Adhesion Characteristics of Fuel Spray Impinging on Flat
Wall for Gasoline Engine(ᖹᖹᯈቨ㠃࡟⾪✺ࡍࡿ࢞ࢯࣜࣥᄇ㟝ࡢᾮ⭷ᙧᡂ࡟㛵ࡍࡿ◊✲)By
㥪ὥு(Hongliang LUO)
Dissertation
Submitted to the Graduate School of Engineeringof Hiroshima University
in Partial Fulfillment of the Requirements
for the Degree ofDoctor of Engineering

Hiroshima University
September, 2018

ABSTRACT
Spray-wall impingement is a widespread phenomenon applied in many fields, inlcuding spraywall cooling precess, spray coating process and fuel spray atomization in internal combustion
engines. Although the spray impingement has been investigated for decades, the mechanisms of
the interaction between spray and wall are still far from being fully understood. Recently, withthe development of the visual technology, a range of interesting behaviors can be observed.
However, researches on impinging spray are not enough, and the mechanisms of fuel adheison
formation are not clear, especailly for gasoline engine. In gasoline direct injection (GDI), it isdifficult to avoid the fuel adhesion on the piston and sylinder surfaces. The wet wall caused by
impingement affects the air-fuel mixture formation process, which is a possible source of
unburned hydrocarbons (UHC) and particulate matters (PM), making it difficult for direct
injection spark ignition (DISI) engines to meeting the future requirements of particle number(PN) regulations.
The novelty of this study lies in the correlation between the impinging spray and fuel adheion
formation together to better understand the spray-wall impingement in gasoline engine. The
high-speed video observation method (Mie scattering technique) was applied to visualize thespray evolution and impingement processes inside a high-pressure and high-temperture constantchamber. Meanwhile, the adhered fuel on the wall was measured by refrective index matching(RIM) under non-evaporation and evaporation conditions considering the effects of different
injection pressures, ambient pressrues, impingement distances, nozzle hole diameters andambient temperatures in experiments.
Furthermore, the microscopic characteristics of impinging spray were investigated using particle
image analysis (PIA). The droplet size and number were compared before and after impingement.
Moreover, the relationship between droplets behaviors and fuel adhesion on the wall was
discussed under the same conditions. Additionally, the spray-wall impingement model anddroplet breakup/ coalescence model were discussed deeply as well.

iii

TABLE OF CONTENTS
ABSTRACT ..................................................................................................................................... i
TABLE OF CONTENTS ............................................................................................................... iii
NOMENCLATURES ................................................................................................................... vii
CHAPTER 1 INTRODUCTION .................................................................................................... 1
1.1 BACKGROUND AND MOTIVATION ............................................................................. 1
1.1.1 Automobiles Development and Envirmental Issues....................................................... 1
1.1.2 Direct-Injection Spark Ignition (DISI) Engines .............................................................. 5
1.1.3 Impinging Spray in DISI Engines ..................................................................................... 8
1.2 OBJECTIVES AND APPROACHES ............................................................................... 10
1.3 OUTLINES ........................................................................................................................ 10
1.4 STUDIES REVIEW........................................................................................................... 11
1.4.1 Spray-Wall Interaction ................................................................................................ 11
1.4.2 Microscopic Characteristics of Spray and Droplet Behaviors...................................... 22
1.4.3 Evoluation of Fule Adheison on the Wall .................................................................... 33
1.4.4 Optical Diagnostics in DISI Engines Conditions ......................................................... 38
1.5 SUMMARY ....................................................................................................................... 45
CHAPTER 2 EXPERIMENTAL APPARATUS AND MEASUREMENT METHODS ............ 47
2.1 CONSTANT HIGH-PRESSURE CHAMBER ................................................................ 47
2.2 ELECTRICAL CONTROLLING SYSTEM ..................................................................... 48
2.3 IMPINGEMENT SYSTEM ............................................................................................... 49
2.4 OPTICAL MEASUREMENT SYSTEMS ........................................................................ 50
2.4.1 Mie Scattering Method ................................................................................................. 50
2.4.2 Refrective Index Matching Method .............................................................................. 52iii

2.4.3 Particle Image Analysis ................................................................................................ 57
2.5 SUMMARY ....................................................................................................................... 61
CHAPTER 3 FUEL ADHESION FORMATION UNDER NON-EVAPORATIONCONDITIONS .............................................................................................................................. 62
3.1 EFFECT OF INJECTION PRESSURE AND AMBIENT PRESSURE ........................... 62
3.1.1 Experimental Conditions .............................................................................................. 62
3.1.2 Spray Impingement Characteristics .............................................................................. 63
3.1.3 Fuel Adhesion Characteristics ...................................................................................... 64
3.2 EFFECT OF SURFACE ROUGHNESS ............................................................................ 69
3.2.1 Experimental Conditions .............................................................................................. 69
3.2.2 Spray Impingement Characteristics .............................................................................. 70
3.2.3 Fuel Adhesion Characteristics ...................................................................................... 75
3.3 EFFECT OF IMPINGEMENT DISTANCE....................................................................... 85
3.3.1 Experimental Conditions .............................................................................................. 85
3.3.2 Spray Impingement Characteristics .............................................................................. 86
3.3.3 Fuel Adhesion Characteristics ...................................................................................... 91
3.4 SUMMARY ........................................................................................................................ 97
CHAPTER 4 FUEL ADHESION FORMATION UNDER EVAPORATION CONDITIONS .. 98
4.1 EFFECT OF AMBIENT TEMPERATURE ....................................................................... 98
4.1.1 Experimental Conditions .............................................................................................. 98
4.1.2 Fuel Adhesion Characteristics .................................................................................... 100
4.1.3 Fuel Adhesion Characteristics .................................................................................... 103
4.2 EFFECT OF NOZZLE HOLE DIAMETER .................................................................... 114
4.2.1 Experimental Conditions ............................................................................................ 114

iv

4.2.2 Fuel Adhesion Characteristics .................................................................................... 116
4.3 SUMMARY ...................................................................................................................... 126
CHAPTER 5 MICROSCOPIC BEHAVIORS OF IMPINGING SPRAY ................................. 128
5.1 EXPERIMENTAL CONDITIONS .................................................................................. 128
5.2 MICROSCOPIC MORPHOLOY OF IMPINGEMENT ON THE WALL ...................... 129
5.2.1 Before Impingement ................................................................................................... 129
5.2.2 After Impingement .................................................................................................... 131
5.3 DROPLET SIZE DISTRIBUTION .................................................................................. 132
5.3.1 Before Impingement ................................................................................................... 132
5.3.2 After Impingement ..................................................................................................... 134
5.4 DROPLET NUMBER DISTRIBUTION ......................................................................... 136
5.4.1 Before Impingement ................................................................................................... 136
5.4.2 After Impingement ..................................................................................................... 139
5.5 COMPARISON OF FUEL ADHEISON FORMATION WITH SINGLE DROPLEANAYSIS ................................................................................................................................... 142
5.6 SUMMARY ...................................................................................................................... 145
CHAPTER 6 CONCLUSIONS .................................................................................................. 147
6.1 FINDINGS OF THE CURRENT STUDY ....................................................................... 147
6.1.1 Non-evaporating Spray Evolution and Fuel Adheison Formation ............................. 147
6.1.2 Evaporating Spray Evolution and Fuel Adhesion Formation..................................... 149
6.1.3 Microscopic Spray Behaviors and Fuel Adhesion Formation .................................... 150
6.2 RECOMMENDATIONS FOR FUTURE WORKS ......................................................... 151
ACKNOWLEDGEMENT .......................................................................................................... 153
LIST OF PUBLICATIONS ........................................................................................................ 154v

REFERENCES ........................................................................................................................... 156

vi

NOMENCLATURES
ASOI

After start of injection

AEOI

After end of injection

BTDC

Before top dead center

Cdroplet

Perimeter of droplet

CCD

Charge-coupled device

CGR

Charged gasoline injection

CFD

Computational fluid dynamics

D

Nozzle hole diameter

Darea

Area diameter

D32

Sauter mean diameterDI

Direct injection

DISI

Direct injection spark ignition

din

Incident droplet diameterdout

Out droplet diameterdflFilm thickness

Dimp

Impingement distance (mm)ECUElectronic control unit

EOI

End of injection

EGR

Exhaust gas recirculationfM (h)

Probability of fuel mass

fps

Frames per second

HC

Hydrocarbon

Hi

Impinging spray height (mm)Hv

Impinging vortex height (mm)h (x, y)Fuel adhesion thickness (μm)

Iwet (x, y)Light intensity of the fuel deposit

Iref (x, y)Light intensity of the dry image

IC

Internal combustion

ICCD

Intensified charge-coupled device

GDI

Gasoline direct injection
vii

Lx

Distance to wall (mm)Lr

Radial distance (mm)L/D

Length to diameter

LCVLight commercial vehicles

LDA

Laser doppler anemometer

LIF

Laser induced fluorescence

Minj

Injection mass (mg)M(h)

Fuel adhesion mass in the thickness (mg)NOx

Oxides of nitrogenOh

Ohnesorge number

PIA

Particle image analysis

Pinj

Injection pressure (MPa)PambAmbient pressure (MPa)PFIPort fuel injection

PN

Particle number

PDA

Phase doppler anemometry

PDPA

Phase doppler particle analysis

PIA

Particle image analysis

PM

Particulate matters

Ra

Arithmetical mean deviation of the profileRe

Reynolds number

Rs

Impinging spray radius (mm)RIM

Refractive index matchingS

Spray tip penetration

SdropletArea of droplet

SI

Spark ignition

SMD

Sauter mean diameter

SOI

Start of injection

Tsat

Saturated temperature of fuel droplet

Tl

Temperature of impinging droplet (K)Tw

Wall surface temperature (K)

TPA

Pure adhesion temperature (K)
viii

TPR

Pure rebound temperature (K)TN

Nakayama temperature (K)TLeid

Leidenfrost temperature (K)TambAmbient temperature (K)td

Injection duration (ms)

TDC

Top dead center

UHC

Unburned hydrocarbon

We

Weber numberθimp

Impingement angle (deg)οܲDifference of injection pressure and the ambient pressure (MPa)ο‫( ܫ‬x, y)

Reduction of the scattered light

3-D

Three dimensionalix

x

CHAPTER 1 INTRODUCTION
1.1 BACKGROUND AND MOTIVATION
1.1.1 Automobiles Development and Environment Issues
The Austrian inventor, firstly put a liquid-fuel internal combustion engine on a simple handcart
which made him the first man to propel a vehicle by means of gasoline. Today, this car is known
as "the first Marcus car". As one of the most important inventions in 19th century, almost alltechnological innovations of internal combustion engines are promoting human progress in themodern history of mankind. Due to its unique high thermal efficiency, wide output and highspeed, high reliability and flexibility, internal combustion engines are the most widely used
power machinery in industrial, agricultural production and transportation [Haworth, 2005]. The
commercial production in automobile began in France in 1890, and it was almost equal to that ofUnited States. One historian said that the cars free common people from the limitations of theirgeography, and they create mobility in human lifestyles. With the development in the economyof the world, the passenger cars are becoming more and more popular in the world.

Figure 1.1 Worldwide car sales in 2017 [JATO Dynamics, 2017]
In 2017, worldwide sales of passenger cars and light commercial vehicles (LCV) increased
2.4%, based on the research from JATO Dynamics. Slightly more than 2 million additional
1

vehicle sales compared to 2016 make for a new total of 86.05 million global car sales. Passenger
car sales were up 2.3% to 82 million and LCV sales were up 4.4% to 4.05 million vehicles. As
shown in Figure 1.1, China remains the largest single market with 25.8 million sales despite
reporting the slowest growth rate in more than a decade. Only North American (although Canada+5%) and S. Korea car sales decreased in the world. Growth in China slowed to just 2.3% from
17% in contrast to 2016. India and Brazil are other engines of growth, as the great increase in theeconomy and population in these countries.

Figure 1.2 Global car sales by fuel type in 2017 [JATO Dynamics, 2017]
Figure 1.2 shows the fuel type of global car in 2017. Among these, gasoline power vehicles
increased to 623.9 million, that occupies the biggest of the market share. According to JATO's
calculations, although the alternative fuel vehicles increase rapidly, the share is quite smaller(about 5%) compared to gasoline and diesel cars. As a result, it is still very important to develop
and investigate on the internal combustion (IC) engines (both gasoline and diesel engines) in theworld.

Figure 1.3 Demand rate of diesel fuel / gasoline forecast [Hitomi M, 2017]2

However, due to the rapid development of IC engines, as the planet on which humankind
depends, its resources such as oil, gas and coal decrease irreparably as they are non-renewableenergy source in the earth. Energy consumption leads to a series of problems, and fossil fuels are
considered overuse. Because the mainly energy is still acquired from coal and oil in the future
decades, although the renewable energy and clear energy is expected to be developed further andmore. Therefore, the energy crisis threatens the development of mankind. Many researchers havespared no effort to find new ways to solve this imminent danger. Especially, this demand willcontinue to increase. Figure 1.3 describes that future demand rate of diesel fuel / gasoline isincreasing due to the transportation consumption. Even in 2050, the demand will be more than 3times compared with current rate due to the booming of the population.
Moreover, while enjoying the development and achievements brought from the IC engine
technology, humans have to face the negative issues such as environmental deterioration andpollutions. The traditional IC engines discharge harmful emissions into the environment toprovide inexhaustible power. As a result, environmental health and safety are a major issue in thedevelopment of human society. In the next half century, it will be the common problem power
facing all the humanity beings.

Figure 1.4 Energy-related CO2 emissions in selected regions [Maria H, 2015]3

Recently, as the climate change, more and more attentions are paid to environmental
protection. In fact, the consumption of the fossil fuel is one of the main indisputable sources ofthe pollutions, which causes up the climate warming, fog, and haze. Dillon et al. [2010] andLacis et al. [2013] have demonstrated that the greenhouse gas emission is the immediate reasonof the world warming. The greenhouse and harmful gases pollute the earth, threatening the ecosystem and the health of human beings. The energy related CO2 emissions, especially caused by
the fossil-fuel combustion in 2014 all around the world is published by International Energy
Agency and shown in Figure 1.4 [Maria van der Hoeven, 2015]. It can be seen that China, North
America, and European Union are the three largest carbon markets in the world. Hence, theimproved technology and method, such as optimizing combustion, enhancing the energy
efficiency, and reducing emission substantially, become very important subject for thesustainable development of the world.

