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EXPERIMENTAL AND SIMULATION STUDY ON THE EFFECT OF

GEOMETRICAL AND FLOW PARAMETERS FOR COMBINED-HOLE FILM

COOLING

HASWIRA BIN HASSAN

A project report submitted in

fulfillment of the requirement for the award of the

Degree of Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

AUGUST 2016

iii

Dedicated to my beloved parents, family, lecturers, housemates, graduated members

and all my friends.

iv

ACKNOWLEDGEMENT

I am grateful to the Allah Almighty God for establishing and giving me

strength to complete this Master Project. I also like to express my deepest appreciation

and sincere thanks to my parents; Hassan bin Jais and Masritah bte Abdullah for their

unceasing encouragement and support. A special and sincere gratitude I give to my

supervisor, Dr. Mohammad Kamil bin Abdullah whose helped me with valuable

guidance and advice in completing this Master Project. His continuing and support

throughout this research is greatly appreciated.

I take this opportunity to record my sincere thanks to my research friends;

Mohd Hazim Fadli bin Aminnuddin and Ahmad Fuad bin Mohd Noor, who kindly

share knowledge, give me support and lend their hand for this Master Project. Last but

not least, many thanks go to all laboratory technicians whose guide and teach me

during the simulation and experimental study.

Lastly, I also place my sense of gratitude to one and all who directly or

indirectly have lent their helping hand in this Master Project. Thank you very much.

v

ABSTRACT

Film cooling method was applied to the turbine blades to provide thermal protection

from high turbine inlet temperatures in modern gas turbines. Recent literature

discovers that combining two cylindrical holes of film cooling is one of the ways to

further enhance the film cooling performances. In the present study, a batch of

simulations and experiments involving two cylindrical holes with opposite compound

angle were carried out and this two cylindrical hole also known as combined-hole film

cooling. The main objective of this study is to determine the influence of different

blowing ratio, M with a combination of different lateral distance between cooling holes

(PoD), a streamwise distance between cooling holes (LoD) and compound angle of

cooling hole (1/2) on the film cooling performance. The simulation of the present

study had been carried out by using Computational Fluid Dynamic (CFD) with

application of Shear Stress Transport (SST) turbulence model analysis from ANSYS

CFX. Meanwhile, the experimental approach makes used of open end wind tunnel and

the temperature distributions were measured by using infrared thermography camera.

The purpose of the experimental approach in the present study is to validate three cases

from all cases considered in the simulation approach. As the results shown, the lateral

coverage was observed to be increased as PoD and 1/2 increased due to the interaction

between two cooling air ejected from both cooling holes. Meanwhile, film cooling

performance insignificantly changed when different LoD was applied. As the

conclusion, a combination of the different geometrical parameters with various flow

parameters produced a pattern of results. Therefore, the best configuration has been

determined based on the average area of film cooling effectiveness. For M = 0.5, PoD

= 1.0, LoD = 2.5 and 1 / 2 = -45o/+45o case is the most effective configuration. In the

case of M = 1.0 and M = 1.5, PoD = 0.0, LoD = 3.5, 1 / 2 = -45o/+45o and PoD = 0.0,

LoD = 2.5, 1 / 2 = -45o/+30o are the best configurations based on the overall

performance of film cooling.

vi

ABSTRAK

Teknik filem penyejukan telah digunakan untuk bilah turbin bagi membekalkan

perlindungan daripada suhu yang tinggi di dalam turbin gas moden. Kajian baru-baru

ini mendapati gabungan dua lubang silinder filem penyejukan adalah salah satu cara

untuk meningkatkan lagi prestasi filem penyejukan. Simulasi dan eksperimen yang

melibatkan dua lubang filem penyejukan berbentuk silinder dengan sudut lubang yang

bertentangan telah dijalankan di dalam kajian ini dan dua lubang silinder tersebut juga

dikenali sebagai filem penyejukan lubang gabungan. Objektif utama kajian ini adalah

untuk mengkaji pengaruh nisbah tiupan, M dengan gabungan jarak antara dua lubang

penyejukan pada paksi z (PoD) dan paksi x (LoD) serta sudut lubang filem penyejukan

(1/2) pada prestasi filem penyejukan. Pendekatan simulasi dalam kajian ini

menggunakan Pengkomputeran Dinamik Bendalir (CFD) dengan mengunakan model

gelora Shear Stress Transport (SST) daripada ANSYS CFX. Manakala, dalam

pendekatan eksperimen, terowong angin jenis hujung terbuka telah digunakan dan

taburan suhu telah diukur dengan menggunakan kamera termografi inframerah.

Eksperimen telah dijalankan dalam kajian ini bagi mengesahkan salah satu kes

daripada semua kes yang dipertimbangkan dalam pendekatan simulasi. Hasil kajian

ini menunjukkan bahawa liputan penyejukan yang lebih luas telah dilihat apabila PoD

dan 1/2 meningkat. Manakala, prestasi filem penyejukan hanya mengalami sedikit

perubahan apabila LoD yang berbeza digunakan. Sebagai kesimpulan, gabungan

parameter geometri yang berbeza bersama parameter aliran menghasilkan corak

keputusan yang berbeza-beza. Oleh itu, konfigurasi yang terbaik telah ditentukan

berdasarkan purata kawasan filem penyejukan yang berkesan. Untuk M = 0.5, PoD =

1.0, LoD = 2.5 dan 1 / 2 = -45o/+45o adalah konfigurasi yang paling berkesan.

