COMPUTATIONAL ANALYSIS OF AIR-PRE HEATER

USING CFD

Presented by,

R.NITHIYANANDAM B.MOHITH RAJ nithidte@gmail.com mohitraj.b@gmail.com Phone: 9791726620

III-YR MECHANICAL ENGINEERING,

V S B ENGINEERING COLLEGE,

KARUR.

SYNOPSIS

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.

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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 preheater 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 preheater 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:

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*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.

Schematic diagram of air preheater

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.

Tubular region 2

Tubular region1

Flue gas outlet

Flue gas inlet

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*Constraints on loading of boiler may occur.

Thus analysis of flow pattern distribution and checking of design parameters play a

vital role to increase the efficiency of the tubular air preheater 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.

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),

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*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.

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

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CFD Techniques in grid generation:

1. Lax-Wendroff’s Technique

2. Maccormack’s Technique

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.

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

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:

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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)

e) Meshing the model

TETRAHEDRAL MESHING IS DONE TO MODEL

Fig. meshed model of air pre heater

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.

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Finite volume method was originally developed as a special finite difference formulation

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.

3. POST PROCESSING

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

*Domain geometry & grid display *Vector plot *Line and shaded contour plots *2D & 3D surface plots *view manipulation (translation, scaling) *Color post script output

RESULTS AND DISCUSSION RESULTS FOR 60%MCR( MAXIMUM CAPACITY RATING )

FIG .VELOCITY DISRIBUTION

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FIG. PRESSURE DISTRIBUTION

RESULTS FOR 110%MCR( MAXIMUM CAPACITY RATING )

FIG .VELOCITY DISRIBUTION

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VELOCITY – VECTOR PLOT FOR 110% MCR:

CONCLUSION:

Modification Suggested

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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-

preheater same as the specification specified by BHEL, Trichy. So our result

can be adopted for design modification.

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