CFD ANALYSIS OF GAS TURBINE

COMBUSTOR

Supervisor

ENGR. NAUMAN JAVED

Submitted by

HASSAAN AHMAD

2011-ME-494

Rachna College of Engineering and Technology, Gujranwala.

A constituent college of

University of Engineering and Technology, Lahore

ii

iii

ACKNOWLEDGEMENT

I want to dedicate my effort to my late mother. May Allah Almighty bless her

with Jannat-ul-Firdous.

The final outcome of this project required a lot of guidance and assistance

from many people and I am extremely fortunate to have got this all along the

completion of my project work. Whatever I have done is only due to such guidance

and assistance and I would not forget to thank them.

Primarily I thank my God for providing me with everything that I required

in completing this project. Then I would like to thank my project advisor Engr.

Nauman Javed, whose valuable guidance has been the one that helped me to

complete my project. His suggestions and instructions have served as the major

contributor towards the completion of the project.

Then I would like to thank my parents and friends who helped me with their

valuable suggestions and guidance that became useful in various phases of the

completion of the project.

Hassaan Ahmad

iv

ABSTRACT

Gas turbine has vast applications in power production due to its high

power to weight ratio. However, one of the difficulties faced in designing

the turbine is high temperature at the exit of combustor, which can cause

melting of the turbine blades. To overcome this problem dilution holes are

made in combustor. The main purpose of this work was to optimize

diameter, position and number of dilution holes for a Can-type combustor.

3D model was made by using reference area approach. Simulations

performed in ANSYS CFX using methane gas (CH4). The results showed

that most appropriate diameter was 30mm, the position had negligible

effect on exit temperature while the optimized number of holes was 5

holes in each row. Two rows of holes in zigzag manner provided the best

results. The structural analysis of the optimized combustor model was also

performed by importing the results of CFD analysis. The results showed

that the model of combustor was structurally stable. The factor of safety

2.95 insured that model was well within safe limits.

v

TABLE OF CONTENTS

Acknowledgement iii

Abstract iv

Table of Contents v

List of Figures vii

List of tables viii

Nomenclature ix

Chapter 1 Introduction 1

1-1 Overview of Gas Turbine 1

1-2 Brayton Cycle 1

1-3 Major Components of Gas Turbine 2

1-3-1 Compressor: 2

1-3-2 Combustion Chamber 3

1-3-3 Turbine 5

Chapter 2 Cooling of Combustion Chamber 6

2-1 Combustion and Dilution 6

2-2 Film Cooling of the liner 7

Chapter 3 Relevant Global Efforts 8

Chapter 4 Geometry 10

4-1 Design Considerations 10

4-1-1 Calculation of Reference Area (Aref) 10

4-1-2 Length of Combustor 11

4-1-3 Diffuser 12

4-2 Modelling of Geometry 13

Chapter 5 Meshing 15

5-1 Patch Independent Method 15

5-2 Mesh Sizing 15

vi

5-3 Named Selections 16

Chapter 6 Numerical Modeling and Boundary Conditions 17

6-1 Numerical Modeling 17

6-1-1 Reynolds Stress Model 17

6-1-2 Eddy Dissipation Model 18

6-1-3 Thermal Energy Model 18

6-2 Boundary Conditions 18

6-2-1 Calculation of Boundary Conditions 18

6-2-2 Applying the Boundary Conditions in ANSYS CFX 20

Chapter 7 Results 21

7-1 Contour Plots 21

7-2 For One Row of Dilution Holes 22

7-2-1 By varying the Number of Dilution Holes 22

7-2-2 By varying the Diameter of Dilution Holes 22

7-2-3 By varying the distance from injector of Dilution Holes 23

7-3 For Two Rows of Dilution Holes 23

7-3-1 By varying the Number of Dilution Holes 23

7-3-2 By varying the Diameter of Dilution Holes 24

7-3-3 By varying the distance from injector of Dilution Holes 24

Chapter 8 Structural Analysis 26

8-1 Boundary conditions 26

8-2 Results 27

Chapter 9 Conclusion 28

Chapter 10 Future recommendation 29

References 30

vii

LIST OF FIGURES

Fig. 1-1 (a) PV diagram. (b) TS diagram for Brayton Cycle 1

Fig. 1-2 Schematic Diagram of Gas turbine Cycle 2

Fig. 1-3 (a) Centrifugal Flow Compressor. (b) Axial Flow Compressor. [3] 3

Fig. 1-4 Air Distribution of combustion chamber [3] 4

Fig. 1-5 (a) Can Type, (b) Can-Annular Type and (c) Annular Type Combustor 5

Fig. 2-1 Distribution between Primary and Dilution Zone [3] 6

Fig. 4-1 Dimension Liner and casing in millimetres 13

Fig. 4-2 Cross-sectional View of Casing liner Assembly in Creo Parametric 2.0 13

Fig. 4-3 Cross-section of Fluid domain 13

Fig. 4-4 Cross Sectional view of Fluid Domain with Dilution Holes 14

Fig. 5-1 Cross-sectional View of Meshed Model 16

Fig. 5-2 Named Selctions of Boundaries 16

Fig. 7-1 Pressure distributions on a plane passing through the central axis of combustor 21

