ANALYSIS OF AIR PRE-HEATER BY USING

COMPUTATIONAL FLUID DYNAMICS (CFD)

A TECHNICAL SEMINAR REPORT

Submitted by

N.VIGNESH (080111810023)

(vigneshmani90@gmail.com)

DEPARTMENT OF MECHANICAL

ENGINEERING.

TAGORE INSTITUTE OF ENGINEERING &

TECHNOLOGY,

DEVIYAKURICHI-636112

ANNA UNIVERSITY OF TECHNOLOGY,

COIMBATORE

DECEMBER-2011

ABSTRACT

The process of heat and mass transfer, fluid flow, chemical reactions etc play a

vital role in a wide variety of Industrial applications. Since all the engineering

activities and process are basically configured on these essential processes the

designer and engineers must be armed with the knowledge and methodology to

predict them quantitatively. This will also enable them to operate existing equipment

efficiently, optimize the results, preclude and or minimize potential dangers.

Throughout most of the twentieth century the study and practice of the fluid

dynamics involved the use of pure theory on one hand and pure experiment on the

other hand. Full scale experimental investigations in most cases are prohibitively

expensive, whereas prototype model do not always simulate all the features of the

actual equipment. Theoretical prediction often employ suitable setup mathematical

model, which represent the physics and chemistry of the system of the behavior using

mathematical equation, embodying relevant scientific and empirical knowledge.

However advent of the high-speed digital computer combined with the

development of accurate numerical algorithm for solving physical problem has

revolutionized the new third approach in fluid dynamics i.e. computational fluid

dynamics.

Thus CFD is poised to make an entry in to the wider industrial community and

helps to reduce development time scales and costs of new design.

2

CONTENTS

CHAPTER

NO

TITLE PAGE

NO

List of Figures 4

1 Introduction 5

2 Types of air pre heater 6

3 Problem definition 7

4 Introduction to CFD 10

5 Basic elements 19

5.1 Solver 19

5.2 Pre processor 20

5.3 Post processing 21

6 Results and Discussion 22

6.1 Velocity distribution results for

60% MCR 22

6.2 Pressure distribution results for

60% MCR 23

6.3 Results for 110% MCR 24

6.4 Velocity-vector plot for 110% MCR 25

7 Conclusion 26

References 27

3

LIST OF FIGURES

FIG.NO DESCRIPTION P.NO

1 Air pre-heater in steam generator 5

2 Schematic of air pre heater 8

3 Meshing techniques 11

4 Tetrahedral meshed model of air pre-heater 20

5 Velocity distribution for 60% MCR 22

6 Pressure distribution for 60% MCR 23

7 Velocity distribution for 110% MCR 24

8 Velocity –vector plot 25

4

Chapter 1

Introduction

An air pre-heater (APH) is a general term to describe any device designed to

heat air before another process (for example, combustion in a boiler) with the primary

objective of increasing the thermal efficiency of the process. They may be used alone

or to replace a recuperative heat system or to replace a steam coil. In particular, this

article describes the combustion air Pre-Heaters used in large boilers found in thermal

power stations producing electric power from e.g. fossil fuels, biomasses or waste.

The purpose of the air Pre-Heater is to recover the heat from the boiler flue gas which

increases the thermal efficiency of the boiler by reducing the useful heat lost in the

flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or

chimney) at a lower temperature, allowing simplified design of the ducting and the

flue gas stack. It also allows control over the temperature of gases leaving the stack

(to meet emissions regulations, for example).

FIGURE: 1 Air Pre-Heater in steam generator

5

Chapter 2

Types of air Pre-Heater

There are two types of air Pre-Heaters for use in steam generators in thermal

power stations: One is a tubular type built into the boiler flue gas ducting, and the

other is a regenerative air Pre-Heater.

2.1 Tubular type:

Tubular Pre-Heaters consist of straight tube bundles which pass through the

outlet ducting of the boiler and open at each end outside of the ducting. Inside the

ducting, the hot furnace gases pass around the Pre-Heater tubes, transferring heat from

the exhaust gas to the air inside the Pre-Heater

2.2 Regenerative air Pre-Heaters:

There are two types of regenerative air Pre-Heaters: the rotating-plate

regenerative air Pre-Heaters (RAPH) and the Stationary-plate regenerative air Pre-

Heaters

a. Rotating-plate regenerative air Pre-Heater:

Typical Rotating-plate Regenerative Air Pre-Heater (Bi-sector type) The

rotating-plate design (RAPH) consists of a central rotating-plate element installed

within a casing that is divided into two (bi-sector type), three (type) sectors containing

seals around the element. The seals allow the element to rotate through all the sectors,

but keep gas leakage between sectors to a minimum while providing separate gas air

and flue gas paths through each sector.

b. Stationary-plate regenerative air Pre-Heater:

The heating plate elements in this type of regenerative air Pre-Heater are also

installed in a casing, but the heating plate elements are stationary rather than rotating.

