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ABSTRACT

JING, WEI. Experimental Investigation of Clean Spray Combustion under Varied Ambient Conditions. (Under the direction of Dr. Tiegang Fang).

In recent decades, environmental pollution becomes the one of the most serious

global issues due to the pollution emissions from the fossil fuel combustion. Soot and NOx

from diesel spray combustion make a large contribution in the pollution emissions from

power and transportation sectors. With the stringent emission standards and the limited fossil

fuel resources, clean spray combustion with a high efficiency needs to be developed in order

to improve the environment and sustain the consumption of fossil fuel.

Experimental spray combustion investigation was conducted in an optically

accessible constant-volume chamber using a single-nozzle fuel injector. The ambient O2

concentration was varied between five discrete values from 10% to 21% and three different

ambient temperatures (800 K, 1000 K, and 1200 K). These conditions simulate different

EGR levels and ambient temperatures in diesel engines. Three fuels (Ultra-low-sulfur Diesel,

Jet-A, and biomass-based BTL) were used in this study.

First, multi-band emission measurement was conducted to illustrate the diesel and

Jet-A flame development under different ambient conditions. The transient and quasi-steady

state analyses were based on measurements using a high-speed video camera and an ICCD

camera, respectively. Soot is seen to be oxidized more quickly for Jet-A than diesel at the end

of combustion, evident by comparing the area of NL, especially under high O2 concentration.

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flame structures were proposed to complement the previous conceptual models for spray

combustion under different combustion modes.

Second, soot concentration (KL factor) and soot temperature were measured under

different ambient conditions for a new biomass-based BTL, ultra-low sulfur diesel, and

Jet-A. A high-speed camera was employed coupled with two bandpass filters (centered at 550

nm and 650 nm, both with 10 nm FWHM) to implement a two-color thermometry method

and measure the soot concentration and temperature simultaneously. More soot is seen in the

near-wall regions under the low-temperature combustion mode while high level soot is

observed in the upstream and midstream for the conventional combustion mode. BTL

combustion generates a lower integrated KL factor and soot temperature compared to diesel

and jet-A fuels under all the experimental conditions. Finally, the characteristics of the

two-color results were further discussed and analyzed.

Finally, the effects of different loads and the two-injection strategy were analyzed

under three typical ambient conditions based on the two techniques mentioned. The results

indicate that low load condition tends to have average lower soot with a similar average soot

temperature compared to high load condition during the whole flame development except the

case that a larger accumulation of soot occurs at the end of combustion under the high

temperature and highly diluted combustion. The two-injection strategy with a pilot injection

can be considered as an effective method to reduce soot and NOx simultaneously compared

to single injections under the high temperature highly diluted combustion and the

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© Copyright 2014 by Wei Jing

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Experimental Investigation of Clean Spray Combustion under Varied Ambient Conditions

by Wei Jing

A dissertation submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Mechanical Engineering

Raleigh, North Carolina

2014

APPROVED BY:

_______________________________ ______________________________

Dr. Tiegang Fang Dr. William L. Roberts

Committee Chair

________________________________ ________________________________

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DEDICATION

To my wife, Huiquan Jiang, my coming daughter, Adela,

and my parents, Zengqiang Jing and Shuling Wang

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BIOGRAPHY

Wei Jing was born in Linfen, Shanxi, China, a historical town in north China. His

father was a manager in a coal bureau and his mother was an accountant in a transportation

company. His parents not only tried their best to give him a better environment filled with

endless happiness in his life, but also set an example for a honest person who has optimistic

and persistent attitude to deal with the problems.

In 2004, he graduated from Linfen No.1 high school, where he learned the basic

knowledge for entering the scientific world. In 2008, he received his bachelor degree in

thermal engineering from Beijing Institute of Technology in Beijing, China. In 2010, he

earned his master degree in power mechanics from Beijing Institute of Technology in Beijing,

China. He focused on the three dimensional simulation of characterization of gas exchange

process of the Hydrogen Internal Combustion Engine in the engine research lab.

In 2010 at STATE, he started his PhD research in spray combustion under the

advisory of Dr. Tiegang Fang. He worked hard and tried plenty of ideas in spray combustion

measurement. During the past four years, he has been fighting with all the problems occurred

during the experiments and data analysis process. PhD life is valuable and indispensable to

some degree, which is very important to help him grow up quickly. Because it is not only

gaining huge knowledge, but also shaping your manners and improving your endurance.

He firstly met his wife, Huiquan Jiang, when he was pursing his bachelor degree in

2005. Two young people were attracted with each other after several months’ communication.

After one year later, it turned into a serious relationship. In 2010, after six years of knowing

each other, they got married with all the wishes right before he came to Raleigh. Meeting his

wife and walking into her life were the most significant and happy timings in his life.

After graduation, he will start a research and development career in automotive

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ACKNOWLEDGMENTS

First and foremost, I would like to express my sincere gratitude to my advisor

Professor Tiegang Fang for his support and guidance during my PhD period at North

Carolina State University. His extensive knowledge, precise and logical research attitude

became a beacon, which is always lighting the way in my research work. His continuous

support and encouragement are the essential driving force for me to overcome all the

difficulties before arriving at the destination.

I would also like to thank my committee members: Dr. William L. Roberts, Dr. Tarek

Echekki, Dr. Alexei V. Saveliev and Dr. Wayne Yuan. Dr. Roberts kindly shared his lab

space and experimental equipment when he worked at NC State University. He also provided

many useful suggestions on my research work, which helped me move faster in the research.

Dr. Saveliev gave me plenty of advices on my research and shared lots of experimental

devices, which is greatly helpful in solving the problems. Dr. Echekki described the fluid

dynamics vividly in his class, which greatly enriched my understanding in this area. Dr.

Yuan shared more lights on my research topics, which is greatly helpful to improve my work.

The effort from all committee members is indispensable for my research and is greatly

appreciated.

I am very lucky to have worked on the same project with Dr. Ji Zhang who shared

with me lots of valuable experience and suggestions on the experiments and data analysis. I

won’t forget the time we worked together and the enjoyment after solving the problems. I

appreciate the help from Zengyang Wu for conducting the experiments during my last year. I

wish him best in his research in the near future.

Daily communication with the smart people in my lab over the past four years was

very important in my PhD life, which was not only the source of valuable knowledge and

opinions to my research work, but also the precious friendship influencing me now and the

future. Parth Shah, Dolanimi Ogunkoya, Dr. Weibo Zhang, Aaron McCullough, Libing

Wang, Shanshan Yao, Pin-jia Chen, Ranjith Kumar A K and Vilas V Jangale are fabulous

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I would express my sincere appreciation to my parents. Their unconditional support

and endless pure love is my essential spiritual support, providing the courage and faith to

overcome all the difficulties. Every time when I stand on the crossing, your smiles are always

shown in my head and accompany with me in my life. In the meantime, I am especially

indebted to my wife, Huiquan Jiang, who always stands by me and shares her most valuable

time with me over the past ten years. You are the fountain of inspiration and hope for my life

in the past, now and the future.