Fig. 7-2 Temperature distribution on plane inside the combustor 21

Fig. 7-3 Graph showing variation of exit temperature with (a) number of holes (b) position

of holes (c) diameter of holes 25

Fig. 8-1 Tetrahedron Meshing of Liner for Structural Analysis 26

Fig. 8-2 Imported Pressure Load to ANSYS Mechanical Solver 26

Fig. 8-3 Equivalent von-Mises stress 27

Fig. 8-4 Total Deformation 27

viii

LIST OF TABLES

Table 2-1 Combustor inlet and exit temperature for modern gas turbines [1] 6

Table 4-1 Parameters of SGT-300 [13] 10

Table 4-2 Values of Aref, Dref, Aft, Dft 11

Table 4-3 Value of LPZ, LSZ 11

Table 4-4 Value of casing’s diffuser area Ao, diameter Do , liner’s diffuser Area As and

Diameter Ds 12

Table 4-5 Diffuser angle & length of diffuser 12

Table 5-1 Number of nodes and elements at mesh size of 2mm 15

Table 6-1 Specifications of Siemens Industrial Gas Turbine SGT-300 [13] 19

Table 7-1 Exit Temperature variation by varying number of holes (one Row) 22

Table 7-2 Variation of exit temperature along the diameter of dilution holes (one Row) 23

Table 7-3 Variation of exit temperature with the distance of dilution holes from fuel injector

(one Row) 23

Table 7-4 Exit Temperature variation by varying number of holes for (Two Rows) 24

Table 7-5 Variation of exit temperature along the diameter of dilution holes (Two Rows) 24

Table 7-6 Variation of exit temperature along the distance from fuel injector (Two Rows) 24

Table 8-1 Results of structural analysis 27

ix

NOMENCLATURE

Subscripts

W Work Output Cyc Cycle

Q Heat Input T Turbine

m Mass flow rate C Compressor

LHV Lower heating value F Fuel

H Enthalpy A Air

Η Efficiency ref Reference

A Area ft Liner

P Pressure PZ Primary zone

D Diameter SZ Secondary zone

ΔP Pressure drop DZ Dilution zone

T Temperature O Diffuser inlet

L Length an Annulus between liner and casing

Ψ Diffuser angle S Swirler

dif Diffuser

1

CHAPTER 1

INTRODUCTION

1-1 Overview of Gas Turbine

Gas turbine is versatile item in turbo machinery and power plants. It can be used in

aviation, power generation, oil and gas sector and small industries. It produces great amount of

energy with small size and weight. Gas turbine is an internal combustion engine which operates

on variety of fuels, which include natural gas, naphtha, diesel fuel, low Btu gases, vaporized

fuel oils and biogas. [1]

Gas turbine has single and two shafts models. Two shaft model has two shafts which

operate at different speeds. One shaft is attached to compressor and driven by turbine and the

other shaft driven by power turbine is connected to external load. Two shaft design is preferred

in industrial gas turbines because it allows the load to be driven at variable speed. [2] [3]

The exhaust gases of gas turbine have very high temperature so gas turbine can be

operated along with steam turbine in combined cycle power plant to give better efficiency. The

exhaust gases can also be used for heating purposes in cogeneration mode. [3]

1-2 Brayton Cycle

The gas turbine is based on Brayton cycle. The cycle consists of two isobaric and two

isentropic processes. Figure below shows P-V and T-S diagram for ideal Brayton Cycle.

[1]

(a) (b)

Fig. 1-1 (a) PV diagram. (b) TS diagram for Brayton Cycle

1-2 is isentropic compression in compressor 2-3 is isobaric heat addition in combustion

3-4 is isentropic expansion in turbine 4-1 is isobaric heat rejection at exhaust

2

Introduction

Figure below shows the schematic diagram of Brayton cycle.

Fig. 1-2 Schematic Diagram of Gas turbine Cycle

Heat added to system.

ctcyc WWW Eq. 1-1

Total work output.

233,2 )( hmhmmLHVmQ afaf Eq. 1-2

The overall efficiency. [1]

3,2Q

Wcyc

cyc Eq. 1-3

1-3 Major Components of Gas Turbine

Gas turbine Cycle involves three main components, compressor, combustion chamber

and turbine. The air from atmosphere is taken at ambient temperature and is compressed to

higher pressure. This highly compressed air is then sent into the combustion chamber where

the fuel is injected into the combustion chamber and is ignited to in the velocity and internal

energy of air. These hot gases are then used to run the turbine which is connected to external

load.

