simulation of different orifice geometries on premix...

SIMULATION OF DIFFERENT ORIFICE GEOMETRIES ON PREMIX INJECTOR

RONNY YII SHI CHIN

A thesis is submitted in

fulfillment of the requirement for the award of the

Degree of Master of Mechanical Engineering with Honours

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

JULY 2016

iii

SPECIAL GRATITUDES TO:

MY BELOVED PARENTS,

Yii Sing Kung

Sim Poh Luang

For their love, patience and support in my whole life

MY HONOURED SUPERVISOR,

Prof. Madya Dr. Amir bin Khalid

For his generosity, advices, trust, support, patience and kindly guide me to complete

this research

MY RESPECTED CO-SUPERVISOR,

Dr. Akmal Nizam bin Mohammed

For his guidance, advices and technical support

MY LOVELY BROTHERS,

Boon Chee Fui

Dr. Raymond Yii Shi Liang

Vincent Yii Shi Choon

For their advices, moral support, patience and encouragement in this study

AND ALL MY COLLEAGUES,

For their help and support direct and indirectly, effort and motivation in this study

Lastly, may God bless you all and thanks you very much.

iv

ACKNOWLEDGEMENT

I would like to express my sincere gratitude to all those who gave me the opportunity

and possibility to finish this thesis and my final year project especially to my beloved

supervisor, Prof. Madya Dr. Amir bin Khalid who has gave plenty of advises and

fully trusted on me for this given title and kindly guide me to complete this research.

Besides, a special thanks to my co-supervisor, Dr. Akmal Nizam bin Mohammed

who has given motivation and support especially in the aspect of experimental work

and thesis writing advises.

I would like to show my sincere appreciation to those who had contributed

directly and indirectly towards to the successful of this research project. I am deeply

indebted to Dr. Azwan bin Sapit, Dr. Hamidon bin Salleh and Dr. Mas Fawzi bin

Mohd Ali from Faculty of Mechanical and Manufacturing Engineering who had gave

a kindness helped, simulating suggestions and invaluable guidance especially in

experimental work, Fluid Mechanics, and Computational Fluid Dynamics.

Moreover, I would like to give my special thanks to a best friend, Ms. Lee

Yet Mian from Faculty of Technology Management and Business who had fully

supported, gave advices, and encouraged me all the time throughout the whole

process of this research work. Especially I am obliged to Mr. Mohamad Rasidi bin

Pairan, Mr. Rosman bin Tukiman, Mr. Latip bin Lambosi, Ms. Mirnah binti Suardi,

Mr. Dahrum bin Samsudin, and Ms. Adiba Rhaodah Andsaler binti Mohd Iskandar

Andrew Abdullah provided me invaluable information, assistance and motivation in

any means to make this research success.

I would like to convey my deepest thanks to my parents Mr. Yii Sing Kung

and Mdm. Sim Poh Luang, my brothers and family members who gave moral

support, encouragement, patience and love to enable me to complete this research.

v

ABSTRACT

The search for higher energy efficiency of industrial scale burners to obtain low

emission especially Nitrogen Oxides (NOx) and Particulate Matter (PM) has

demanded experimental studies that are complemented with Computational Fluid

Dynamics (CFD) simulations. Active and passive emission control methods are

common techniques in controlling emission. However, the aim of this research is

focused on the passive control techniques with a specifically modification on burner

orifice’s geometry and different fuel properties are used to study the prediction of

combustion modeling. Prediction is based on the nozzle orifice flow behaviour and

spray characteristics as well as fuel-air mixture formation in mixing chamber through

ANSYS FLUENT simulation. Multiphase of Volume of Fluid (VOF) and turbulence

model of Transition Shear Stress Transport (SST) are used throughout whole

simulation analysis. Other than that, Eulerian-Lagragian multiphase approach is used

for the steady simulations and spray generation. Furthermore, experimental spray

droplets sizes at selected locations from nozzle outlet are taken by direct

photography method and validated with the simulated droplets size. The rapid

mixing region is predicted at the area near to fuel and air inlet. Besides, the cavitation,

spray dispersion and Sauter Mean Diameter (SMD) are predicted increased when

L/D dropped from 2.5 to 1.667. Furthermore, cavitation is predicted reduced but

spray dispersion and SMD increased when orifice coefficient increased from K = 0 to

K = 20. Meanwhile, high viscosity and density of Crude palm oil (CPO) biodiesel

blends is believed to cause high cavitation inside orifice, wider spray dispersion and

high SMD.

vi

ABSTRAK

Kajian bagi menghasilkan tenaga yang efisien dan mempunyai tahap pelepasan asap

pencemaran Nitrogen Oksida (NOx) dan Particulate Matter (PM) yang rendah perlu

melibatkan kajian eksperimen dan Computational Fluid Dynamics (CFD) terutama

system yang berskala industri. Kaedah-kaedah aktif dan pasif untuk kawalan asap

pencemaran adalah merupakan teknik asas dalam mengawal kadar pencemaran asap

tersebut. Walau bagaimanapun, tujuan kajian ini hanya memandang berat kepada

kaedah kawalan pasif khususnya pengubahsuaian pada orifis geometri dengan

penggunaan pelbagai bahan api yang berbeza untuk mengkaji model pembakaran.

Model kajian ini adalah berdasarkan tingkah laku aliran dalam orifis dan ciri-ciri

semburan serta pra-campuran bahan api dengan udara dalam kebuk pergaulan

melalui simulasi ANSYS FLUENT. Multiphase Volume of Fluid (VOF) dan model

gelora bernama Peralihan SST (Transition Shear Stress Transport) telah digunakan

bagi simulasi dan analisis. Selain itu, jenis berbilangfasa Eulerian-Lagragian telah

digunakan untuk kajian kestabilan simulasi dan pelbagai pengenerasi semburan. Bagi

tujuan pengesahan model, saiz butiran semburan dikajikan melalui eksperimen dan

kaedah fotografi langsung di lokasi yang terpilih daripada orifis telah disahkan

dengan saiz butiran simulasi. Rantau pencampuran pesat diramalkan berlaku di

kawasan berdekat bahan api dan kemasukan udara. Selain itu, peronggaan,

penyebaran semburan dan SMD (Sauter Mean Diameter) diramalkan meningkat

apabila L/D menurun daripada 2.5 ke 1.667. Di samping itu, peronggaan diramalkan

berkurang tetapi penyebaran semburan dan SMD meningkat apabila pekali orifis

meningkat daripada K = 0 hingga K = 20. Sementara itu, kelikatan dan ketumpatan

yang tinggi untuk biodiesel minyak sawit campuran dengan diesel adalah dipercayai

sebagai punca menyebabkan peronggaan tinggi di dalam orifis, penyebaran

semburan lebih luas dan juga SMD tinggi.