Manakala dalam kes M = 1.0 dan M = 1.5, PoD = 0.0, LoD = 3.5, 1 / 2 = -45o/+45o

dan PoD = 0.0, LoD = 2.5, 1 / 2 = -45o/+30o adalah konfigurasi yang terbaik

berdasarkan prestasi keseluruhan filem penyejukan.

vii

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS xvi

LIST OF APPENDICES xvii

CHAPTER 1 INTRODUCTION 1

1.1 Introduction 1

1.2 Background of study 4

1.3 Problem statement 6

1.4 Important of study 7

1.5 Objective of study 7

1.6 Scope of study 8

viii

CHAPTER 2 LITERATURE REVIEW 9

2.1 Introduction 9

2.2 Geometrical effect on the film cooling method 9

2.2.1 Film cooling hole design 10

2.2.2 Length and inclination angle of film

cooling hole 12

2.2.3 Streamwise angle for film cooling hole 14

2.3 Flow setting effect on the film cooling method 15

2.4 Lift-off phenomena of the cooling air in the

cylindrical film cooling hole 17

2.5 Combined-hole film cooling method with

compound angle 18

2.6 Aerodynamic performance of film cooling 23

2.7 Thermal performance of film cooling 25

CHAPTER 3 METHODOLOGY 29

3.1 Introduction 29

3.2 Flow chart 29

3.3 Experimental setup in the present study 32

3.3.1 Main source of the mainstream air 33

3.4.2 Main source of the cooling air

3.4.3 Experimental procedure

3.4 Simulation setup in the present study

3.4.1 Computational domain setup

ix

3.4.2 Boundary conditions and flow parameters

3.4.3 Performance indicator applied

3.4.4 Mesh dependency test

CHAPTER 4 RESULTS AND DISCUSSION 52

4.1 Introduction 52

4.2 CFD results validation 52

4.3 Effects of lateral distance between two holes, PoD 55

4.4 Effects of streamwise distance between two holes,

LoD 68

4.5 Effects of compound angle, 1 / 2 of combined-hole

film cooling 80

4.6 Total pressure loss coefficient, Cp 89

4.7 Area average film cooling effectiveness 91

CHAPTER 5 CONCLUSION AND RECOMMENDATION 96

5.1 Conclusion 96

5.2 Recommendation 98

REFERENCES 99

APPENDICES 104

x

LIST OF TABLES

3.1 Flow setting applied in the experimental approach 39

3.2 List of all computational domain at different PoD,

LoD and

42

3.3 List of all computational domain names at

different geometrical parameters

44

3.4 Flow setting applied in three different blowing

ratios

48

3.5 Elements and nodes details in different type of

mesh

50

xi

LIST OF FIGURES

1.1 Modern gas turbine [1] 1

1.2 T-s and P-ν diagram for Brayton cycle [2] 2

1.3 Turbine inlet temperature of MHI gas turbines series

[3]

3

1.4(a) Turbine blade with external cooling techniques [4] 5

1.4(b) Turbine blade with internal cooling techniques [4] 5

2.1(a) Geometry of cylindrical film cooling hole design [22] 11

2.1(b) Geometry of laidback cylindrical film cooling hole

design [22]

11

2.1(c) Geometry of fan-shaped film cooling hole design [7] 11

2.1(d) Geometry of laidback fan-shaped film cooling hole

design [7]

11

2.2(a) Flow streamlines model for counter flow [25] 14

2.2(b) Flow streamlines model for short hole with

unrestricted plenum [25]

14

2.2(c) Flow streamlines model for co-flow [25] 14

2.2(d) Flow streamlines model for long hole with

unrestricted plenum [25]

14

2.3(a) Cylindrical film cooling hole with streamwise angle,

α [6, 26-30]

15

2.3(b) Fan-shaped film cooling hole with streamwise angle,

α [6, 26-30]

15

2.4 The kidney vortex formation of single cylindrical film

cooling hole [35]

18

2.5 The kidney vortex formation of combined-hole film

cooling with opposite compound angle [37]

20

2.6 Film cooling arrangement in Ahn et al. [10] 21

xii

2.7(a) Combined-hole configurations by Kusterer et al. [38] 21

2.7(b) Combined-hole configurations by Wright et al. [39] 21

2.8 Film cooling arrangement in Javadi et al. [42] 22

2.9 Total pressure loss coefficient contour at x/D = 8 for

low blowing ratio [43]

24

2.10 Total pressure loss coefficient graph at x/D = 8 for

different streamwise angle [43]

24

2.11 Pitchwise distributions of total pressure loss

coefficient at different blowing ratios [44]

25

2.12 Film cooling effectiveness distributions from the

experimental study of Mayhew et al. [45]

26

2.13 Centreline film cooling effectiveness comparison at

low blowing ratio, M = 0.5 [45]

26

2.14 Film cooling effectiveness distributions for different

film cooling hole design at high blowing ratio [16]

27

2.15 Laterally averaged film cooling effectiveness results

at high blowing ratio of Wright et al. [39]