1-3-1 Compressor:

Compressors are used in gas turbine to compress the inlet air for combustion chamber.

For gas turbine only continuous flow compressor are used i.e. centrifugal and axial compressors.

Axial flow compressor first accelerates the working fluid and then diffuses it to obtain

a pressure increase. The working fluid is accelerated by a row of rotor (moving blades) then its

pressure is increased by row of stator blades these stator blades remain stationary all the time.

The row of stator and rotor blade combine to form one stage of axial compressor and give 1.1:1

3

Introduction

to 1.4:1 compression ratio per stage. Multiple stages are used to give overall compression ratio

of 40:1. Axial compressor gives high flow but low head.

In centrifugal flow compressor the fluid is compressed by forcing the fluid through

rapidly rotating the impeller blades. In this way, the velocity of fluid is converted into pressure

partially in diffuser and partially in impeller. The centrifugal compressors give high

compression ratio 1.3:1 to 13:1 per stage. Air initially enters the impeller at the inducer, inducer

is very much like rotor of axial compressor, then air goes 90° turn in diffuser to give the

pressure increase. [1]

(a) (b)

Fig. 1-3 (a) Centrifugal Flow Compressor. (b) Axial Flow Compressor. [3]

1-3-2 Combustion Chamber

In combustors the fuel is burnt almost at almost stoichiometric condition with one third

or less discharge air from compressor. The compressed air at velocity up to 500ft/s from

compressor is fed into the combustion chamber, after diffusing through diffuser to decrease the

velocity up to 80ft/s. The temperature of air varies from 454°C to 649°C in typical industrial

gas turbines. The turbine combustion chambers have three basic features: a recirculation zone,

a burning zone and a dilution zone. Ideally all the fuel should be burnt completely within the

burning zone. In dilution zone air is mixed with burnt gases to lower the mixture temperature

4

Introduction

to a level that turbine can handle. Figure below shows the distribution of these zones and air

distribution. Normally the combustion temperature is about 1927°C. [1]

Fig. 1-4 Air Distribution of combustion chamber [3]

There are three main types of combustion chambers i.e. can type, annular type and can-

annular type.

Can type combustors are mainly used in industrial applications. In this multiple cans

are used. In some books this type is also known as multiple combustors. The gases during

combustion cannot transfer from one can to other. But instead gases are mixed after exiting

from combustor.

Can-annular combustors are the combustors which have arrangement similar to can

type combustor but in addition to that the gases can also transfer from one can to other these

cans form a complete circle and provide a uniform outlet gaseous mixture. These type of

combustors are also known as turbo-annular combustors and are used in industrial and aviation

applications.

Annular combustors are mainly used in air-crafts where weight is primary concern. In

this type gases single hollow circular combustor is made with many fuel injectors and air inlets.

Burnt gases form a uniform mixture and a uniform temperature is achieved throughout the

periphery of turbine.

5

Introduction

Fig. 1-5 (a) Can Type, (b) Can-Annular Type and (c) Annular Type Combustor

1-3-3 Turbine

Turbine uses the energy of hot gases to produce torque. There are two types of turbines

used in gas turbine; radial inflow and axial flow turbine.

Radial inflow turbines operate in reversed manner of centrifugal compressor. Hot

moving gases drive that turbine by flowing inwards in radial direction and this rotates turbine.

Radial inflow turbines are not widely used because they don’t have greater frontal area as

compared to axial flow turbine. However, axial turbines are longer than radial turbine but the

design of radial turbines in very difficult that’s why axial flow turbine are preferred. [1]

Axial flow turbine are reverse of axial flow compressors they have stator and rotor blades

stator blades direction the hot gases onto rotor blades to maximize the rotation of rotor blades

turbine blade rotate with very high velocity and due to high temperature of hot gases turbine

blades have to bear thermal stresses.

6

Cooling of Combustion Chamber

CHAPTER 2

COOLING OF COMBUSTION CHAMBER

Our concern is thermal stresses produce in combustion chamber of gas turbine. For this

purpose, only a small portion of air is used for burning fuel while rest of the air is used for

mixing and cooling. 28% volume of a typical combustor is used for burning and rest of 72%

of volume consists of dilution zone where relatively cooler air is mixed with the hot burning

gases to keep the wall of combustor at lower temperature. Figure below shows the distribution

between primary and dilution zone. [1]

Fig. 2-1 Distribution between Primary and Dilution Zone [3]

Table 2-1 Combustor inlet and exit temperature for modern gas turbines [1]

2-1 Combustion and Dilution

Properly developed combustion chambers have equivalence ratio of 1.0 in primary zone.