Instead the air ducts in the Pre-Heater are rotated so as to alternatively expose sections

of the heating plate elements to the upflowing cool air.

6

Chapter 3

Problem definition

The tubular air pre-heater (TAPH) of the 20MW bagasse fired boiler, supplied

by BHEL, trichy is one of the continuously available equipment of the power station

at BANNARI AMMAN COGENERATION PLANT. At the economizer exit of the

boiler, the flue gas duct passage into two sections, one heater section in first bend and

other heater in the other bend. The flue gas duct passage is arranged in such a way to

preheat the supplied primary air.

The tubular air Pre-Heater are build and designed with two section named as

tubular region 1 and tubular region 2. Each block is made up with carbon steel .Gas

flows through the air heater at the rate of 254 t/hr, 229.5 t/hr,132.4t/hr at feed rates of

110% MCR, 100% MCR and 60% MCR respectively.

Flue gas is entered into the air pre-heater at the temperatures of 271ºc, 262ºc,

231ºc and leaves at 165ºc, 160ºc, and 150 º c for feed rates of 110% MCR, 100%

MCR, and 60% MCR respectively. Thus gas temperature is dropped to 242ºc. For air

it entered into the air Pre-Heater at the temperature of 33ºc and leaves at 102 ºc. Thus

the heater air Temperature raised to 200ºc, 196ºc, and 165 º c respectively for various

MCR.

Periodic inspection of the plant reveled localized ash deposition in the air pre-

heater. In air pre-heater both LHS and RHS were opened and inspected during the

shut down for vapour pipe modification work.

The observations are recorded below:

*On LHS block of APH 85% of the area ash is clear. Only 15% of the area

towards front duct wall was covered by ash.

*On RHS, it had heaps of the ash settlement, blockage of air pre-heater.

7

FIGURE: 2 Schematic of air Pre-Heater

The Blockage of Air-Pre heater leads to,

*Temperature gradients in flue gas zones leading to temperature differences in steam

circuit

*Differences in the exit gas temperatures and non uniform loading of ID fan.

*It will mislead in the operation of bypass air dampers of FD fans.

*Constraints on loading of boiler may occur.

Tubular region 2

Tubular region1

Flue gas outlet

Flue gas inlet

8

Thus analysis of flow pattern distribution and checking of design parameters

play a vital role to increase the efficiency of the tubular air Pre-Heater performances.

In this current paper CFX software was used to model and analyze of a tubular air

pre-heater performance.

The task to sort out the reasons for ash settling, the task of examining the

effect of flow geometry and Design parameter for a given set of operating condition-

using CFD (computational fluid dynamics) analysis is instituted as the first step in

understanding the phenomenon at work. For this the entire tubular air heater was,

modeled and processed through the three stages

Pre-processing

Solving

Post processing

The results of the above analysis in the form of vector plots, contour plots,

graphs, etc viewed and studied.

9

Chapter 4

Introduction to CFD

CFD constitutes a new third approach in the philosophical study and

development of the whole discipline of Fluid Dynamics. The advent of high speed

digital computer combined with a development of accurate numerical algorithms for

solving physical problems on these computers has revolutionized the way we study

and practice Fluid Dynamics today.

CFD is today an equal partner with pure theory and experiment in the analysis

and solution of Fluid Dynamics problem.

CFD is predicting what will happen quantitatively, when fluids flow, often

with the complications of:

Simultaneous flow of heat,

Mass transfer (e.g. perspiration, dissolution),

Phase change (e.g. melting, freezing, boiling),

Chemical reactions (e.g. combustion, rusting),

Mechanical movement (e.g. Pistons, fans, rudders),

Stresses in and displacement of immersed or surrounding solids

This computer based simulation technique is very powerful and spans wide

range of industrial and non-industrial application areas.