Figure 1.5 Light-duty regulations for tailpipe emissions in major markets: (a) the concurrent
tightening of criteria pollutant and CO2 emission limits in the USA; (b) by 2023, the proposed
limits in China will be tighter than those in Europe [Joshi A, 2017]
What comes with pollutions is the increasingly stringent emission standards. Increasingly
stringent emission standards have been introduced to reduce gasoline automotive emissions suchas NOX, particulates, CO2, in the world. The light-duty regulation for emissions in major markets
for gasoline car is exhibited in Figure 1.5.

4

1.1.2 Direct-injection Spark Ignition (DISI) Engines
As shown in Figure 1.6, the development of injection system from carburetor to port fuelinjection (PFI) to direct injection (DI) system.
In early gasoline engines, the mixture formation was created by an aid of a carburetor,
which causes a worse engine-out emissions and fuel consumption. Thereafter, the PFI system
was introduced into gasoline engines with the development of electrical controlling. Although
PFI engines can ensure the equivalence ratio Φ=1.0 in chamber, which is required to the work of
three-way oxidation/reduction catalysts, they still have several disadvantages. The relative lowpressure fuel injector installed in the intake port may cause fuel adhesions on the port walls, evenon the intake valves during cold start (excess of fuel requires to form favorable fuel vapor mixing
in cylinder for robust ignition). Large-scale swirl and tumble flow induced by the intake
movement leads to intense turbulent flows in cylinder, thus decreasing cycle-to-cycle variability
owing to the distortion of flame kernel formation. For preventing the in-cylinder knocking, thecompression ratio is limited to 10:1. Furthermore, a decrease in fuel economy at part load occurs
in the consequence of the pumping (throttling) losses [Maly RR,1994; Drake et al., 2007].

Figure 1.6 The development of fuel injection system [Romain N, 2013]5

(a) Stratified charge(b) Homogeneous charge

Figure 1.7 Homogeneous and stratified-charge modes of DISI engine [Suresh et al., 2015]
Therefore, direct-injection spark ignition (DISI) engines as a greatly potential alternative toconventional PFI gasoline engines become more and more widely used for their significant
advantages: (1) volumetric efficiency, because of the decrease in in-cylinder temperature induced
by fuel direct injection; (2) enhanced thermal efficiency due to less pumping losses during intake
stroke; (3) increased specific output at part load owing to stratified mixture formation andcombustion in cylinder; (4) improved fuel economy throughout precise fuel injection in transient
response. [Zhao et al., 1999; Moreira et al., 2010; He et al., 2012].
Meanwhile, two basic charge modes are implemented in the DISI engines: homogeneousand stratified charge homogeneous charge mode, as shown in Figure 1.7 [Suresh et al., 2015].
The homogeneous charge is used at full loads. In that case the fuel is injected duringintake stroke so that the early injection timing allows for a homogeneous mixture forming
throughout the cylinder [Kume et al., 1996]. In homogeneous charge condition the managing ofengine load is by throttling while in stratified charge condition it is by air-fuel equivalence ratio[Spicher et al., 2000].
The stratified charge is used at partial loads. It occurs when fuel is injected late in thecycle (0-90 degrees BTDC) creating a non-homogeneous mixture of air, fuel and exhaust gas
6

recirculation (EGR). Due to the late injection timing, the injected fuel remains close to the spark
plug and does not penetrate the cylinder as far as in homogeneous combustion. This creates a
stratified mixture of fuel and air with a high air-fuel ratio at the spark plug electrode. The result
is a local ignitable mixture near the spark plug with a globally lean air-fuel mixture in the rest of
the cylinder. It allows for the injection of the minimal amount of fuel required to achieve a power
output or engine speed. Due to this reason, stratified combustion is usually applied for idle, lowengine loads and speeds, and transition from these points to higher loads and speeds [Peery,1975].
In stratified mode, three combustion processes are used to form ignitable mixture. These arespray guided, wall guided and air guided combustion processes as shown in Figure 1.8.

Figure 1.8 Classification of the GDI combustion systems [Preussner C, 2006]
Spray guided combustion process: Here the fuel is injected near the spark plug where it
evaporates. The spray guided combustion theoretically has highest efficiency. This process
requires advanced injection systems. The spray guided combustion has the following advantages.
These include less wall wetting, large stratified operation region, reduced emissions, and lesssensitive to cylinder to cylinder variation and in-cylinder air flow. Recognized limitations
include high sensitivity to changes in injection and ignition timing and spark plug reliability[Cathcart et al., 2001]. Mercedes-Benz devised a sophisticated spray guided combustion system.
It is a stratified charged gasoline injection (CGI) engine with piezo injection technology. The
spray guided injection achieves better efficiency than the conventional wall guided injection
process. The advantage of CGI engine is attained at the stratified operating mode. In this modethe engine runs with excess air hence high fuel efficiency is obtained. High rpm and higher loads
can be driven using multiple injections. During each and every compression stroke a series of7

injections takes place space a fraction of seconds apart. This permits good mixture formation,
good combustion and low fuel consumption [Baumgarten C, 2006].
Wall guided combustion process: In this case a specially designed piston will be used totransfer the fuel to the spark plug. The fuel will not evaporate completely because the fuel isinjected on the surface of the piston resulting in high emissions and fuel consumption. Usually
this system will be used along with air guided combustion process [Anon et al., 2002].
Air guided combustion process: The fuel is injected onto the flow of air which moves thefuel sprayed near the spark plug. With a special shape the inlet ports provide the air flow and the
speed of the air is controlled using air perplexes in the manifold. Here the fuel does not wet thepiston and the cylinder. Most of the CGI DISI engines use a large scale of air motion as well as
specially shaped piston to keep the fuel spray compact and to move it to the spark plug [Park etal., 2012]. In air guided and wall guided combustion systems the injector is isolated to the spark
plug. Volkswagen DISI system is a combination of two systems. The resulting system is lesssensitive against the cyclic variations of airflow. As shown in Figure 1.8 (b), injector is placed
intake-side. The fuel is injected into the chamber but onto the piston with a specific angle. Piston
has two bowls; fuel bowl and air bowl. The fuel bowl is on the intake valve side and air bowl ison the exhaust valve side. Tumble or swirl flow is obtained by special shaped intake port [Kim etal., 2008; Alkidas et al., 2007]. The fuel is guided simultaneously via air and fuel bowl to sparkplug.

1.1.3 Impinging Spray in DISI Engines
Owing to the short impingement distance and high injection pressure, spray impingementon the combustion chamber wall and piston head in gasoline engine is quite difficult to avoid at
both stratified charge and homogeneous charge conditons. However, the impingement affects thefuel-air mixture prior to combustion, which is a possible source for unburned hydrocarbon(UHC)and particulate matters [Kalantari and Tropea, 2007]. Although it has been demonstrated that the
port-injection generally produces a thicker fuel adhesion than DI owing to the lower injection
pressure and ambient temperature when compared to DI engines [Zhao et al., 1999], it is alsosuggested that the soot emissions of DI engines is more strongly dependent on the fuel adhesionon the wall, especially considering the future regulatory requirements regarding the particle
number (PN) [Moreira et al., 2010; He et al., 2012]. Tanaka et al., [2017] showed the8relationship between the deterioration ratio of engine our PN emissions and fuel adhesion
volume, indicating that PN emissions increases with the increasing of fuel adhesion on the wall,as shown in Figure 1.9.

Figure 1.9 Relationship between PN emission and fuel adhesion on the wall [Tanaka et al., 2017]
The stratified charge DISI engines increases high fuel efficiency at part load, but have someproblems to create the proper stratified-charge fuel distribution to ensure reliable ignition, proper
combustion phasing, and minimum engine emissions. During this process, the fuel sprays impact
the piston and form adhesions of liquid fuel on the wall [Drake et al., 2003]. The fuel adhesion
formation and evaporation, which results in cylinder pool fires, unburned hydrocarbon andsmoke emissions, have been studied extensively experimentally and computationally, but manyquestions remain [Oh et al., 2010]. For homogeneous charge DISI engines, an optimization of
injector spray pattern can reduce liner and piston wetting which leads to a reduction in oil
dilution. It also reduces soot emissions over a wide window of fuel injection timing, however,the soot emission problem is still serious [Lai et al., 2011]. As a result, it is important to get a
deeper understanding on the fuel adhesion formation along with the impinging spraycharacteristics.
9

1.2 OBJECTIVES AND APPROACHESThe target of this study is to investigate the characteristics of gasoline spray impingement andfuel adhesion formation experimentally. The specific objectives of this study are shown as
follows:
Analyze the impinging spray evolution characteristics which belongs to the single-holenozzle under the non-evaporation and evaporation conditions.
Illustrate the influence of dynamic factors (injection pressure and injection quantity),geometrical factor (hole diameter) and impingement factor (impingement distance and roughnessof wall surface) on the difference in the spray behaviors and fuel adhesion formation.
Model the fuel adhesion formation under the non-evaporation and evaporation conditions toprovide an insightful understanding of the interaction between spray and wall.
Focus on the droplets diameter and velocity before and after impingement to characterize
the microscopic spray under different injection and ambient pressure.
Correlate the spray evolution and fuel adhesion results with droplets behaviors undergasoline engine like condition.
In this study, the Mie Scattering technique was implemented to obtain the observations ofgasoline spray emerging from single hole injector under the different condition. In addition, therefractive index matching (RIM) technique was adopted to analyze the formation process of fueladhesion on the wall qualitatively and quantitatively under both non-evaporation the evaporation
condition. Moreover, the microscopic characteristics of impinging spray were obtained using
particle image analysis (PIA) to investigate the influences of breakup and coalescence on thedroplet behavior.

1.3 OUTLINES
To present this work, the thesis is organized as follows: First of all, a review of previous works
on the development of DISI engines, impinging spray and fuel adhesion formation as well as its
relative experimental methods are presented in Chapter 1. Chapter 2 introduces the experimental
apparatus implemented in this study, such as constant high-pressure chamber, electrical control
system, impingement system and the observation techniques applied in this investigation. The
spray characteristics and fuel adhesion evolution are experimentally investigated under non10

evaporation conditions in Chapter 3. The effect of pressure, roughness of the wall surface,
impingement distances are tested in this chapter. After that, the evaporation conditions are
considered in Chapter 4, and the effects of ambient temperatures and hole diameters on the spray
impingement and fuel adhesion formation are studied. Furthermore, the different mechanisms infuel adhesion formation will be discussed in this chapter. To correlate the spray behaviors withthe fuel adhesion, Chapter 5 illuminates the microscopic characteristics of impinging spray under
different conditions, then, the droplets diameter and velocity will be reported before and after
impingement. Finally, the relationships between droplets and fuel adhesion are summarized andcompared with the single droplets behavior in this chapter.

1.4 STUDIES REVIEW
1.4.1 Spray-Wall Interaction
A schematic description of a full-cone high-pressure spray is given in Figure 1.10. The graphic
shows the lower part of an injection nozzle with needle, sac hole, and injection hole. Modern
injectors for passenger cars have hole diameters of about 0.18 mm and less, while the length of
the injection holes is about 1 mm.
Immediately after leaving the nozzle hole, the spray jet starts to break up into a conicalspray. This first break-up of the liquid is called primary break-up and results in large ligaments
and droplets that form the dense spray near the nozzle. In case of high-pressure injection,
cavitation and turbulence, which are generated inside the injection holes, are the main break-up
mechanisms. The subsequent break-up processes of already existing droplets into smaller ones
are called secondary break-up and are due to aerodynamic forces caused by the relative velocity
between droplets and surrounding gas. The aerodynamic forces decelerate the droplets. The
drops at the spray tip experience the strongest drag force and are much more decelerated thandroplets that follow in their wake.

11

Figure 1.10 Typical schematic of spray structure [Baumgarten C, 2006]
During the secondary break-up, the more and more liquid droplets are formed and movetowards downstream of the spray. The droplets at spray tip also lost their momentum gradually,
then the following droplets with higher momentum probably purse the precedent ones and pushthem forward. Therefore, the spray propagates continuously downwards, and the large ambientgas vortex is induced by the spray plumes which further enhances the spray atomization.
However, the droplet collide with each other can produce the droplet coalescence effect andresult in a formation of larger droplet. The liquid fuel at the downstream spray is mainly
influenced by the ambient gas conditions, such as pressure, temperature and density. When theambient temperature increases over the room temperature, the liquid droplet set out to vaporize.
In the well-used DISI engines of the wall-guided concept, when the gasoline is lately injected
into the wall to form the stratified mixture, it is inevitable that a fraction of fuel would impingeon the piston crown and probably form a fuel film on that wall. This would cause an inferior
UHC and soot emission as describe above. Therefore, the interaction of liquid droplet and piston
wall needs to be investigated to reduce the impact of the spray impingement.

12With the great progress on the optical diagnostic technique, it is possible to study the spray
break-up and evolution more intuitively by the high-speed imaging. The characteristics of spray,such as spray morphology, droplet’s size distribution and equivalence ratio distribution havebeen studied aggressively, and numerous semi-empirical equations and theories also have beensummarized. The spray tip penetration of the spray was examined by Hiroyasu et.al [1990]. They
separated the spray evolution into two aspects. First, the penetration length is proportion to thetime as the liquid core region moves without break-up. Second, during the spray evolution, themomentum of liquid droplet is influenced by the surrounding gas in the result of the slower
droplets at the tip region. The equations which describe these two regimes can be summarized asfollows:
0.5

S§ 2'P ·¸
0.39  ¨
¨ U ¸
f
©
¹

S

§ 'P ·¸¸  D  t 0.5
2.95  ¨¨© Ua ¹

t

(1-1)

0.5

tbreak

(1-2)

28.65  U f  D(1-3)

U a  'P 0.5

Where ΔP is the pressure drop at the nozzle hole exit [Pa], ρf is the liquid density [kg/m3], D isthe nozzle hole diameter [m]. The influence of the ambient temperature Tamb was taken into
consideration and the increase in the ambient gas temperature can reduce the length of the spray
penetration. The improved equation is shown as:

S§ 'P ·¸¸
3.07  ¨¨
U
© a¹

0.25

 Dt

0.5

§ 294 ·¸¸ ¨¨T© amb ¹

0.25

(1-4)

The sauter mean diameter (SMD) is usually used to quantitatively characterize the droplet
size apart from the spray tip penetration. It is defined as the diameter of a sphere that has thesame volume/surface area ratio as a particle of interest [Wang et al., 2013]. The definition of
SMD is described as follow.