This reduces visible smoke. After combustion the burning mixture flows in between the rows

of jet row entering the combustor these row of jets helps to keep the wall temperature to a lower

level. There are dilution holes in liner to make temperature appropriate for blade of turbine and

to increase the volume flow of gases for the outlet of combustor.

Pressure Ratio Combustor-inlet Temperature Combustor-exit Temperature

(°C) (°C)

New Industrial 17:1 to 35:1 541-614 927-1593

New Aircraft Engine upto 45:1 541-925 1593

7

Cooling of Combustion Chamber

In practice it is found that the holes in primary zone should not be greater than 0.1 of

liner diameter and dilution holes diameter can be adjusted to obtain desired exit temperature of

combustor. Dilution holes not only responsible for exit temperature but they also reduce

thermal NOx emissions.

2-2 Film Cooling of the liner

To increase liner’s life it is necessary to decrease the liner temperature. For this purpose

air film cooling methods are used to reduce temperature of the inside and outside of liner.

Metallic rings are mounted around the complete circumference of liner with small holes. These

holes form a film of air around hot burning gases. But these are not called dilution holes.

Dilution holes are the holes through which larger amount of air enters the liner. [1]

8

Relevant Global Efforts

CHAPTER 3

RELEVANT GLOBAL EFFORTS

Turbine inlet temperature should be low enough to save turbine blades from melting.

Ideally speaking, it is the temperature at the exhaust of combustion chamber. For this purpose

dilution holes are made in combustion chamber which reduce the exhaust gases temperature to

a value that turbine can bear.

Bhupendra Khandelwal et al. (2013) observed substantial variation in swirler

performance by changing vane angle, vane number and mass flow. Four different types of axial

and radial velocity profiles observed. He concluded that turbulence distribution pattern shows

double peaks at all positions and reduces with increasing axial distance. [4]

Ana Costa Conrado et al. (2004) calculated for the dimensions of the casing, the liner,

the diffuser, and the swirler using reference area approach and conducted CFD analysis. He

concluded that this method is able to provide a combustor design that attends gas turbine

operation conditions [5]

Leilei Xu et al. (2014) observed that apparent flow separation occurred on pre-diffuser

wall when pre-diffuser wall angle amplified to certain degree. The pre-diffuser exit flow was

distorted, indicating that the uniform exit conditions typically assumed in the diffuser design

were violated. [6]

Fagner Luis Goular Dias et al. (2014) carried out numerical analysis using ANSYS CFX

according to reference area and concluded that some changes in the reference area calculated

by Lefebvre produces better results, especially by improving the burning process and the

behaviour of the flame. [7]

Guenther C. Krieger (2012) performed numerical analysis in the inner flow by using

RSM turbulence model and compared results with experimental data and concluded that the

results are accurate with 2% deviation of temperature. [8]

DING Guoyu, HE Xiaomin, ZHAO Ziqianga, AN Bokun, SONG Yaoyu, ZHU Yixiao

(2014) observed experimentally that if dilution holes are located at 0.6H (H is height of liner

dome) ignition is very difficult and combustion efficiency was higher if placed at 0.9H in a

triple swirler combustor. [9]

9

Relevant Global Efforts

S N Sing et al (2006) has analysed flow in annular combustor by using k-ε turbulence

model and found that uniform velocity distributions are obtained in liner passage area. [10]

P Saravan Kumar et al. (2013) performed CFD analysis on ANSYS CFX using SST

turbulence model of an annular combustor and observed quality velocity and temperature

distributions. [11]

Firoj H Pathan et al. (2012) conducted CFD analysis on can type combustor by using

Eddy Dissipation Combustion Model to reduce the emissions and realized that the Combustion

model is fairly accurate. They changed different parameters air-fuel ratio, swirler angle axial

position of dilution holes and observed variations of emission with parameters. [12]

10

Geometry

CHAPTER 4

GEOMETRY

Can combustor was selected for the analysis. The dimensions of combustor geometry

calculated. The calculations were based on reference area calculation by considering

aerodynamic approach. The method used for calculations was taken from conference

proceedings Basic Design Principles for a Gas turbine Combustor [5]. Both liner and casing

dimensions were calculated and based on these dimensions fluid domain in prepared for the

analysis. Geometry is modelled in PTC Creo Parametric 2.0

4-1 Design Considerations

The design of combustor was done by considering reference area approach this method

involves the calculation of reference area for combustion chamber. Reference area (Aref) is

maximum cross sectional area of casing in absence of liner. This area gives us the diameter of

casing and other dimensions are calculated in term of Aref.

In order to calculate the reference area Aerodynamic approach was used. The formulae

were taken from conference proceedings [5]. Input parameters were based on SGT-300 which

are as follows.