Last but not the least, this work is supported by, or in part by, the Natural Science

Foundation under Grant No. CBET-0854174, and the U.S. Army Research laboratory and the

U.S. Army Research Office under Grant – W911NF-10-1-0118. The funding sources are the

important factors in my PhD research work, which is greatly appreciated. Any opinions,

findings, and conclusions or recommendations expressed in this material are those of the

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TABLE OF CONTENTS

LIST OF TABLES ... x

LIST OF FIGURES ... xi

1. Introduction ... 1

Motivation ... 1

1.1. Research objective and approaches ... 2

1.2. Figures ... 5

1.3. 2. Literature Review ... 8

Spray, combustion and emission of liquid fuel ... 8

2.1. 2.1.1. Liquid sprays ... 8

2.1.1.1. Break-up length ... 8

2.1.1.2. Spray tip penetration ... 9

2.1.1.3. Spray cone angle ... 9

2.1.1.4. Drop size distribution ... 9

2.1.1.5. Fuel air entrainment ... 10

2.1.2. Spray Combustion ... 11

2.1.2.1. Ignition delay ... 11

2.1.2.2. Lift-off length... 11

2.1.3. Emissions ... 12

2.1.3.1. Chemiluminescence ... 12

2.1.3.2. Soot luminosity ... 13

2.1.3.2.1. Soot formation and oxidation... 13

2.1.3.2.2. Effects of physical parameters on soot ... 15

In-cylinder soot reduction in spray combustion ... 16

2.2. 2.2.1. Flame temperature effect on soot reduction ... 16

2.2.2. Equivalence ratio effect on soot reduction ... 18

2.2.3. Fuel type effect on soot reduction ... 19

Soot measurement techniques ... 21

2.3. 2.3.1. Two-color measurement ... 21

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2.3.1.2. Implementation ... 22

2.3.1.2.1. Selection of ... 22

2.3.1.2.2. Selection of wavelength ... 22

2.3.1.2.3. Calibration... 22

2.3.1.3. Accuracy ... 23

2.3.2. Light extinction measurement... 23

2.3.2.1. Principle ... 24

2.3.2.2. Implementation of the light extinction method ... 25

2.3.3. Laser induced incandescence (LII) ... 25

2.3.3.1. Principle ... 25

2.3.3.2. Implementation of LII ... 27

Summary ... 27

2.4. Figures ... 28

2.5. 3. Experimental setup ... 31

Combustion chamber ... 31

3.1. 3.1.1. Background ... 31

3.1.2. Chamber body configuration ... 31

3.1.3. Chamber body Stress evaluation ... 32

3.1.3.1. Peak temperature and pressure calculation ... 32

3.1.3.2. Chamber stress calculation ... 33

3.1.3.3. Quartz window stress calculation ... 33

Premixing gas supply system ... 33

3.2. Fuel delivery system ... 34

3.3. Ignition system ... 35

3.4. Data acquisition and control system ... 35

3.5. 3.5.1. Hardware and software configuration ... 35

3.5.2. Protection configuration... 36

Experimental sequence ... 37

3.6. Tables and figures ... 38

3.7. 4. Spray combustion investigation based on multi-band emission measurement ... 42

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4.2. Diagnostics and Measurements ... 46

4.3. RESULTS AND DISCUSSION ... 47

4.3.1. Diesel spray combustion analysis ... 47

4.3.1.1. Transient analysis... 48

4.3.1.2. Quasi-steady analysis ... 50

4.3.1.3. Analysis of ambient oxygen concentration effect on flame structure ... 52

4.3.1.4. Spatially integrated intensity and image area during quasi-steady combustion ... 55

4.3.2. Spray Combustion of Jet-A and Diesel Fuels ... 57

4.3.2.1. Transient Analysis ... 57

4.3.2.2. Quasi-steady State Analysis ... 60

4.3.2.3. Comparison of Jet-A and Diesel ... 62

4.3.2.4. Further Discussion on Spray Flame Structure ... 65

4.4. Conclusions ... 69

4.5. Tables and figures ... 72

5. Soot measurement based on two-color pyrometry ... 94

5.1. Introduction ... 94

5.2. Diagnostics and Measurements ... 95

5.3. Results and discussion ... 97

5.3.1. Soot KL Factor and Soot Temperature Distribution ... 98

5.3.2. Time Resolved Results Analysis ... 100

5.3.3. Quasi Steady State Process Analysis ... 101

5.3.4. Further Discussion on the Two-color Results ... 103

5.4. Conclusions ... 105

5.5. Tables and figures ... 107

6. Effect of loads on the spray combustion process ... 119

6.1. Introduction ... 119

6.2. Diagnostics and Measurements ... 120

6.3. Results and discussion ... 121

6.3.1. Multi-band emissions measurement ... 121

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6.3.1.2. Time resolved results (Three emissions development of BTL combustion for two

typical loads under two different ambient conditions) ... 124

6.3.1.3. Time resolved results (Effective area trend of three emissions for two typical loads under two ambient conditions)... 126

6.3.1.4. Time resolved results (The trend of different loads for each emission under two ambient conditions) ... 127

6.3.1.5. Transient distribution analysis ... 129

6.3.2. Two-color measurement ... 130

6.3.2.1. Transient analysis... 130

6.3.2.2. Time resolved results (The trend of different loads for BTL combustion under three ambient conditions) ... 131

6.3.2.3. Time resolved results (Comparison of three fuels under two typical loads) ... 132

6.4. Conclusion ... 133

6.5. Tables and figures ... 135

7. Effect of pilot injection on the spray combustion ... 155

7.1. Introduction ... 155

7.2. Diagnostics and Measurements ... 156

7.3. Results and discussion ... 157

7.3.1. Multi-band emissions measurement ... 157

7.3.1.1. Transient analysis... 157

7.3.1.2. Time resolved results ... 158

7.3.2. Two-color measurement ... 159

7.3.2.1. Transient analysis... 159

7.3.2.2. Time resolved results ... 161

7.4. Conclusion ... 162

7.5. Tables and figures ... 164

8. CONCLUSION ... 177

8.1. Conclusions ... 177

8.2. Major contributions ... 180

8.3. Future work... 181

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LIST OF TABLES

Table 3. 1 Initial mixture parameters for online equilibrium calculation ... 38

Table 3. 2 The composition of exhaust ... 38

Table 3. 3 Maximum allowable stresses at different positions on chamber ... 38

Table 4. 1 Details for the common rail injection system and injection parameters ... 72

Table 4. 2 Experimental ambient conditions for diesel spray combustion ... 72

Table 4. 3 Selected properties for Jet-A and diesel ... 72

Table 4. 4 Optical thickness of the neutral density filter (D) for Andor ICCD camera. ... 73

Table 4. 5 Optical thickness of the neutral density filter (D) for Phantom v4.3 camera ... 73

Table 4. 6 Parameter setup of the three cameras ... 73

Table 5. 1 Experimental ambient conditions for different fuels ... 107

Table 5. 2 Selected properties of BTL and diesel ... 107

Table 6. 1 Fuel quantities of different loads for three fuels ... 135

Table 6. 2 Ambient conditions for two different measurements ... 135

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LIST OF FIGURES

Figure 1. 1 Atmospheric emissions by source in the European Union (EU15). ... 5

Figure 1. 2 The percent contribution from source sectors across the common pollutants in US. ... 5