13

¦ d
¦ d
n

SMD

i 1
n

i 1

3
i

(1-5)

2
i

The semi-empirical equation of SMD also was presented by Hiroyasu et al. [1990] as below:

SMD
D

0.38  Re0.25

 0.32
l

 We

§P ·
 ¨¨ l ¸¸© Pa ¹0.37§U · ¨¨ l ¸¸© Ua ¹

0.47

(1-6)

Moreover, other researchers also pointed their models, such as Wakuri et al. [1957], Narberand Siebers 1996].
Although the characteristics about spray have been well studied, the interactions between
wall and spray are seldom considered. Recently the combustion chamber in a gasoline engine
tends to be small in order to reduce fuel consumption, and injection pressure tends to increase ascompared with a conventional engine. Wall impingement of the spray might occur due todownsizing of the engine and high pressure of fuel injection, and injected fuel was adhered on
the wall of cavity. The impingement spray causes the emission of hydrocarbon (HC) and sootfrom the diesel engine. Therefore, it is important to understand the spray-wall interaction andadhesion characteristics of impinging spray.
Spray-wall interactions occur if a spray penetrating into a gaseous atmosphere impacts awall, which can be the backside of the intake valve or the wall of the combustion chamber incase of a direct injection engine. Two main physical processes can be involved: wall-spraydevelopment and wall adhesion evolution. Both processes may strongly influence combustion
efficiency and the formation of pollutants. Whether wall impingement occurs or not depends on
the penetration length of the spray and on the distance between injection nozzle and wall. High
injection pressures as well as low gas densities and temperatures increase penetration and thepossibility of wall impact.

14

Figure 1.11 Typical schematic of impinging spray [Heil et al., 2001]

Figure 1.12 Typical flow around the impinging spray [Heil et al., 2001]
Figures 1.11 and 1.12 illustrate the schematic of imping spray and a typical gas flow fieldaround the spray. When the fuel jet impinged on the wall, a large number of droplets is formed at
the periphery of the spray tip, which develops along the wall. Whether the drops stick to the walland continue to evaporate, spread out to form a liquid film, reflect, or break up into smaller
droplets, depends on the kinetic energy of the incident drops and on the wall temperature, A
15

large-scale vortex is formed around the wall spray entraining a considerable amount of hot gas
into the spray. In the case, the velocities of droplets and air are changed by the vortex.
Depending on wall temperature and the amount of liquid deposited on the wall, spray-wallinteraction can have both negative and positive effects. In the case of low wall temperature,under cold starting conditions for instance, the formation of a liquid wall adhesion cansignificantly increase the unburned hydrocarbon and soot emissions because of partial burning
due to the very slow evaporation of the wall adhesion. On the other hand, shattering of dropletsat the wall may intensify droplet disintegration and increase the total spray surface. The
formation of a large-scale gas vortex near the wall may also promote air entrainment andenhance mixture formation. Furthermore, contact with a hot wall intensifies evaporation. In someDI engines, the fuel spray has been intentionally impinged on a surface as a method to control
combustion.

Figure 1.13 Typical flow around the impinging spray [Senda et al., 1997]
Senda et al. [1997] proposed a new model for spray-wall interaction by consider thesaturated temperature of the fuel and wall surface temperature, see in Figure 1.13. In the case of
Tw < Tsat, fuel film breakup process and dispersion of the droplets are considered. At the start ofspray impingement, a fuel film is formed in the impingement region by the impinging droplets.
Here, it is assumed that all impinging droplets spread as a radial film and they remain on thesurface, until a wall wetting film as a pool is formed on the wall. After the fuel film is
16

completely formed in the whole impingement region as a pool, the film is disintegrated into
droplets with simple model as splash process by further impingement of droplets on the film.
Then, the newly formed droplets disperse in a calculated direction, based on the assumption thatmass and momentum of the impinging droplet is conserved during the film breakup process. In
the case of Tw ı Tsat, sub-model for heat transfer process from the wall surface to the droplets,
the breakup, process of impinging droplets due to the boiling phenomena and dispersion processof droplets are considered on the basis of previous experimental results [Senda et al., 1982; 1988;
1994]. Thus, droplets impinging on the hot surface breakup into small droplets due to dynamic
unbalance and vapor formed at the liquid-solid interface; then, they disperse and evaporate. In
this case, the droplets just disperse on the wall with the velocity estimated from his previous
experimental data [Senda et al., 1982]. And, during the residence time on the surface, heat istransferred from the surface to the impinging droplet. They can even estimate the spray
dispersion for the case of high Tw, such as heavy load operation in engines, using this model.

Figure 1.14 Structure of the spray impinged on the wall [Park et al., 2004]
Recently, researchers were interested in the observation of impinging spray to characterize
it. An experimental study was performed to investigate the macroscopic behavior andatomization characteristics by Park et al. [2004] using the spray visualization system composedof a Nd: YAG laser and an intensified charge-coupled device (ICCD) camera. Figure 1.14
17

depicts the structures of spray impinged on the wall. He reported that     impinged spray has the longer radial penetration at higher injection pressures, shorter wall
distances from the injector, and smaller angles of wall inclination. However, the radial
penetration decreases with the increase in ambient pressure. The effect of wall temperature on
the radial penetration is negligible in comparison to the effects of other parameters. Alger et al.
[2001], Andreassi et al. [2007] and Schulz et al. [2014] did the similar research on impinging
spray.

Figure 1.15 The contribution of each dimensionless number to the Rw variation [Yu et al., 2004]
As shown in Figure 1.15, Yu et al. [2016] investigated the spray-wall impingement
characteristics by testing different fuels. The impingement spray radius (Rw) and height (Hw)were obtained experimentally so as to evaluate the impingement characteristics. Results showedthat the spray impingement momentum had the greatest effect on Rw, followed by air entrainment,18

while the impact of the fuel viscosity and surface tension (or spreading resistance) were limited.
Lahane et al. [2014] also studied the wall impingement for biodiesel-diesel blend.
Du et al. [2018] studied the different phase on the spray structure after wall impingement by
obtaining the behavior of liquid and vapor phase impingement spray, respectively. Figure 1.16
shows the processing method for liquid and vapor image. Results show that higher injection
pressures cause higher liquid-phase spray diffusion rates after wall impingement.

(a) Liquid phase(b) Vapor phase

Figure 1.16 Processed image of impinging spray [Du et al., 2018]
Liu et al. [2018] studied the spray-wall impingement with different wall temperatures andheat transition of the spray, as shown in Figure 1.17. Results show that the liquid phases of thespray do not reach the wall except in the condition of low wall temperature and high injection
pressure. With the decrease of wall temperatures, the high-concentration regions are enlarged at
the near wall regions. However, at the injection pressure of 1600 bar, the influence of wall
temperatures on the equivalence ratio is small. The decreasing wall temperature deteriorates the
mixing process of the fuel and ambient gas, but the effect is weakened with the increase of
injection pressure. Arcoumanis et al. [1993;1998], Panao et al. [2009] and Zhou et al. [2018] did
the similar work to study the heat transfer of spray impingement on piston surface.

19

Figure 1.17 Typical flow around the impinging spray [Liu et al., 2018]
Pizziol et al. [2018] tested the multiple impinging jet atomization in Figure 1.18. In multiple
impinging jets sprays, considering 2-,3-and 4-jets, the air flow rate assisting atomization allows
some control over the average droplet size.
The researches above focus on the flat wall impinging spray, however, Zhang et al. [2018]
investigated the corner angle of the expanding corner on the evolution of the impinging sprayand found that the expanding corner angle remarkably affects the impinging spray structure.
Figure 1.19 shows the spray structures. Ko et al. [2003] did the similar work before. The spraywall interaction has been widely investigated, however, the research correlation the spray
behavior and fuel adhesion together is seldom reported in the literatures. Besides, more studies
should be done for the mechanisms of impinging spray.

20

Figure 1.18 Injector and spray produced by the multiple impinging jets atomizer [Pizziol et al.,2018](a) θc = 45e21

(b) θc = 60e
Figure 1.19 Image of the impinging spray under different corner angles [Zhang et al., 2018]

1.4.2 Microscopic characteristics of Spray and Droplet Behaviors
It is well known that the microscopic characteristics of fuel spray are very important foratomization and mixture formation. The droplet size and number distributions reveal the qualityof spray and atomization, which affects the subsequent combustion and emissions characteristics[Guan et al., 2015]. In the near spray region, the transition from the injector flow to the fully
developed spray is modeled as a three-step mechanism, see Figure 1.20, consisting of film
formation, sheet break-up, and, final disintegration into droplets.

Figure 1.20 Break-up of the liquid sheet into ligaments and droplets [Baumgarten C, 2006]

22

Comprehensive experimental investigations have been carried out to deeper understand themicroscopic spray characteristics. Both Payri et al. [1996] and Lacoste et al. [2003] discussed theshear force and coalescence behavior of droplets in diesel free spray. Manin et al. [2014]
reported direct observations of the liquid structures or droplets in the near-nozzle region of asingle-hole injector, as shown in Figure 1.21. Their observations showed that the surface tensionof high-grade n-dodecane fuel was evident when fuel was injected into a low-ambient pressure
condition, whereas the surface tension decreased significantly under higher ambient-pressure
condition as fuel ligaments and droplets breakup. Lee et al. [2014] focused on the spray breakup
and atomization processes of GDI injectors. Their results showed that an increase in injection
pressure led to a decrease in Sauter mean diameter (SMD, also known as D32), but there is alimitation on droplet breakup up to 20 MPa. Guan et al. [2015] examined the effect of different
fuels on the microscopic spray characteristic during free spray. They reported that lower
viscosity and surface tension improved the spray characteristics by comparisons of droplet
number density and droplets size distribution. Wang et al. [2015] investigated the microscopic
characteristics of split injection spray, and their results showed that higher injection pressure led
to better dispersion and atomization. Moreover, the strong collision between split injection
caused larger droplets than those obtained with single injection. Crua et al. [2015] used longrange microscopy to investigate the primary atomization of diesel, biodiesel and kerosene fuelsin the near-nozzle region, both at atmospheric and realistic engine conditions. Their findings
confirm that fuel can remain trapped in the injector holes after the end of injection. Although we
could not measure the hydrocarbon content of the trapped vapourised fluid, we observed that its
density was lower than that of the liquid fuel, but higher than that of the in-cylinder gas, as
shown in Figure 1.22.

23

Figure 1.21 The liquid structures or droplets in the near-nozzle region [Manin et al., 2014]24

Figure 1.22 Effect of injection pressure on the initial stage of spray [Crua et al., 2015]25

Feng et al. [2016] experimentally studied the spray and atomization characteristics of fuels
blends in common rail injection system. They found that the droplet size decreased as gasoline
blending ratio increased in diesel fuel, indicating that the spray atomization was promoted. Guo
et al. [2017] visualized the gasoline microscopic spray under idle and wide-open throttle
conditions in GDI engine. It was revealed that the SMD decreased on increasing the fueltemperature from 20 to 60 qC, and the reduction in viscosity and surface tension should be themain reasons for that. Liu et al. [2017] reported on the droplet characteristics of gasoline sprayunder high-temperature (up to 376 qC). They found that the droplet size dropped significantly
when the fuel temperature approximated the critical point (278 qC), and that large droplets
disappeared when the temperature exceeded 173 qC. Jing et al. [2017] investigated the spraycharacteristics of dieseline fuel. Macroscopic spray characteristics in terms of spray morphology
and penetration were investigated in a constant volume vessel under elevated ambient pressuresusing high-speed imaging system, while the microscopic spray characteristics including droplet
size and velocity distribution were measured at atmospheric condition using Phase Doppler
Particle Analyzer (PDPA) system. The results have shown that the gasoline/diesel blending ratio
played an important role in affecting the spray atomization process, as shown in Figure 1.23.

26

Figure 1.23 Droplet size distribution of gasoline [Jing et al., 2017]
A series of studies have been extensively conducted in terms of free spray processes under
different pressures, temperatures and test fuels. However, wall-impingement behavior has notbeen sufficiently studied. After impingement, the fuel spray velocity changes significantly, andthe energy interaction between the fuel and air becomes more violent, resulting in the mechanismof spray and atomization becoming more complicated, especially under high ambient andinjection pressures. Therefore, more researches are required to clarify the mechanism of wallimpinging spray. First, the single droplets experiment was done by Mundo et al. [1995], heobserved the droplets depositing on the wall and splashing off the wall, then gave the schematics
of both, see Figure 1.24 and 1.25.

(a) Deposition(b) Splashing

Figure 1.24 Imping droplets behaviors [Mundo et al.,1995]

27

Bai et al. [2002] proposed the Bai-Gosman model considers four impingement regimes:
stick, rebound, spread and splash, shown schematically in Figure 1.26. They identify the various
impingement regimes that an impinging droplet may undergo under different conditions as listed:“Stick” ü in which the impinging droplet adheres to the wall in nearly spherical form. Thisoccurs when the impact energy is very low and the wall temperature Tw is below TPA (pureadhesion temperature, below it adhesion occurs at low impact energy).
“Spread” ü where the droplet impacts with a moderate velocity onto a dry or wetted walland spreads out to form a wall film for a dry wall, or merges with the pre-existing liquid film fora wetted wall.
“Rebound” ü in which the impinging droplet bounces off the wall after impact. This
regime is observed for two cases: (1) on a dry wall when Tw ı TPR, (pure rebound temperature,
above which bounce occurs at low impact energy), in this case contact between the liquid droplet
and the hot surface is prevented by the intervening vapor film; (2) on a wetted wall, when theimpact energy is low, and the air film trapped between the droplet and the liquid film causes low
energy loss and results in bouncing.
“Rebound with break-up” ü where the droplet bounces off a hot surface (Tw ˘ TPR),
accompanied by breakup into two or three droplets.
“Boiling-induced breakup” ü in which the droplet, even at very low collision energy,
disintegrates due to rapid liquid boiling on a hot wall whose temperature lies near the Nakayama
temperature TN (is the Nakayama temperature at which a droplet reaches its maximum
evaporation rate).
“Break-up” üwhere the droplet first undergoes a large deformation to form a radial filmon the ’hot’ surface (Tw > TPA), then the thermo-induced instability within the film causes thefragmentation of the liquid film in a random manner.
“Splash” üin which, following the collision of a droplet with a surface at a very highimpact energy, a crown is formed, jets develop on the periphery of the crown and the jets
become unstable and break up into many fragments.
28

The existence of these impingement regimes is governed by a number of parameterscharacterizing the impingement conditions. These include incident droplet velocity, size,
temperature, incidence angle, fluid properties such as viscosity, surface tension, wall temperature,surface roughness, and if present wall film thickness and gas boundary layer characteristics inthe near-wall region. Quantitative criteria for the regime transitions for both the dry- and wetwall situations at sub-boiling temperatures derived from the earlier study Bai and Gosman [1995]
and refined in the present work are presented in Figure 1.27 and Table 1.1.