Table 4-1 Parameters of SGT-300 [13]

Quantity Value Units Symbol

Air mass flow rate for one combustor 4.8 kg/s m3

Inlet air temperature 620 K T3

Inlet total pressure of air 1.3×106 Pa P3

Mass flow rate of fuel for one combustor 0.079 kg/s m f

Inlet fuel temperature 400 K

Inlet total pressure of fuel 1.3×106 Pa

Percentage Pressure loss 6.0 % P loss

Pressure loss 7.8×104 Pa ∆P3-4

4-1-1 Calculation of Reference Area (Aref)

Generally for multiple can combustor reference area is given by Eq. 4-1

4

2

ref

ref

DA

Eq. 4-1

11

Geometry

Formula for calculation of Aref is given by Eq. 4-2

21

3

43

432

3

33

P

P

q

P

P

TmRA ref

aref

Eq. 4-2

Percentage pressure loss (ΔP3-4/P3) for can combustor is assumed to be 6%, quantity

(ΔP3-4/Qref) is taken as 30 and Ra=143.5J.kg-1.K-1 as described in research paper.

Putting values in Eq. 4-2

Table 4-2 Values of Aref, Dref, Aft, Dft

Aref Dref Aft Dft

22268.93mm2 170 mm 15888.60 mm2 145 mm

Where refft AA 7.0 and Aft is are cross sectional area of liner and Dft is diameter of

liner.

4-1-2 Length of Combustor

Length of primary zone (LPZ) is ¾th of Dft and length of Secondary zone (LSZ) is ½ of

Dft this gives following values

Table 4-3 Value of LPZ, LSZ

LPZ LSZ

110mm 75mm

The length of Dilution zone (LDZ) is function of Temperature Transverse Quality (TQ).

Where,34

4max

T– T

T– T = TQ assuming suitable values of Tmax and T4 is gives a value of TQ 0.225

For ΔP3-4/Qref =40 the relation for LDZ/Dft is given by Eq. 4-3

23.1386.996.2 TQTQD

L

ft

DZ Eq. 4-3

Substiting values of TQ and Dft ,LDZ was evaluated 200 mm. Total length of Combustor became

410 mm. Total length of liner became 385 mm from the dome.

12

Geometry

4-1-3 Diffuser

If Ao is the area of diffuser at the inlet, Aan is the annular area between Aref and Aft, m3 is

the mass flow rate of air and the man the mass flow rate at the annulus between liner and casing

then Ao is given by

anan

o

m

m

A

A

3 Eq. 4-4

Similary As can be given by Eq. 4-5, where ms is mass flow rate of air at inlet to liner’s

diffuser

30 m

m

A

A ss

Eq. 4-5

Table 4-4 Value of casing’s diffuser area Ao, diameter Do , liner’s diffuser Area As and

Diameter Ds

Ao Do As Ds

9079.20 mm2 107.5 mm 2269.80 mm2 54 mm

The diffuser angle ψ is given by Eq. 4-6

2

3

2

3

22.12

3

33

3

1)(tan

75.1

A

A

AP

TmR

P

Pa

dif Eq. 4-6

Where ΔPdif/P3 is 1% and 1.75×Ra is 504.2 J.kg-1K-1 putting the value in Eq. 4-6 gives

diffuser angle. ψ=17°

The length of diffuser is given by Eq. 4-7

tan

3RRL o

dif

Eq. 4-7

Where Ro and R3 are the radii at o and 3. Putting values in Eq. 4-7 gives

Table 4-5 Diffuser angle & length of diffuser

Diffuser angle (ψ) Length of Diffuser (Ldif)

17° 54 mm

13

Geometry

4-2 Modelling of Geometry

The geometry based on the calculations is modelled in PTC Creo Parametric 2.0. The

casing and liner is made and fluid domain is extracted from the assembly of liner and casing.

The geometry’s dimension are shown in Fig. 4-1

Fig. 4-1 Dimension Liner and casing in millimetres

Fig. 4-2 Cross-sectional View of Casing liner Assembly in Creo Parametric 2.0

The extracted fluid domain is show in Fig. 4-3

Fig. 4-3 Cross-section of Fluid domain

14

Geometry

Geometry is then exported to ANSYS Workbench 15.0 as an IGES file. Up till now it

contained primary holes and secondary holes, whereas dilution holes are made in Workbench’s

Design-modeller where Position, Diameter and Numbers of holes are set as Input parameters.

Fig. 4-4 Cross Sectional view of Fluid Domain with Dilution Holes

15

Meshing

CHAPTER 5

MESHING

Meshing was performed in ANSYS Mechanical Meshing. The tetrahedron patch

independent mesh method was used.

5-1 Patch Independent Method

The patch independent method is technique of meshing in which faces and boundaries

are not respected. This technique enables us to make uniform mesh size. Patch independent

mesh method is a type of tetrahedron mesh, in which only tetrahedron elements are generated.

The quadrilaterals elements are more efficient than tetrahedrons but the quadrilaterals cannot

be generated for complex geometry.