10

4.1 Different Meshing Techniques:

Meshing

Finite Difference Finite Volume Finite Element

Basic Derivation

of Finite difference Basic Derivation of

order of accuracy Finite Volume

Equations

Finite difference

Equations,

Truncation Errors

Types of solutions -

Explicit and Implicit

Stability Analysis

FIGURE: 3 Meshing techniques

CFD Techniques in grid generation

1. Lax-Wendroff’s Technique

2. Maccormack’s Technique

11

4.2 Background and history of CFD

The fundamental bases of almost all CFD problems are the Navier–Stokes

equations, which define any single-phase fluid flow. These equations can be

simplified by removing terms describing viscosity to yield the Euler equations.

Further simplification, by removing terms describing vorticity yields the full potential

equations. Finally, these equations can be linearized to yield the linearized potential

equations.

Historically, methods were first developed to solve the Linearized Potential

equations. Two-dimensional methods, using conformal transformations of the flow

about a cylinder to the flow about an airfoil were developed in the 1930s.[1] The

computer power available paced development of three-dimensional methods. The first

paper on a practical three-dimensional method to solve the linearized potential

equations was published by John Hess and A.M.O. Smith of Douglas Aircraftin

1967. This method discretized the surface of the geometry with panels, giving rise to

this class of programs being called Panel Methods. Their method itself was simplified,

in that it did not include lifting flows and hence was mainly applied to ship hulls and

aircraft fuselages. The first lifting Panel Code (A230) was described in a paper written

by Paul Rubbert and Gary Saaris of Boeing Aircraft in 1968. In time, more advanced

three-dimensional Panel Codes were developed atBoeing (PANAIR,

A502), Lockheed (Quadpan), Douglas (HESS), McDonnell

Aircraft (MACAERO),NASA (PMARC) and Analytical Methods (WBAERO,

USAERO and VSAERO). Some (PANAIR, HESS and MACAERO) were higher

order codes, using higher order distributions of surface singularities, while others

(Quadpan, PMARC, USAERO and VSAERO) used single singularities on each

surface panel. The advantage of the lower order codes was that they ran much faster

on the computers of the time. Today, VSAERO has grown to be a multi-order code

and is the most widely used program of this class. It has been used in the development

of many submarines, surface ships, automobiles, helicopters , aircraft, and more

recently wind turbines. Its sister code, USAERO is an unsteady panel method that has

also been used for modeling such things as high speed trains and racing yachts.

12

NASA PMARC code from an early version of VSAERO and a derivative of

PMARC, named CMARC, is also commercially available.

4.3 Methodology:

In all of these approaches the same basic procedure is followed.

During preprocessing

o The geometry (physical bounds) of the problem is defined.

o The volume occupied by the fluid is divided into discrete cells (the

mesh). The mesh may be uniform or non uniform.

o The physical modeling is defined – for example, the equations of

motions + enthalpy + radiation + species conservation

o Boundary conditions are defined. This involves specifying the fluid

behaviour and properties at the boundaries of the problem. For

transient problems, the initial conditions are also defined.

The simulation is started and the equations are solved iteratively as a steady-

state or transient.

Finally a postprocessor is used for the analysis and visualization of the

resulting solution.

4.3.1 Discretization methods:

The stability of the chosen discretization is generally established numerically

rather than analytically as with simple linear problems. Special care must also be

taken to ensure that the discretization handles discontinuous solutions gracefully.

The Euler equations and Navier–Stokes equations both admit shocks, and contact

surfaces.

Some of the discretization methods being used are:

4.3.2 Finite volume method:

The finite volume method (FVM) is a common approach used in CFD

codes. The governing equations are solved over discrete control volumes. Finite

volume methods recast the governing partial differential equations (typically the

Navier-Stokes equations) in a conservative form, and then discretize the new

13

equation. This guarantees the conservation of fluxes through a particular control

volume. The finite volume equation yields governing equations in the form,

Where Q is the vector of conserved variables, F is the vector of fluxes (see Euler

equations or Navier–Stokes equations), V is the volume of the control volume

element, and   is the surface area of the control volume element.

4.3.3 Finite element method:

The finite element method (FEM) is used in structural analysis of solids, but is

also applicable to fluids. However, the FEM formulation requires special care to

ensure a conservative solution. The FEM formulation has been adapted for use with

fluid dynamics governing equations. Although FEM must be carefully formulated to

be conservative, it is much more stable than the finite volume approach . However,

FEM can require more memory than FVM.

In this method, a weighted residual equation is formed:

Where Ri is the equation residual at an element vertex i, Q is the conservation

equation expressed on an element basis, Wi is the weight factor, and Ve is the volume

of the element.