vii

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

CONTENTS vii

LIST OF TABLE xii

LIST OF FIGURE xiii

LIST OF SYMBOLS AND ABBREVIATIONS xix

LIST OF APPENDIX xxiii

CHAPETER 1 INTRODUCTION 1

1.1 Research background 1

1.2 Problem statement 3

1.3 Objective 4

1.4 Scope 5

1.5 Significance of study 5

CHAPTER 2 LITERATURE REVIEW 7

2.1 Introduction 7

viii

2.2 Premix injector 7

2.3 Fuel-air premixing 9

2.4 Effects of swirling flow 10

2.5 Cavitation 14

2.6 Nozzle orifice geometries of the

injector 16

2.6.1 Nozzle orifice diameter 16

2.6.2 Nozzle hope shape 20

2.6.3 Conical nozzle 20

2.6.4 Cylindrical nozzle 23

2.7 Influences of the hole shaped nozzles 25

2.8 Spray characteristics 25

2.8.1 Spray Penetration 27

2.8.2 Spray angle and dispersion 28

2.8.3 Spray droplets 29

2.8.4 Sauter mean diameter 30

2.9 Influence of fuel on spray

characteristics 32

2.10 Spray analysis 34

2.11 Computational Fluid

Dynamics (CFD) 38

2.12 Mesh density 38

2.13 Turbulence model 39

ix

2.14 Eulerian-Lagragian multiphase

approach result 41

2.15 Crude palm oil 42

2.15.1 Properties of CPO 43

2.15.2 Palm oil biodiesel (POB) 46

2.15.3 Results of previous researches

about Crude Palm Oil (CPO) 46

CHAPTER 3 METHODOLOGY 51

3.1 Introduction 51

3.2 Experiment setup 53

3.3 Image processing 55

3.3.1 Image J 55

3.4 Design and geometry of premix injector

and spray chamber 55

3.5 Grids 57

3.6 Boundary condition 58

3.7 Simulations 60

3.8 Procedure for a complete simulation by

using ANSYS Fluent 61

3.9 Flow rate calculation 62

3.10 Grid independency study 62

CHAPTER 4 RESULT AND DISCUSSION 64

4.0 Introduction 64

4.1 Experimental droplets size analysis 74

4.1.1 Measurement areas selection 65

x

4.2 Image processing analysis 67

4.3 Thresholding adjustment 67

4.4 Model selection and validation 68

4.5 Simulation analysis of rapid mixing

in the mixing chamber of a premix

swirl injector 73

4.5.1 Introduction of rapid swirl

injector 73

4.5.2 Internal swirling mixing

analysis 73

4.5.3 Velocity profile formation 75

4.5.4 Volume fraction between air

and fuel phase mixing 79

4.5.5 Summary result of rapid mixing

in the mixing chamber 83

4.6 Prediction of cavitation and spray

behaviour in single orifice with different

nozzle characteristics of the premix injector 83

4.6.1 Influence of ratio orifice’s length

to diameter (L/D) on flow and

cavitation 84

4.6.2 Spray performances of different

ratio of orifice’s length to

orifice’s diameter 94

4.6.3 Influence of nozzle orifice

coefficient (K) on the flow

and cavitation 103

4.6.4 Spray performances of nozzle

orifices with different

orifice coefficient, K 111

4.6.5 Summary of the orifice

coefficient analysis 117

xi

4.7 Prediction of cavitation and spray

characteristics with different fuel

properties used 118

4.7.1 Static pressure distribution along

the orifice with diesel and biodiesel

blends, B10 and B20 118

4.7.2 Radial velocity at orifice outlet

for diesel and biodiesel blends,

B10 and B20 121

4.7.3 Turbulence kinetic energy at

orifice outlet for diesel fuel and

biodiesel blends, B10 and B20 123

4.7.4 Diesel phase volume fraction at

orifice outlet for diesel fuel and


4.7.5 Spray performances of different

fuel properties; Diesel fuel and

biodiesel CPO blends, B10 and B20 125

4.7.6 Summary of the different fuel

properties used analysis 130

CHAPTER 5 CONCLUSION AND RECOMMENDATION 132

5.1 Conclusion 132

5.2 Recommendation 135

REFERENCES 136

APPENDIX 149

xii

LIST OF TABLES

2.1 Dimensional specifications of the nozzle 17

2.2 The physical characteristics of CPO, RBD

and WCO with ASTM standard 44

2.3 The properties of STD and bleanding ratio

biodiesel at ambient temperature 44

2.4 Properties of blending ratio biodiesel at

various temperature 45

2.5 Fuel properties of RBDPO blends and

Diesel Fuel 45

2.6 Properties of diesel fuel and and palm

oil biodiesel (POB) 46

3.1 Summaries of boundary conditions

and parameters 53

3.2 Input values of air and diesel fuel, biodiesel B10

and B20 mass flow rate at the inlet of premix

injector 61

3.3 Fuel properties of CPO blends and

standard diesel 61

4.1 Calculated percentage differences

between simulated droplets size with

different drag coefficients and

experimental droplets size 71

4.2 Detailed information of different nozzle

orifice designs 84

4.3 Detailed information of different nozzle’s

orifice coefficients designs 103

xiii

LIST OF FIGURES

2.1 Fuel-water internally rapid mixing type of

injector 8

2.2 Axial velocity distribution and stream lines

(m/s) 11

2.3 The velocity vector of (a) standard k – ε,

and (b) realizable k – ε turbulence model 12

2.4 Streamline patterns for two different swirling

numbers 13

2.5 Contour of vorticity magnitude for two swirling

numbers 13

2.6 Iso-vorticity surface for two swirling numbers 14

2.7 Cavitation formation inside nozzle 15

2.8 Liquid velocity generated by two different

nozzle diameters with different L/D ratio

(a) D = 3 mm and (b) D = 1.5 mm 18

2.9 Injection pressure (bar) against spray angle (°) 18

2.10 Nozzle flow characteristics under different L/D:

(a) mass flow rate and flow coefficient;(b) average

volume of vapour; and (c) average velocity at outlet

and average turbulent kinetic energy 19

2.11 Types of nozzle hole shape 21

2.12 Schematic diagram of the cylindrical nozzle

head and exit 23

2.13 The graph of discharge coefficient versus

the Reynolds number 24

2.14 The effects of increasing flow rate on spray

characteristics for swirl injector 26

2.15 Three dimensional spray flow field and droplets 27

2.16 The graph of penetration distance (m) against

various biofuel blends 27

2.17 Spray penetration, angle and area of diesel-water

emulsified against various equivalent ratio 28

xiv

2.18 Droplets density distribution for dispersion

prediction with diesel and biodiesel fuel 29

2.19 The graph of particles size of droplets

against biofuel blends 30

2.20 A typical spray pattern from spray nozzle 30

2.21 The graph of fuel influences to the spray angle 32

2.22 Pattern of diesel fuel spray 32

2.23 Pattern of EMW20 fuel spray 33

2.24 Predicted SMD for simulation and empirical

against various biodiesel blends with diesel 34

2.25 The graph of spray tip penetration against time 35

2.26 The radial profiles of the normalised Sauter mean

diameter D32, the normalised volume flux V,

and the normalised droplet density Dρ 36

2.27 SMD at six different axial locations 37

2.28 Radial profile particle velocity at three

different axial locations 38

2.29 Different mesh sensitivity in the central

cross section of hole 39

2.30 Turbulence flow structure 40

2.31 Energy cascade 40

2.32 Dynamic viscosity against the temperature

for pure CPO 45

2.33 Particle sizes of PM (a) and particle-bound

PAHs (b) by using PB0, PB30 and PB40 47

2.34 Projected Area comparison 48

2.35 The storage duration versus viscosity and

CO emission 49

2.36 Spray SMD at downstream 40mm with various

Blends 50

3.1 Methodology flowchart 52

3.2 Schematic diagram of experimental setup 54

3.3 Detail dimensions of premix injector mixing

chamber and orifices 56

3.4 Computational domain geometry of mixing

chamber for premix injector 56

3.5 Meshing of premix injector and its spray chamber 57

3.6 Meshing of premix injector with nozzle

connected to the spray chamber 58

xv

3.7 Wall set in the model geometry 59