28

3.1 Flow chart involving experimental and simulation

approaches

31

3.2 Overall view of the experimental setup 32

3.3 Schematic diagram of the experimental setup 33

3.4 Open loop low speed win tunnel details 34

3.5 Dimensions of the modified test section 34

3.6 Pitot tube used in the experimental approach 35

3.7 Overall view of the blower 36

3.8 Specification of the blower’s motor 36

3.9 Air heater stored in the Bakelite box container 37

3.10 Venturi meter applied to measure the flow rate of the

cooling air

37

3.11 Dimensions on the test plate 38

3.12 Air flow meter used to measure the velocity of the

mainstream air

40

xiii

3.13 Two way valve used to turn the cooling air flow inside

the plenum

40

3.14 Different value of PoD, LoD and applied in the

simulation approach

42

3.15 Detail of computational domain name for different

cases

43

3.16 The example of computational domain considered in

the simulation approach

44

3.17 Dimensions on the side view of the computational

domain

45

3.18 Dimensions on the top view of the computational

domain

45

3.19 Hybrid meshing applied on the computational domain

in the simulation approach

46

3.20 Structured meshing applied on the critical region of

the computational domain

46

3.21 Boundary conditions applied on the computational

domain

47

3.22 Mesh dependency test result 51

4.1 Lateral average film cooling effectiveness

comparison at M = 0.5

53



54



55

4.4 Film cooling effectiveness distribution at different

PoD for M = 0.5

56

4.5 Lateral average film cooling effectiveness at

different PoD for M = 0.5

57

4.6 Vorticity and film cooling effectiveness for AY4545

case at M = 0.5

58

4.7 Vorticity and film cooling effectiveness for BY4545

case at M = 0.5

59

xiv

4.8 Vorticity and film cooling effectiveness for CY4545

case at M = 0.5

60


PoD for M = 1.0

61



63


PoD for M = 1.5

64



65

4.13 Isosurface of film cooling effectiveness, fce = 0.5 at

different cases for M = 1.5

66


LoD for M = 0.5

68



69


LoD for M = 1.0

71



72

4.18 Contour plane with vector for AX3030 case at M =

1.0

73

4.19 Contour plane with vector for AY3030 case at M =

1.0

74

4.20 Contour plane with vector for AZ3030 case at M =

1.0

75


LoD for M = 1.5

75

4.22 Vorticity and film cooling effectiveness for AX3030

case at M = 1.5

76

4.23 Vorticity and film cooling effectiveness for AY3030

case at M = 1.5

77

xv

4.24 Vorticity and film cooling effectiveness for AZ3030

case at M = 1.5

78


different cases

79


compound angle for M = 0.5

81

4.27 Vorticity distribution on the plane x/D = 18 at

different compound angle for M = 0.5

81

4.28 Lateral average film cooling effectiveness for

different compound angle, 1/2 at M = 0.5

83



84



86



87



88

4.33 Total pressure loss coefficient at 1/2 = -30/30 cases 89

4.34 Total pressure loss coefficient at 1/2 = -45/30 cases 90

4.35 Area average film cooling effectiveness at 1/2 = -

30/30 cases

92


45/45 cases

93


60/60 cases

93


45/30 cases

94


60/45 cases

95

xvi

LIST OF SYMBOLS AND ABBREVIATIONS

q - Heat

v - Velocity

w - Work

ρ - Density

∞ - Mainstream flow

Taw - Adiabatic wall temperature

T∞ - Mainstream temperature

T2 - Cooling air temperature

T3 - Turbine inlet temperature

T4 - Turbine outlet temperature

Tu - Turbulence intensity

η - Film cooling effectiveness

Cp - Total pressure loss coefficient

CFD - Computational Fluid Dynamic

D - Hole diameter

y1 - Compound angle of upstream cooling hole

y2 - Compound angle of downstream cooling hole

LoD - Streamwise distance between two holes of combined-hole

PoD - Lateral distance between two holes of combined-hole

Re - Reynolds number

M - Blowing ratio

RMSE - Root mean square error

xvii

LIST OF APPENDICES

A Cases matrix 105

B Mass flow rate calculation for cooling air 106

C Film cooling effectiveness distributions 108

D Lateral averaged film cooling effectiveness 114

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Gas turbine is one of the most popular heat engine used in power generation industry.

One of the attractive features is capability of the gas turbine to operate using various

fuels such as crude oil, liquefied natural gas (LNG), naphtha, kerosene and diesel.

Besides its application in electricity generation, gas turbines also used to generate

power for aircrafts, ships and trains. Figure 1.1 shows the example of modern gas

turbine involving four main sections; compressor, combustion chamber, turbine and

exhaust.

Figure 1.1: Modern gas turbine [1]

2

In principal, gas turbine make used of Brayton cycle as its working cycle. As

shown in Figure 1.2, the cycle involves four processes; compression, heat addition,

expansion and heat rejection. In the compression process, the fresh air is drawn into

the compressor where the temperature and pressure of the air is raised. The compressed

air is then introduces into the combustion chamber and mixed with the fuel which is

injected through the nozzles. The fuel-air mixture then ignited under constant pressure

conditions to complete the heat addition process through converting the chemical

energy of the fuels to flow energy of the combustion gases. Hot combustion gases then

expands through the turbine while producing mechanical energy in terms of rotating

shaft. The expanded combustion gas is then rejected to the ambient.

Figure 1.2: T-s and P-ν diagram for Brayton cycle [2]

The overall thermal efficiency of Brayton cycle which applied on gas turbine

can be determine through Eq. 1.1. Based on the equation, the overall thermal efficiency

is directly proportional to the turbine inlet temperature. The increase in the turbine

inlet temperature not only improve the overall efficiency, but also allow higher power

output of the gas turbine.

3

𝜂𝐵𝑟𝑎𝑦𝑡𝑜𝑛 =𝑤𝑛𝑒𝑡

𝑞𝑖𝑛 = 1 −

𝑇4

𝑇3

where, wnet - net work [Watt]

qin - total heat input [Watt]

T3 - turbine inlet temperature [K]

T4 - turbine outlet temperature [K]

Figure 1.3 shows the pattern of turbine inlet temperature for different gas

turbine produces by Mitsubishi Heavy Industries (MHI) along the year 1962 to 2010.

The turbine inlet temperature has increased tremendously in comparison with its early

design. During its early days in 1940s, the turbine inlet temperature was limited to be

around 540 oC. This limitation is due to the metallurgical reason whereby the available

blade material at that time inhibit higher operational temperature of the gas turbine.

Figure 1.3: Turbine inlet temperature of MHI gas turbines series [3]

The early progress in achieving higher overall thermal efficiency of a gas

turbine via increase in turbine inlet temperature have been propelled by the

introduction of new materials which capable to sustain at higher temperature. These

(1.1)

*Adapted from Mitsubishi Heavy Industries

4

materials allow the gas turbine manufacturer to have much more durable blades to be

applied in gas turbine. In the late 90’s, the introduction of cooling technique into

turbine blade design helps to further increase the turbine inlet temperature.