Figure 1.26 Schematics of different impact regimes [Bai et al., 2002]

Figure 1.27 Regime transition conditions [Bai et al., 1995]
29

Table 1.1 Regime transition conditions [Bai et al., 2002]

Senda et al. [1997] divided the impact model into three parts by considering the ratio of out
droplets diameter to incident droplets diameter and the ratio of film thickness to incident droplets
diameter, as shown in Figure 1.28. Thus, the breakup form was classified into 3 types: Rim typebreakup, Cluster type breakup and Column type breakup with their probability. Briefly, thesebreakup modes for impingement onto a liquid film included breakup or droplet ejection of one ora few droplets at the outer edge of the film (Rim type). Breakup into clusters of many small
droplets (Cluster type). And breakup into one or a few droplets from a column of fluid at thecenter of the spreading droplet resulting from the surface waves reflecting back to their source(Column type). Moreover, he also pointed out that the spray impingement model on hightemperature wall and droplets life time on the wall with consideration of the superheat, shown in
Figures 1.29 and 1.30.

Figure 1.28 Change in non-dimentional droplet diameter ratio dout /di with non-dimentionalfilm thickness dfl/din [Senda et al., 1997]30

Figure 1.29 Spray impingement model on high temperature wall [Senda et al., 1994]

31

Figure 1.30 Droplet life time with superheat [Senda et al., 1994]
Papers in the current literature mainly focus on experimental work, although a few
numerical works have also been conducted. Allocca et al. [2016] described the spatial andtemporal evolution of liquid and vapor phases on a heated wall of GDI spray to investigate thestructure of spray–wall interaction. Montanaro et al. [2016] studied the spray-wall impact of both
GDI multi-hole spray and a single-hole spray over a cold and hot wall to evaluate the droplet
behavior after impingement. Piazzullo et al. [2017] investigated the heat transfer effect on wall–
film formation in GDI spray, and a good agreement was obtained between the experimental andnumerical results. Zhao et al. [2017] studied the spray morphology before and after impingementof spray, and showed that the larger droplets tend to contribute to the fuel film formation morethan the smaller ones. Cen et al. [2018] studied the dynamic of jet breakup on the wall with heattransfer. The droplet falling height was set from 1 cm to 9 cm with interval of 0.5 cm, withcorresponded Weber number around from 6.94 to 102.12, as shown in Figure 1.31.

32

(a) At the height of 1.0 cm(b) At the height of 1.5 cm(c) At the height of 2.0 cm
Figure 1.31 Phenomenon of jet breakup [Cen et al., 2018]

1.4.3 Evolution of Fuel Adhesion on the Wall
Fuel droplets tend to get deposited on the wall, resulting in “wet wall” on the piston head andcylinder wall [Yu et al., 2017; Gold et al., 2001]. This affects the air–fuel mixture formation
process, which is a major source of excessive soot and unburned hydrocarbons (UHC), making it
difficult for DISI engines to meet the subsequent requirements of particle number (PN)
regulations [Montanaro et al., 2012; Zhao et al., 1999; He et al., 2012]. In particular, thetendency of reduction in engine size and increase in rail pressure makes it increasingly difficult33

to prevent the occurrence of adhered fuel. Therefore, it is essential to understand theimpingement behavior and formation of fuel adhesion to improve the performance of gasoline
engine and make it more environmentally friendly [Schulz et al., 2017].
However, since it is hard to measure fuel adhesion in a real working engine, comprehensive
experimental investigations have been carried out in a constant volume chamber. Specificquantitative studies were performed on fuel adhesion, out of which mainly three methods are
summarized. Akop et al. [2013;2013;2014;2014] and Yu et al. [2017] calculated the mass of fueladhered to an impingement disk wall under different conditions. This method is limited tocharacterization of the total fuel adhesion mass, not the evolution and thickness of adhesion, as
shown in Figures 1.32 and 1.33.

Figure 1.32 Adhered mass characteristics [Akop et al., 2013;2014]34

Figure 1.33 Wall film ratio under conditions [Yu et al., 2017]
In the laser-induced fluorescence (LIF) technique, a fluorescent tracer is adjusted to acertain concentration with a given quantum yield and a molar absorption coefficient, and it isexcited with a predefined radiation. The adhesion thickness is then determined by measuring thefluorescence intensity. The spatial distribution of fuel adhesion was captured by Senda et al.
[1999], Okamoto et al. [2012], Cheng et al. [2010], and Schulz et al. [2015; 2016; 2017].
Especially, Schulz et al. [2016;2017] not only measure the fuel film of the single hole, but
multiple holes. And some results are shown in Figure 1.34. However, this method is limited to aperfectly smooth impinging wall, which differs from the real roughness of an engine piston,especially one with carbon depositing after combustion.

35

(a) Single fuel adhesion(a) Multiple fuel adhesion
Figure 1.34 Wall adhesion evolution [Schulz et al., 2016; 2017]36

A more widespread and well-observed technique for measuring the fuel adhesion on a flatwall with roughness similar to that of a real engine piston is the refractive index matching (RIM)
method. Drake et al. [2002; 2003; 2007] developed this optical technique, which was then usedby Yang and Ghandhi [2007], Maligne and Bruneaux [2011], Zheng et al. [2012], Henkel et al.
[2016], and Luo et al. [2017; 2018] to measure the fuel adhesion under different impingement
conditions. In recent times, Ding et al. [2018] has also used the RIM method to measure fueladhesion in a stratified-charge SI engine, some results from them are shown in Figure 1.35.

Figure 1.35 Normalized adhesion images of selected cycles [Ding et al., 2018]37

A series of extensive studies have been conducted on fuel adhesion characteristics under
non-evaporation conditions. However, the air temperature in a real engine during operation isquite high, especially when the engine is under high load. Maligne and Bruneaux [2011]
observed “discrete pocket” areas of the gasoline fuel that evaporate faster than areas of“continuous film”. Zheng et al. [2012] studied the mass and area of gasoline fuel adhesion underconditions of high temperature of the air–fuel mixture, and compared the results withcomputational fluid dynamics (CFD) calculations. Schulz et al. [2017] investigated the effect of
flash-boiling on fuel adhesion formation using a multi-hole gasoline injector, and discovered that
increasing the fuel temperature to a certain value does not lead to a constant reduction in theadhered fuel on the wall owing to the collapse of spray jets and fuel accumulation in a single
region. Piazzullo et al. [2017] compared the gasoline footprint at the wall between measured andsimulated results under evaporation condition by evaluating the fuel mass adhering to the walland the wall heat flux. Until now, the formation of fuel adhesion on the wall is still ambiguous
and requires more research.

1.4.4 Optical Diagnostic Technique in DISI Engines Conditions
In order to get thorough understand of the impinging sprays, the optical diagnostic techniques
have been widely applied in the study. Most of the techniques have already been summarized
excellently by Zhao and Ladommatos [2001].
1.4.4.1 Spray observation
In this section, some typical spray visualization diagnostic techniques will be reviewed briefly
based on their work introduced in Part 1.4.3, and Table 1.2 shows the list of these techniques.
Table 1.2 Summary of optical techniques for spray evolution measurement
Technique

Applications

Advantages

Mie
Scattering

Liquid fuel distribution

Simple setup

Schlieren andShadowgraphObservation of overall
spray

Simple setup

38

Limitations
Sensitive to large
droplets
Sensitive to both
liquid and vapor
phasesBecause of the simplicity and explicitness of Mie scattering method, shown in Figure 1.36(a), it is mainly applied to detect the spray liquid phase. According to the Lorenz-Mie theory,when the droplets are spherical and their diameter is larger than 1 μm, the scattering intensity ismainly determined by the droplets concentration and the square of droplet diameter [Kim et al.,2002]. Hodges et al., [1991] applied ensemble-scattering polarization ratio method, based on theMie scattering, to measure the droplet size and concentration. One year after that, the spray fuelconcentration was measured by Kosaka et al., [1992] quantitatively by applying a 2-D Mie
scattering technique. However, compared with recently developed techniques, the accuracy ofthe droplet diameter concentration measured from the Mie scattering experiment is not highenough. As a result, this technic is usually applied to detect the spray structure [Gulder et al.,1992] and the liquid phase penetration [Siebers DL, 1998].

Figure 1.36 Sketch of optical techniques arrangement. (a) Mie scattering for liquid length
measurements and (b) focused shadowgraph for vapor penetration measurements [Pastor et al.,
2011]
39

As two of the earliest optical diagnostics Schlieren (shown in Figure 1.37) and shadowgraph(shown in Figure 1.36 (b)) can be used to observe the spray liquid and vapor phase,simultaneously. Normally, they are also usually adopted to measure the spray structure, becauseof the limitation of quantitative analysis. However, compared with Mie scattering method, theirsuperiority is that they can not only detect the spray liquid phase but also distinguish the vapor
phase. Furthermore, the interface between liquid and vapor phase can be evaluated through thegradient of intensity roughly. As a result, it also has been used to identify the evaporating sprayboundary [Siebers, 1998; Pickett et al., 2009].

Figure 1.37 Sketch of optical techniques arrangement for Schlieren [Lee et al., 2014]
1.4.4.2 Droplets detection
For droplets velocity and diameter measurement, two methods are widely used: PDPA and
PIA, as summarized in Table 1.3.
The Laser doppler velocimetry has been widely applied and well developed, including theLaser Doppler Anemometer (LDA) and Phase Doppler Anemometry (PDA), which is also
known as the name of Phase Doppler Particle Analysis (PDPA) techniques. Based on the40

Doppler shift of the laser light, which is scattered from the small particles within the moving
fluid, LDA is usually applied to measure the particle size and velocity. Figure 1.38 shows the
PDPA system including an argon ion laser, a beam separator, a Fiber Flow transmitting optics, aFiber PDA receiving optics, a photoelectric converter, a BSA P80 doppler signal analyzer with180 MHz frequency and a 3-D motion traverse system. Laser power used in this study was 1300
mW and lase wave length ranged from 488 nm to 514. 5 nm. The focal lengths of transmitter andreceiving optics were both 500 mm and they were installed in an L-type 3-D motion traverse
system to achieve stable light path. The measuring range of droplet size was set from 0 μm to
236

μm

with

resolution

of

±0.05

μm.

Meanwhile,

dropletvelocity

measuring

range was set from 151.95 m/s to 238.77 m/s.
Table 1.3 Summary of optical techniques for droplets measurement
Technique

Applications

Advantages

Limitations

PDPA

Droplets velocity anddiameterTime-resolved data

Complex setup

PIA

Droplets velocity anddiameter

Simple setup

Single shot data

Figure 1.38 Typical PDPA system for analyzing droplets size and velocity [Pei et al., 2017].

41

Previously, Kashdan et al. [2007] and Berg et al. [2006] compared the PIA technique withthe well-established technique PDPA and the results show that PIA is a reliable technique tomeasure the size of spherical and non-spherical droplets. Hence Guan et al. [2015] employed the
PIA system to investigate the microscopic spray characteristics, shown in Figure 1.39. An Nd:
YAG laser with the wavelength of 532 nm, along with a diffusor attachment, was employed forthe homogeneous illumination. The diameter of the lens at the head of the diffusor is 120 mm,
which is larger than the window of the chamber, to ensure the homogeneous illumination of thespray flow field. CCD camera (ImagerProSX 5M) connected with a long-distance microscope(Queststar QM1) and a magnifying lens with an amplification factor of 2 was used to capture thedroplets. The camera and microscope were mounted on an electric positioner MC600. The
precise displacement control (1 μm) ensures the accurate scanning of the measurement position.
The calibration was carried out via a scaling plate with minimum scale of 25 μm.

Figure 1.39 The sketch of PIA system [Guan et al., 2017].
1.4.4.3 Fuel adhesion measurement
As I introduced in Part 1.4.3, Specific quantitative studies were performed on fuel adhesion,out of which mainly three methods are summarized: Weighting; LIF and RIM, as summarized inTable 1.4.

42

Table 1.4 Summary of optical techniques for fuel adhesion measurement
Technique

Applications

Advantages

Limitations

Weighting

Liquid fuel adhesion

Simple setup

Measure the mass only

LIF

Liquid fuel adhesion

Simple setup;
Thickness distribution

Difficult to calibrate;
Flat-wall must be
smoothLiquid fuel adhesion

Simple setup;
Thickness distribution˗Different roughness of thewall can be used

Difficult to calibrate;

RIM

The procedure for measurement of adhering fuel mass is illustrated in Figure 1.40. The
mass of the dry impingement disk was measured before fuel injection. Then, the impingement
disk was set normally to the injector. After spray impinged on the disk, the disk was removed
from the high-pressure vessel and adhered fuel mass was measured together with the mass of
disk by using a precision balance with sensitivity of 0.01 mg. Adhered fuel mass was derived by
the difference of the disk masses of before and after impingement

Figure 1.40 Measurement procedure for weighting adhesion [Akop et al., 2013]
Laser Induced Fluorescence (LIF) method has been applied for measuring temperature, thinfilm thickness, concentration and so on using the changes of fluorescence lifetime and intensity.
An explanation of LIF is shown in Figure 1.41. LIF uses an electronic absorption and emission
43

process which can produce relatively strong signal with high spatial resolution [Andresen et al.,1990]. When the atom or molecule absorbs the specified laser energy, the electronic state energy
is excited to an upper level. Because of the instability, the electronic goes back to state ground
immediately accompanied with the emission light, which is usually called as fluorescence.
Therefore, the fluorescence intensity can be used to estimate the fuel concentration because it is
proportional to the molecular density. As a result, LIF is widely used to measure mass
distribution [Yeh et al., 1994; Bruneaux, 2002]. However, fluorescence intensity is very sensitive
to the quenching effect, and the impingement plat must be smooth, which is different with thereal roughness of the piston head in the engine.