Figure 1. 3 World commercial energy consumption. ... 6

Figure 1. 4 World Oil Consumption by Sector from 2003 to 2030. ... 6

Figure 1. 5 The evolution of EPA emission standards... 7

Figure 1. 6 Energy reserves for fossil fuel. ... 7

Figure 2. 1 Chemiluminescence spectrum during the early stages of diesel autoignition under the ambient condition: 900 K, 16.6 kg/m3. ... 28

Figure 2. 2 Chemiluminescence spectrum for the 0.25 mm spectrometer entrance. ... 28

Figure 2. 3 Natural combustion emission spectrum for sooting combustion in the diffusion flame. ... 29

Figure 2. 4 Effect of stoichiometry of the fuel side on soot formation in ethylene and propane opposed jet counterflow diffusion flames. ... 29

Figure 2. 5 Different combustion regimes in ϕ-T map. ... 30

Figure 2. 6 Schematic layout for the light extinction measurement. ... 30

Figure 3. 1 Experimental system: 1.fuel injector; 2.exhaust line; 3.chamber body; 4.quatz window; 5. plug/window retainer; 6. pressure transducer; 7.intake line; 8.metal plug; 9. spark plug; 10. combustion chamber. ... 39

Figure 3. 2 The diagram configuration of gas supply system. ... 39

Figure 3. 3 The actual hardware configuration of gas supply system. ... 40

Figure 3. 4 The diagram configuration of fuel delivery system. ... 40

Figure 3. 5 The actual hardware configuration of fuel delivery system. ... 41

Figure 4. 1 Experimental system: 1.fuel injector; 2.exhaust line; 3.chamber body; 4.quatz window; 5. plug/window retainer; 6. pressure transducer; 7.intake line; 8.metal plug; 9. spark plug; 10. combustion chamber; 11.natural density filter; 12. band pass filter(310nm,430nm and 470nm, 10 nm FWHW); 13.high speed cameras(Kodak, Phantom v4.3 or Andor ICCD camera) ... 74

Figure 4. 2 The major compositions of the two fuels based on GC analysis: diesel (top) and Jet-A (bottom). ... 74

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Figure 4. 4 Spatially integrated intensity of NL (SINL) under three temperatures with

different O2 concentrations: (a) 800 K; (b) 1000 K; (c) 1200 K. ... 76

Figure 4. 5 Spatially integrated intensity of OH* (SIOH*) under three temperatures with different O2 concentrations: (a) 800 K; (b) 1000 K; (c) 1200 K. ... 77

Figure 4. 6 Images of OH* distribution and average OH* intensity profiles along the nozzle axial direction for five different O2 concentrations under 1200 K. ... 78

Figure 4. 7 Details near the nozzle tip of average OH* intensity along the nozzle axial direction for five different O2 concentrations under 1200 K, the distance of the red circles

present the lift-off length for each O2 concentration condition. ... 79

Figure 4. 8 The comparison of lift-off length in all the experimental conditions. ... 79 Figure 4. 9 The comparison of four different signals areas in the quasi-steady state under 1200 K with various O2 concentrations. ... 80

Figure 4. 10 (a) Contour plots for Ψ=1 with different ambient oxygen concentrations;

Contour plots under three ambient oxygen concentrations with five different Ψ values of 0.5, 1, 2, 3 and 5: (b) 21% of O2 concentration; (c)15% of O2 concentration; (d) 10% of O2

concentration. ... 81 Figure 4. 11 The comparison of steady flame length for 21% O2 concentration conditions

between model value and experimental values with three threshold ratios for different

temperatures. ... 82 Figure 4. 12 Spatially integrated intensity, spatial area and intensity per pixel for OH* with different O2 concentrations and temperatures. ... 82

Figure 4. 13 Spatially integrated intensity, spatial area and intensity per pixel for NL with different O2 concentrations and temperatures. ... 82

Figure 4. 14 Spatially integrated intensity, spatial area and intensity per pixel for band A with different O2 concentrations and temperatures. ... 83

Figure 4. 15 Spatially integrated intensity, spatial area and intensity per pixel for band B with different O2 concentrations and temperatures. ... 83

Figure 4. 16 The comparison of spatial area of four different signals for diesel under different O2 concentrations and temperatures. ... 83

Figure 4. 17 Spatial distribution at different times of Natural Luminosity (left) and OH* chemiluminescence (right) under different ambient temperatures with three O2

concentrations: 10% (left two columns); 15% (middle two columns); 21% (right two

columns); (a) 1000 K; (b)1200 K; (c)800 K. ... 84 Figure 4. 18 Spatially integrated intensity of NL (SINL) by using under three temperatures with different O2 concentrations: (a) 800 K; (b) 1000 K; (c) 1200 K... 85

Figure 4. 19 Spatially integrated intensity of OH* (SIOH*) chemiluminescence under three temperatures with different O2 concentrations: (a) 800 K; (b) 1000 K; (c) 1200 K. ... 86

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Figure 4. 22 Spatially integrated intensity, spatial area and intensity per pixel for NL with different O2 concentrations and temperatures. ... 88

Figure 4. 23 Spatially integrated intensity, spatial area and intensity per pixel for band A with different O2 concentrations and temperatures. ... 88

Figure 4. 24 Spatially integrated intensity, spatial area and intensity per pixel for band B with different O2 concentrations and temperatures. ... 89

Figure 4. 25 The comparison of spatial area of four different signals for Jet-A under different O2 concentrations and temperatures. ... 89

Figure 4. 26 Spatial distribution at different times of OH* chemiluminescence for Jet-A (left) and diesel (right) under different ambient temperatures with 21% O2 concentration: 800 K

(left two columns); 1000 K (middle two columns); 1200 K (right two columns). ... 89 Figure 4. 27 Spatial distribution at different times of OH* chemiluminescence for Jet-A (left) and diesel (right) under different ambient temperatures with 10% O2 concentration: 800K

(left two columns); 1000K (middle two columns); 1200 K (right two columns). ... 90 Figure 4. 28 Spatial distribution at different times of Natural Luminosity for Jet-A (left) and diesel (right) under different ambient temperatures with 21% O2 concentration: 800 K (left

two columns); 1000 K (middle two columns); 1200 K (right two columns). ... 90 Figure 4. 29 Spatial distribution at different times of Natural Luminosity for Jet-A (left) and diesel (right) under different ambient temperatures with 10% O2 concentration: 800 K (left

two columns); 1000 K (middle two columns); 1200 K (right two columns). ... 90 Figure 4. 30 Difference of spatially integrated intensity, spatial area and intensity per pixel for OH* between Jet-A and diesel with different O2 concentrations and temperatures. ... 91

Figure 4. 31 Difference of spatially integrated intensity, spatial area and intensity per pixel for NL between Jet-A and diesel with different O2 concentrations and temperatures. ... 91

Figure 4. 32 Difference of spatially integrated intensity, spatial area and intensity per pixel for NL between Jet-A and diesel with different O2 concentrations and temperatures. ... 91

Figure 4. 33 Difference of spatially integrated intensity, spatial area and intensity per pixel for NL between Jet-A and diesel with different O2 concentrations and temperatures. ... 92

Figure 4. 34 . Boundaries of the four flame emissions for Jet-A under two ambient

conditions: (a) 800 K and 10% O2; (b) 1000 K and 21% O2 ... 92

Figure 4. 35 OH* intensity along the radial direction at three locations from nozzle for jet-A: 30mm, 50mm and 70mm under two conditions: (a) 800 K and 10% O2 ; (b) 1000 K and 21%