3.8 Inlet mass flow of the premix injector 59

3.9 Pressure outlet of the model 60

3.10 Velocity profile along the mixing chamber

simulated by different grid sizes 63

4.0 Spray image at equivalent ratio equal to 1.0 with

the reference scale 65

4.1 Measurement location of 20 mm and 30 mm

from the orifice injector 66

4.2 Selected measurement area for droplets

size analysis 66

4.3 Image cropped at 20 mm location with

threshold intensity adjusted 68

4.4 Image cropped at 30 mm location with threshold

intensity adjusted 68

4.5 Straight line drawn on measurement location of

(a) 20 mm and (b) 30 mm 69

4.6 Comparison between simulation and experimental

at measurement location of 20 mm 70

4.7 Comparison between simulation and experimental

at measurement location of 30 mm 70

4.8 Internal computational domain of mixing chamber

with swirler and four different labels of

measurement areas 75

4.9 Tangential velocity profile for four different

measurement areas along the diameter of

mixing chamber 76

4.10 Velocity vector vortex structure with scales

at measurements area (a) y = 10mm and

(b) y = 20mm 77

4.11 Velocity vector vortex structure with scales

at measurements area (c) y = 30mm and

(d) y = 40mm 78

4.12 3-D velocity streamline distribution in the

mixing chamber with coloured scale 79

4.13 Diesel fuel phase volume fraction at different

measurement areas along the mixing chamber 80

4.14 Contour of diesel fuel phase volume fraction

at measurement areas of (a) y = 7.6mm and

(b) y = 8mm 81


xvi

at measurement areas of (c) y = 9mm and

(d) y = 10mm 82


at measurement areas (e) y = 13mm 83

4.17 Static pressure distribution along the orifice

(a) L/D = 2.5, (b) L/D = 2 and (c) L/D = 1.667 86

4.18 Static pressure distribution against the orifice

length for three different L/D designs 87

4.19 Radial velocity against normalize radius of

orifice outlet 88

4.20 Contours of velocity along the orifices of

(a) L/D = 2.5, (b) L/D = 2 and (c) L/D = 1.667 89

4.21 Contours of velocity distribution on nozzle outlet

for (a) L/D = 2.5, (b) L/D = 2 and

(c) L/D = 1.667 90

4.22 Turbulence kinetic energy at orifice outlet for

three different L/D orifice designs 91

4.23 Diesel fuel phase volume fraction at orifice

outlet for three different L/D orifices 93

4.24 Contours of diesel fuel phase volume fraction

at orifice outlet of (a) L/D = 2.5, (b) L/D = 2 and

(c) L/D = 1.667 94

4.25 Mass density of droplets at 20 mm downstream

from orifice outlet 96

4.26 Mass density of droplets at 30 mm downstream

from orifice outlet 96

4.27 Spray breakup and atomization 98

4.28 Prediction of spray droplets density for three

different orifices (a) L/D = 2.5, (b) L/D = 2 and

(c) L/D = 1.667 99

4.29 Prediction SMD of spray development at

downstream 20 mm from orifice outlet 100

4.30 Prediction SMD of spray development at

Downstream 30 mm from orifice outlet 100

4.31 Prediction of SMD layout with coloured

scale (in unit m) for three different orifices of

(a) L/D = 2.5, (b) L/D = 2 and (c) L/D = 1.667 102

4.32 Static pressure distribution along the orifice of

(a) K = 0, (b) K = 10 and (c) K = 20 104

4.33 The graph of static pressure distribution

throughout the orifice for three different orifice

coefficient’s designs 105

xvii

4.34 Radial velocity at orifice outlet for three

different orifice coefficient designs 106

4.35 Contours of velocity inside the orifice with

different orifice coefficients (a) K = 0, (b) K = 10

and (c) K = 20 107

4.36 Velocity contours at orifice outlet for orifice

coefficient (a) K = 0, (b) = K = 10 and (c) K =20 108


different orifice coefficients 109

4.38 Diesel phase volume fraction at orifice outlet

for orifices with different orifice coefficients 111

4.39 Spray droplets density at measurement location

of 20 mm downstream from orifice outlet for

three different orifice coefficients 113

4.40 Spray droplets density at measurement location

of 30 mm downstream from orifice outlet for

three different orifice coefficients 114

4.41 Sauter mean diameter of spray droplets at

measurement location of 20 mm downstream

away from orifice outlet 115

4.42 Sauter mean diameter of spray droplets at

measurement location of 30 mm downstream

away from orifice outlet 116

4.43 Static pressure distribution along the orifice

for diesel fuel and biodiesel blends of B10 and B20 119

4.44 Two dimensional static pressure contours for

(a) diesel fuel, (b) biodiesel blends, B10 and

(c) biodiesel blends, B20 120

4.45 Radial velocity at orifice outlet for diesel and


4.46 Two dimensional velocity contours in the orifice

for (a) diesel fuel, (b) biodiesel blends B10 and

(c) biodiesel blends B20 122


diesel fuel and biodiesel blends B10 and B20 123

4.48 Diesel phase volume fraction at orifice outlet

for diesel fuel and biodiesel blends, B10 and B20 124

4.49 Spray droplets density at downstream 20 mm

away from orifice outlet for diesel fuel and


4.50 Spray droplets density at downstream 30 mm

away from orifice outlet for diesel fuel and

xviii


4.51 Droplets Sauter mean diameter at downstream

20mm away from orifice outlet for diesel fuel

and biodiesel CPO blends, B10 and B20 128

4.52 Droplets Sauter mean diameter at downstream

30mm away from orifice outlet for diesel fuel

and biodiesel CPO blends, B10 and B20 130

xix

LIST OF SYMBOLS AND ABBREVIATIONS

m - Metre

s - Seconds

K - Kelvin

°C - Celsius

% - Percentage

cm - Centimetre

mm - Millimetre

µm - Micrometre

ms - milliseconds

cP - Centipoise

t/h - Tonnes per hour

m/s - Metre per seconds

MWe - Megawatt electrical

MPa - Mega pascal

bar - Pressure Bar

cST - Centistokes

ppm - Parts per million

mPas - Millipascal seconds

kg/liter - Kilogram per litre

kJ/kg - Kilojoules per kilogram

kg/s - Kilogram per seconds

kg/m3 - Kilogram per cubic metres

g/cm3 - Gram per cubic centimetres

ɸ - Equivalent ratio

xx

- Diameter

D - Diameter

L - Length

V - Normalised volume flux

A - Area

Q - Volume flow rate

K - Orifice coefficient

Kcrit - Critical cavitation number

- Mass flow rate

mL - Milliliter

ρfuel - Density of fuel

CN - Cavitation Number

MW - Molecular weight

DM - Discrete Multicomponent

Pi - Injection pressure

Pb - Ambient pressure

Pvapor - Vaporization pressure

Pv - Vapour pressure

Re - Reynolds Number

Vexit - Averaged flow velocity of nozzle exit

μfuel - Viscosity of fuel

Cd - Discharge coefficient

Ath - Cross sectional area

Ca - Area contraction coefficient

Cv - Velocity coefficient

w/o - Water in oil

D32 - Symbol represent Sauter mean diameter

Dρ - Normalised droplet density

i - Size range considered

xxi

Ni - Number of drops in size range i

Di - Middle diameter of size range i

k – ε - K-epsilon

AFstoich - Air-fuel ratio stoichiometry

AFexp - Air-fuel ratio experiment

L/D - Ratio orifice’s length to diameter

r/D - Ratio of orifice’s radius to diameter

PM - Particulate matter

CO2 - Carbon dioxide

CO - Carbon monoxide

HC - Hydrocarbon

NOx - Nitrogen oxide

NO - Nitrogen monoxide

R&D - Research and development

SOx - Sulphur oxide

Vol - Volume

CTM - Continuous Thermodynamics

OFA - Over air flow

ISF - Interaction shear flows

CFD - Computational Fluid Dynamics

CRZ - Central Recirculation Zone

DNS - Direct Numerical Simulation

SST - Shear Stress Transport

GUI - Graphical User Interface

LSB - Low-swirl burner

LES - Large-eddy-simulation