Figure 1.41 Typical LIF system for fuel adhesion [Okamoto et al., 2012]
Drake et al. [2002; 2003; 2007] developed an optical method to quantitatively investigate
the fuel film on the piston of a direct-injection gasoline engine, as shown in Figure 1.42. The
technique is based on the variation of light scattered from a roughened surface when a liquid ofrefractive index that matches the window is in contact with the surface. This method enabled
time-resolved quantitative information to be derived from the entire fuel film under vaporizing
conditions with high spatial and temporal resolution. Their experimental results showed thatliquid fuel film volumes were approximately 1% of the injected fuel for a hollow-cone swirl
injector and only 0.1% for a multi-hole injector. The liquid fuel films, with gasoline fuel, wereobserved to have a maximum local film thickness of around 3 μm and area-average film
44

thickness of around 1 μm. These results were found to correlate very well with engine-out smoke
emissions from an optimized wall-controlled SIDI gasoline engine under warmed-up stratifiedcharge operating conditions.

Figure 1.42 Typical RIM system for fuel adhesion [Drake et al., 2007]

1.5 SUMMARY
In this chapter, the background and motivation of this study is introduced firstly. After that, theoutline is shown before a series of study review. From the spray-wall interaction to droplets
behaviors and fuel adhesion formation, a lot of classic theories developed form experimental andnumerical researches were described in detail. After that, the corresponding technical approaches
were also introduced one by one.
Based on the review of previous study about the gasoline spray system, impinging spray forgasoline was well investigated under high injection pressure conditions. However, the researchon fuel adhesion is not enough, especially for the formation of fuel adhesion on the wall.
Therefore, a single-hole nozzle is originally applied in the fundamental research to provide
insights into the spray characteristics and fuel adhesion in this study. Meanwhile, previous
research has also shown some interesting report about droplets, but most of them focused on thefree spray. The microscopic of impinging spray is necessary to study, and correlation between

45

the droplets characteristics and fuel adhesion can give us a clear understanding about fueladhesion formation.
In the following chapters, the experimental investigations method applied in this study andthe results about the impinging spray under non-evaporation and evaporation conditions will bepresented.

46

CHAPTER 2 EXPERIMENTAL APPARATUS AND
MEASUREMENT METHODS
2.1 CONSTANT HIGH-PRESSURE CHAMBER
In order to acquire the engine-like ambient condition and high quality of visualization of sprayevolution, a special high pressure and high temperature chamber was applied.

Figure 2.1 Schematic of the high-pressure chamber
As shown in Figure 2.1, it has four available windows, and according to the specificexperiment the utilized window was variable. The quartz glass with diameter of 100 mm was
mounted in the utilized window adaptor. Two thermocouples were fixed by the thermocouple
adaptor, which can detect the ambient temperature and the near wall temperature. A kanthal alloy
heating unit was placed inside the chamber to produce the engine like high temperature and hightemperature condition as high as 500 K. The internal and external surfaces of the chamber arecovered by the thermal insulator to avoid the heat conduction and maintain the stableenvironment. There were cooling water circulations inside the injector adaptor to avoid

47

overheating. The high-pressure condition inside the chamber was produced by applying a gas
cylinder, which can increase the chamber pressure to above 1.5 MPa.

2.2 ELECTRICAL CONTROLLING SYSTEM
In the experiments, the injection timing, imaging timing, and laser induced timing are allcontrolled by the electrical controlling system. Specifically, the injector was controlled by the
injector electronic control unit (ECU). Injection timing, injected mass and image taken timing
were controlled by a delay pulse generator (Stanford Inc., DG 535 or DG645). The basic
information of the signal for injector, camera is shown in Figure 2.2.

Figure 2.2 Timing chart of signal
However, it can be seen that there is a delay between the injection signal and the fuelinjection, which can cause the difficulties in deciding the start of injection and time after start of
injection by the high-speed images. In order to find the start of injection timing as accurately as48

possible, the injection delay measurement was conducted by comparing the pulse signal and theinjection rate signal during the injection rate measurement experiments. The imaging time of thecamera and laser induced setting are changing according to the injection delay variation.

2.3 IMPINGEMENT SYSTEMAs shown in Figure 2.3, a quartz glass (Sigma Koki, DFSQ1-50CO2) with a diameter of 50 mm
and thickness of 2 mm, was set under the injector as the flat wall. The impingement angle was 45°
and the impingement distance was 22 mm from the nozzle exit to the impingement point of theflat wall along the spray axis. The coordinate system is defined, and the intersection point o of
the nozzle center axis and the wall is determined as the impingement point. The positive y axis isalong the lateral direction of the spray after impingement, and the positive z axis is parallel to theinjector axis, and the x axis is perpendicular to the yz plane, pointing out of the figure. Moreover,the surface roughness was measured by a portable high-performance surface roughness andwaviness measuring instrument (Kosaka Laboratory Ltd., SE300), whose resolution is 0.0064
μm.

Injector

y

o

45°

2 mm

Lw

=2
2

m

m

z

Φ 50 mm Flat Wally

Fuel FilmoxFigure 2.3 Schematic of injector and flat wall49

2.4 OPTICAL MEASUREMENT SYSTEMS
2.4.1 Mie Scattering Method
2.4.1.1 Experiment apparatus
In order to observe the spray development process, a Mie scatting experiment was performed
using a single-hole injector, and the specific experimental apparatus are shown in Figure 2.4. It
consists of a constant high-pressure chamber, an injection system, and an optical system.
Toluene fuel was directed into the mini sac injector by a high-pressure injection system. A highspeed video camera (Photron FASTCAM SA-Z) was utilized to observe the spray and was set ata frame rate of 20000 frames per second (fps) and at a frame size of 512 × 512 pixels. A xenon
lamp (Ushio SX-131 UID501XAMQ) was set in a direction perpendicular to the camera toilluminate the spray, and both were in the same plane. The injector and camera weresynchronized by a delay generator. Two windows of the constant volume chamber were used.
The layout of the lamp and the camera is shown in Figure 2.4 (b).

N2

ECU

Injector

High-Speed Video
CameraConstant High-Pressure Chamber

Delay Generator
Computer(a) Front view

50

Xenon Lamp

Xenon Lamp

Constant High-Pressure Chamber

High-Speed Video
Camera(b) Side view
Figure 2.4 Experimental apparatus for spray observation.
2.4.1.2 Image processing method
The parameters such as impinging spray radius (Rs), impinging vortex height (Hv), spray tip
penetration (S) and impinging spray height (Hi) are widely used to investigate the characteristicsof the spray-wall impingement (Park and Lee, 2004; Andreassi et al., 2007; Zhang et al., 2014;
Yu et al., 2016). These values were experimentally obtained from raw images by determining theedge of the impinging spray using inhouse code created by ourselves in MATLAB software asshown in Figure 2.5.
The dotted line in Figure 2.5 represents the wall surface. The focus of the front and sideviews is on the impingement point plane and spray axis plane, respectively. Rs and Hv are definedfrom the front view. Rs is the maximum horizontal distance from the spray center to the furthestedge of the spray. Hv is the maximum distance from the wall surface to the edge of the spray
vortex. S and Hi are defined from the side view. The spray tip penetration is defined as the
distance from the nozzle exit to the spray tip (Guo et al., 2015). Generally, before impingement,
the spray tip penetration is just the distance from the nozzle exit to the spray tip. However, after
wall impingement, the penetration is not only the distance from the nozzle to the wall, but also
the radial distance (Yu et al., 2012; Guo et al., 2016). Thus, the spray tip penetration S is definedas the sum of the distance to wall Lx and the radial distance Lr,. The impinging spray height Hi is51

the maximum distance from the wall surface to the edge of the impinging spray. Results of thefront and side views were obtained from a clear spray by binarizing the images. The fueladhesion data were obtained from the bottom view. All the results were calculated three times
under each specific set of experimental conditions, and the average values were presented.

Figure 2.5 Experimental extraction of Rs, Hv, S and Hi

2.4.2 Refractive Index Matching (RIM) Method
2.4.2.1 Experiment apparatus
The RIM experimental equipment in the current study is shown in Figure 2.6. The high-pressure
constant volume chamber and injection system were the same as those for the Mie scatteringexperiment. Only the optical system had some differences. The xenon lamp was positioned at theside window to emit continuous and high intensity light that passed through the window of thechamber with an incident angle about 5°. A reflection mirror was placed directly beneath theimpingement surface. Fuel adhesion images were captured with a high-speed video camera
through the mirror, as shown in Figure 2.6 (b). Further, the high-speed video camera was set at aframe rate of 10,000 fps and at a frame size of 512 × 512 pixels. The layout of the lamp and thecamera is shown in Figure 2.6 (c).
52

ECU

N2

Injector
Flat Wall
Light Out
High-Speed
Video Camera

Light InV

MirrorConstant High-Pressure Chamber

Delay Generator
Xenon Lamp
ComputerWindow(a) RIM experiment setup(b) Internal structure

Spray

High
Speed
High-Speed
Video
Video Camera

Window

Y

X

Xenon Lamp(c) Optical arrangement

Figure 2.6 Experimental setup for fuel adhesion measurement
2.4.2.2 Image processing method
The RIM technique measures the spatial distribution of the fuel adhesion thickness, from which
the adhered adhesion area and mass can be calculated. In this method, the difference in refractive
index between the quartz glass and air results in the scattering of light from the roughened53surface that is modified by the presence of the fuel closely matching the refractive index of theimpingement glass.
Figure 2.7 describes the light transfer in the quartz glass without and with fuel adhesion on
the wall. When fuel adheres on the wall, the transmitted light becomes strong, while the diffusedreflection light becomes weak, which results the reduction in the light from the fuel image.

(a) Without fuel adhesion

Transmitted Light

Incident Light

Diffused
Reflection Light

Fuel Adhesion

SpecularReflection Light(b) With fuel adhesion on the wall
Figure 2.7 The schematics of light transfer in the quartz glass
Drake et al. (2002) showed the relation between the fuel adhesion thickness h x, y and theintensity reduction of the scattered light ΔI x, y . The reduction through the glass is written as
follows:54

ΔI x, yI x, y1  wetI ref x, y

(2.1)

where I ref x, y is the intensity of the scattered light in the dry image at the location of (x, y), andI wet x, y is the intensity of the fuel deposit at the location of (x, y).

After the calibration, a function that is the correlation between h x, y and ΔI x, y can bebuilt:

h x, y

f ΔI

(2.2)

To obtain the calibration curves, a calibration experiment was carried out under atmosphericconditions without the injection system. Two different fuels were selected because tridecane has
low volatility and high viscidity, but toluene has high volatility and low viscidity. Moreover, theirrefractive index is similar to that of the quartz glass. A much thinner thickness of fuel adhesion
can be obtained with a liquid mixture of these fuels. The characteristics of the fuels and quartz
glass are summarized in Table 2.1.
Table 2.1 Characteristics of fuels and quartz glass
Parameter

Refractive
Index

Density(kg/m3)

Boiling
Point (K)

Kinematic
Viscosity(10-6 m2/s)

SurfaceTension(N/m)

Toluene1.49

866

382.750.68

0.0285Tridecane

1.43

756

507.582.35

0.0303Gasoline

1.42

737

-

0.46

0.022

Quartz Glass

1.46

-

-

-

-

A mixture (10% volume of tridecane and 90% volume of toluene) was used for thecalibration procedure. The fuel mixture was dripped on the dry window by means of a syringe
and the reduction in scattering light increased from 0 to the maximum value. After a certain
volume of the mixture was dripped on the rough quartz, the droplet rapidly expanded, and thearea of fuel increased rapidly. During this time, toluene, which is the high volatility component,
quickly evaporated; however, there was only a slight increase in the scattered light. Once theadhesion area reached a certain value, tridecane, which is the low volatility component, begins to55

evaporate, and the scattered light changed significantly. There are two hypotheses in this case:one stating that all the toluene has evaporated, but all the tridecane has not yet evaporated;
another stating that the thickness of fuel adhesion is uniform. Thus, h x, y can be calculated
because the tridecane volume was calculated as 10% of the mixture, and the averaged reference
dry image was obtained before the liquid deposited on the glass. Then ΔI x, y can be calculatedusing Equation (2.1). Eventually, one point with h x, y and ΔI x, y was obtained. By varying
the mixture volume from 0.1 μL to 10 μL, the calibration curve was acquired by Equation (2.1).
The calibration curves are plotted in Figure 2.8. The horizontal axis is the reduction in scattered
light, and the vertical axis is fuel adhesion thickness. Although it is difficult to calibrate the fueladhesion thickness below 0.2 μm and above 1.4 μm owing to the limitations of the experimental
instruments, extrapolation was applied to calculate the fuel adhesion thickness in the full range

Fuel Adhesion Thickness h(μm)

with acceptable accuracy (Drake and Haworth, 2007; Zheng et al., 2012).

1.6
1.4
1.2

h=1.2366ΔI2+1.6283ΔI

1.0
0.8
0.6
0.4
0.2
0

0

0.1

0.2 0.3 0.4 0.5 0.6 0.7
Reduction of Scattered Light ΔI

0.8

Figure 2.8 Calibration curve.
After the calibration calculation, the fuel adhesion thickness can be measured through the
RIM method. The image processing of the RIM experiment is shown in Figure 2.9. First, a dry
image was acquired. Then, it was subtracted by the wet image to obtain only the adhered fuelimage, and the ΔI x, y can be obtained by this image. Finally, the thickness distribution was
calculated through the calibration curve. The adhered fuel area and mass can also be integrated

56

from the pixels of each thickness. Additional details about RIM method can be found in ourprevious studies (Luo et al., 2017; 2018).