O2 ... 92

Figure 4. 36 Boundaries of the four flame emissions for diesel under two ambient conditions: (a) 800 K and 10% O2; (b) 1000 K and 21% O2 ... 93

Figure 4. 37 Low temperature (a) and conventional (b) high-load quasi-steady state spray flame structure based on four emissions: OH*, Band A, Band B, and natural luminosity. The flame structures are not in scale. ... 93

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plug; 10. combustion chamber; 11. Double-image lens; 12. Narrowband filters (550 nm and 650 nm, both 10 nm FWHM); 13.high speed cameras. ... 108 Figure 5. 2 Camera calibration curves for aperture F# of 11 at two wavelengths. ... 108 Figure 5. 3 Soot temperature and KL factor at the initial stage under 1000K ambient

temperature for 10%, 15% and 21% O2 concentration conditions: diesel (top), BTL (middle),

and Jet-A (bottom). ... 110 Figure 5. 4 Soot temperature and KL factor distribution at the developing stage under 1000K ambient temperature for 10%, 15% and 21% O2 concentration conditions: diesel (top), BTL

(middle), and Jet-A (bottom). ... 111 Figure 5. 5 Soot temperature and KL factor distribution at the quasi-steady state under 1000K ambient temperature for 10%, 15% and 21% O2 concentration conditions: diesel (top), BTL

(middle), and Jet-A (bottom). ... 112 Figure 5. 6 Soot temperature and KL factor distribution at the end of combustionunder

1000K ambient temperature for 10%, 15% and 21% O2 concentration conditions: diesel

(top), BTL (middle), and Jet-A (bottom). ... 114 Figure 5. 7 Time resolved results for average soot temperature under all the experimental conditions: BTL (top) and Diesel (bottom). ... 114 Figure 5. 8 Time resolved results for average KL factor under all the experimental

conditions: BTL (top) and Diesel (bottom). ... 115 Figure 5. 9 Quasi-steady state comparison for BTL and diesel under all experimental

conditions: (a) average KL factor, (b) average KL factor, (c) integrated KL factor, (d) soot area and (e) high temperature area. ... 116 Figure 5. 10 Integrated KL factor and soot temperature under all the experimental conditions for the two fuels. ... 117 Figure 5. 11 Distribution of soot temperature and KL factor differentiated by the radiance of the wavelength of 550 nm for BTL under different ambient conditions: 800 K, 1000 K, and 1200 K (top to bottom); 10%, 15%, and 21% (left to right). ... 117 Figure 5. 12 Distribution of soot temperature and KL factor differentiated by the radiance ratio of two wavelengths for BTL under different ambient conditions: 800 K, 1000 K, and 1200 K (top to bottom); 10%, 15%, and 21% (left to right). ... 118 Figure 5. 13 The relation between radiance ratio and soot temperature. ... 118

Figure 6. 1 Three emissions development of BTL flame under two ambient conditions for the low load condition (0.5 ms): OH (left two columns), Band A (middle two columns), and Band B (right two columns); 1400 K and 10% O2 (left), 1000 K and 21% O2 (right). ... 136

Figure 6. 2 Three emissions development of BTL flame under two ambient conditions for the medium load condition (1.0 ms): OH (left two columns), Band A (middle two columns), and Band B (right two columns); 1400 K and 10% O2 (left), 1000 K and 21% O2 (right). ... 136

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Figure 6. 4 Three emissions development of diesel flame under two ambient conditions for the low load condition (0.5 ms): OH (left two columns), Band A (middle two columns), and Band B (right two columns); 1400 K and 10% O2 (left), 1000 K and 21% O2 (right). ... 137

Figure 6. 5 Three emissions development of Jet-A flame under two ambient conditions for the low load condition (0.5 ms): OH (left two columns), Band A (middle two columns), and Band B (right two columns); 1400 K and 10% O2 (left), 1000 K and 21% O2 (right). ... 137

Figure 6. 6 Intensities of three emissions of BTL flame under two ambient conditions at two loads: low load (top) and high load (bottom)... 138 Figure 6. 7 Effective areas of three emissions of BTL flame under two ambient conditions at two loads: low load (top) and high load (bottom). ... 138 Figure 6. 8 Intensities of three emissions of diesel and Jet-A flames under two ambient conditions at two loads: low load (top two rows) and high load (bottom two rows). ... 139 Figure 6. 9 Effective areas of three emissions of BTL flame under two ambient conditions for low load: 1400 K and 10% (top), 1000 K and 21% (bottom). ... 140 Figure 6. 10 Effective areas of three emissions of diesel and Jet-A flames under two ambient conditions for low load: diesel (top), Jet-A (bottom). ... 140 Figure 6. 11 Intensities of three emissions of BTL flame for different loads under two

ambient conditions: 1400 K and 10% (top), 1000 K and 21% (bottom). ... 141 Figure 6. 12 Effective areas of three emissions of BTL flame for different loads under two ambient conditions: 1400 K and 10% (top), 1000 K and 21% (bottom). ... 141 Figure 6. 13 Intensities per effective area of three emissions of BTL flame for different loads under two ambient conditions: 1400 K and 10% (top), 1000 K and 21% (bottom). ... 142 Figure 6. 14 Intensities of three emissions of diesel and Jet-A flames for different loads under the condition of 1400 K and 10%: diesel (top), Jet-A (bottom). ... 142 Figure 6. 15 Intensities of three emissions of diesel and Jet-A flames for different loads under the condition of 1000 K and 21%: diesel (top), Jet-A (bottom). ... 143 Figure 6. 16 Effective areas of three emissions of diesel and Jet-A flames for different loads under the condition of 1400 K and 10%: diesel (top), Jet-A (bottom). ... 143 Figure 6. 17 Effective areas of three emissions of diesel and Jet-A flames for different loads under the condition of 1000 K and 21%: diesel (top), Jet-A (bottom). ... 144 Figure 6. 18 Intensities per effective area of three emissions of diesel and Jet-A flames for different loads under the condition of 1400 K and 10%: diesel (top), Jet-A (bottom). ... 144 Figure 6. 19 Intensities per effective area of three emissions of diesel and Jet-A flames for different loads under the condition of 1000 K and 21%: diesel (top), Jet-A (bottom). ... 145 Figure 6. 20 Five emissions of BTL flame under the condition of 1000 K and 21% O2... 145

Figure 6. 21 Five emissions of BTL flame under the condition of 1400 K and 10% O2... 146

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Figure 6. 25 KL factor images at the developing stage of combustion for two loads under three ambient conditions: low load (top), high load (bottom). ... 148 Figure 6. 26 Soot temperature images at the end of combustion for two loads under three ambient conditions: low load (top), high load (bottom). ... 148 Figure 6. 27 KL factor images at the end of combustion for two loads under three ambient conditions: low load (top), high load (bottom). ... 149 Figure 6. 28 Average soot temperature of BTL flame for different loads under three ambient conditions. ... 149 Figure 6. 29 Average KL factor of BTL flame for different loads under three ambient

conditions. ... 150 Figure 6. 30 Integrated KL factor of BTL flame for different loads under three ambient conditions. ... 150 Figure 6. 31 Soot area of BTL flame for different loads under three ambient conditions. ... 150 Figure 6. 32 Average soot temperature of three fuels for low and high loads under three ambient conditions. ... 151 Figure 6. 33 Average KL factor of three fuels for low and high loads under three ambient conditions. ... 152 Figure 6. 34 Average soot temperature of three fuels for low and high loads under three ambient conditions. ... 153 Figure 6. 35 Average soot temperature of three fuels for low and high loads under three ambient conditions. ... 154