5-2 Mesh Sizing

Mesh sizing was another critical parameter, because if mesh size is very small it will give

a large number of elements solution will be more accurate but more time consuming. Whereas

large size means that small number of elements and nodes, less accurate solution but less time

for computation. That’s why considering an optimization analysis, in which a large number of

simulations have to be performed, a moderate size selection was necessary. Therefore, a global

element size of 2mm was selected, producing 61964 nodes and 321782 elements as show in

Table 5-1

Table 5-1 Number of nodes and elements at mesh size of 2mm

Mesh Size (mm) Nodes Elements

2 61964 321782

16

Meshing

Fig. 5-1 Cross-sectional View of Meshed Model

5-3 Named Selections

The three faces were selected and named, in order to make the process of boundary

conditions specification easy. The faces were end of combustion chamber which was named as

outlet. The starting face was named as inlet. The face enclosed by swirler vanes was named as

fuel injector.

Fig. 5-2 Named Selctions of Boundaries

The meshed model is then exported to ANSYS CFX using workbench.

17

Numerical Modeling and Boundary Conditions

CHAPTER 6

NUMERICAL MODELING AND BOUNDARY

CONDITIONS

6-1 Numerical Modeling

ANSYS CFX was used for analysis. The ANSYS CFX is a general purpose

Computational Fluid Dynamics (CFD) software suite that combines an advanced solver with

powerful pre- and post-processing capabilities.

Following models were selected for analysis in CFX.

Reynolds Stress Turbulence Model (RSM).

Eddy Dissipation Combustion Model.

Thermal Energy Model for Heat Transfer.

6-1-1 Reynolds Stress Model

Reynolds Stress model may be used in the following types of flow:

Free shear flows with strong anisotropy, like a strong swirl component. This includes

flows in rotating fluids.

Flows with sudden changes in the mean strain rate.

Flows where the strain fields are complex, and reproduce the anisotropic nature of

turbulence itself.

Flows with strong streamline curvature.

Buoyant flow.

Since combustor geometry produces the swirling flows and strong streamlines curvature are

also to be observed. The RSM was best suited for the analysis. Also according to Guenther C,

Krieger et al (2012) the RSM produced the best approximated experimental results for flow

characteristics in combustor.

The Reynolds averaged momentum equations for the mean velocity are

18

Numerical Modeling and Boundary Conditions

Miji

jii

j

j

i

j

ji

j

i Suuxx

p

x

U

x

U

xUU

xt

U

" Eq. 6-1

The static (thermodynamic) pressure by

kx

Upp

3

2" Eq. 6-2

6-1-2 Eddy Dissipation Model

The eddy dissipation model is based on the concept that chemical reaction is fast relative

to the transport processes in the flow. When reactants mix at the molecular level, they

instantaneously form products. The model assumes that the reaction rate may be related directly

to the time required to mix reactants at the molecular level. In turbulent flows, this mixing time

is dominated by the eddy properties and, therefore, the rate is proportional to a mixing time

defined by the turbulent kinetic energy, k, and dissipation, ε.

krate

Eq. 6-3

This concept of reaction control is applicable in many industrial combustion problems

where reaction rates are fast compared to reactant mixing rates.

6-1-3 Thermal Energy Model

This models the transport of enthalpy through the fluid domain. It differs from the Total

Energy model in that the effects of mean flow kinetic energy are not included. It consequently

reproduces the same results as the Total Energy model when kinetic energy effects vanish, and

is therefore adequate for low speed flows where kinetic effects are negligible.

6-2 Boundary Conditions

Boundary conditions for the analysis is first calculated and then set in ANSYS CFX.

6-2-1 Calculation of Boundary Conditions

Since the no practical data was available for boundary conditions. Boundary conditions

were calculated for single can-type combustor based on specifications of Siemens Industrial

Gas Turbine SGT-300

19

Numerical Modeling and Boundary Conditions

Table 6-1 Specifications of Siemens Industrial Gas Turbine SGT-300 [13]

Mechanical drive 8.2mw (11,000bhp)

Fuel Natural Gas

Efficiency 34.6%

Heat rate 10,400kj/kwh (7,350 btu/bhph)

Turbine speed 11,500rpm

Compressor pressure ratio 13:1

Exhaust gas flow 29.0kg/s (63.9lb/s)

From Eq. 1-2 & Eq. 1-3

LHVm

W

f

c

Eq. 6-4

LHV

Wm

c

f

mf=0.47 kg/s where LHV for methane gas is 50MJ/kg and W and ηc are taken from

Table 6-1. mf is total mass flow rate for 6 combustors.

The mass flow rate of fuel for one combustor (mf) 0.079 kg/s

Approximated mass flow rate air can be calculated from exhaust air flow rate.