Combination of various cooling techniques such as impingement cooling, convection

cooling and tip cap cooling lead to a highly sophisticated cooling system which can be

commonly found in today modern gas turbine. The modern gas turbine operates in the

range of 1200 oC to 1600 oC which is expected to further increase in the coming future.

1.2 Background of study

Nowadays, the modern gas turbine operates in very high temperature range and is

expected to increase in upcoming future. This high operating temperature has

improved the overall efficiency and allow higher power output of the gas turbine.

However, due to high operating temperature, the turbine components particularly the

blades are exposed to high thermal loads which compromise its operational durability.

Therefore, enhancements of thermal protection on critical surfaces are required to

ensure not only the reliability but also lowering the maintenance cost of the gas turbine

engine.

Cooling techniques have been introduced and studied in the last few decades

to improve thermal protection of a turbine blade. Various cooling techniques have been

introduced such as internal cooling which involves convection cooling, impingement

cooling and transpiration cooling, while film cooling as external cooling. Each cooling

technique has their own advantages and the combination of these techniques developed

a sophisticated cooling system of the turbine blades.

Film cooling is an external cooling technique which provides a thin layer of

cooling air on the critical section of the blade surfaces. This layer also prevents direct

contact between the high temperature stream and the blade surface. The protective

layer allows the turbine to operate at very high temperature thus improve the overall

performance of the gas turbine. Figure 1.4 shows the example of turbine blade together

with details on internal and external cooling techniques.

5

Figure 1.4: Turbine blade with various cooling techniques [4]

The early introduction of film cooling hole involves a discrete cylindrical hole.

This particular hole design has been widely used even in the modern gas turbine blade

design due to its manufacturability [3-5]. Other designs of film cooling hole have been

also introduced aims to improve the effectiveness of film cooling hole. Fan-shaped

film cooling hole, laidback film cooling hole and trench film cooling hole [6-9] are

some example of other design of film cooling hole.

Fan-shaped and laidback film cooling hole designs are characterized by the

expanded cooling hole exit in comparison to cylindrical film cooling hole. Cooling air

ejected from these types of cooling holes appears to have better lateral spread thus

producing better performance in comparison with the cylindrical hole. On the other

hand, trench film cooling hole design has a small ditch which placed above the film

cooling holes with specific width and depth. By adding the small ditch (trench),

cooling air was covering the surface in streamwise and spanwise direction and

improving the cooling performance.

a) External cooling b) Internal cooling

6

In addition to the aforementioned designs, researchers also study the effect of

combined-hole film cooling. Among them is Ahn et al. [10] where the research focus

on the arrangement of two rows of the cylindrical hole with opposite orientation angle.

Kusterer et al. [11] also combined the cylindrical hole with various compound angle

and distance between two cylindrical holes. Both of the works indicate better film

cooling performance compared to the other film cooling hole design. Hence, many

studies [12, 13, 14] were carried out related to the combined-hole film cooling because

its involved combination of two simple cylindrical holes which simpler to manufacture

in comparison with the laidback or fan-shaped film cooling hole.

1.3 Problem statement

Among the early works on combined-hole film cooling is the work by Han et al. [15].

It has been reported that combined-hole film cooling with opposite compound angle

produced better performance by providing wider lateral coverage. Kusterer et al. [16]

also indicate that combined-hole film cooling provides better lateral coverage after

comparing with fan-shaped film cooling hole.

The combined-hole film cooling was introduced by combining two cylindrical

holes with opposite compound angle to overcome and weaken the formation of kidney

vortex while improving the film cooling performance. By combining two cylindrical

hole, the anti-kidney vortex was produced and counter the formation of kidney vortex

which attracts hot mainstream air to the bottom of cooling air. In combined-hole film

cooling system, it is observed that the anti-kidney vortex produced are controlled by

the distance between cooling holes and compound angle of the cooling holes. The

distance between cooling holes determines the formation of anti-kidney vortex led by

the interaction between two vortices, while the compound angle affects the strength of

each branch of the anti-kidney vortex.

In the work of Han et al. [15], the geometrical parameter of the distance

between cooling holes and compound angle of the cooling holes were considered and

varied to observe the effects on the film cooling performance. However, both

geometrical parameters lead to an asymmetrical flow structure due to the uncertain

7

behaviour of the anti-kidney vortex. This condition also causes the film cooling

effectiveness along the streamwise direction inclined towards one side and leave

another side less protected. Therefore, some improvement were needed to overcome

the asymmetrical flow structure and improve the overall performance of combined-

hole film cooling. Further improvements could possibly achieve by adding and varying

the geometrical and flow parameters of combined-hole film cooling. It is possible if

the geometry parameters and flow parameter match well on the local flow structure.

1.4 Important of study

The present study intended to explore various geometrical and flow parameters to

provide more information on the thermal performance of combined-hole film cooling.

Simulation and experimental approach were consider in the present study to compare

and validate the results between the two different approaches. The information of film

cooling performance, flow structure and optimal arrangement of combined-hole film

cooling will be used as the reference for the future study.

1.5 Objective of study

The objectives of this study are:

a) To predict the film cooling effectiveness of combined-hole using

Computational Fluid Dynamic (CFD);

b) To validate the predicted film cooling effectiveness by CFD with the

experimental results;

c) To determine the influence of geometrical parameters on the film cooling

performance of combined-hole;

d) To determine the influence of flow parameters on the film cooling performance

of combined-hole

8

1.6 Scope of study

The scopes of the present study were divided into two parts; experimental and

simulation. For the experimental work, the open end wind tunnel was considered as

the main source of the mainstream air. Mainstream air flowed inside the wind tunnel

was set at 22.0 m/s which produce Reynolds number of 4200. For the cooling air, a

blower was used to supply the cooling air with a moderate increase of pressure. The

cooling air was supplied through the air heater and venturi meter before entering the

plenum and flow out through the film cooling hole. The infrared thermography camera

was used in the present experimental work to observe the temperature change on the

test plate surface.