(a)Dry Image(b) Wet Image

Thickness (μm)
2.5
2.0
1.5
1.0
0.5
0
(d)Thickness Distribution Image (c)Background Removed Image
Figure 2.9 Image processing

2.4.3 Particle Image Analysis (PIA)
2.4.3.1 Experiment apparatus
Previously, both Berg et al. (2006) and Kashdan et al. (2006) certified that particle image
analysis (PIA) was a reliable technique to measure the size of spherical and non-spherical
droplets. Hence the PIA system was employed to investigate the microscopic spraycharacteristics. The experimental equipment is shown in Figure 2.10. It consisted of a constantvolume chamber, an injection system, and an optical system. Toluene fuel was directed into themini-sac injector by a high-pressure injection system. An Nd: YAG laser with a wavelength of
532 nm was used as the light source. The energy of the laser pulse was in the order of 10 mJ, anda duration of approximate 6 ns. The light beam was expanded from 8 to 100 mm to provide
homogeneous illumination over a sufficiently large area. A charge-coupled device (CCD) camera(Flowtech Research Inc., FtrNPC) with a long-distance microscope was employed to visualize
the spray structure. As shown in Figure 2.9, two teleconverters (Kenko Tokina, N-AF 1.4X
57

TELEPLUS MC4) were connected with the CCD camera by a bellows to enlarge the spray
image. The camera location was collinear with the laser axis. The image was captured at 1200 μs
and 900 μs after the start of injection under Pinj = 10 and 20 MPa, respectively, when the spray
showed the quasi-steady state period. Guan et al. (2014) and Feng et al. (2016) selected similar
periods for image taking in their work. The resolution was approximately 2.008 μm per pixel,
and the frame size was 1600 × 1200 pixels.
CCD Camera Teleconverter
ECU

Bellows

Injector

Lens

Flat Wall

CCD Camera

Diffuser

Nd: YAG laser

Window
Constant High-pressure Chamber

Delay Generator
ComputerFigure 2.10 PIA experimental setup.
The measurement regions and coordinate system are shown in Figure 2.11. The field ofview is 3.2 × 2.4 mm2. The exit of the nozzle was defined as the origin point. The positive x-axis
is along the lateral direction of the spray after impingement, and the positive y-axis is the verticaldirection along the injector axis. There are many droplets in the impingement region, thus
making it difficult to recognize droplets in the dense area. The regions before and after
impingement are selected near the real impingement region, whose center point coordinates are(10,10) and (22,14) mm. The solid line represents the surface of the wall after impingement. In

58

Chapter 5, all results are calculated for these two regions under different conditions to compare
the microscopic behaviors before and after impingement on the wall.

2.4 mm

3.2 mm(0,0)
X(10,10)3.2 mm

2.4 mm(22,14)Y

Beforeimpingement

After impingement
C

Surface of the wall
Figure 2.11 Definition of before and after impingement
2.4.3.2 Image processing method
Twenty injection events under each condition were recorded to detect more than 10,000
validated particles. To detect the droplets correctly, refined criteria were used. Droplets appear
sharp on the focal plane and others that are away from the focal plane appear blurred. The
threshold was set at 25/255 (10% of the maximum pixel intensity), and all pixels whose
intensities are above 25 can be detected. Droplets with pixel numbers less than 4 are rejected,
thereby requiring droplets to consist of at least four pixels for detection.
Moreover, the roundness of the droplets was defined as follows:

Roundness

4S  Sdroplet
2
Cdroplet

×100

(2.3)

where S droplet and Cdroplet are the area and perimeter of the droplet. A roundness value of 100
implies that the droplet has a spherical shape. The roundness of the droplet was set from 80 to59

100 to ensure that all the selected droplets were nearly round. More detailed algorithms of thedroplet sizing can be found in a previous paper (Fujimoto et al., 2008; Luo et al., 2018).
The area diameter Darea of the droplets was defined as follows:

Darea

4 u S droplet

(2.4)

π

Droplets with the pixel numbers less than 4 are rejected, such that detected droplets consistsof at least 4 pixels. With a resolution of 2.008 μm/pixel, the pixel number was set from 4 to 2500;
as a result, the area diameter of droplets was limited to 100 μm, as discussed in the next section.
The refined criteria are presented in Table 2.2.
Table 2.2 Refined criteria.
Threshold Value

25/255

Roundness

80 – 100

Pixel Number

4 – 2500

Area Diameter4 – 100 μm

Figure 2.12 illustrates the droplet detection procedure. A commercial software package,
FtrPIA-Cruise, was used to process the shadow images. Before impingement, all the droplets
were above the wall. First, the raw image was acquired using the CCD camera. The grayscale
image was then inverted for better recognition. Using the refined criteria listed in Table 2.2, thedroplets were filtered, as indicated by red points in Figure 2.12. However, after impingement, inorder to ensure that the surface of the wall is taken in the image, the image necessarily includes aregion of high droplet density near the wall. It is difficult to correctly distinguish the droplets inthis region, such that only the droplets at least 0.6 mm from the surface of the wall can bedetected. Moreover, the dense region is limited, and we are confident that the specific spray
region after impingement is sufficient and meaningful for discussion in Chapter 5.

60

0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
(mm)

0
0.4
0.8
1.2
1.6
2.0
2.4
(mm)

Raw image

Reversed color

Filtered droplet(a) Before impingement00 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
(mm)

0.4
0.8
1.2
1.6
2.0
2.4
(mm)

Surface of the wall
Raw image

Surface of the wall
Reversed color

Surface of the wall
Filtered droplet(b) After impingement
Figure 2.12 Droplet detection procedure

2.5 SUMMARY
In this chapter, the experimental apparatus applied in this dissertation are introduced in detail, forexample, high-pressure and high-temperature ambient condition acquiring system, electricalcontrolling system and so on. Moreover, the measurement technology, including the Miescattering, RIM and PIA technical methods, which are corresponding to the non-evaporation andevaporation condition measurements, respectively, are also introduced in this chapter. All theapparatus and technic introduced here will be applied to investigate the spray evolution and fueladhesion formation processes inside the chamber. In next chapter, the detailed results obtainedby these measurements in this study will be discussed.

61

CHAPTER 3 FUEL ADHEISON FORMATION UNDER NONEVAPORATION
3.1 EFFECT OF INJECTION AND AMBIENT PRESSURE
3.1.1 Experimental Conditions
The experimental conditions, shown in Table 3.1, were determined in consideration of the real
operating conditions of passenger car gasoline engines.
Table 3.1 Experimental conditions
Injection Conditions
Fuel
Injection Mass (Minj)

Toluene
3.0 mgInjection Pressure (Pinj)

10, 20, 30 MPa

Injector Type

Mini-Sac, Single-HoleNozzle Hole Diameter (d)0.135 mm

Injection Duration (td)

2.4, 1.65, 1.36 ms

Ambient Conditions
Ambient Gas

NitrogenAmbient Pressure (Pamb)

0.1, 1.0 MPa

Ambient Temperature (Tamb)

300 K

Impingement Conditions
Impingement Plate

Quartz Glass

Impingement Distance (Lx)22 mm

Impingement Angle

45 deg

Surface Roughness

Ra7.5 μm

The ambient pressures ranged between 0.1 and 1.0 MPa, and the injection pressures varied
from 10 to 30 MPa. The experiments were carried out under room temperature with nitrogen. A
mini-sac injector with a nozzle hole of 0.135 mm diameter was used, and toluene was employed
as a surrogate fuel for gasoline. The injection mass was kept constant at 3.0 mg with different
conditions, resulting in the different injection durations of 2.4, 1.65 and 1.36 ms at different
62injection pressures. The impingement angle was 45 deg and the impingement distance was 22mm from the nozzle exit to the impingement point of the quartz glass along the spray axis. Glass
plates with surface roughness of Ra7.5 μm, was investigated in this study.

3.1.2 Spray Impingement Characteristics
Figure 3.1 shows spray tip penetrations under three conditions. The horizontal axis is the timeafter start of injection, and the vertical axis is spray tip penetration S. For all conditions, the spraydevelopment can be divided into two stages. It increases almost linearly before impingement,
and the gradient of S decreases after spray impingement. The splashing due to the wall
impingement, drag force from the wall and ambient gas can be regarded as the main reasons forthis phenomenon. More importantly, under different ambient pressures, S of 0.1 MPa is larger
than that of 1.0 MPa, and impingement time of 0.1 MPa is earlier than 1.0 MPa. It can be
expected that higher ambient pressure decreases S by transfer the spray momentum to theambient gas, and stronger air resistance force is expected with higher ambient pressure. Under
different injection pressures, S of 30 MPa is larger than that of 10 MPa, and impingement time of
30 MPa is earlier than that of 10 MPa due to higher spray velocity.

Spray Tip Penetration (mm)

40
35
30
25

Impingement Distance
Pinj
Pamb
0.1 MPa-10 MPa
1.0 MPa-10 MPa
1.0 MPa-30 MPa

20
15
10
5
0

Injection Duration0

0.2 0.4
0.6 0.8
1.0 1.2
Time after Start of Injection (ms)Figure 3.1 Spray tip penetration (S).

63

Impinging Spray Height (mm)

8

Pinj
Pamb
0.1 MPa-10 MPa
1.0 MPa-10 MPa
1.0 MPa-30 MPa

7
6
5
4
3
2

Injection Duration=1.36 ms
Injection Duration=2.4 ms

1
0
0

0.4
0.8
1.2
1.6
2.0 2.4
Time after Start of Injection (ms)Figure 3.2 Impinging spray height (Hi).
Figure 3.2 presents impinging spray heights under three conditions (the same as spray tip
penetration). The horizontal axis is the time after start of injection, and the vertical axis isimpinging spray height Hi. In contrast to ambient pressures, Hi of 0.1 MPa is larger than that of
1.0 MPa. Although increasing the pressure can increase the spray angle, resulting more droplets
above the wall. Higher ambient pressure increases the density of the ambient atmosphere,
resulting in a decrease in the droplets splashing, thus decreasing Hi. In comparison with theinjection pressures, Hi of 30 MPa is larger than that of 10 MPa, it can be expected that theincreased injection pressure results in much more droplets above the wall than 10 MPa.

3.1.3 Fuel Adhesion Characteristics
Figure 3.3 shows the evolution of the adhered fuel on wall. The pseudocolor represents the fueladhesion thickness varying from 0 to 2.5 μm. Further, the cross symbol is the impingement point.
All cases at different time show similar structures. It is clear that the area and thickness increase
with time, and it can be expected that there are still some droplets impinging on the wall. The
fuel adhesion thickness is not uniform in the initial stage. The non-uniform tendency is more
obvious with time. The thicker fuel adhesion area is located at the upstream. The increased
injection and ambient pressure decrease the maximum thickness of the adhesion.

64

-10

10 (mm) -1000

10 (mm) -100

10 (mm) -100

10 (mm)
2.5

0

2.0
10

1.5
1.0

20

0.5
30

0
10 ms ASOI
20 ms ASOI
29 ms ASOI Thickness(μm)(a) Pamb=0.1 MPa, Pinj=10 MPa
010 (mm) -100
10 (mm) -100
10 (mm)
10 (mm) -10(mm) 3.5 ms ASOI
-10

0

0
10
20
30

(mm) 3.5 ms ASOI
-100

10 ms ASOI
20 ms ASOI
29 ms ASOI(b) Pamb=1.0 MPa, Pinj=10 MPa
010 (mm) -100
10 (mm) -100
10 (mm)
10 (mm) -10

0
10
20
30

(mm) 3.5 ms ASOI

10 ms ASOI
20 ms ASOI(c) Pamb=1.0 MPa, Pinj=30 MPa

29 ms ASOI

Figure 3.3 Fuel adhesion evolution.
The fuel adhesion mass and area were calculated and depicted in Figure 3.4 and 3.5. The
horizontal axis is the time after start of injection, and the vertical axis is fuel adhesion mass andarea, respectively. For all conditions, when the spray starts, both the fuel mass and area increase.
Comparing with different conditions, it is clear that an increase of the ambient pressure decreases
the fuel adhesion mass and area, whereas increasing the injection pressure increases those. Thiscan be argued that the increased ambient pressure coalesces the droplets after impingement, thenthe bigger droplets are difficult to drop on the wall with the effect of vortex. However, the65increased injection pressure makes the droplets breakup into smaller ones, which causes more
droplets dropping on the wall after impingement.

Fuel Adhesion Mass (mg)

0.7
0.6

Injection Duration=1.36 ms
Injection Duration=2.4 ms

0.5
0.4
0.3

Pinj
Pamb
0.1 MPa-10 MPa
1.0 MPa-10 MPa
1.0 MPa-30 MPa

0.2
0.1
0

5
10
15
20
25
Time After Start of Injection (ms)30

Figure 3.4 Fuel adhesion mass.

Fuel Adhesion area (mm2)

1000
900

Injection Duration=1.36 ms
Injection Duration=2.4 ms

800
700
600
500

Pinj
Pamb
0.1 MPa-10 MPa
1.0 MPa-10 MPa
1.0 MPa-30 MPa

400
300
200
0

5
10
15
20
25
Time After Start of Injection (ms)30

Figure 3.5 Fuel adhesion area.
Time-resolved results of the maximum fuel adhesion thickness under different conditions
are presented in Figure 3.6. The horizontal axis is the time after start of injection, and the verticalaxis is the fuel adhesion maximum thickness. It can be seen the maximum thickness decrease66with an increase of ambient pressure. Stronger drag force decelerates the droplets greatly. As aresult, the droplets drop on wall more easily, and the fuel adhesion becomes thicker. On thecontrary, an increase of injection pressure increases the maximum fuel adhesion thickness due tothe enhancement of breakup and atomization.

