.2’ /&DEVC XYZ=0.1,0, .8 , QUANTITY=’THERMOCOUPLE’, ID=’0.8’ /&DEVC XYZ=0.1,0, .4 , QUANTITY=’THERMOCOUPLE’, ID=’0.4’ /

Thermocouples.&DEVC XYZ=0.1,0,2.0 , QUANTITY=’FED’, ID=’FED’ /

FED calculation.&DEVC XB=0.1,0.1,0,0,0.0,2.4,

QUANTITY=’LAYER HEIGHT’, ID=’layer_h’ /Layer height calculation.

&ISOF QUANTITY=’TEMPERATURE’, VALUE(1)=60.0 /3D contours of temperature at 60°C.

&ISOF QUANTITY=’VISIBILITY’, VALUE(1)=10.0 /3D contours of VISIBILITY 10 m.

&SLCF PBX= 0.1, QUANTITY=’TEMPERATURE’, VECTOR=.TRUE. /&SLCF PBY= -3., QUANTITY=’TEMPERATURE’, VECTOR=.TRUE. /

Vector slices colored by temperature.&SLCF PBX= 0.1, QUANTITY=’VISIBILITY’ /


&SLCF PBZ= 2.0, QUANTITY=’VISIBILITY’ /Visibility slices.

&SLCF PBX= 0.1, QUANTITY=’VOLUME FRACTION’,SPEC_ID=’carbon monoxide’ /

&SLCF PBX= 0.1, QUANTITY=’VOLUME FRACTION’,SPEC_ID=’carbon dioxide’ /

&SLCF PBX= 0.1, QUANTITY=’VOLUME FRACTION’,SPEC_ID=’oxygen’ /species slices.

&BNDF QUANTITY=’WALL TEMPERATURE’ /&BNDF QUANTITY=’NET HEAT FLUX’ /&BNDF QUANTITY=’ADIABATIC SURFACE TEMPERATURE’ /

Quantities at all solid obstructions.

&TAIL / end of file


Figure 15.2: The entered geometry

Figure 15.3: Heat release rate curve


Figure 15.4: Temperatures measured by the thermocouples at the center of thecar parking area. The effect on temperatures of the glazing breaking is noticeable.

Figure 15.5: Gas temperatures measured in front of the window panes vs time.It is prescribed that the glazing breaks effectively at 300°C. The SE glass is thenearest to the fire and breaks at about 370 s; the NE glass breaks at about 612 s.The others follow later.


Figure 15.6: FED vs time is shown here. At FED=.3 an occupant standing atthe center of the car parking is incapacitated by toxic gases.

Figure 15.7: Layer height vs time is shown. The smoke evacuation is not enoughto keep the smoke layer near the ceiling.


Figure 15.8: AST (Adiabatic Surface Temperature) boundary file at 1200 s.

Figure 15.9: Visibility slice file at 120 s.


Figure 15.10: Visibility (10 m) isosurface at 300 s.

Figure 15.11: Net heat flux on boundary surfaces at 1200 s.

Chapter 16

Using a GUI

An open source, multi platform Graphical User Interface (GUI) for FDS5 is cur-rently being developed.

Figure 16.1: A Blender session

This FDS GUI will be based on the Blender project (http://www.blender.org), anopen source 3D modeler available for Linux, Windows and MacOS X platforms.It will be released. . . when it’s ready!

141

http://www.blender.org

142 CHAPTER 16. USING A GUI

Bibliography

[Hottel 1984] H C Hottel. Stimulation of Fire Research in the United StatesAfter 1940 (A Historical Account). Combustion Science andTechnology, 39:1–10, 1984.

[FDS5 user’s guide] McGrattan K, Hostikka S, Floyd J, “Fire dynamics simulator(version 5) User guide”, NIST Special Publication 1019-5,2008.

[FDS5 technical reference] McGrattan K, Bryan K, Hostikka S, Floyd J, “Firedynamics simulator (version 5) Technical guide”, NIST Spe-cial Publication 1019-5, 2008.

[Smokeview user’s guide] Forney G P, “User’s Guide for Smokeview Version 5 -A Tool for Visualizing Fire Dynamics Simulation Data”, NISTSpecial Publication 1017-1, 2007.

[FDS5+EVAC 2008] Korhonen T, Hostikka S, “Fire dynamics simulator withevacuation, version 5”, VTT Research notes, Espo (Finland),2008.

[Rehm 1978] R.G. Rehm and H.R. Baum. The Equations of Motion forThermally Driven, Buoyant Flows. Journal of Research of theNBS, 83:297–308, 1978.

[NUREG 1824] Verification and Validation of Selected Fire Models for Nu-clear Power Plant Applications. NUREG 1824, United StatesNuclear Regulatory Commission, Washington, DC, 2007.

[Grosshandler 1993] W. L. Grosshandler. “RADCAL: A Narrow-Band Model forRadiation Calculations in a Combustion Environment”, NISTTN 1402; 52 p. April 1993.

143

144 BIBLIOGRAPHY

[Wickstrom 2007] U. Wickstrom, D. Duthinh, and K.B. McGrattan. AdiabaticSurface Temperature for Calculating Heat Transfer to FireExposed Structures. In Proceedings of the Eleventh Inter-national Interflam Conference. Interscience Communications,London, 2007.

[Purser 2002] D.A. Purser. SFPE Handbook of Fire Protection Engineer-ing, chapter Toxicity Assessment of Combustion Products.National Fire Protection Association, Quincy, Massachusetts,3rd edition, 2002.

[Mulholland 2002] G.W. Mulholland. SFPE Handbook of Fire Protection Engi-neering, chapter Smoke Production and Properties. NationalFire Protection Association, Quincy, Massachusetts, 3rd edi-tion, 2002.

Index

Adiabatic Surface Temperature (AST),122

BURN_AWAY, 102Direct Numerical Simulation (DNS), 4EXPOSED, 85Finite-rate approach, 46Fire Dynamics Simulator, version 5 (FDS5),

7Heat Release Rate (HRR), 12, 24, 66,

82, 112, 115, 120INERT, 77INSULATED, 85Large Eddy Simulation (LES), 3Light extinction coefficient, 120Mass extinction coefficient, 120Message Passing Library (MPI), 21MIRROR, 78Mixture fraction model, 45OPEN, 78Reynolds-averaged form of the Navier-

Stokes equations (RANS), 3SAWTOOTH, 100Sign convention, 80, 81, 86, 127Smokeview, 7Visibility factor, 121VOID, 85Zone models, 2, 121

145

intro to fire sim-027

Documents