The mass flow rate of air for one combustor (ma) 4.8 kg/s

The temperature of inlet air is calculated by formula of isentropic process and by taking

efficiency of compressor as 80%.

For isentropic process

1

1

2

1

2

P

P

T

T Eq. 6-5

Where, T2 is ideal temperature of inlet air to combustor.

T1 is temperature of atmospheric air. 300K

P2/P1 is compression ratio of compressor. From Table 6-1

γ is 1.4 for air.

Ideal temperature of inlet air to combustor. 624.3K

The actual temperature of air was calculated by incorporating isentropic efficiency of

compressor.

20

Numerical Modeling and Boundary Conditions

12

12

TT

TT

Eq. 6-6

Actual temperature of inlet air to combustor. 705.3K

The fuel temperature is consider equal to atmospheric temperature.

6-2-2 Applying the Boundary Conditions in ANSYS CFX

The calculated conditions were then applied in ANSYS CFX. The imported mesh of fluid

domain of combustor. A material with name methane air mixture is created and default

combustion reaction from library of CFX is selected. The fluid domain was assigned methane

air mixture. And the reference pressure of 13atm was selected. In fluid model tab. RSM

turbulence, Thermal Energy heat transfer model, and Eddy dissipation Reaction Model was

selected.

Firstly, a boundary was created at named selection AIR INLET, and specified as inlet.

The mass flow rated of 4.8 kg/s and temperature of 705.3K was specified. The Mass fraction

of O2, CO2 and H2O were set to 0.232, 0.01 and 0.01 respectively and remaining is N2.

Secondly, a boundary was created at named selection FUEL INJECTOR, and specified

as inlet. The mass flow rate of 0.079 kg/s and temperature of 300.0K was specified. The Mass

fraction of CH4, O2 and H2O were set to 1, 0 and 0.

Thirdly, boundary was created at named selection OUTLET, and specified as outlet. The

average gauge pressure of 0 Pa was selected.

In solution control the 500 number of iterations are selected and solution is converged

with the convergence criterion of 10-3 of RMS residual. The plot of residual of with number of

iterations is shown.

The results were imported in CFD POST. Where a user defined expression (Exit Temp) was

created. Which was equal to the average temperature at the Outlet of Combustor. Syntax of

expression is shown Eq. 6-7. The expression was then set as workbench Output parameter.

outletetemperaturaveExittemp @)( Eq. 6-7

Input parameters mentioned in 4-2, were varied a large number of simulations where

performed and the results were obtained.

21

Results

CHAPTER 7

RESULTS

7-1 Contour Plots

The pressure, temperature and velocity distributions were obtained from CFD post.

Fig. 7-1 Pressure distributions on a plane passing through the central axis of combustor

Fig. 7-2 Temperature distribution on plane inside the combustor

Temperature distributions shows air enters the combustor and the reaction starts as air

and fuel (methane) are mixed together. The temperature at low velocity zone i.e. the

recirculation zone is highest but the air entering from primary and secondary air holes keep the

walls of liner cool. Then to reduce the exit temperature the dilution air enters through dilution

holes due to the difference of pressure.

22

Results

There were two cases of analysis.

1. One by using one row of dilution holes.

2. By using two rows of dilution holes in zigzag manner.

The simulations are performed for both the cases by changing three parameters

1. Position of dilution holes.

2. Diameter of dilution holes.

3. Number of dilution holes.

7-2 For One Row of Dilution Holes

Diameter, Number and position of dilution holes is varied by making one row of dilution

holes.

7-2-1 By varying the Number of Dilution Holes

The analysis was performed by varying number of dilution holes from 2 to 9 while

keeping the Diameter 20mm and Distance of 230 mm from fuel injector. Results are as shown

in the Table 7-1

Table 7-1 Exit Temperature variation by varying number of holes (one Row)

No. Of Holes Exit Temp

K

2 1327.19

3 1315.36

4 1329.83

5 1301.05

6 1273.29

7 1255.36

8 1239.39

9 1222.75

7-2-2 By varying the Diameter of Dilution Holes

The analysis was performed by varying diameter of dilution holes from 5 to 30 mm while

keeping the 4 holes in each row and Distance of 230 mm from fuel injector. Results are as

shown in the Table 7-2

23

Results

Table 7-2 Variation of exit temperature along the diameter of dilution holes (one Row)

Diameter of Hole Exit temperature

Mm K

5 1353.26

10 1347.13

15 1349.5

20 1326.01

25 1292.67

30 1264.4

7-2-3 By varying the distance from injector of Dilution Holes

The analysis was performed by varying distance of dilution holes from 190 to 300 mm

while keeping 4 holes in each row and diameter 20m. Results are as shown in the Table 7-3.