4.3.4 Finite difference method:

The finite difference method (FDM) has historical importance and is simple to

program. It is currently only used in few specialized codes. Modern finite difference

codes make use of an embedded boundary for handling complex geometries, making

these codes highly efficient and accurate. Other ways to handle geometries include

use of overlapping grids, where the solution is interpolated across each grid.

Where Q is the vector of conserved variables, and F, G, and H are the fluxes in

the x, y, and z directions respectively.

14

4.3.5 Boundary element method:

In the boundary element method, the boundary occupied by the fluid is divided

into a surface mesh.

4.4. High-resolution discretization schemes:

High-resolution schemes are used where shocks or discontinuities are present.

Capturing sharp changes in the solution requires the use of second or higher-order

numerical schemes that do not introduce spurious oscillations. This usually

necessitates the application of flux limiters to ensure that the solution is total variation

diminishing.

4.4.1 PDF Methods:

 (PDF) methods for turbulence, first introduced by Lundgren, are based on

tracking the one-point PDF of the velocity,  , which gives the

probability of the velocity at point   being between   and  . This approach is

analogous to the kinetic theory of gases, in which the macroscopic properties of a gas

are described by a large number of particles. PDF methods are unique in that they can

be applied in the framework of a number of different turbulence models; the main

differences occur in the form of the PDF transport equation. For example, in the

context of large eddy simulation, the PDF becomes the filtered PDF. PDF methods

can also be used to describe chemical reactions, and are particularly useful for

simulating chemically reacting flows because the chemical source term is closed and

does not require a model. The PDF is commonly tracked by using Lagrangian particle

methods; when combined with large eddy simulation, this leads to a Langevin

equation for subfiler particle evolution.

4.4.2 Vortex method:

The vortex method is a grid-free technique for the simulation of turbulent

flows. It uses vortices as the computational elements, mimicking the physical

structures in turbulence. Vortex methods were developed as a grid-free methodology

that would not be limited by the fundamental smoothing effects associated with grid-

based methods. To be practical, however, vortex methods require means for rapidly

computing velocities from the vortex elements in other words they require the

solution to a particular form of the N-body problem 

15

A breakthrough came in the late 1980s with the development of the fast

multipole method (FMM), an algorithm by V. Rokhlin (Yale) and L. Greengard

(Courant Institute). This breakthrough paved the way to practical computation of the

velocities from the vortex elements and is the basis of successful algorithms. They are

especially well-suited to simulating filamentary motion, such as wisps of smoke, in

real-time simulations such as video games, because of the fine detail achieved using

minimal computation.

Software based on the vortex method offer a new means for solving tough

fluid dynamics problems with minimal user intervention. All that is required is

specification of problem geometry and setting of boundary and initial conditions.

Among the significant advantages of this modern technology;

It is practically grid-free, thus eliminating numerous iterations associated with

RANS and LES.

All problems are treated identically. No modeling or calibration inputs are

required.

Time-series simulations, which are crucial for correct analysis of acoustics, are

possible.

The small scale and large scale are accurately simulated at the same time.

4.4.3 Vorticity confinement method:

The vorticity confinement (VC) method is an Eulerian technique used in the

simulation of turbulent wakes. It uses a solitary-wave like approach to produce a

stable solution with no numerical spreading. VC can capture the small scale features

to within as few as 2 grid cells. Within these features, a nonlinear difference equation

is solved as opposed to the finite difference equation. VC is similar to shock capturing

methods, where conservation laws are satisfied, so that the essential integral quantities

are accurately computed.

4.5 Two-phase flow:

The modeling of two-phase flow is still under development. Different methods

have been proposed.The Volume of fluid method has received a lot of attention

lately for problems that do not have dispersed particles.

16

But the level set method and front tracking are also valuable approaches. Most

of these methods are either good in maintaining a sharp interface or at conserving

mass. This is crucial since the evaluation of the density, viscosity and surface tension

is based on the values averaged over the interface. Lagrangian multiphase models,

which are used for dispersed media, are based on solving the Lagrangian equation of

motion for the dispersed phase.

4.6 Solution algorithms:

Discretization in space produces a system of ordinary differential equations for

unsteady problems and algebraic equations for steady problems. Implicit or semi-

implicit methods are generally used to integrate the ordinary differential equations,

producing a system of (usually) nonlinear algebraic equations. Applying

a Newton or Picard iteration produces a system of linear equations which is

nonsymmetric in the presence of advection and indefinite in the presence of

incompressibility. Such systems, particularly in 3D, are frequently too large for direct

solvers, so iterative methods are used, either stationary methods such as successive

overrelaxation or Krylov subspace methods. Krylov methods such as GMRES,

typically used with preconditioning, operate by minimizing the residual over

successive subspaces generated by the preconditioned operator.