PIV - Particle image velocimetry

PVC - Precessing vortex core

SCR - Selective catalytic reduction

xxii

SNCR - Selective non-catalytic reduction

RANS - Reynolds-Averaged Navier-Stokes

DSLR - Digital Single Lens Reflex

VOF - Volume of fluid

TKE - Turbulence kinetic energy

SMD - Sauter mean diameter

TAB - Taylor Analogy Breakup

ETAB - Enhanced Taylor Analogy Breakup

WME - Waste cooking oil methyl esters

EME - Emulsified WME

POB - Palm oil biodiesel

CDF - Commercial Diesel Fuel

MCPO - Mixed crude palm oil

PB0 - Palm oil blends 0 vol% with diesel



MMAD - Mass median aerodynamic diameters

Dg-aMCPO - Degummed-deacidified mixed crude palm oil

RBDPO - Refined, Bleached and Deodorized Palm oil

CPO - Crude Palm Oil

B10 - Biodiesel 10% blends with 90% diesel fuel

B20 - Biodiesel 20% blends with 80% diesel fuel

xxiii

LIST OF APPENDIX

APPENDIX TITLE PAGE

A Sample steps of using image J 150

B Calculation for diesel fuel 155

C Calculation for Biodiesel CPO

blends, B10 and B20 157

D Detail drawing of the injector’s

nozzle 161

E Detail drawing of the injector’s

swirler 162

F Spray droplets density for different


G Spray droplets SMD for different


H Spray droplets density for

different fuel properties, diesel fuel

and biodiesel CPO blends, B10

and B20 165

I Spray droplets SMD for different

fuel properties, diesel fuel and

biodiesel CPO blends, B10 and B20 166

CHAPTER 1

INTRODUCTION

1.1 Research background

The global environmental change and global warming effects have been become a

famous issue and also the major interest in the world. In order to face the new

challenges at coming new century, the progress of the reduction of harmful gases and

improvement of burner system needs to be priority [1]. The burner system is an

unique, patented and higher-efficiency system that works with a specially designed

oil burner to create ultra-efficient combustion that reduces oil use and greenhouse

gases and other harmful emissions. The search for higher energy efficiency of

industrial scale burners to obtain low emission especially Nitrogen Oxides (NOx) has

demanded experimental studies that are complemented with Computational Fluid

Dynamics simulations. Many methods have been developed to reduce emission,

while these emission control methods can be divided into two major categories,

which are active and passive controls method. Active control techniques include

flame characteristics modification by external influence on the combustion system.

Modification of flame properties by acoustic forcing or magnetic fields are examples

of active control methods. Moreover, passive control techniques involve adjustments

of flame dynamics by changing the initial conditions of the combustion system such

as different fuels selection, by premixing fuel and oxidizer in different proportions,

and by modifying the burner geometry [2]. However, the main focus in this study is

on the passive control techniques which is investigate the effects of different fuel

properties used and specific modification on premix injector.

Nitrogen Oxides (NOx) have many unfavourable environmental effects such

as ozone hole, acid rain, and photochemical smog, which are international concern.

Besides, many governments and organisations who responsible for

2

environmental protection had established various stringent regulations and

legislations for emissions control. However, great attention on well regulate

combustion modeling during last 20 years [3], which can effectively control the

emission produced from burner combustion. Furthermore, many technologies have

been innovated for controlling emissions from burner, such as low-NOx burner, over

air flow (OFA), swirl burner, selective non-catalytic reduction (SNCR) and selective

catalytic reduction (SCR) [4]. Huang et al. conducted a study on the effect of OFA

on the air flow and coal combustion in a 670 t/h wall-fired boiler to obtain the

optimised OFA parameters and arrangements [5]. They found that the NOx

concentration at the outlet of the furnace is reduced. Zeng et al. investigated the

combustion characteristics and NOx emissions for a 300 MWe utility boiler and they

found a nonlinear relationship between NOx emission and outer secondary-air vane

angles [6]. Hao Zhou et al. conducted a numerical simulation about optimisation of a

single low-NOx swirl burner and analysed the effects of the size and structure of the

primary air pipe of the swirl burner on flow, combustion, and NOx formation. They

found a reduction of 39.8% in NOx emission [4].

Nevertheless, great interest on investigating the interaction of turbulence

combustion and its consequences, including deciding which turbulence model is the

most appropriate depending on the specificity of the application. The burner orifice

modelling usually does not take part of the analysis [3]. However, the geometrical

burner is one of the method in controlling emission and practically playing a

significant role on performance of spray combustion, atomization and formation of

fuel-air mixture in order to improve combustion performance, and decrease some

pollutant products from burner system [1, 3]. Few researchers had concluded that the

burner geometry contribution to the refinement of combustion and emission

reduction. Praveen Hariharan et al. conducted an experimental study on turbulent

hydrogen flames from two different geometrical burners, circular and elliptic burners

with varying degrees of premixedness (diffusion, fuelrich, stoichiometric, and fuel-

lean) [2]. Graham E. Ballachey and Matthew R. Johnson investigated the effects of

burner geometry, swirl, and fuel composition in prediction of blowoff in a fully

controllable low-swirl burner burning alternative fuels. They used three different sets

of inner/outer low-swirl burner (LSB) nozzles, exit diameters of 38.1 mm, 50.8 mm,

and 76.2 mm and constant inner to outer diameter ratio of 0.8 to investigate scaling

effects [7]. B. Miller studied the effect of fuel quality on burner design and ignition

3

stability in reducing emissions, specifically NOx emissions [8]. Therefore, the

geometrical burner orifice is particularly influences to the combustion performance

and emission reduction. Further investigation on premix swirl burner orifice is

valuable and precious in order to minimise the emission production and it is fewer

found in previous literature.

Apart from that, growing population and overuse of fossil fuel is also a

critical problem nowadays. Improvement of technology had led to a greater use of

hydrocarbon fuels. Thus, it is necessitates the search for alternative oil as energy

source to replace or reduce the usage of hydrocarbon fuels [9, 10]. Besides,

renewable energy sources are greatly developed all around the world due to attractive

oil prices and limited greenhouse gas emissions. Biodiesel is an alternative fuel with

esters of vegetable oils and animal fats act as the renewable sources to made

biodiesel [9, 11]. Biodiesel has more beneficial combustion emission profile

compared to the petroleum-based diesel such as low emissions of particulate matter,

unburned hydrocarbons and carbon monoxide (CO). Photosynthesis process has

taken part to recycle the carbon dioxide produced by the combustion of biodiesel in

order to minimize the influences of biodiesel combustion on the greenhouse effect

[12-14].