For the simulation work, the analysis of ANSYS CFX from Computational

Fluid Dynamics was considered with the application of Shear Stress Transport (SST)

turbulence model. Steady state simulation was applied in the present simulation work

and the auto timescale was used together with RMS residual target value of 1 × 10-4.

Minimum iterations were set at 100 iterations while maximum iterations were set at

300 iterations to complete the simulation process. There is some constant variable

considered in the present simulation work; Reynolds number set at 4200, mainstream

air temperature set at 300K and cooling air temperature set at 310K.

For both experimental and simulation works, several flow and geometrical

parameters was considered. For flow parameter, the blowing ratios are varied at three

different value; 0.5, 1.0 and 1.5. Similarly three geometrical parameters were

considered as below:

a) Lateral distance between two holes of combined-hole unit divide by hole

diameter, PoD was set at 0.0D, 0.5D, 1.0D

b) Streamwise distance between two holes of combined-hole unit divide by hole

diameter, LoD was set at 2.5D, 3.0D, 3.5D

c) Compound angle of cooling hole, 1 / 2 was set at -30o/+30o, -45o/+45o, -

60o/+60o, -45o/+30o, -60o/+45o

9

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Gas turbines are widely used as the power generators for electricity and

transportations. Good performance of a gas turbine was required to allow high power

output and increase the overall performance of the gas turbine. In the aims to improve

the overall performance of a gas turbine, higher turbine inlet temperature is required.

Due to very high temperature, the turbine components particularly the blades are

exposed to high thermal loads. The high temperature also has compromised the

durability and the original structure of the blades. Therefore, the cooling techniques

were introduced to protect and increase the durability of the turbine blade. Film

cooling which is one of the cooling techniques allows to increase the turbine inlet

temperature while providing thermal protection on the blade from hot stream gas.

2.2 Geometrical effect on the film cooling method

Film cooling method which is an external cooling technique was introduced to provide

thermal protection on the turbine blade surfaces. The original structure of the turbine

blade are exposed to the high thermal loads during the operation of high turbine

temperature while keeping the turbine in high performance. By supplying a certain

amount of cooling air through small holes embedded on the turbine blade, film cooling

10

provides thermal protection with a thin layer of cooling air on the blade surfaces and

prevent direct contact between the high temperature stream and the blade surface.

2.2.1 Film cooling hole design

Film cooling hole design plays an important role in determining the cooling air

structure that covered the surface of the turbine blades. In the early development of

film cooling hole, the single cylindrical hole was introduced as a simple and basic

design of film cooling. This film cooling hole widely used in the gas turbine due to

low manufacturing and maintenance cost. Therefore, some research based on the

single cylindrical hole were carried out. In the research of Goldstein [17], the single

cylindrical hole was applied on two and three-dimensional studies to discuss and

provide the further information about the film cooling mechanism of single cylindrical

film cooling hole. As one of the pioneers in this field, Goldstein also provides the

fundamental understanding of film cooling. However, much work must be done

experimentally during the 1970’s to understand the effects of the cooling hole design.

In the other research, Brown and Saluja [18] had studied the performances of

a single cylindrical hole and a row of the cylindrical holes with different pitch diameter

experimentally. Various pitch diameter was set to compare the advantages of a row of

the cylindrical hole with a single cylindrical hole on the film cooling performance. The

results show that the performance of a row of cylindrical hole is better than the single

cylindrical hole. Meanwhile, better film cooling performance was observed at the low

pitch diameter compared to high pitch diameter.

Fan-shaped film cooling hole is the other design introduced since the

commencement of this research field. Differed with the cylindrical hole, large exit area

of fan-shaped film cooling hole was designed to improve the cooling air distribution.

Based on several studies of film cooling [19-21], the expanded exit hole spread the

cooling air wider and produce good film cooling performance. Therefore, to expand

the exit area of the cooling hole more, the laidback geometry of film cooling was

introduced. Figure 2.1 presents the general details of different designs of film cooling

hole aforementioned before. By applying laidback geometry to cylindrical and fan-

11

shaped film cooling holes, the laidback cylindrical and laidback fan-shaped film

cooling hole were produced. These two configurations significantly have wide exit of

cooling hole in comparison with the cylindrical and fan-shaped film cooling holes.

Figure 2.1: Geometry of different film cooling hole design [7, 22]

Goldstein et al. [20] carried out an experimental study comparing three

different types of film cooling hole. The fan-shaped hole, long cylindrical hole and

single cylindrical hole were arranged in a row across the span with three different pitch

diameter. Between all three configurations, fan-shaped hole produces better cooling

air spread at near hole region thus enhanced the lateral coverage of film cooling

effectiveness. Similar with Hyams and Leylek [21], larger exit area of cooling hole

produced better film cooling performance in comparison with the other film cooling

holes. The large exit area of the fan-shaped hole caused the velocity of the cooling air

decrease. Consequently, cooling air stays closer and covering the wall rather than

penetrating into the mainstream flow.

Barigozzi et al. [22] discussed the effects of the cylindrical and laidback

cylindrical film cooling hole on the aero-thermal performance of a film cooling

a) Cylindrical hole b) Laidback cylindrical hole

c) Fan-shaped hole d) Laidback fan-shaped hole

12

endwall. Based on the aerodynamic results, higher thermodynamics losses was

observed on the laidback cylindrical film cooling hole for a constant mass flow rate,

MFR value. However, the minimum loss has been observed at medium MFR value for

both cylindrical and laidback cylindrical film cooling holes. Based on the adiabatic

film cooling effectiveness, cylindrical film cooling hole shows optimum film cooling

coverage at MFR = 0.75%. Meanwhile, laidback cylindrical film cooling hole

produced better film cooling coverage with greater MFR value at 1.5%. By comparing

both results, better cooling air distribution was observed in the case of laidback

cylindrical film cooling hole even with the high MFR value to have better film cooling

performance.