Figure 7. 1 The three emissions development under two ambient conditions for three injection types: OH (left two columns), Band A (middle two columns), and Band B (right two columns); 1400 K and 10% O2 (left), 1000 K and 21% O2 (right); (a) two-injection, (b)

single injection with the injection width of 0.8 ms, (c) single injection with the injection width of 1.0 ms. ... 165 Figure 7. 2 Intensities of three emissions under two ambient conditions for three injection types. ... 166 Figure 7. 3 Effective areas of three emissions under two ambient conditions for three

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Figure 7. 7 KL factor images at the developing stage of combustion for the three injection types under ambient conditions: two-injection (top), single injection A (middle), single injection B (bottom) ... 171 Figure 7. 8 Soot temperature images at the end of combustion for the three injection types under ambient conditions: two-injection (top), single injection A (middle), single injection B (bottom)... 172 Figure 7. 9 KL factor images at the end of combustion for the three injection types under ambient conditions: two-injection (top), single injection A (middle), single injection B

(bottom)... 173 Figure 7. 10 Soot developments for the three injection types in terms of average soot

temperature, average KL factor, integrated KL factor and soot area under the condition of 800 K and 10% O2. ... 174

Figure 7. 11 Soot developments for the three injection types in terms of average soot temperature, average KL factor, integrated KL factor and soot area under the condition of 1400 K and 10% O2. ... 175

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1.

Introduction

Motivation

1.1.

As the global economic increases dramatically in recent decades, environmental

pollution becomes the one of the most serious global issues to be addressed either in the

industry production process or in the academic research. Pollution emissions to the

atmosphere have been paid more attention due to their greater importance for environmental

pollution and health (they are more readily discernible and more widely through the

environment). Combustion process is considered as the major source leading to the

atmosphere air pollution, and spray combustion has been involved in plenty of applications

including mobile and fuel combustion, such as on-road vehicles, off-road vehicles, and

industrial boilers. Figure 1.1 shows the main sources of emissions of selected pollutants in

the European Union [1]. It clearly shows that combustion represents one of the most

important emission processes for many pollutants, not only from industrial sources, but also

from low-level sources such as motorized vehicles and domestic chimneys, as well as indoor

sources such as heating and cooking in the home or workplace. Additionally, more

comprehensive data of pollutions from EPA of USA has been shown by different source

sectors in Figure 1.2 [2]. Mobile and fuel combustion are the two major sources from human

activities for these pollutions, particularly for PM (particle matter), Nitrogen Oxides, Carbon

Monoxide. In order to reduce pollutions and make a healthier environment, it is necessary to

achieve a clean combustion process.

On the other hand, oil as one of the major energy carriers has been widely used in our

industry and daily life and it will continue to play an important role in the near future. Figure

1.3 shows the world energy consumption for different fuels [3]. Oil consumption takes more

than one quarter of the total energy consumption. What’s more, transportation and industrial

are the two major parts in the oil consumption, as shown in Figure 1.4 [4]. Undoubtedly,

(22)

transportation and industries. It is illustrated that well controlled spray combustion with less

pollution will mitigate the worsening environment greatly.

Another factor should be always put in mind. The emission regulation is becoming

more and more stringent [5], which forces the in-cylinder combustion process to be cleaner.

The evolution of EPA emission standards is shown in Figure 1.5. The Tier 4 emission

standards are currently employed which was started from 2008, featuring substantial

reductions of NOx (for engines above 56 kW) and PM (above 19 kW) as well as more

stringent HC limits. In order to meet those requirements, a clean and highly efficient spray

combustion need to be developed.

Last but not the least, as we all know, fossil fuel reserves can run out for a certain

period. Three major fossil fuels are plotted in Figure 1.6 [6]. Oil deposits will be gone by

2052, which is calculated based on our consumption rate of 4 billion tons a year. It also

pointed out that coal deposits we know about will give us enough energy to take us as far as

2088. This urgent energy situation drives two solutions. First, fuel consumption needs to be

reduced as much as possible in order to extend the timeline, which can be achieved by

increasing combustion efficiency. On the other hand, renewables offer us another way to

avoid this energy time bomb, which should be also paid more attention.

All in all, spray combustion process applied in transportation and industries needs to

be well understood in order to achieve clean spray combustion with a high efficiency.

Meanwhile, renewable fuels should be studied by comparing with fossil fuels in order to

implement them into the current combustion devices without major changes. Combining the

two aspects, the environment can be improved and human disease led by pollution can then

be avoided.

Research objective and approaches

1.2.

This study is mainly focused on the spray combustion process for different fuels

based on the multi-band emission measurement and two-color pyrometry. The major content

(23)

Chapter 2 reviews the basic definitions of physical parameters involved in the spray

combustion process, including spray, combustion, and emissions parameters. The research

activities associated with these parameters are also provided in details. Soot reduction

mechanisms are emphasized in three aspects. Soot measurement techniques are discussed in

the end. Particularly, the two-color pyrometry is described in details in order to implement

the soot concentration and soot temperature measurement.

Chapter 3 describes the experimental configurations of combustion chamber,

premixing gas supply system, fuel delivery system, ignition system, and data acquisition and

control system. Experimental sequence is shown in the end to illustrate how a complete

combustion event occurs.

Chapter 4 focuses on the chemiluminescence and soot luminosity measurement based

on the multi-band emission measurement technique. OH*, Band A, Band B and Natural

Luminosity were compared for three fuels (ultra-low-sulfur diesel, Jet-A, and biomass to

liquid (BTL) fuel) under varied ambient conditions. The effect of available oxygen ratio on

the flame structure was analyzed under different ambient conditions. Quasi-steady state

flame structures were proposed based on the four flame emissions distribution boundaries.

Chapter 5 presents the two-color results for the three fuels under different ambient

conditions. Soot temperature and soot concentration (KL factor) were compared in terms of

transient image analysis and time resolved results among three fuels under those ambient

conditions. The characteristics of the two-color results were further discussed and analyzed.

Chapter 6 studies the effect of different loads, namely fuel injection quantity, on

spray combustion based on the two techniques (multi-band emission measurement and

two-color pyrometry). Three typical ambient conditions were measured. Low load and high load

were emphasized to show the difference of three fuels.

Chapter 7 demonstrates the effect of the pilot injection on the main injection during

spray combustion process under three typical ambient conditions. Two single injections

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two-injection or the main two-injection. The results from multi-band measurement and two-color

pyrometry were compared and analyzed for different parameters.

Chapter 8 presents the important conclusions and summarizes the major contributions

of this work based on the current study. Meanwhile, future work is suggested in the end.

(25)

Figures

1.3.

Figure 1. 1 Atmospheric emissions by source in the European Union (EU15).