However, many experiments have been conducted in respect of combustion

and emissions using biodiesel, mainly by the experimental standpoint. Minor

frequent detailed studies on burner injection and spray atomization characteristics in

previous literature for biofuels [15].

1.2 Problem statement

High levels of emission emitted from combustion are harmful to our environment

and even pose a threat to human respiratory systems. Lower and reduce the emission

levels is responsible to every aspects of department and also human being.

Geometrical burner injector orifice had mentioned as an important parameter in

controlling combustion with linear relationship of emission produced, especially NOx.

However, there is another problem facing by experimental group which is costly for

conducting experiments due to many parameters need to be tested and hardly to

differentiate the significant changes from the spray characteristics. Besides, the

critical behaviour inside the experimental instrument is invisible through the

4

experiment work. Therefore, the aim of this research is to use CFD simulation to

obtain the flow inside orifice and spray characteristics of the premix swirl injector

and to predict the preliminary data prior to the actual design. In addition, the

influences of injector’s orifice geometries to the spray are studied. The behaviour of

fluid flow inside orifice, spray characteristics regarding Sauter mean diameter and

droplets density are analysed from the simulation result.

Other than burner injector geometry, biodiesels are renewable energy sources

which have properties similar to petroleum diesel fuel, while its usage contributed

pros and cons to the environment. Biodiesel provided benefits of lower emission of

matter (PM), carbon dioxide (CO2) and unburnt hydrocarbons (HC). Besides, reusing

of biodiesel can reduce the impact to the environment due to some biodiesel are

made from wasted oil. Moreover, biodiesel can be produced from the vegetable oil

which can cause a reduction of the usage of conventional fuels. However, there are

also some issues about its disadvantages, such as increase in nitrogen oxide (NOx)

emissions still is an issue for the research and development (R&D) group. Moreover,

high viscosity of biodiesel can gives significant impacts to the spray characteristics,

flame formation and combustion. Fuel characteristics of biodiesel still is an unsolved

problem due to its oxidation stability, stoichiometric point, biofuel composition,

antioxidants on the degradation which can cause more emission of NOx and PM [16].

Two types of alternative fuel blends, B10 and B20 are requires to test in order to

predict its combustion behaviour through nozzle flow and spray analysis.

1.3 Objective

The objectives of this research are:

(i) Simulate the physics flow pattern of rapid mixing with tangential velocity and

volume fraction between standard diesel fuel and air in the mixing chamber

of premix swirl injector.

(ii) Determine the nozzle flow and spray characteristics for different ratio of

orifice’s length to diameter and orifice coefficients on single orifice of the

premix swirl injector by using CFD.

5

(iii)Comparing two types of CPO biodiesel blends, B10 and B20 to the standard

diesel fuel (B0) with a standard orifice (1 mm) on nozzle flow and spray

characteristics by using CFD.

1.4 Scope

In this project, the behaviours of mixture formation between standard diesel fuel and

air inside the mixing chamber are studied by using premix fuel-air concept swirl

injector. Three different nozzle diameters with 0.8 mm, 1.0 mm, 1.2 mm and nozzle

hole shapes such as cylindrical shape (K = 0) and conical shapes with k-factor of 10

and 20 were the manipulate geometries in this simulation. The nozzle orifice injector

angle used is fixed at 45° with an equivalent ratio at 1. The working fluids were air,

standard diesel fuel, alternative fuel blends of crude palm oil (CPO) B10 and B20.

Last but not least, the mixture formation results are analysis base on tangential

velocity, and physics flow pattern inside the mixing chamber at four (4) different

positions along y-axis. Meanwhile, volume fraction at five (5) different critical

positions are analysed along the y-axis in mixing chamber. Besides, the variant types

of geometrical injector’s orifice are study in terms of behaviour of nozzle flow and

spray characteristics to predict the combustion and production of emission. Last but

not least, all the simulations were completed by software of ANSYS FLUENT.

1.5 Significance of study

A new application, innovation and modification of an existing design tool will lead

to the improvement of quality product. Therefore, study of nozzle characteristics of

the injector in fuel injection system can bring a significant affection to the air

pollutant and benefit to the environment. However, premix swirl injector orifice

geometries can gives impact to the combustion profile with affecting the spray

characteristics of diesel fuel and alternative fuels. Other than that, many researchers

reported that the biodiesel affects the injection system with respect of nozzle flow

and spray characteristics [17-23]. Thus, few types of biodiesel blends are applied and

compared with the diesel fuel.

6

A specific modification of the premix injector nozzle geometry can improve

the combustion performance significantly. Therefore, differences of nozzle shapes

and diameters are studied in this simulation. Cylindrical and conical of nozzle orifice

shapes were chosen as part of simulation in the category of nozzle shape. Apart from

that, orifice diameter was also the other category of nozzle characteristics in this

study. Nevertheless, using CFD simulation is always gives a good visualise result

compared to the experimental observations. Besides, the simulation result was

validated with the experimental result using droplets size analysis in order to obtain a

medium for further simulation. Nevertheless, CFD simulation also provides

advantages in respect of saving energy, time and cost.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The fuel injection system is widely used in the field of a burner system nowadays.

However, previous research showed that the nozzle characteristics influenced the

spray characteristics, especially nozzle shape and diameter of the injector. Besides,

many literatures also concluded that the alternative fuel, crude palm oil and its blends

also effects to the performance of combustion, while direct affect to emission

produce. This chapter describes the relevant information from previous literatures

like premix injector, fuel-air mixing characteristics, effects of swirling flow for the

fuel-air mixing and cavitation inside the nozzle orifice Moreover, types of nozzle

orifices such as orifice shape and orifice diameters and their effects to the spray and

nozzle flow, spray characteristics, and Computational Fluid Dynamics (CFD) also

reviewed. Other than that, literature review about crude palm oil (CPO), properties of

biodiesel, effects of biodiesel and biodiesel blends, specifically on CPO also been

revised in this chapter.

2.2 Premix injector

Tomoaki Yatsufusa et al. was innovated a new injector to introduce water directly

into the combustion field to achieve the water-fuel emulsion concept [24]. Water-

emulsified fuel can lower flame temperature while it leads to NOx reduction and PM

emissions [24-29]. The new injector was designed based on the concept of fuel-water

internally rapid mixing. Atomization air in the small mixing chamber was forced fuel

and water mixed rapidly and then injected from the chamber. The mixture of water,

fuel and air are introduced directly into the combustion field.

8

The new injector was named as “fuel-water internally rapid mixing type of

injector”. Furthermore, fuel and water were inserted separately and independently

into to a small mixing chamber inside of the injector. Figure 2.1 illustrates the detail

of the injector in cross sectional view and its photographs. The capacity of the

mixing chamber is 3.9 mL. Fuel was supplied from a ring-shaped slit with an outer-

diameter of 5.5 mm and slit-width of 0.45 mm while water was supplied from

another center hole with a diameter of 2.0 mm as shown in the Figure 2.1. Moreover,

the function of swirler used in the injector was generated the air swirl flow in the

mixing chamber in order to improve mixing [24].

