Hand Calculations, Zone Models and CFD

Areas of Disagreement and Limits of Application in practical Fire Protection

Engineering

Wojciech WęgrzyńskiBuilding Research Institute (ITB)

SFPE Polish Chapter

Piotr TofiłoThe Main School of Fire Service

SFPE Polish Chapter

Rafał PorowskiThe Main School of Fire Service

SFPE Polish Chapter

Presentation• Introduction

• Illustration of the issue – plume theory

• Strengths & Weakness

• Practical examples

• Opportunities & Threats

• Correct workflow of an engineering work:

Analysis of theoretical

boundaries of the study

Choice of applicable tools for the design

Preliminary studywith simple tools

Verification of the assumptions and

the tools

Proposal of a solution

Advanced studyof the solution

Verification of the study with simple

tools

SolutionProblem

Introduction

Redesign?

Introduction• Realistic workflow of an engineering work…

What doesengineer feel,

that should work

CFDSolutionProblem Plenty of pictures

to support the first case

If it does not work, it is a problem…

The Knowledge• Fire Safety Engineers should have a reasonable background, to

judge on basic solutions, know the boundaries of their tools andhow to use them.

• Engineers are responsible for their own V&V• Other participants of the process may not be that competent in this

field – they need a simple tool for verification of the engineers work• Education falls short in providing solution to this issue – there are

far more people involved on different stages of the design, than wecan educate!

The Knowledge

Why lack of knowledge is a problem?

One man can ask more stupid questions, than twenty wiseman can answer…

The Knowledge

Why lack of knowledge is a problem?• With insufficient ability to verify the design on the other side, it

sometimes feels like a waste of time to explain the solution• Engineer takes a risk, that the solution will not be understood, and

possibly reverted• Engineer may be forced into a sub-optimal solution or unnecessary

simulations, required only for explanation but not for the solutionitself

Toolbox

So what is in our toolbox?• Simple „hand calculation” models that origin from experimental

studies and relate to particular phenomena;• Zone (or lumped) models that relate to a global view on the fire, but

in a simplified geometry and discretization• CFD and other numerical tools, that may give the most detailed

answer but are time and resource consuming

• Often the CFD is viewed as „oracle” for the design – and due to itscost is done at the end, becoming a tool of acceptance and not thedesign;

• Engineers may be forced to do the CFD because it is the easiesttool to prove your solution for AHJ

• Tools are misused or used beyond their scope of application, butuser is not aware, as he is not verifying his solution with otherapproach

Toolbox

Is There a Problem with the Choice of Tools?

To illustrate that, we have chosenthe axisymmetric plume theory,which is one of most investigatedfire related phenomena, and onethat is used in almost any firerelated design.

Is There a Problem with the Choice of Tools?

What Are the Differences between Models?

Q = 2500 kW, Q” = 250 kW/m² Q = 5000 kW, Q” = 250 kW/m²

What Are the Differences between Models?

Q = 2500 kW, Q” = 1000 kW/m² Q = 5000 kW, Q” = 1000 kW/m²

What Are the Differences between Models?

What Are the Differences between Models?

The Explanation• Hand models are based on different experiments, with their own

limitations. The closer to the experiment, the better the results;• In some areas, the results of CFD are matching the hand-calc closely,

but there are areas where the differences are enormous;• Choosing a tool may be a choice of model, user is not aware of;• Can a hand model be a verification tool for more complex tool, given the

error?• Will anyone beside a Fire Safety Engineer be aware of these issues?

Our Idea• We need a good guideline on how to choose our models, where are

their limits and what is their scope of application• We have good guidelines on V&V (SFPE, NIST, NUREG etc.), but some

parties will not go into a complex methodology, as they cannot afford thetime or lack the knowledge

• The first step for this – what are the strengths, weakness, opportunitiesand threats connected with our tools?

Strong Points of Our ToolsHand calculations/simple models• steady state calculations unless the time is directly connected to the investigated

phenomena (i.e. sprinkler activation, surface flame spread);• finding realistic limits for a phenomena (i.e. maximum temperature that can occur);• initial check, preliminary studies in which instant result is required;• spatial domain directly connected to the size of phenomena investigated; • level of details described by the method is directly connected to the size of the

domain.

Strong Points of Our ToolsZone Fire Models• transient simulations;• situations in which the zone averaged output is enough and the detailed local

conditions are not in the main scope of the analysis;• quick computational time makes it a suitable tool for probabilistic studies;• model made with modular geometry of compartments, with hard limitations in their

size;• flows between compartments may be analyzed;• possible use of sub models (plume, ceiling jet), but have the same limitations as

the hand calculations.