From all aforementioned film cooling hole design, the more expanded exit of

the cooling hole produced better performance in comparison with cylindrical film

cooling hole. However, the cylindrical film cooling hole was more considered in

comparison with the expanded exit film cooling hole such as fan-shaped and laidback

fan-shaped film cooling holes due to low maintenance and manufacturing cost. The

cylindrical hole is simpler to manufacture in comparison with a complicated design of

fan-shaped film cooling hole in the real application on the gas turbine which

consequently increases the manufacturing and maintenance cost.

2.2.2 Length and inclination angle of film cooling hole

Kohli and Bogard [23] study the effect of the inclination angle of the film cooling hole

at constant length experimentally. In terms of aerodynamic performance of the film

cooling, the work reported that larger the inclination angle of the film cooling hole

produces greater cooling air diffusion at near hole region in comparison with a low

inclination angle of the film cooling hole. The diffusion was observed due to vigorous

interaction and mixing between the cooling air with the mainstream air and the cooling

air diffusion becomes a primary cause of the decreased film cooling effectiveness of

the larger inclination angle configuration.

Differed with the previous studies, Yuen and Botas [24] study the effect of a

different inclination angle of film cooling hole on the film cooling performance. The

13

low inclination angle case shows better performance at different blowing ratios in

comparison with the larger inclination angle case. For low inclination angle, the

cooling air remained close to the wall and continued further downstream. Although

the larger inclination angle shows higher film cooling effectiveness at near hole region

for low blowing ratio, the performance is rapidly reduced as the blowing ratio increase.

The cooling air was released vertically further from the wall and not covering the wall

along the streamwise direction. From overall observation, the low inclination angle

case produced better film cooling performance further downstream at various blowing

ratio applied. Compared to the large inclination angle, cooling air only covering 1/5 of

the wall further downstream when blowing ratio increased.

Burd and Simon [25] presents the effect of different types of cooling air

delivery; counter flow, co-flow, short hole with unrestricted plenum and long hole with

unrestricted plenum on the film cooling performance. Counter flow means the cooling

air inside the plenum has an opposite direction to the mainstream flow. Meanwhile,

for co-flow, the cooling air flow inside the plenum has same direction with the

mainstream flow direction. Unrestricted plenum means the cooling air is delivered

directly into the cooling hole without through any plenum. Thus, short and long hole

with unrestricted plenum differentiate the length of the cooling hole with unrestricted

plenum.

Figure 2.2 shows the flow streamline model at different cooling air delivery.

In the case of counter flow and short hole with the unrestricted plenum, the cooling air

was kept to the wall at near hole region due to the redirection of the cooling air after

the interaction with the mainstream air. Meanwhile, the co-flow and long hole with

unrestricted plenum which have great cooling air momentum produced low

effectiveness at near hole region due to cooling air detachment. However, the cooling

air was improved at further downstream. The work also reported that plenum geometry

has negligible effect on the film cooling performance while film cooling hole length

effected the results more appreciably. As shown in Figure 2.2 (b) and (d), the different

of cooling hole length enhancing the cooling air contact on the wall and improved the

lateral spread further downstream.

14

Figure 2.2: Flow streamlines model for (a) counter flow, (b) short hole with

unrestricted plenum, (c) co-flow and (d) long hole with unrestricted plenum [25]

2.2.3 Streamwise angle of film cooling hole

Streamwise angle was introduced to change the orientation of film cooling hole thus

alter the cooling air spread further downstream. The streamwise angle was measured

in positive or negative orientation from the mainstream direction as shown in Figure

2.3, which α is the streamwise angle applied on both films cooling hole designs. As

studied by Sen et al. [26-27], the research considered the application of streamwise

angle on a row of cylindrical film cooling hole. The results were compared with the

other two cases of a row of cylindrical and laidback cylindrical film cooling hole.

Between all three configurations, the cylindrical hole with streamwise angle produced

higher heat transfer rates thus reduced the overall film cooling performances. Followed

by Nasir et al. [28], the research also covered the streamwise angle on a row of

cylindrical film cooling hole. Differed with the previous study of Sen et al., steeper

inclination angle was applied in this study. High heat transfer coefficient also produced

as the streamwise angle applied on the cylindrical film cooling hole and provide low

film cooling performance.

15

a) Cylindrical hole b) Fan-shaped hole

Figure 2.3: Film cooling hole with streamwise angle, α [6, 26-30]

Other works on streamwise angle have also been reported by Ligrani and Lee

[29] and McGovern and Leylek [30], which covered the cylindrical hole with a various

streamwise angle. From both studies, the lateral spread of cooling air was improved

and cooling air covering the wall better than cylindrical hole without a streamwise

angle. However, as streamwise angle increase, the lateral spread was turned into one

side. The kidney vortex also becomes asymmetrical and fundamentally alters the

interaction of the cooling air and mainstream flow. Same as the Brittingham and

Leylek [31] and Brauckmann and Wolfersdorf [6], the spread of cooling air also turned

to one side as the streamwise angle increase on a fan-shaped film cooling hole and on

a row of laidback fan-shaped film cooling holes. Although the application of

streamwise angle altered the cooling air distribution, another method should be

developed to cover the wall laterally and improved the one-sided wall covering.

2.3 Flow setting effect on the film cooling method

Besides the various geometrical parameter applied in film cooling field, several flow

parameters; blowing ratio, density ratio and turbulence intensity also considered in

some research to observe the change of the aerodynamic and thermal performance. In

the early research of Bons et al. [32], the work considered the experimental study of a

single row of cylindrical film cooling hole with 35o of cooling air injection to the

mainstream flow. The elevated blowing ratio applied in the experimental study shows

reduced in the film cooling effectiveness. The blowing ratio, M of 0.7 has better film

cooling performance in comparison with a higher blowing ratio. In the other work of

mainstream direction

16

Abu Talib et al. [33], the cylindrical film cooling hole with an inclination angle of 45o

also produced better film cooling performance at blowing ratio, M = 0.64 in

comparison with the high blowing ratio.