Figure 1. 2 The percent contribution from source sectors across the common pollutants in US.

0% 20% 40% 60% 80% 100%

P

er

ce

n

t con

trib

ut

ion

fr

om

sour

ce

sect

or

s BiogenicsSolvent

Mobile Miscellaneous Fuel_Combustion Industrial_Processes Dust

(26)

Figure 1. 3 World commercial energy consumption.

1. .8

Figure 1. 4 World Oil Consumption by Sector from 2003 to 2030.

0% 20% 40% 60% 80% 100%

2003 2010 2015 2020 2025 2030

Electricity

Transportation

Industrial

Commercial

(27)

Figure 1. 5 The evolution of EPA emission standards.

(28)

2.

Literature Review

Spray, combustion and emission of liquid fuel

2.1.

2.1.1.

Liquid sprays

2.1.1.1.

Break-up length

The injected liquid does not break up instantly after injection. The unbroken portion

is defined as the liquid break-up length. It characterizes a point of discontinuity, where the

spray changes from a densely packed zone of liquid (bulk liquid, or interconnected ligaments

and droplets), to a finely atomized regime of droplets. The following expression of break-up

length was derived from experimental data in ref [7].

( ) ( )

( ) ( ) (2-1)

where D is the diameter of the nozzle hole, L is the length of the nozzle hole, r is the round

radius at the entrance of the nozzle, is the injection velocity, is the surrounding pressure,

is the cavitation number used to represent the factor of internal turbulence in the nozzle.

is the ratio of surrounding density to liquid density. This expression is valid for any

complete spray region.

The break-up length of diesel sprays was measured under a high pressure

environment by Yule and Pilipovic [8]. The results showed the breakup length of diesel

sprays were consistently about 35% of the spray penetration length. In addition, the breakup

length slightly increased for a larger nozzle diameter but it decreased with increasing

injection pressure. In a high speed small direct injection diesel engine of a light duty vehicle,

break-up length of the fuel spray and the distance from the nozzle to the combustion chamber

wall are almost the same. Therefore, the break-up length may have a great effect on the

spatial distribution of the liquid fuel and the formation of the fuel-air mixture in that small

(29)

2.1.1.2.

Spray tip penetration

The spray tip penetration is defined as the maximum distance from the nozzle to the

tip of the spray at any given time and is one of the most important parameters which affect

the spray combustion process. Hiroyasu and Arai [7] proposed the formulas of spray tip

penetration based on their experimental results, which were divided into two parts:

( ) ( ) (2-2a)

( ) ( ) ( ) (2-2b)

where

( ) .

During the first period, the spray behaved as a steady jet with a tip velocity equal to

the jet exit velocity. After the break-up, the jet disintegrated to form a spray, with a tip

penetration proportional to .

2.1.1.3.

Spray cone angle

The spray angle does not remain constant as spray penetration increases, which can

be measured by optical or mechanical methods, such as shadowgraphy, extinction

tomography, and Mie Imaging. The spray angle tends to collapse or diverge with increasing

distance from the orifice. An experimental correlation for the spray angle was developed by

Hiroyasu et al. [7]. They found that spray angles were widened by a reduction in liquid

viscosity and an increase in injection pressure.

( ) ( ) ( ) (2-3)

2.1.1.4.

Drop size distribution

A spray is occasionally defined as a system of droplets plunging through a gaseous

continuous phase. Two different mean diameters of droplets are commonly used in Particle

Dynamics Analysis studies: the arithmetic mean diameter ( ) and the Sauter mean diameter.

The Sauter mean diameter of a spray represents the diameter of the droplet that has the same

(30)

and get a better spray atomization. The expressions of the two diameters were proposed by

Mugele and Evans [9].

, and ∑

In order to simplify the description of droplet size distribution, mathematical

expressions were employed in two ways: normal distribution function is based on the random

occurrence of a given drop size. It is usually expressed in terms of a number distribution

function ( ) that gives the number of particles of a given diameter d:

( )

√ * ( ) + (2-4)

where is a measure of the deviation of values of from a mean value , which is referred

to as standard deviation.

The other one names Log-Normal distribution, where the logarithm of the particle

diameter is used as the variable. It is shown as follows:

( )

√ * ( ) + (2-5)

where is the number geometric mean drop size.

2.1.1.5.

Fuel air entrainment

When a fuel jet penetrates into ambient air, fuel vapor mixes with the air. During the

quasi-steady stage, the mixture stoichiometry or the air/fuel ratio (A/F) can be quantified

approximately as a non-reacting isothermal jet at any position along its axis as [10],

( ) √ ( ) (2-6)

where √ √

( ) and ( ) [( )

] is the ratio of the

ambient gas to fuel density, d is the diameter of injector nozzle in mm, is a characteristic

length scale for the jet, is the jet spreading angle, is the axial distance normalized by the

(31)

value of 0.86, and the spreading angle constant is assumed to have a value of 0.265. Both

and are based on the interpolation from refs [10, 11].

2.1.2.

Spray Combustion

2.1.2.1.

Ignition delay

The ignition delay in a diesel engine is defined as the time interval between the start

of injection and the start of combustion. This delay period consists of physical delay, wherein

atomization, vaporization and mixing of air fuel occur and of chemical delay attributed to

pre-combustion reactions. Physical and chemical delays occur simultaneously. A common

expression for ignition delay in diesel combustion is the Arrhenius expression [12]:

( ) (2-7)

where A is the pre-exponential constant, E is the global activation energy, R is the universal

gas constant, represents the ambient density, and is the stoichiometric mixture fraction.

Additionally, ignition delay was divided into first ignition delay time and overall

delay time by Kumar and Sung [13, 14]. Ignition delay correlations for Jet-A/oxidizer

mixtures were proposed in the form that embodies the effects of various physic-chemical

parameters, the general form of the correlation is given as follows:

[ ] [ ] ( ) (2-8)

where is the compressed charge pressure, is the compressed gas temperature, is the

fuel mole fraction, and is the oxygen mole fraction, and is the activation temperature,

A is a constant.

2.1.2.2.

Lift-off length

Lift-off length is defined as the farthest upstream location of high temperature

combustion, and it is a characteristic parameter used to describe the location where

the high temperature reactions begin, which can be used to determine the amount of air

entrainment and fuel air premixing that occurs upstream of any combustion in a DI diesel

(32)

upstream location of combustion. The interpolation of the measured lift-off length H for a

selected temperature T and density is given as below [15]:

( ) ( ) (2-9)

where is the measured lift-off length for an ambient gas temperature and density ,

( )

, ( ) , the terms of , and are the temperature,

pressure and density of the ambient gas at the time of the start of fuel injection, is 1.31.

Meanwhile, a more general development of the lift-off length scaling, following the approach

of Peters [16], but accounting for the possibility of changes in the growth rate of a jet with

changes in the ambient gas density, was proposed by Siebers et al. [17]:

( ) ( ) (2-10)

where is the fuel injection velocity, is the stoichiometric fuel mixture fraction, is

the thermal diffusivity, ( ) is the laminar flame speed of a stoichiometric fuel air mixture,

is the spray spreading half-angle which can be found in equation (2-6).

2.1.3.

Emissions

2.1.3.1.