Strong Points of Our ToolsCFD• transient or steady state simulations of complex flows of heat and mass in almost any geometry;• location of features within the model influences the results – can be used in assessment of complex

ventilation system and their optimization;• the level of details that can be investigated is limited mostly by the time and space discretization;• overall size and complexity of the model is limited only by the preparation time and computational

power available;• investigation of far field results without focusing on details of complex phenomena (i.e. to assess

smoke movement we substitute pyrolysis and flamelet model into volumetric source of heat and smoke)

Weak Points of Our Tools

• The weak points we present in the article can be attributed to themost of user errors in the modelling process,

• Moving out of the scope of model application (geometry, size offire, air velocity, turbulent flow etc.)

• Trying to model phenomena too complex for the model – eachadditional „complication” into the model increases the uncertaintyand resources require to process it!

A Good Practice• An illustration on how to benefit from knowledge of strength and

weakness of each model is shown on four different examples• At times, a single tool may not be enough to solve the issue, and at

other times using more tools may actually simplify the wholeprocess

• It is always good to take a look back and verify your results with adifferent, even not ideal, tool

Example 1

Flow out of a small compartment, estimationof the temperature:• possible to do both with a CFD• estimation of the flow – Zone model• estimation of the temperature – hand model

Example 1

Scenario Mass flow (CFD) Mass flow (B-Risk)B1 13,8

13,39

B2 13,3B3 13,15B4 13,6B5 13,6B6 13,5

Flow out of a small compartment, estimation of the temperature

Example 2Flow out of a small compartment through a very small opening

Example 2

• Modelling the compartment fire a priori with a CFD is a difficult andvery resource consuming task, with the results being burdened withlarge, unknown, error…

• Modelling the flow our of small opening with a zone model isimpossible…

• Solution – use both, transpone the mass flow estimated with Zonemodel through the opening into a boundary condition in the CFD

Example 3Jet fan system design in a car park

Example 3Jet fan system design in a car park:• CFD is too expensive to use as a general design tool for simple car

parks, while it is the best tool for this case• Simple hand models can allow first dimensioning of the system, but

are unable to provide a proof to AHJ that system works. Thesemodels may seem difficult so the designers often go with ownjudgment based on experience, as these projects have a „feel” ofrepeatability, while this routine may lead to a fault design!

A solution is to pre-design with hand-calc, and verify with CFD

Example 4Tunnel ventilation system design

Example 4Tunnel ventilation system design:• boundaries of the system will strongly influence the results of the

analysis;• It is extensive effort to make a 3-D model of a whole system in order

to resolve the pressure at the boundaries• There are 1-D models that may be used for a tunnel ventilation

design, in order to simplify this problem and provide the designer withcorrect values of pressure loss at the tunnel ends

The results of this study on applicability of various models is summarizedthe best by presenting opportunities and threats connected to eachmodel.

A user is responsible for his V&V, but there is a need for morecomprehensive guidelines on applicability of the models, which can beused by not only engineers, but also AHJ, investors, third party orstudents.

Summary

OpportunitiesHand calculations/simple models Zone Fire Models CFD• methods presented in standards

are usually hand calculation/simple models – these methods are usually straightforward accepted by AHJ;

• quickest method, can be used to find initial assumptions for further studies and thus reduce amount of complex analysis to carry (i.e. calculation of the thrust of jet-fans required in a tunnel);

• solution of complex small scale phenomena to become a boundary condition for CFD modeling;

• probabilistic search for worst case scenario for further research;

• great academic tool to present visualization of the theoretical aspects;

• almost limitless creation of numerical domain, including many details hidden in different approach - solution suiting the project;

• cost optimization studies due to possibility of using smaller safety margins (i.e. combination of smoke and wind simulation may lead to optimization of NSHEV);

ThreatsHand calculations/simple models Zone Fire Models CFD• while in their scope of application, that

usually is limited by the original experimental validation, expected results should be correct, using these methods outside of their range may lead to large error;

• relying on maximum theoretical values of parameter in the design leads to unnecessary overdesign, far bigger than required margin of safety (i.e. temperature of steel calculation from EC-1);

• going over limits of the model may yield reliable results, but the real area of application depends on the case and generally remains unknown;

• local turbulence in the flow of smoke may be critical for mixing air and smoke, and lead to failure of ventilation system – it is a phenomena not possible to witness in zone model;

• due to high cost of the method, long computational time and difficulties in verification, CFD simulations require high work ethics and great knowledge – the computation is as good, as the engineer behind it;

• user error may cause fault results, that can be overlooked if not checked with additional means;

• often used as the “confirmation” and not the “design” study – errors shown by analysis at late stage of the project may be intentionally hidden or neglected;

Thank You for Your Attention!Piotr Tofiło

The Main School of Fire ServiceSFPE Polish Chapterpiotr.tofilo@gmail.com

+48 728 109 723

Rafał PorowskiThe Main School of Fire Service

SFPE Polish Chapterrporowski@gmail.com

Wojciech WęgrzyńskiInstytut Techniki Budowlanej

w.wegrzynski@itb.plfire@itb.pl

+48 696 061 589