Figure 2.1: Fuel-water internally rapid mixing type of injector [24]

Nevertheless, M. Farid Sies et al. introduced this kind of pre-mixing injector

fuel and water-fuel emulsion into the application of open burner. The working fluids

are CPO biodiesel blends with diesel, standard diesel fuel and mixed with water for

comparison. They found that the spray formations depend on the equivalent ratio or

mass flow rate, while CPO biodiesel has longer penetration length and spray area

than diesel, but the spray angle is smaller than diesel. Besides, they observed the high

density fuel and additional water content in the fuel can produce longer spray

penetration and small spray angle. The increase of water content in fuel will increase

the spray penetration while decreased in spray area [30].

Additionally, Amir Khalid et al. also applied premix injector into an external

burner system to investigate the effects of diesel-water emulsification on mixture

9

formation, burning process and flame development in the burner system by using

direct photography method. They found that the higher penetration length and lower

spray angle higher generated due to water contents caused higher viscosity influences

and thus predominantly lower combustible mixture and flame penetration which is

good agreement in estimation of reduction emission produced from combustion

process [31].

2.3 Fuel-air premixing

Mixing is a slow physical process and it does not force by convection. Convection

represented large-scale transport while turbulence means large-scale to small-scale

transport. Turbulent mixing is the most important rule in all practical fluid flows

which having scales larger than the millimetre. All the ultra-low-emissions

combustors rely on the achievement of near-perfect mixture homogeneity before the

combustion can be success. A homogeneous combustible mixture has the added

benefits in greatly reduces the possibility of autoignition. High degree of mixture

homogeneity is essential, not only for the attainment of low NOx emissions, but also

to alleviate the problem of autoignition [32]. Furthermore, a successful turbulent

flow of mixing can be done from the mixing of water, fuels, and gases. Mixture

formation of fuel and air is extremely important in contribution of combustion

modelling and it is a complicated process which can be accomplished in many ways

using a wide variety of technologies.

A number of researches investigated spray fuel-air mixing since pass 20 years

started from great spray researcher H. Lefebvre [33]. He was used various types of

atomizer to create fuel and air mixing in the application of either internal combustion

or external combustion. However, all the atomizers developed were found good

agreement in term of fuel air mixing, and yet these atomizers were great

manufactured all around the world for any application involving spray injection. As

world attaining advances, the fuel-air mixing is no longer depends on the atomizer.

Many external techniques had introduced for enhancement of fuel-air mixing, such

as turbulence generators, fractal grid, swirling flow, high pressure injection of fuel

into air, and scramjet. T. S. Cheng et al. is performed a study the effects of initial

fuel-air mixing on NOx and CO emissions in swirling methane jet flames. Where the

major parameters are swirl number used to modify the initial fuel-air mixing ahead of

10

the swirling flame, the fuel-air momentum flux ratio, and the location of fuel

injection. The results showed that strong and rapid mixing of the strongly

recirculating flame, which mixture homogeneity increased and shortens the

characteristic time for NOx formation, results in a lower NOx emission index [34].

Furthermore, M. Stohr et al. investigated the fuel–air mixing and reaction in a

lean partially-premixed turbulent swirl flame using simultaneous particle image

velocimetry (PIV) and planar laser-induced fluorescence of OH and acetone with a

repetition rate of 10 kHz. They concluded the enhancement of fuel–air mixing is

strongly induced by the precessing vortex core (PVC) in the shear layer of the inner

recirculation zone with significant contributed to the stabilization of the flame [35].

Moreover, Tomoaki Yatsufusa et al. studied tried to apply biomass fuel to produce a

clean burner combustion under high load conditions by water addition used a newly

developed injector mixes fuel rapidly with water inside of the injector with support

of atomizing air. They pressured the air-flow into the injector and swirl flow is

generated by swirler in the mixing chamber to improve mixing. Meanwhile, water

and fuel are pumped into the mixing chamber to achieve rapid mixing with fuel, air

and water. The result showed this premix swirl injector emitted exceedingly less

particulate matters at high load and NOx emission is strongly dependent on water

flow rate [24]. Other than that, Amir Khalid et al. studied on the relation between

mixture formation and flame development of swirl premix burner combustion using

optical visualization technique and image processing technique. However, they found

that changes in equivalent ratio was not appeared beneficial to the physical fuel-air

mixing but chemical reaction improved and promoted greater quantity fuel prepared

for combustion due to higher luminosity [36].

2.4 Effects of swirling flow

Swirling flows have been commonly used by the previous researchers and industries

for a number of years, purposed for stabilization the high intensity combustion

processes [37, 38]. Swirling flow plays an important role in creating an internal

recirculation zone, which produced the low velocity regions where the flame can be

stationary [38, 39]. The recirculation zone has several beneficial effects to the

improvement of fuel/air mixing and the turbulent flame speed can be increased by

augmenting the level of turbulence [38].

11

Cunxi Liu et al. investigated the effects of gap of nozzle shroud on the

combustion stability and the spray characteristics with some of combustion

performance in a swirl-cup combustor. The performance they determined was flame

patterns, ignition and lean blowout performances. They observed that the ignition

performance was improved greatly with fuel/air ratio at lower 18% of ignition limit.

Figure 2.2 shows the detailed flow structures inside the venture and simulated non-

reacting flow field inside the model combustor. The swirling air and jetting air

streams were interacted from the purge holes and it brought a significant impact to

the flow field structure. The Central Recirculation Zone (CRZ) moved downstream

and the reverse flow velocity decreased when increasing h from 0.5 mm to 2.5 mm

[38].

Figure 2.2: Axial velocity distribution and stream lines (m/s) [38]

M. M. Rahman et al. analysed numerically with the commercial CFD about

the performance of the separator which created the orthogonal driving flow to cause

swirling for vapour-water two phase flow separation. In their analysis, the turbulence

models of standard k − ε and realizable k − ε were implemented. Moreover, Figure

2.3 represents the results of velocity vector for standard k − ε and realizable k – ε

from their simulation. From the Figure 2.3, the turbulence model of realizable k – ε

showed it consists of high velocity swirling flow around the center while the low

velocity of weak swirling flow was near to the wall. The strong swirling flow (at

center) created the centrifugal forces to propel the liquid phase (water) to the outer

12

free vortex region. The water was resided and collected in the weak swirling

intensity region [40].

(a) (b)

Figure 2.3: The velocity vector of (a) standard k – ε, and (b) realizable k – ε

turbulence model [40]

Ying Huang and Vigor Yang investigated numerically about the effect of

inlet swirl on the flow development and combustion dynamics in a lean-premixed

swirl-stabilized combustor by using a large-eddy-simulation (LES) technique. The

result showed the swirl number increased, the recirculation zone moved to upstream

and merged with the wake recirculation zone which located behind the centerbody.

Besides, a higher swirl number tends to increase the turbulence intensity, and

consequently the flame speed reduced the flame surface area. However, excessive

swirl might cause the central recirculating flow to penetrate into the inlet annulus and

lead to the occurrence of flame flashback. Figure 2.4, 2.5 and 2.6 shows the

streamline result, contour of vorticity magnitude and so-vorticity surface for two

different swirl numbers [41].