Chemiluminescence

CH and CH2O were considered as the major contributors to the chemiluminescence

signal at the low temperature reactions prior to the onset of soot formation [18]. The

measurement of chemiluminescence spectrum of diesel autoignition was conducted by Dec

and Espey [19]. The results showed that a broad peak centered at about 430 nm

corresponding to the strongest emission band of CH, as shown in Figure 2. 1. Formaldehyde

has several bands ranging from 340 nm to 520 nm. The peak locations of formaldehyde are

indicated by the narrow vertical-axis lines in Figure 2. 2. Even though OH and C2 have the

strongest signal at 310 nm and 516 nm respectively, all the emissions can be minimized as

the measurement is conducted at an early stage during the spray combustion. Consequently,

for the cool combustion chemistry conditions, CH and formaldehyde and CH emission tend

(33)

OH emission is considered as the marker of high temperature reaction. A large

amount of OH emission is observed normally in the downstream of spray flame, where has a

higher equivalence ratio around 2 to 4 [20]. The strongest OH emission band is located at

310 nm.

2.1.3.2.

Soot luminosity

Sooting-combustion spectrum was acquired by Dec and Espey [19], as shown in

Figure 2. 3. The sooting combustion spectrum is totally different from the

chemiluminescence spectrum. A strong gray-body emission dominates the spectrum at

wavelengths longer than about 340 nm. The weak signals from CH and CH2 would be easily

covered by the strong gray-body soot emission.

2.1.3.2.1.

Soot formation and oxidation

Soot is impure carbon particles resulting from the incomplete combustion of

hydrocarbons, particularly for the incomplete diesel spray combustion. Newly formed

particles have the highest hydrogen content with a C/H ratio as low as one, but as soot

matures the hydrogen fraction decreases. The density of soot is reported to be 1.8470.1

g/cm3 by Choi et al. [21]. The typical soot formation process can be considered in Figure 2.1

[22]. The processes shown may occur in a spatially and temporally separated sequence in a

laminar diffusion flame or all of the processes may occur simultaneously as in a well-stirred

reactor. In practical combustion systems the sequence of processes may vary between these

two extremes.

Pyrolysis is the process of organic compounds, such as fuels, altering their molecular

structure in the presence of high temperature without significant oxidation even though

oxygen species may be present. Pyrolysis reactions are highly dependent on temperature [23]

and fuel concentration, resulting in the production of some species which are precursors or

building blocks for soot.

Nucleation or soot particle inception is the formation of particles from gas-phase

reactants. The smallest identifiable solid particles in luminous flames have diameters in the

(34)

temperatures vary from 1300 to 1600 K [24-27]. These particle nuclei do not contribute

significantly to the total soot mass, but do have a significant influence on the mass added

later, because they provide sites for surface growth. Three soot nucleation routes were

provided by Bryce et al. [28]. (1) Cyclization of chain molecules into ring structures. An

example of this is acetylene molecules combining to form a benzene ring. (2) A direct path

where aromatic rings dehydrogenate at low temperature and form polycyclics, and (3)

breakup and recyclization of rings at higher temperatures.

Surface growth is the process of adding mass to the surface of a nucleated soot

particle. There is no clear distinction between the end of nucleation and the beginning of

surface growth and in reality the two processes are concurrent. The majority of the soot mass

is added during surface growth and thus, the residence time of the surface growth process has

a large influence on the total soot mass or soot volume fraction.

Coalescence occurs when particles collide, decreasing the number of particles and

holding the combined mass of the two soot particles constant. Agglomeration occurs when

individual or primary particles stick together to form large groups of primary particles.

Exhaust soot from diesel engines consist of spherical primary particles in long chain-like

structures. The size of primary soot particle varies depending on operating condition, injector

type, and injector conditions, which was reported in the range of 20 to 70 nm [29-32].

Soot oxidation is the conversion of carbon or hydrocarbons to combustion products,

which can occur at any time during the soot formation process. Once carbon has been

partially oxidized to CO, the carbon will no longer evolve into a soot particle even if entering

a fuel-rich zone. It suggests that soot oxidation occurs when the temperature is higher than

1300 K [33]. It also pointed out that soot oxidation has two stages. First, chemical attachment

of oxygen to the surface (absorption), and second, desorption of the oxygen with the attached

fuel component from the surface as a product. OH radical was considered to be an important

role in soot oxidation under fuel-rich and stoichiometric conditions, while under lean

conditions, soot is oxidized by both OH and O2. It was reported 10-20% of all OH collisions

(35)

2.1.3.2.2.

Effects of physical parameters on soot

Temperature has the greatest effect on soot formation and oxidation by increasing all

of the reactions rates involved in spray combustion process. As the discussion above, soot

inception occurs from 1300 to 1600 K while burnout ceases below 1300 K [34].

Changing pressure will lead to the changes to the temperature, flow velocity, flame

structure, and thermal diffusivity. Thus the effects of pressure on soot could be hard to isolate.

In the premixed flames, a dramatic soot formation can be found as pressure increases [35]. A

P2 dependence of the final soot volume fraction was observed with the pressure varied from

1 to 5 bar [36]. In the diffusion flames, a power-law relationship between soot concentration

and pressure is confirmed by several investigations [37-40] which yield a pressure exponent

of 1.1-1.26. Ranjith et al. [41] pointed out that the dependence of peak hydrocarbon species

concentrations on pressure increases with the molecular weight of the species.

The effect of oxygen on soot formation is complex. High oxygen content in either

the fuel or through premixing will help to oxidize soot during the spray combustion;

on the other hand, higher oxygen leads to a higher flame temperature which is in favor of the

increase for both soot formation and oxidation. Hara and Glassman [42] showed the effect of

oxygen on soot formation in counter flow diffusion flame in Figure 2. 4. As additional

oxygen is added, both fuels show increased soot formation reaching a peak before dropping

off to zero. More recently G ̈lder [43] measured soot in axisymmetric laminar diffusion

flames. It was concluded that when oxygen is added to the fuel side of the flame it can either

enhance soot formation through the production of H atoms and hydrocarbon radicals or

reduce soot formation by attack on aromatic radicals and aliphatic hydrocarbons.

The prevailing consensus in the literature shows that fuel composition plays an

important role in soot formation in all types of flames [35, 44, 45] while fuel structure is

important in diffusion flames but less important or not important at all in premixed flames

[34, 46, 47]. It is more likely to produce soot with a higher carbon molecular weight [35, 44].

The importance of fuel structure to soot formation in diffusion flames was demonstrated in

(36)

and fused cyclic molecules are the most prolific sooters. For non-aromatic fuels, the main

chain length or ring circumference (number of carbon atoms) and the number, position, and

length of side chains have secondary structural effects that tend to increase sooting tendency.

However, the fuel structure does not play a role in premixed flames by showing that two

different structures like benzene and decane have the same critical sooting equivalence ratio

at a given flame temperature [34].

In-cylinder soot reduction in spray combustion

2.2.

Exhaust soot is a heavily regulated emission for diesel engines [49], and while

effective aftertreatment systems have been developed for its mitigation, in-cylinder soot

reduction techniques remain attractive alternatives to reduce or eliminate the aftertreatment

burden. The soot formation is dramatically increased under the high temperature and

fuel mixture conditions [47], as shown in Figure 2. 5. High level soot is located in the

rich-fuel zone with a relative high flame temperature around 2100K, while high level NOx can be

observed near the equivalence ratio area with very high flame temperature from 2400 K all

the way to 3000 K. A desirable path can be easily identified between the two high level

emissions, providing a possible solution to achieve the simultaneous reduction of soot and

NOx.