Maximum thickness (μm)

2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
0

Pinj
Pamb
0.1 MPa-10 MPa
1.0 MPa-10 MPa
1.0 MPa-30 MPa
Injection Duration=1.36 ms
Injection Duration=2.4 ms
5
10
15
20
25
30
Time after Start of Injection (ms)Figure 3.6 Maximum fuel adhesion thickness.
In order to get more details about fuel adhesion thickness, the probability of fuel adhesion
thickness is depicted in Figure 3.7. The horizontal axis is fuel adhesion thickness, and thevertical axis is the probability of fuel adhesion thickness. The dotted lines show fuel adhesion
thickness of Pamb = 0.1 MPa, and the solid lines show that of Pamb = 1.0 MPa.
The probability based on fuel adhesion thickness is about the thickness of each pixel, andthe probability of fuel adhesion thickness must satisfy the normalization conditions:

¦

f
i 0

f M hi1(3.1)

where the sum of fuel adhesion mass in the thickness fraction between h - Δh and h is defined as

M h , f M h is the probability of M h , and Δh is 0.05 μm.
Firstly, the results at the ambient pressure of 0.1 MPa are examined. There are two peakvalues at 0.3 μm and 1.7 μm, respectively. Moreover, the thickness range is from 0 to 2.2 μm andinjection pressure has less effect on that. Then the results at the ambient pressure of 1.0 MPa are
examined. It is interesting to find that only one peak value exists under 1.0 MPa, and the peak67

value is larger than that of 0.1 MPa. This indicates that an increase of ambient pressure improves
the uniformity of adhesion thickness. Increasing ambient pressure under a certain temperature
raises the density of ambient gas, leading to an increase of the momentum transfer between thespray and the ambient gas. As a result, the spray angle increases and the droplet velocity
decreases. Consequently, the droplets drop on the wall more evenly, resulting in more uniform
fuel adhesion.

0.08

Pinj
Pamb
0.1 MPa-10 MPa
1.0 MPa-10 MPa
1.0 MPa-30 MPa

0.07

Probability

0.06

29 ms ASOI

0.05
0.04
0.03
0.02
0.01
0

0

0.2
-10

0.4
0

0.6

10 (mm)0.8
1
1.2 1.4 1.6
Fuel Adhesion Thickness (μm)-1010 (mm)

0

-10

0

1.8

2

10 (mm)

0

2.2

2.5
2.0
1.5

10

1.0
20

0.5
0
Thickness(μm)

30

(mm)

(a) Pamb=0.1 MPa,
Pinj=10 MPa(b) Pamb=1.0 MPa,
Pinj=10 MPa(c) Pamb=1.0 MPa,
Pinj=30 MPa

Figure 3.7 Probability of fuel adhesion mass along thickness.

683.2 EFFECT OF SURFACE ROUGHNESS
3.2.1 Experimental Conditions
Table 3.2 Experimental conditions
Injector Conditions
Injector Type

Mini-Sac, Single-Hole

Hole Type

Straight-Hole without Counterbore

L/D Ratio

4.8

Nozzle Hole Diameter (d)0.135 mm

Injection Conditions
Fuel

TolueneInjection Mass (Minj)

3.0 mgInjection Pressure (Pinj)

10, 20, 30 MPa

Injection Duration (td)

2.4, 1.65, 1.36 ms
Ambient Conditions

Ambient Gas

NitrogenAmbient Pressure (Pamb)
Ambient Temperature(Tamb)
Ambient Density (ρamb)

0.1, 1.0 MPa
300 K
1.19, 11.9 kg/m3

Impingement Conditions
Impingement Plate

Quartz Glass

Impingement Distance (Lw)

22 mm

Impingement Angle

45 deg

Surface Roughness

Ra2.5, Ra7.7 μm

The experimental conditions, given in Table 1, were determined considering the real operating
conditions of passenger car gasoline engines. The injection pressures varied from 10 to 30 MPa
and the ambient pressures ranged between 0.1 and 1.0 MPa, which means that the ambient gas
density changed between 1.19 and 11.9 kg/m3. The experiments were conducted with nitrogen atroom temperature. A mini-sac injector with a nozzle hole of diameter 0.135 mm was used; thenozzle is a conventional straight-hole without a counterbore. The length-to-diameter (L/D) ratio69

is 4.8. Toluene was employed as a surrogate fuel for gasoline. The injection mass was kept
constant at 3.0 mg under different conditions, resulting in different injection durations of 2.4,
1.65, and 1.36 ms at different injection pressures. The impingement angle was 45 deg and theimpingement distance was 22 mm from the nozzle exit to the impingement point of the flat wallalong the spray axis. The surface roughness of the new piston used in the engine isapproximately Ra1.0, but it may increase to Ra10.0 or more owing to combustion (Maligne andBruneaux, 2011). To investigate the effect of wall roughness, glass plates with two different
surface roughnesses, Ra2.5 and Ra7.7 μm, were chosen. Moreover, the surface roughness wasmeasured by a portable high-performance surface roughness and waviness measuring instrument(Kosaka Laboratory Ltd., SE300), whose resolution is 0.0064 μm.

3.2.2 Spray Impingement Characteristics
3.2.2.1 Spray impingement
To discuss the spray characteristics under different wall roughnesses, the ambient pressure wasset at 1.0 MPa and the injection pressure was set at 30 MPa. The spray and adhered fuel adhesion
images from 0.15 to 10 ms after start of injection (ASOI) with a wall roughness of Ra2.5 arepresented in Figure 3.8. The sprays are shown in false-color images acquired from the Miescattering experiment, and fuel adhesions are shown in the raw images obtained from the RIM
experiment. Owing to the Mie scattering of droplets above the wall, the bright area in the raw
images is the fuel spray during injection. The fuel adhesion, spray, and splash edge can be
identified by image edge detection due to different intensities of light. However, due to theoverlap of spray droplets cloud in the impinging region, the detection of fuel adhesion edge inraw images of RIM is still far from being achieved with accuracy during impingement.
Before impingement, the sprays are asymmetric because of the internal flow of the injector.
Wavy edges of the spray were observed that might be attributed to the air resistance force andturbulence in the spray (Wang et al., 2016). During impingement, there are fuel spray above thewall. At 0.6 ms ASOI, the fuel adhesion tip is shorter than the spray tip, indicating that most of
the fuel droplets splash above the wall instead of spreading on it. Fuel spray above the wall canalso be seen from the bottom-view image. At 1.4 ms ASOI, equivalent to 0.06 ms after end of
injection (AEOI), the fuel spray in the fuel adhesion image still can be observed. From 3.5 ms

70

ASOI, the fuel adhesion is formed on the wall without bright regions, but some droplets still
exist in the false color image.
Figure 3.9 shows spray and the adhered fuel images at a wall roughness of Ra7.7. The
tendencies of spray and fuel adhesion formation are the same as that at Ra2.5. The radial distanceafter impingement at Ra7.7 is smaller, and the impinging spray height is larger due to thestronger drag force of the wall. At 3.5 ms ASOI, more droplets are observed in the air above thewall from the false color images. This might be caused by two reasons. First, the stronger dragforce decelerates the spray, so that more droplets are left. Second, a higher roughness intensifies

False Color Image (Mie Scattering)
15

False Color Image (Mie Scattering)
15

False Color Image (Mie Scattering)
15

300

10

10

10

200

5

5

5

150

0

0

0

(mm) 1050-5(mm) 10-10 -1550-5-10 -15

Raw Image (RIM)

Raw Image (RIM)

Spray Edge

Spray Edge(a) 0.15 ms ASOI(b) 0.3 ms ASOI(mm) 10

5

0

-5

0

Raw Image (RIM)

Fuel Film
Splash Edge(c) 0.6 ms ASOI

False Color Image (Mie Scattering)
15

False Color Image (Mie Scattering)
15

10

10

10

5

5

5

0

0

5

0

-5

-10 -15

Raw Image (RIM)

Fuel Film(d) 1.4 ms ASOI(mm) 105

0-5-10 -15

Raw Image (RIM)

100
50-10 -15

False Color Image (Mie Scattering)15(mm) 10250

Light Intensity [A.U. ]the splashing process, resulting in droplets in the air.

0(mm) 105

0-5-10 -15

Raw Image (RIM)

Fuel Film

Fuel Film(e) 3.5 ms ASOI(f) 10 ms ASOI

Figure 3.8 Spray impingement and fuel adhesion images on Ra2.5 roughness wall (Pamb = 1.0
MPa, Pinj = 30 MPa).

71

False Color Image (Mie Scattering)
15

False Color Image (Mie Scattering)
15

10

10

10

5

5

5

0

0

0

(mm) 1050(mm) 10-10 -15-550(mm) 10-10 -15

-5

5

0

-5

Raw Image (RIM)

Raw Image (RIM)

Raw Image (RIM)

Spray Edge

Spray Edge

Fuel Film
Splash Edge(a) 0.15 ms ASOI(b) 0.3 ms ASOI

-10 -15

False Color Image (Mie Scattering)
15

10

10

10

5

5

5

0

0

0

0

-5

-10 -15

Raw Image (RIM)

Fuel Film(d) 1.4 ms ASOI(mm) 105

0-5-10 -15

Raw Image (RIM)

200
150
100
50

(c) 0.6 ms ASOI

False Color Image (Mie Scattering)
15

5

250

0

False Color Image (Mie Scattering)15(mm) 10300

Light Intensity [A.U. ]False Color Image (Mie Scattering)15(mm) 105

0-5-10 -15

Raw Image (RIM)

Fuel Film

Fuel Film(e) 3.5 ms ASOI(f) 10 ms ASOI

Figure 3.9 Spray impingement and fuel adhesion images on Ra7.7 roughness wall (Pamb = 1.0
MPa, Pinj = 30 MPa).
3.2.2.2 Spray tip penetration
Figure 3.10 shows spray tip penetrations under three conditions, and the dotted line is theimpingement distance. Three conditions were investigated (Pamb = 0.1 MPa, Pinj = 10 MPa, Pamb
= 1.0 MPa, Pinj = 10 MPa, and Pamb = 1.0 MPa, Pinj = 30 MPa). Generally, the spray development
can be divided into two stages. It increases almost linearly before impingement, and the gradientof S decreases after spray impingement. The splashing due to the wall impingement, drag force
from the wall and ambient gas can be regarded as the main reasons for this phenomenon.
However, the gradient decreases before impingement when Pamb = 1.0 MPa, and the air
resistance of high ambient density should be responsible for it (Li et al., 2012). More importantly,
after impingement, S of Ra2.5 is larger than that of Ra7.7 under all conditions. Due to stronger
drag force on Ra7.7 wall, spray velocity decreases more rapidly. Besides, under different
ambient pressures, S of 0.1 MPa is larger than that of 1.0 MPa, and impingement time of 0.1
72

MPa is earlier than 1.0 MPa that agrees well with other results (Wang et al., 2016). It can be
expected that higher ambient pressure decreases S by transferring the spray momentum to theambient gas, and stronger air resistance force is expected with higher ambient pressure. Under
different injection pressures, S of 30 MPa is larger than that of 10 MPa, and impingement time of
30 MPa is earlier than that of 10 MPa due to higher spray velocity.
40

Ra2.5
Ra7.7

35

Spray Tip Penetration (mm)Spray Tip Penetration (mm)

40
30
25

Impingement Distance

20
15
10
Injection Duration = 2.4 ms

5
0

0

0.2
0.4
0.6
0.8
1.0
Time after Start of Injection (ms)(a) Pamb = 0.1 MPa, Pinj = 10 MPa

Spray Tip Penetration (mm)

40

1.2

Ra2.5
Ra7.7

35
30
25

Impingement Distance

20
15
10
Injection Duration = 2.4 ms

5
0

0

0.2
0.4
0.6
0.8
1.0
Time after Start of Injection (ms)(b) Pamb = 1.0 MPa, Pinj = 10 MPa

1.2

Ra2.5
Ra7.7

35
30
25

Impingement Distance

20
15
10
Injection Duration = 1.36 ms

5
0

0

0.2
0.4
0.6
0.8
1.0
Time after Start of Injection (ms)(c) Pamb = 1.0 MPa, Pinj = 30 MPa

1.2

Figure 3.10 Spray tip penetration under different conditions.
3.2.2.3 Impinging spray height
Figure 3.11 presents impinging spray heights under three conditions (the same as spray tip
penetration). The horizontal axis is the time after start of injection, and the vertical axis is theimpinging spray height Hi.
73

Ra2.5
Ra7.7

7
6
5
4
3
2

Injection Duration = 2.4 ms

1
0

0

0.4

0.8

1.2

1.6

2.0

Impinged Spray height (mm)Impinged Spray Height (mm)

8

2.4

8

Ra2.5
Ra7.7

7
6
5
4
3
2

Injection Duration = 2.4 ms

100Time after Start of Injection (ms)(a) Pamb = 0.1 MPa, Pinj = 10 MPa

Impinged Spray height (mm)

8

0.4

0.8

1.2

1.6

2.0

2.4

Time after Start of Injection (ms)(b) Pamb = 1.0 MPa, Pinj = 10 MPa

Ra2.5
Ra7.7

7
6
5
4
3
2

Injection Duration = 1.36 ms

1
0

0

0.4

0.8

1.2

1.6

2.0

2.4

Time after Start of Injection (ms)(c) Pamb = 1.0 MPa, Pinj = 30 MPa

Figure 3.11 Impinging spray height under different conditions.
Under all conditions, Hi of Ra7.7 is larger than that of Ra2.5. Owing to higher wallroughness, the wall has a stronger drag force, so the number of droplets formed from
impingement increases, resulting in an increase of Hi. In contrast to ambient pressures, Hi of 0.1
MPa is larger than that of 1.0 MPa. Although increasing the pressure can increase the spray angle(Guan et al., 2015), resulting in more droplets above the wall. Higher ambient pressure increases
the density of the ambient atmosphere, resulting in a decrease in the droplet splashing, thus
decreasing Hi. In comparison with the injection pressures, Hi of 30 MPa is larger than that of 10
MPa, it can be expected that the increased injection pressure results in much more droplets above
the wall than 10 MPa. It is noteworthy that the difference caused by varying the roughness is74

larger when ambient pressure is lower. It can be observed that the cases with low ambientpressure show higher sensitivity to the variation of wall roughness due to stronger splashingprocess.