13

Figure 2.4: Streamline patterns for two different swirling numbers [41]

Figure 2.5: Contour of vorticity magnitude for two swirling numbers [41]

14

Figure 2.6: Iso-vorticity surface for two swirling numbers [41]

2.5 Cavitation

Cavitation is one of the important methods to reduce the emissions from combustion

at source is enhances the spray breakup and introducing smaller droplets size inside

the combustion chamber. The physical flow inside the injector nozzle has significant

influences on the spray formation. The high pressure in the injector sudden drop

when across the injector nozzle which tends to accelerate the liquid within the small

nozzle, while the extreme high velocity and low pressure regions creating the

cavitation as shown in the Figure 2.7. Nevertheless, cavitation inside the nozzle has

been resulted as one of the important parameters which can affect fuel spray

atomization.

15

Figure 2.7: Cavitation formation inside nozzle [42]

However, cavitation has positive effects to the development of the fuel spray

breakup. There are many researchers concentrated on using both the experimental

and numerical methods to gain a better knowledge of this phenomenon and its

relation on the durability and performance of diesel fuel injection. According to the

geometric-induced cavitation and string cavitation from experimental, it was found

that the spray angle increases significantly with the existence of string cavitation

while it appear in the nozzle [43]. Gavaises concluded that the cavitation will

damage the areas of bubble collapse, while the engine exhaust emissions increased

by the association of string cavitation structure inside nozzle [44]. Payri et al.

conducted an experimental study about the effects of geometric parameters on

cavitation phenomena within a visual closed vessel by using laser technologies. The

results of their study showed that the jet angle has noticeable affected by the

cavitation behaviours [45]. J.M. Desantes et al. conducted an experimental study

about the influences cavitation phenomena at near-nozzle field through visualization

technique to detect cavitation bubbles injected in a chamber pressurized with liquid

fuel. The noticeable increment of spray cone angle and spray contour irregularities

has been found which related with the presence of cavitation bubbles at the orifice

outlet. They assumed this fact was an indicator of atomization improvement induced

by the collapse of cavitation bubbles at the nozzle exit [46]. Payri et al. studied the

Pinj Pback

Pinj

Pback

Pv

Cavitation onset

Nozzle hole

Theoretical Real pressure

distribution

16

effect of cavitation phenomenon to the spray cone angle, found a significant

increment involved with cavitation appearing [47]. Meanwhile, a similar result was

obtained by Hiroyasu by visualizing separately internal flow and macroscopic spray

from different large-scaled nozzles [48]. Akira Sou et al. also investigated the

cavitation flow in an injector nozzle numerically. They purposed the combination

equations used such as Large Eddy Simulation (LES), Eulerian–Lagrangian Bubble

Tracking Method (BTM), and the Rayleigh–Plesset (RP) to simulate an incipient

cavitation, in which only cavitation bubble clouds appear. In this case, they found a

good agreement with the prediction of cavitation using combination equations [49].

2.6 Nozzle orifice geometries of the injector

Nozzle hole gives a significant impact to the flow inside the nozzle orifice, spray

characteristics, cavitation and turbulence levels and influences to the combustion

indirectly [1, 23, 43, 50]. Nevertheless, there are few types of nozzle orifice are

famous among the researchers such as differences in nozzle hole shape (cylindrical

and conical) and orifice’s diameter.

2.6.1 Nozzle orifice diameter

Li-jun Yang et al. found that the atomization behaviour was affected by the nozzle

diameter, which can be obtained by comparing the spray characteristics of two

different nozzle diameter injectors. The size of swirl chamber and the tangential

inlets are the same for the two injectors. Comparing two injectors under the same

pressure drop, the spray angles of injector with the nozzle diameter of 1.5 mm are

smaller than that of the injector with nozzle diameter of 2 mm. Under the same

pressure drop, a rotating fluid jet is produced from the swirl injector (nozzle diameter

= 1.5 mm). On the contrary, the 2 mm nozzle diameter of injector is swing to form a

twisted ribbon-like structure under the larger swirl strength [51]. Masatoshi Daikoku

and Hitoshi Furudate conducted a studied on effect of the generation and collapse of

cavitation in a nozzle on the breakup of liquid using 3 different ratio nozzle’s length

to diameter as showed in the Table 2.1 below. Their results showed the disturbance

occurring inside nozzle disintegrates as it flows through the large L/D ratio nozzle,

and it did not promote atomization of the issuing liquid jet. On the contrary, a small

17

L/D ratio nozzle, the cavitation generated was not susceptible to the effect of the

downstream flow. It was generated although at low injection pressures, which

promoted the liquid jet breakup. Besides, all transitions are affected by the

generation and collapse of cavitation in small L/D ratio nozzle, and consequently the

flow velocity diminished at which a transition from wavy jet to spray occurs [52].

The liquid velocity against L/D ratio for nozzle diameter of 3 mm and 1.5 mm as

shown in the Figure 2.8 which resulted slow velocity generated with small nozzle

diameter.

Table 2.1: Dimensional specifications of the nozzle [52]

D (mm) L (mm) L/D

3 9

3 1.5 4.5

0.5 1.5

3 36

12 1.5 18

0.5 6

3 60

20 1.5 30

0.5 10

18

(a) D = 3 mm

(b) D = 1.5 mm

Figure 2.8: Liquid velocity generated by two different nozzle diameters with

different L/D ratio (a) D = 3 mm and (b) D = 1.5 mm [52]

Apart from that, the characteristic of cone shape spray was studied and the

efficient atomization or fuel combustion was determined. Other than that, by

referring to the Figure 2.9, it can be concluded that the spray angle increasing while

injection pressure increased, which is in agreement with experimental results by

Halder et al. [53, 54].

Figure 2.9: Injection pressure (bar) against spray angle (°) [54]

19

Similarly, Zuo-Yu Sun et al. conducted numerically investigation on the

influences of nozzle’s geometric parameters on the flow and the cavitation

characteristics within injector’s nozzle. Indeed, the ratio of nozzle’s length to

orifice’s diameter was one of the nozzle parameter to be investigated. They noticed

that narrowing orifice’s diameter can significantly enhances the drag force on the

flow, which induced a sharp reduction of the flow velocity within nozzle.

Additionally, they also concluded for as nozzle’s diameter decreases with a specific

nozzles length, the distributions of the concerned physical fields are obviously varied.

Under different nozzle’s diameter, the different locations at where the low (even

negative) pressure occurs, which results the origins cavitation locations was different.

Consequently, subsequent development and extinction of cavitation were diverse

under different diameter [55]. Moreover, Figure 2.10 illustrates their result published

under different ratio of length to diameter.

Figure 2.10: Nozzle flow characteristics under different L/D: (a) mass flow rate and

flow coefficient; (b) average volume of vapour; and (c) average velocity at outlet and

average turbulent kinetic energy [55]

With the same D With the same L

With the same D

With the same L

With the same D

With the same L

20

2.6.2 Nozzle hope shape

There are two common nozzle shapes used to determine the injection behaviour of

fuel, fuel-air mixing or fuel-air-water mixing. That two common nozzle shapes are

conical and cylindrical nozzle orifices. Many previous literatures [15, 56-58] used

these two types of nozzle hope shape to make a comparison for investigate the

cavitation, flow efficiency (discharge coefficient) and exit velocity, spray

characteristics, and mass flow rate.