Multigrid has the advantage of asymptotically optimal performance on many

problems. Traditional solvers and preconditioners are effective at reducing high-

frequency components of the residual, but low-frequency components typically

require many iterations to reduce. By operating on multiple scales, multigrid reduces

all components of the residual by similar factors, leading to a mesh-independent

number of iterations.For indefinite systems, preconditioners such as incomplete LU

factorization, additive Schwarz, and multigrid perform poorly or fail entirely, so the

problem structure must be used for effective preconditioning.[16] Methods commonly

used in CFD are the SIMPLE and Uzawa algorithms which exhibit mesh-dependent

convergence rates, but recent advances based on block LU factorization combined

with multigrid for the resulting definite systems have led to preconditioners that

deliver mesh-independent convergence rates.

17

4.7 Advantages of CFD over experimental based approaches to fluid

system design:

*Substantial reduction of lead times and costs of new design.

*Ability to study systems where controlled experiments are difficult or impossible

to perform (e.g. very large systems).

*Ability to study systems under hazardous condition at and beyond their normal

performance limits (e.g. safety and accident scenarios)

*Practically unlimited level of detail of results.

18

Chapter 5

Basic elements

CFD codes are structured around the numerical algorithm that can tackle fluid

flow problems.

All CFD CODES contain three main elements

1. Pre-processor

2. Solver

3. Post processor

5.1 Pre-Processor:

Pre-processor consists of the input of a flow problem to a CFD program by

means of an operating-friendly interface and the subsequent transformation of this

input into a form suitable for use by the solver:

The user activities at the preprocessing stages involve:

a) Definition of the geometry of the region of interest-the computational domain

Note: we had drawn the model with the help of co-ordinate system

b) Selection of the physical and chemical phenomena that need to be modeled.

C) Definition of fluid property

d) Specification of appropriate boundary condition (i.e. input and output

conditions)

19

e) Meshing the model

FIGURE: 4 Tetrahedral meshed model of air pre heater

5.2 Solver:

The numerical method that forms the basis of the solver performs the

following steps:

a) Approximation of the unknown flow variables by means of simple functions.

b) Discrimination by substitution of the approximations into the governing flow

equation and subsequent mathematical manipulation.

c) Solution of the algebraic equations.

Finite volume method was originally developed as a special finite difference

formulation.

20

Why finite volume method?

Finite volume method doesn’t demand a uniform, rectangular grid for

computation, such finite volume calculations can be made directly in the physical on a

non-uniform mesh i.e. no transformation is necessary.

5.3 Post Processing:

CFD package are now equipped with versatile data visualization tools. These

includes

Domain geometry & grid display

BVector plot

Line and shaded contour plots

2D & 3D surface plots

view manipulation (translation, scaling)

Color post script output

21

Chapter 6

Results and Discussion

6.1 Velocity distribution Results for 60% MCR (Maximum Capacity

Rating)

FIGURE: 5 Velocity distribution

22

6.2 Pressure Distribution Results for 60% MCR (Maximum Capacity

Rating):

FIGURE: 6 Pressure distribution

23

6.3 Results for 110% MCR (Maximum Capacity Rating):

Velocity Distribution

FIGURE: 7 Velocity distribution

24

6.4. Velocity – Vector plot for 110% MCR

FIGURE: 8 Velocity-Vector plot

25

Chapter 7

Conclusion

Modification Suggested

To prevent ash deposition, certain modifications have to be made either in the

geometry or the fluid flow parameter, if there is any alteration in the mass flow

rate ,then boiler input feed rate ,supply of air also be varied. The plant being an

existing one, change of the operational conditions is impossible.

Result Validation

The result we got is validated; we got the output pressure of air-Pre-Heater

same as the specification specified by BHEL, Trichy. So our result can be adopted for

design modification.

26

References

1. Fundamentals of Compressible Flow with Aircraft & Jet Propulsion - S.M.Yahya,

New Age International Publishers.

2. Computational Fluid Dynamics-T.J. Chung, Cambridge university press.

3. Hills (1996), Power From the Wind, Cambridge University Press.

4. Maxwell, James Clerk (1868), Proceedings of the Royal Society of London.

27