Ligrani and Lee [29] study the effect of high blowing ratio, M in the range of

1.5 – 4.0 on a row of cylindrical film cooling holes with a streamwise angle of 50.5o.

The larger streamwise angle was considered to imitate the arrangement of rotating

blades on the first stage of the operating gas turbine engine. In the study, the film

cooling performance was reduced at elevated blowing ratio value, similarly with the

previous study of Bons et al. [32]. The lift-off effect is the main cause of the separation

of cooling air from covering the blade surfaces while lowering the overall film cooling

performance. Therefore, increase the blowing ratio to M = 4.0 for cylindrical film

cooling hole with a streamwise angle not producing better film cooling performance

in comparison with the film cooling performance at M = 1.5.

In the other research of film cooling, Gritsch et al. [7] had focused on the effect

of various blowing ratio on three different film cooling hole; cylindrical, fan-shaped

and laidback fan-shaped film cooling hole. As the results shown, increased the blowing

ratio cause the elevated heat transfer coefficient for all three different geometries.

However, laidback fan-shaped cooling hole significantly improved film cooling

performance as the heat transfer coefficient and film cooling effectiveness produced

is better in comparison with cylindrical and fan-shaped film cooling hole.

Turbulence intensity of mainstream air is one of the flow parameter applied in

the simulation and experimental study of the film cooling hole. Turbulence indicates

the fluctuation of mainstream air flow. Meanwhile, turbulence intensity is a scale

which characterized the turbulence value and expressed as a percent. A steady flow of

air would have low turbulence and an unsteady flow of air would have higher

turbulence value. In Bons et al. [32] study, besides discussing the effects of blowing

ratio, they also reported the effect of high freestream turbulence on film cooling

effectiveness. Similar with Brown and Saluja [18], the work also reported the effect of

different freestream turbulence intensity on a row of cylindrical hole. High freestream

turbulence intensity significantly reduces the film cooling effectiveness while low

freestream turbulence intensity produces better film cooling performance. The

accelerated spanwise diffusion of the cooling air which produces an early interaction

17

of the cooling jets gives an advantage of high turbulence intensity. Improved film

cooling coverage in the region between injection holes was observed as turbulence

increase. However, the results also indicate that elevated freestream turbulence

reduced the overall film cooling performance due to enhanced mixing.

Schmidt and Bogard [34] also carried out the experimental study of cylindrical

film cooling hole with applying the high freestream turbulence. As an agreement to

the Bons et al. [32] results, increasing the freestream turbulence reduces the film

cooling performance. In summary, blowing ratio and turbulence intensity are

important flow parameters that determine the performance of film cooling hole. The

appropriate combination of blowing ratio and turbulence intensity can be applied at

different film cooling hole design and configuration to optimize the overall film

cooling performance.

2.4 Lift-off phenomena of the cooling air in the cylindrical film cooling hole

Lift-off phenomena is a phenomenon occur in film cooling field which influenced the

film cooling performance along the mainstream flow direction. Lift-off phenomena

indued by the vortex formation due to the interaction between the cooling air and the

mainstream flow. In the cylindrical film cooling hole, a pair of vortex or known as a

kidney vortex was formed with opposite direction of rotation. Kidney vortex attracts

hot mainstream air to the bottom of cooling air ejection from both sides and lift-off the

cooling air from the wall thus leaving it unprotected. Figure 2.4 shows the kidney

vortex formation of cylindrical film cooling hole with opposite direction of rotation

which has been presented by Fric and Roshko [35].

18

Figure 2.4: The kidney vortex formation of single cylindrical film cooling hole [35]

In addition, the lift-off effect also depends on the momentum of the cooling

air. Studied by Leylek and Zerkie [36], the greater size of the vortex was formed and

faster lift-off of cooling air has been observed as the momentum increase. The high

momentum of the cooling air jet fails to attach to the surface and penetrates into the

mainstream air. As the conclusion, lift-off phenomena occurred due to the entrainment

of mainstream air occupying the area beneath the vortices and mixed with the vortices

and consequently, lowering the overall film cooling performance of film cooling.

2.5 Combined-hole film cooling method with compound angle

Combined-hole is a combination between two or more film cooling hole. Meanwhile,

the compound angle is the combination of opposite streamwise angle applied on the

combined-hole film cooling. This combined-hole film cooling configuration with the

application of compound angle was introduced base on an idea to overcome and

weaken the kidney vortex formation while improving the film cooling performance.

By introducing the anti-kidney vortex, the lift-off effect can be reduced and cooling

air able to attach and cover the blade surfaces in a lateral direction.

19

Figure 2.5 presents the concept of kidney vortex formation from combined-

hole film cooling with opposite compound angle which introduced by Han and Ren

[37]. By adding a positive or negative compound angle on the cylindrical film cooling

hole, one of the vortices existed in the kidney vortex formation was weakened while

the other one was enhanced. Therefore, the anti-kidney vortex was produced by

combining two pairs of asymmetrical vortices formed by combined-hole film cooling.

A proper pitch distance applied on the combined-hole film cooling ensure the vortex

is pushed to the wall by shear stress led by the rotation of the other vortex. The

weakened vortex tends to spread the cooling air in lateral direction thus produced wide

film cooling coverage along the streamwise direction.