2.2.1.

Flame temperature effect on soot reduction

Based on the analysis of Figure 2. 5, high level soot can be avoided by moving the

combustion event into either the right side or left side. The left hand region represents the

low temperature combustion mode, and the right hand leads to a very high combustion mode.

As we all known, the low temperature combustion (LTC) concept is widely used to

suppress soot formation and NOx simultaneously in spray combustion process, as mentioned

in the Introduction. It usually covers three specific combustion modes, HCCI, PCCI, and

RCCI. Since HCCI and RCCI have a very low equivalence ratio, it will be discussed in the

following section. In order to achieve the LTC mode, two different injection strategies are

employed. Very early direct-injection LTC has the potential of better mixing quality and a

(37)

densities of charges, leading to the low temperature combustion. However, optimized

injection needs to be developed to avoid the wall wetting. Various injectors and techniques

were used to improve the mixing quality for the early injection scheme [50-52]. High level of

cooled EGR has been normally used in the early injection method to slow the autoignition

process [53, 54]. Reduced compression ratio and late intake valve closing were also

considered to prolong the mixing time before premixed combustion occurs [54, 55].

On the other hand, near-TDC direct-injection LTC was proposed firstly as the name

of MK (modulated kinetics) combustion [56]. It was demonstrated both for injection timings

earlier and later than the one in conventional diesel combustion [56, 57]. The trend of soot

emission is more complex as this injection has been implemented coupled with the EGR

addition. Akihama pointed out that soot emission increases first and then decreases as EGR

level increases [47]. The initial increase of EGR leads to a lower O2 concentration and a

lower combustion temperature, resulting in a lower soot oxidation rate, while the soot

formation rate is reduced significantly as the EGR level continues to increase due to a very

low combustion temperature. But it should be taken more care to prevent a fuel consumption

increase. Higher HC emissions were reported for near-TDC injection compared to

conventional diesel combustion [58, 59], however, HC emission of near-TDC injection

method is lower than that of the early injection method.

At the right hand side of the high soot region in Error! Reference source not found., igh temperature combustion with highly diluted mixture shows a possibility to reduce soot

and NOx effectively. An early study on high combustion temperature for soot reduction was

conducted by Kamimoto et al. [60], indicating a simultaneous soot and NOx reduction. It

was demonstrated by Zhang et al. [61] that lower soot levels can be found for high

temperature and highly diluted ambient conditions (10% and 1400 K). The fuel-rich region is

realized by using high EGR rate, where the definition of equivalence ratio should include the

effect of EGR on the total mixture. From this view, high level of HC and CO emissions and

low combustion efficiency, which are the major concerns in the applications of LTC and

(38)

2.2.2.

Equivalence ratio effect on soot reduction

Besides the combustion temperature, equivalence ratio is another governing factor in

soot formation, as seen in Figure 2. 5Error! Reference source not found.. The typical ombustion modes, namely HCCI, have been widely studied to realize the soot reduction in

spray combustion process [62-65]. Since these combustion modes have a very lean mixture,

the typical combustion temperature is maintained in a relative low level. The very dilute

mixture is generated either by being lean with equivalence ratio less than 0.45, or through the

use of high levels of EGR for equivalence ratio up to stoichiometric [64, 65].

In principle, HCCI is an ideal combustion process for internal-combustion engines,

since it can deliver high thermal efficiencies, comparable with those of conventional diesel

engines, and extremely low NOx and PM emissions. However, there are two limits needs to

be considered as HCCI is used. For HCCI engines, decreased combustion efficiency and

increased CO emission can be observed under very low load condition, such as equivalence

ratio below 0.2 [66]. That’s because the mixtures are so dilute that combustion temperatures

are too low (typically below 1500 K) for the bulk gas reactions to go to completion before

they are quenched by the expansion stroke [67]. One effective strategy is charge stratification,

featuring a locally richer mixture with a globally low equivalence ratio, and significantly

increasing combustion efficiency [66, 68]. Another limitation is the high load condition

where an excessive pressure rise rate (PRR) occurs if equivalence ratio increases to 0.3 [69].

Since the high PRR is resulted from the intense heat release of larger amount of mixture with

a higher equivalence ratio, combustion occurs sequentially with several zones would be help

to suppress the high PRR. The thermal stratification is verified as an effective way to realize

this sequential autoignition [69], however, significant enhancement of this thermal

stratification is hard to be implemented without less influence on temperature distribution in

the bulk gases. Another way to decrease PRR is retard the combustion-phasing [70, 71]. It

pointed out that combustion-phasing retard increases the thermal stratification, and in turn

reduces the PRR. Although combustion-phasing retard is very effective for slowing the PRR

(39)

stability and eventually misfire [72]. Therefore, the combustion-phasing must be maintained

between the knocking and stability/misfire limits, which makes more difficult on the control

of the combustion-phasing at high loads [71].

Recently, RCCI is a dual fuel engine combustion technology that was developed at

the University of Wisconsin-Madison Engine Research Center laboratories. RCCI is a variant

of HCCI, which can be used to improve the performance of HCCI by enlarging the load

range and realizing a flexible combustion phase control, featuring an ultra-low soot and NOx

emissions with higher combustion efficiency [73]. The reduction of soot and NOx are

resulted from the avoidance of high equivalence ratio and high temperature regions in the

combustion chamber. Lower UHC and CO emissions can be maintained by advancing

injection timing with dual injection strategies [74]. However, a higher PRR, ring intensity (RI)

and combustion losses were observed in RCCI [75].

2.2.3.

Fuel type effect on soot reduction

Beyond the scope of Figure 2. 5, fuel type is another factor being used to reduce soot

emission in the spray combustion. Different fuels have different physical and chemical

characteristics which can significantly affect the spray, combustion and emission

performance. Many investigations have been focused on the effect of fuel type on the

reduction of soot, which can be divided into three categories: the effect of single alternative

fuel, the effect of blends or additives. Generally, a significant soot reduction can be achieved

by using different fuels coupled with advanced combustion concepts (LTC or HCCI).

The effect of oxygen atom within fuels on soot formation is demonstrated in the

previous section. Therefore, BTL as an attractive alternative fuel with additional oxygen

atoms has received a great attention for applications in diesel engines

[76-78][76-78][76-78][76-78], which can be considered as one typical application of using the effect of single

fuel. Ethanol (one typical alcohol fuel) is one of BTLs for diesel replacement. Generally, it is

applied as a supplementary fuel with diesel or gasoline. It has a higher octane number which

can allow the compression ratio to be higher, featuring high combustion efficiency with

Figure

Figure 1. 2 The percent contribution from source sectors across the common pollutants in US
Figure 1. 3 World commercial energy consumption.
Figure 2. 1 Chemiluminescence spectrum during the early stages of diesel autoignition under the ambient condition: 900 K, 16.6 kg/m3
Figure 2. 4 Effect of stoichiometry of the fuel side on soot formation in ethylene and propane opposed jet counterflow diffusion flames
+7

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