Table 7-3 Variation of exit temperature with the distance of dilution holes from fuel

injector (one Row)

Distance from fuel

injectors Exit Temperature

mm K

190 1300.48

200 1314.64

210 1333.83

220 1317.56

230 1327.20

240 1317.13

250 1321.10

260 1338.28

270 1325.12

280 1323.51

290 1338.28

300 1326.08

7-3 For Two Rows of Dilution Holes

Diameter, Number and position of dilution holes were varied by making two row of

dilution holes. The distance between two rows was set equal 30mm.

7-3-1 By varying the Number of Dilution Holes

The analysis was performed by varying number of dilution holes from 2 to 9 while

keeping the Diameter 20mm and Distance of 230 mm from fuel injector. Results are as shown

in the Table 7-4

24

Results

Table 7-4 Exit Temperature variation by varying number of holes for (Two Rows)

No. Of Holes Exit Temp

K

2 1299.14

3 1272.67

4 1240.05

5 1207.17

6 1193.70

7 1182.01

8 1162.21

9 1144.98

7-3-2 By varying the Diameter of Dilution Holes

The analysis was performed by varying diameter of dilution holes from 5 to 30 mm while

keeping the 4 holes in each row and Distance of 230 mm from fuel injector. Results are as

shown in the Table 7-5

Table 7-5 Variation of exit temperature along the diameter of dilution holes (Two Rows)

Diameter of Hole Exit temperature

mm K

5 1262.57

10 1255.60

15 1252.89

20 1245.20

25 1233.16

30 1223.41

7-3-3 By varying the distance from injector of Dilution Holes

The analysis was performed by varying distance of dilution holes from 200 to 270 mm

while keeping 4 holes in each row and diameter 20m. Results are as shown in the Table 7-6

Table 7-6 Variation of exit temperature along the distance from fuel injector (Two Rows)

Distance from fuel

injectors Exit Temperature

mm K

200 1228.68

210 1239.53

220 1235.4

230 1230.94

240 1230.71

250 1238.46

260 1243

270 1243.43

25

Results

Fig. 7-3 Graph showing variation of exit temperature with (a) number of holes (b) position

of holes (c) diameter of holes

(a)

(b)

(c)

26

Structural Analysis

CHAPTER 8

STRUCTURAL ANALYSIS

The structural analysis was also necessary to check the stability of liner. The analysis

was performed on same geometry of liner using ANSYS Mechanical APDL solver. The

pressure load was imported from CFX. The support were defined at outer periphery of liner.

8-1 Boundary conditions

Meshing was done for optimized diameter and number of holes which produced the best

results. A fine tetrahedron mesh was generated using aluminium alloy as liner material. Meshed

model is show in Fig. 8-1.

Fig. 8-1 Tetrahedron Meshing of Liner for Structural Analysis

Pressure load was imported from CFX solution to the structural analysis. The analysis

was done by defining small supports at periphery of liner. The Fig. 8-2 shows imported

pressure in Mechanical.

Fig. 8-2 Imported Pressure Load to ANSYS Mechanical Solver

27

Structural Analysis

The results showed that the model was stable and very low deformation was observed.

Fig. 8-3 Equivalent von-Mises stress

Fig. 8-4 Total Deformation

8-2 Results

Results of structural analysis are shown in Table 8-1.

Table 8-1 Results of structural analysis

Quantity Magnitude

Maximum Total Deformation 7.41×10-6 m

Maximum Equivalent Stress 2.79×107 Pa

Minimum Factor of Safety 2.95

28

Conclusion

CHAPTER 9

CONCLUSION

From this work following conclusions can be deduced.

By observing the graphs between exit temperature and number of holes it is clear that the

exit temperature decreases with the increase in number of holes. But the number of holes cannot

be increased beyond a limit otherwise the combustor liner will become weak. The most

appropriate number of dilution holes is 5 holes in each row.

Two rows of holes in zigzag manner give better results than single row of holes. Two

rows of allow more air to enter through the holes. That’s why the temperature at the exit is

lowered.

Diameter of holes has inverse relation to the exit temperature. The diameter can also not

be increased beyond the limit considering that it may weaken the liner. For the analysed

combustor geometry and dimensions the combustor the most appropriate diameter is 30mm.

The position of dilution holes does not affect the exit temperature because it does not

affect the amount of air entering through the dilution holes.

The structural analysis of combustor showed that the model is structurally stable with

very low deformation and safety factor of 2.95 which is acceptable.

29

Future recommendation

CHAPTER 10

FUTURE RECOMMENDATION

The CFD analysis requires a very high performance hardware, analysis was performed

on a Dual Core with 2 GB Ram and in limited time. So, for better and faster results the analysis

should be performed on a high performance computer.

Experimental setup for combustor requires a high cost. But experimental investigation is

necessary for the actual optimization of dilution holes.

The analysis was performed on calculated dimensions of combustor casing and liner. For

better results, it is recommended that the analysis and experiments should be performed on a

combustor with actual dimensions.

30

References

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