In early study of Ahn et al. [10], the research focus on the arrangement of the

cylindrical hole in two different rows with opposite streamwise angle as shown in

Figure 2.6. It was originally a study on a row of the cylindrical hole with a streamwise

angle. However, the addition of another row of the cylindrical hole with opposite

streamwise angle produce the effects of combined-hole film cooling. In this study, the

arrangement of the film cooling holes was varied with the application of various

blowing ratios. As the results shown, the film cooling performance was increased at

near hole region. Better lateral coverage also covering the wall along the mainstream

direction thus increased the overall film cooling performance of the film cooling hole.

Kusterer et al. [38] focused on the combined-hole film cooling with a

compound angle as shown in Figure 2.7 (a) which γ1 and γ2 are the compound angle

applied in the combined-hole film cooling. This study has been followed by Han and

Ren [37] which also discussed the combined-hole film cooling with various

geometrical and flow parameter. From both studies, the researchers indicate that

applying larger compound angle on combined-hole film cooling provide better lateral

coverage. While reducing the compound angle below 11o limit the lateral spread of the

cooling air [11]. In comparison with fan-shaped film cooling hole, combined-hole film

cooling produced a better result. Kusterer et al. [16] determined that anti-kidney vortex

produced by combined-hole film cooling leads to a better cooling air distribution with

high film cooling effectiveness. The combined-hole film cooling also produces

slightly wide lateral coverage of cooling air in comparison with the fan-shaped film

cooling hole. Generally, combined-hole film cooling produces better film cooling

effectiveness in comparison with the other film cooling hole designs; cylindrical, fan-

20

shaped and laidback film cooling holes. With the application of two cylindrical hole,

the combined-hole film cooling is more preferred due to manufacturability of this

cooling hole design.

Figure 2.5: The kidney vortex formation of combined-hole film cooling with

opposite compound angle [37]

21

Figure 2.6: Film cooling arrangement in Ahn et al. [10]

Figure 2.7: Different combined-hole configurations between (a) Kusterer et al. [38]

and (b) Wright et al. [39]

Wright et al. [39] have also reported on the performance of combined-hole film

cooling. However, the work has considered different combined-hole film cooling

arrangement which is the hole was arranged side by side as shown in Figure 2.7 (b)

and γ1 and γ2 are the compound angle applied on the combined-hole film cooling

arrangement. Comparing the results experimentally with cylindrical and fan-shaped

film cooling hole, the results show that combined-hole film cooling only succeeds to

improve the film cooling effectiveness at near hole region. However, the performance

mainstream

(a) (b)

mainstream

mainstream

22

of fan-shaped film cooling hole shows better results and cover the wall more lateral

compared to the combined-hole film cooling by Wright et al. Although the anti-kidney

vortex existed in the case of combined-hole film cooling, narrow coverage was

observed along streamwise direction due to the non-effective arrangements of

combined-hole and reduced the overall film cooling performance of this combined-

hole film cooling.

Besides combining two cylindrical holes, researchers also combined more than

two cylindrical holes as a film cooling. Schulz et al. [40] combined three cylindrical

holes known as an anti-vortex film cooling holes. Meanwhile, sister holes introduced

by Ely and Jubran [41] is a combination of three to five cylindrical hole with different

film cooling hole diameter. Both types of cooling holes produced the anti-kidney

vortex which constrained the lift-off of cooling air and enhanced film cooling

effectiveness. Both design also succeeds to improve film cooling effectiveness along

the streamwise direction and produce more lateral film cooling coverage in

comparison with the other film cooling hole.

Figure 2.8: Film cooling arrangement in Javadi et al. [42]

In Javadi et al. [42], the study covered a perpendicular injection of cooling air

with used of the rectangular hole. This simulation study also considered a combination

of one main rectangular hole with two small rectangular holes located beside the main

hole as shown in Figure 2.8. The outcome of this study shows that the combination of

three rectangular holes enhanced the film cooling effectiveness in the streamwise

direction in comparison with a single rectangular hole. The formation of vortices

originated from two small rectangular holes reduced the interaction and mixing

23

process with the hot mainstream air. The vortices also produced great momentum of

cooling air to spread and covered the wall in the lateral direction. Therefore, the

formation of other vortices is important in this study to enhance the film cooling

performance in lateral and streamwise direction.

2.6 Aerodynamic performance of film cooling

The aerodynamic performance of film cooling usually characterizes by the qualitative

result of total pressure loss coefficient contour or distribution and the quantitative

result of total pressure loss coefficient graph. Figure 2.9 shows the total pressure loss

coefficient contour in the research of Lee et al. [43]. The contour was taken from the

plane of x/D = 8 for low blowing ratio, M = 0.5 in the case of cylindrical film cooling

hole at a different streamwise angle. As shown in Figure 2.9, the large streamwise

angle case of 90o produced larger total pressure loss coefficient in comparison with the

other two cases at low blowing ratio. However, the insignificant change was produced

on the total pressure loss coefficient graph as shown in Figure 2.10. Significant change

only can be observed in the case of high blowing ratio of 1.0 and 2.0 where high total

pressure loss coefficient was produced which indicate the low aerodynamic

performance of film cooling hole.

Figure 2.11 shows the pitchwise distributions of total pressure loss coefficient

from the suction side to pressure side at different blowing ratios with the Reynold

number of 1.45 x 106, turbulence intensity of 10% and cooling air temperature of 60oC.

The result of the total pressure loss coefficient was taken from the other field of

research which is the performance of turbine airfoil on the suction and pressure side

by Drost and Bolcs [44]. As shown in Figure 2.11, the high blowing ratio produced

relatively high total pressure loss coefficient with the application of high turbulence

intensity. Consequently, low aerodynamic performance was produced on the turbine

airfoil.

24

Figure 2.9: Total pressure loss coefficient contour at x/D = 8 for low blowing ratio

[43]

Figure 2.10: Total pressure loss coefficient graph at x/D = 8 for different streamwise

angle [43]

99

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experimental and simulation study on the effect …eprints.uthm.edu.my/9925/1/haswira_hassan.pdf ·...

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