An experimental study was conducted by Benajes et al. to analyse the effects

of conical and cylindrical nozzle orifices to the injection rate behaviours of a fuel

injection system on a cavitation test. The observation of the experiment was taken to

compare a cylindrical orifice and a conical orifice in order to increase flow efficiency

(discharge coefficient), cavitation reduction, and exit velocity. However, the fuel

injection rate is minimized due to the smaller exit area [56]. Payri et al. was observed

that the smothery conditions with cylindrical nozzles, meanwhile for the mass flow

rate of conical nozzle was relative to the square root of pressure drop referential

absence of cavitation at the nozzle exit. Other than that, they also found that was an

increasing in injection velocity due to the existence of vapour at the orifice outlet for

the cylindrical nozzle [15]. Furthermore, Han et al. found that the primary breakup

region is relatively influenced by nozzle geometry when compared to the conical and

cylindrical nozzles [58]. Besides, he also examined the effects of different nozzle

orifice geometry on spray penetration, liquid length, and cone angle [23, 58] .

2.6.3 Conical nozzle

The conical nozzle is a cone shape nozzle which designed by the cone half angle as

viewed from the side. The conicity of nozzle is determined by the formula (2.1) of

orifice coefficient, K [17, 23, 42, 55, 59]. The conicity of nozzle and is defined as the

ratio of the difference between nozzle’s inlet (Din) and nozzle’s outlet (Dout) to a

constant10 µm [55, 60]. If orifice coefficient, K is positive value, the orifice gives a

conical pattern. Contrarily if K is negative value, the orifice gives an inverted conical

pattern.

m

DDK outin

10

_ (2.1)

21

Where, K : Conicity of the nozzle hole

Din : Inlet diameter of the nozzle in unit μm

Dout: : Outlet diameter of the nozzle in unit μm

Figure 2.11 shows several types of different nozzle shape with their value of

Kfactor. Positive Kfactor has a larger inlet diameter and smaller outlet diameter to form a

convergent nozzle. Zero Kfactor is a kind of nozzle that has the same diameter for both

inlet and outlet. Meanwhile, the negative Kfactor nozzle has a smaller inlet diameter

but larger outlet diameter, which can be form a divergent nozzle [42]. However,

there are only two types of nozzle hole shape use in this study, cylindrical and

conical nozzles.

Figure 2.11: Types of nozzle hole shape

There are some advantages for using a conical nozzle, such as the nozzle

design is simple which just like cone shape, and it is no any inflection when

propellants are dismissed from the combustion chamber. No inflection means that the

propellant is coming out straight line from the nozzle of the throat to the exit.

Therefore, a conical nozzle is always used for solid and hybrid propellant types due

to the lack of inflection.

Meanwhile, there are also some disadvantages of a conical nozzle. One of it

is the fact that conical nozzle more divergence loss significantly at the exit. The

propellant is flow parallel with the centerline of the nozzle as it exits. It would rather

22

at the cone angle if the flow exit is not parallel because of the exit angle of a conical

nozzle is the same as the cone angle. The flow will experiences divergence loss

which can cause energy loss, when there is an angle at the exit. Energy loss will turn

a loss of nozzle efficiency indirectly. The exit angle a conical nozzle is large and

therefore it maximized the divergence loss. Another disadvantage is a conical nozzle

is heavier because of it contains more materials, even it is in the same design with

others nozzle.

A pintle-type nozzle can form the conical-shaped spray and the homogeneous

mixture is created for the purpose of low exhaust emissions are achieved in the

premixed diesel combustion. As compared to a nozzle jet burner, the conical shape of

the nozzle can well-controlled the level of partial premixing and highly stabilized

partially premixed flames [61]. The lean mixture with high homogeneity could

achieve by conical-spray injector in the cylinder for advanced diesel injection.

Furthermore, by impinging the fuel jets on the guide wall shortly before the nozzle

exit will improve the conical-spray nozzle forming a conical shape spray [43].

Moreover, the conical nozzle has higher exit density, velocity, and discharge

coefficient which could contributed to the suppression of cavitation for the nozzle.

Besides, its fuel injection rate is lower due to a smaller exit diameter.

Ming Jia et al. conducted a numerical simulation with ANSYS Fluent on the

cavitation within conical-spray nozzle, the results indicated that the cavitation

influences to the film thickness and fuel velocity at nozzle’s exit and also the spray

angle [43]. K. Pougatch et al. investigated numerically the influence of conical

nozzle attachments on spray dispersion in a fluidized bed. They found the small

angles have a stabilizing effect on spray, while large angle caused destabilising [62].

Michele Battistoni et al. studied cavitation flow and spray through CFD simulation

with two types of nozzle, cylindrical and conical and two different fuel properties,

diesel and biodiesel. They concluded that the fuel type was not much affected to the

extent of cavitation regions, whereas it is strongly dependent on the nozzle shape.

However, cylindrical nozzle generated high cavitating flow, while conical nozzle has

the tendency to reduce cavitation inside the nozzle. Other than that, diesel fuel was

observed produced higher liquid penetration in the conical convergent nozzle [17].

A.M. Elbaz et al. investigated the stabilization mechanism, stability limits, and the

flow field structure of highly stabilized partially premixed methane flames in a

concentric flow conical nozzle burner with air co-flow and observed that the stability

23

is improved by added air co-flow. Most of the stable flames were approached fully

premixed when the air co-flow velocity increased [61].

2.6.4 Cylindrical nozzle

The standard nozzle shape is in cylindrical which having same diameter of nozzle

inlet and outlet. Cylindrical nozzle is a type of nozzle with cylindrical shaped head of

nozzle exit. Besides, cylindrical nozzle is much more inclined to cavitate compared

to conical nozzle because of it has a small conicity and low values of the rounding

radii [57]. Therefore, people tend to use both of these different shaped nozzles as

comparison in the experiment. Figure 2.12 below shows the geometry of cylindrical

nozzle head and its output.

Figure 2.12: Schematic diagram of the cylindrical nozzle head and exit [63]

The mass flow rate of cylindrical nozzle increases proportionally with the

root of the pressure differential until the point where it achieved its stability. This

situation occurred is because of the appearance of the cavitation phenomenon. The

critical cavitation number could be associated the point which the mass flow rate

starts to stabilize. Since cylindrical nozzle is a cavitating nozzle, thus its critical

cavitation number is defined as Kcrit, which corresponding of cavitation starts in the

injector orifice to the pressure drop. This phenomenon occurs when it is detected by

the stabilization of the mass flow rate across the orifice at a given value of the

injection pressure. Moreover, this phenomenon causes the further decrease in

24

discharge pressure which has another name called as choking. Hence, cavitation

occurs if the cavitation number that corresponds to these pressure conditions is lower

than the critical value of KcritT [47]. The values of the critical cavitation number can

be calculated using the equation:

bi

vi

PP

PPK

(2.2)

Where Pv represent the vapour pressure.

F. Payri et al. was determined the influence of cavitation on the internal flow

and the macroscopic behaviour of the spray in Diesel injection nozzles with two

different nozzles, cylindrical and conical nozzles. They observed there was an

exception for the cylindrical nozzle, which showed it decreased in the discharge

coefficient for each injection pressure with marked Reynolds number as shown in the

Figure 2.13. The decreased of the discharge coefficient can be explained with the

collapse of the mass flow rate due to the cavitation. In this case of the cylindrical

nozzle, the increasing of the discharge coefficient was cut short when the moment

that cavitation appears. Therefore, the discharge coefficient depends on the cavitation

number for that moment [47].

Figure 2.13: The graph of discharge coefficient versus the Reynolds number [47]

Masatoshi Daikoku and Hitoshi Furudate studied the disintegration of liquid

jet by cavitation inside a cylindrical nozzle with different ratio nozzle’s length per

simulation of different orifice geometries on premix...

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