3.2.3 Fuel Adhesion Characteristics
3.2.3.1 Fuel adhesion evolution
Figure 3.12 shows the evolution of the adhered fuel on walls of different roughnesses under anambient pressure of 1.0 MPa and injection pressure of 30 MPa. To observe the results clearly,
fuel adhesion images undergo 90° clockwise rotation. The fuel adhesions formed on the Ra2.5
and Ra7.7 walls at 3.5, 10, 20, and 29 ms ASOI are shown. The pseudo color represents the fueladhesion thickness varying from 0 to 2.5 μm. Further, the cross symbol is the impingement point.
-100

10 (mm) -100

10 (mm) -100

10 (mm) -100

10 (mm)
2.5

0

2.0
1.5

10

1.0
20

0.5

30

(mm) 3.5 ms ASOI
-100

10 ms ASOI

0
29 ms ASOI Thickness (μm)

20 ms ASOI(a) Ra2.5
10 (mm) -100

10 (mm) -100

10 (mm) -100

10 (mm)
2.5

0

2.0
10

1.5
1.0

20

0.5
30

(mm) 3.5 ms ASOI

10 ms ASOI

20 ms ASOI

0
29 ms ASOI Thickness (μm)(b) Ra7.7
Figure 3.12 Fuel adhesion evolution on different roughness walls (Pamb = 1.0 MPa, Pinj = 30
MPa).

For Ra2.5, all cases at different time show similar structures. In the initial stage, thethickness is almost uniform; however, from 3.5 ms ASOI fuel adhesion becomes moderately
thicker at the downstream. At the bottom region, there is a cluster of droplets caused bysecondary breakup of fuel adhesion on the impingement wall surface (Zama et al., 2014). For
Ra7.7, fuel adhesion thickness is not uniform in the initial stage. The non-uniform tendency is75

more obvious with time. The thicker fuel adhesion area is located at the upstream. For both
Ra2.5 and Ra7.7, the fuel adhesion areas are almost symmetric, and they increase with time. The
increased area can be expected that there are still some droplets impinging on the wall even after
the end of injection that can also be seen from the Mie scattering results in Figure 3.7 and 3.8.
The difference of thicker adhesion areas between Ra2.5 and Ra7.7 might be due to the different
drag forces of the wall. For Ra2.5, high velocity droplets impinge on the wall then splash andrebound. Some droplets above the wall collide with others and velocity directions change;
because of the vortex, they drop on the wall again, and this might result in some thicker area on
the downstream. However, when the wall roughness increases, the velocities of the splashing andrebounding droplets decrease considerably (Mundo et al., 1995). After collision, velocity
directions of these droplets change and they drop on the wall readily, and this might beresponsible for the fuel thicker area on the upstream.
3.2.3.2 Fuel adhesion mass and area
The fuel adhesion mass and area are calculated and depicted in Figure 3.13. The horizontal
axis is the time after start of injection. The left vertical axis of the above figure is the fueladhesion mass, and the right vertical axis is the ratio of fuel adhesion mass to total fuel mass.
The left vertical axis of the below figure is fuel adhesion area.
For all conditions, when the spray starts, both the fuel mass and area increase. Finally, themass and area of the fuel adhesion at Ra7.7 are higher than those at Ra2.5. One possible reason
is that when fuel spray impinges on a wall with higher roughness, the splashing velocity ofdroplets become smaller due to the reduction of the horizontal velocity component (Mundo et al.,1995), resulting in more fuel left on the wall. Comparing with different conditions, when Pamb =0.1 MPa and Pinj = 10 MPa, the fuel adhesion mass and area of Ra7.7 are moderately larger thanthose of Ra2.5, as shown in Figure 3.13 (a). However, such tendency becomes weaker when theambient pressure is increased to 1.0 MPa, as shown in Figure 3.13 (b). On the contrary, thetendency becomes stronger when the injection pressure is increased to 30MPa, as shown in
Figure 3.13 (c). This indicates that increasing the ambient pressure decreases the differences in
the fuel mass and area between two kinds of walls, whereas increasing the injection pressure
increases the differences. It can be argued that the increased ambient pressure decelerates thedroplets after the splashing and rebounding processes for two types of walls that causes more76

droplets to promptly drop on the wall. However, the increased injection pressure accelerates thedroplets, which causes fewer droplets to drop on the wall after the splashing and rebounding
processes. Further, this accelerating effect is more significant for the wall with a smallerroughness. More importantly, both mass and area of the fuel adhesion increase even AEOI for all
the conditions, and the droplets in the air impinging on the wall should be the reason for that.

13.3%

0.3

10%

0.2

6.6%

0.1

3.3%

0
800
700
600
500
400
300 Injection Duration = 2.4 ms
2000

5
10 15 20 25 30
Time after Start of Injection (ms)(a) Pamb = 0.1 MPa, Pinj = 10 MPa

Fuel Adhesion Mass (mg)

0.4

Fuel Adhesion Area (mm2)

16.6%

0

Fuel Adhesion Area (mm2)

20%

77

0.6
0.5

Ra2.5
Ra7.7

20%
16.6%

0.4

13.3%

0.3

10%

0.2

6.6%

0.1

3.3%

0

0.5

Ra2.5
Ra7.7

0
800
700
600
500
400
300 Injection Duration = 2.4 ms
2000

5
10 15 20 25 30
Time after Start of Injection (ms)(b) Pamb = 1.0 MPa, Pinj = 10 MPa

Adhesion Mass/Total ×100%

0.6

Adhesion Mass/Total ×100%

Fuel Adhesion Mass (mg)

The same phenomena can also be seen in (e) and (f) of Figures 3.8 and 3.9.

Fuel Adhesion Mass (mg)

16.6%

0.4

13.3%

0.3

10%

0.2

6.6%

0.1

3.3%

0

Fuel Adhesion Area (mm2)

0.5

20%

Ra2.5
Ra7.7

0
800

Adhesion Mass/Total ×100%

0.6

700
600
500
400
300 Injection Duration = 1.36 ms
2000

5
10 15 20 25 30
Time after Start of Injection (ms)(c) Pamb = 1.0 MPa, Pinj = 30 MPa

Figure 3.13 Fuel adhesion mass and area under different conditions.
3.2.3.3 Fuel adhesion thickness
Time-resolved results of the maximum fuel adhesion thickness under different conditions arepresented in Figure 3.14. The horizontal axis is the time after start of injection, and the verticalaxis is the fuel adhesion maximum thickness.
It can be seen that the maximum thickness values of Ra7.7 are above 1.6 μm, whereas thoseof Ra2.5 are below 1.6 μm. This is due to the effect of wall surface roughness on droplets.
Stronger drag force decelerates the droplets substantially. As a result, the droplets drop on thewall readily, and the fuel adhesion becomes thicker. Furthermore, it is remarkable that anincrease of ambient pressure decreases the maximum fuel adhesion thickness. One possible
reason is that the increased spray angle results in a uniform distribution of droplets on the wall.
On the contrary, an increase of injection pressure increases the maximum fuel adhesion thickness
due to better dispersion (Wang et al., 2016).
In order to obtain more details about fuel adhesion thickness, the probability of fueladhesion thickness is depicted in Figure 3.15. The horizontal axis is fuel adhesion thickness, and78

the vertical axis is the probability. The dotted lines represent fuel adhesion thickness of Ra2.5,
and the solid lines represent that of Ra7.7.

Maximum Thickness (μm)

2.2
Pamb

2.0

Pinj

0.1 MPa-10 MPa
1.8

1.0 MPa-10 MPa
Ra7.7

1.0 MPa-30 MPa

Ra2.5

0.1 MPa-10 MPa

1.6
1.4

1.0 MPa-20 MPa
1.0 MPa-30 MPa

1.2
1.0

0

5

10
15
20
25
Time after Start of Injection (ms)30

Figure 3.14 Maximum thickness of fuel adhesion under different conditions.

Probability of Thickness

0.16
0.14 Pinj = 10 MPa
Pinj = 20 MPa
0.12
Pinj = 30 MPa
0.10

Ra2.5 Ra7.7

29 ms ASOI

0.08
0.06
0.04
0.02
0

0

0.2

0.4

0.6 0.81
1.2 1.4 1.6
Fuel Adhesion Thickness (μm)(a) Pamb = 0.1 MPa

79

1.8

2

2.2

Probability of Thickness

0.16
0.14 Pinj = 10 MPa
Pinj = 20 MPa
0.12
Pinj = 30 MPa
0.10

Ra2.5 Ra7.7

29 ms ASOI

0.08
0.06
0.04
0.02
0

0

0.2

0.4

0.6 0.81
1.2 1.4 1.6
Fuel Adhesion Thickness (μm)

1.8

2

2.2

(b) Pamb = 1.0 MPa
Figure 3.15 Probability of fuel adhesion thickness (29 ms ASOI).
Firstly, the results at the ambient pressure of 0.1 MPa are examined. For Ra2.5, there are
two peak values at 0.7 μm and 1.1 μm, respectively. For Ra7.7, there are also two peak values at
0.3 μm and 1.7 μm. Moreover, the thickness range of Ra7.7 is from 0 to 2.2 μm that is wider thanthat of Ra2.5. Two peak values of Ra2.5 are considerably bigger than that of Ra7.7. This
indicates that the fuel adhesion is uniform for a wall of smaller roughness. One possible reason isthat for small roughness of wall, some droplets impinge on the wall, and then spread out readily.
Whereas, for the wall with larger roughness, uniform spread is difficult. This result can also beseen in Figure 3.12.
The results at the ambient pressure of 1.0 MPa are then examined. It is interesting to findthat only one peak value exists at 1.0 MPa, and the peak value is larger than that of 0.1 MPa.
This indicates that an increase in ambient pressure improves the uniformity of adhesion thickness.
Increasing ambient pressure under a certain temperature increases the density of ambient gas,leading to an increase in the momentum transfer between the spray and the ambient gas. As aresult, the spray angle increases and the droplets velocities decrease (Guan et al., 2015). The
increased spray angle leads to better atomization and more fuel impinging on fuel uniformly.
Moreover, the droplets with a decelerated velocity impinge on the wall and spread out easily to80form a fuel adhesion. Even the splashing droplets with decelerated velocity can impinge on thewall again readily with the help of vortex, resulting in more uniform fuel adhesion.
To explain the wall roughness effect on fuel adhesion uniformity more clearly, a cumulative
distribution function F(x), as shown in Eq. (3.2) is used.

F(x)

¦

x

i 0

(3.2)

f M hiIn Figure 3.16, the horizontal axis is fuel adhesion thickness, and the vertical axis is F(x).
F(x) increases with fuel adhesion thickness. The dotted lines represent Ra2.5, and the solid lines
represent Ra7.7. If the fuel adhesion is completely uniform, the cumulative distribution function
F(x) is a straight line perpendicular to the horizontal axis, shown as in a dashed dotted line. If thefuel adhesion is completely non-uniform, the cumulative distribution function F(x) is a straightline with a certain slope to the horizontal axis, shown as in a dashed double-dotted line.

Completely Uniform
Completely Non-uniform

F(x)

110%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

Ra2.5 Ra7.7
Pinj = 10 MPa
Pinj = 20 MPa
Pinj = 30 MPa
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Fuel Adhesion Thickness (μm)(a) Pamb = 0.1 MPa

81

Completely Uniform
Completely Non-uniform

F(x)

110%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

Ra2.5 Ra7.7
Pinj = 10 MPa
Pinj = 20 MPa
Pinj = 30 MPa
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Fuel Adhesion Thickness (μm)(b) Pamb = 1.0 MPa
Figure 3.16 Cumulative distribution function curves (29 ms ASOI).
For the ambient pressure of 0.1 MPa, the curves of Ra2.5 increase readily than that of Ra7.7.
The curves of Ra2.5 reach to 100% at about 1.3 μm, whereas the curves of Ra7.7 reach to 100%
at about 2.0 μm. Furthermore, the curves of Ra2.5 are closer to the completely uniform curve,
whereas the curves of Ra7.7 are closer to the completely non-uniform curve, indicating that thefuel adhesion of Ra2.5 is considerably more uniform than that of Ra7.7. The curves withdifferent injection pressures are similar to each other, indicating that the injection pressure hasless effect on the uniformity of fuel adhesion thickness than wall roughness.
For an ambient pressure of 1.0 MPa, the tendencies are similar to those of 0.1 MPa, exceptthat the curves for Ra2.5 under 1.0 MPa are substantially closer to the completely uniform curve
and the curves for Ra7.7 under 1.0 MPa are considerably farther from the completely nonuniform curve, which also proves that the ambient pressure promotes uniformity in the fueladhesion thickness.
Further investigation was conducted to clarify the effect of wall roughness on fuel adhesion
thickness along different lines. In Figure 3.17, the coordinate system is defined, and the origin ois determined as the impingement point. The positive y axis is along the direction of fuel splash
after impingement, and positive x axis is perpendicular to the y axis, pointing to the left. Because82fuel adhesion is almost symmetric about x = 0 axis, we only discuss the adhesion thickness alongx = 0 line. For horizontal axis, we divided fuel adhesion by y = 0, 10 and 20 mm lines that
represent the upstream, midstream, and downstream of the fuel adhesion.
x (mm)
0

-10

10

-5
y = 0 mm

0y (mm)

Upstream10

y = 10 mm
Midstream20

y = 20 mm
Downstream30

x = 0 mmFigure 3.17 Definition of fuel adhesion thickness along different lines.

-5
0
5

15

x = 0 mm

y (mm)

1020
25

Fuel Adhesion Thickness (μm)

Ra2.5
Ra7.7

2.0
1.5
1.0
0.5
0

Ra2.5
Ra7.7
y = 0 mm

2.0
1.5
1.0
0.5
0

Ra2.5
Ra7.7
y = 10 mm

2.0
1.5
1.0
0.5
0

Ra2.5
Ra7.7
y = 20 mm

30
0 0.5 1.0 1.5 2.0
Fuel Adhesion Thickness (μm)

-15

-10

-5

0
x (mm)

51015(a) Fuel adhesion thickness along x = 0 line (b) Fuel adhesion thickness along y = 0, 10 and 20 mm lines

Figure 3.18 Fuel adhesion thickness along different lines (29 ms ASOI, Pamb = 1.0 MPa, Pinj =30 MPa).
83

To express concisely, the fuel adhesion thickness along x = 0 line was defined as x0. The
fuel adhesion thickness along y = 0, 10, and 20 lines were defined as y0, y10, and y20,
respectively. In the following discussion, the terms of x0, y0, y10 and y20 are used to represent
the fuel adhesion thickness along these different lines. The experimental condition discussed inthis section is at 29 ms ASOI, Pamb = 1.0 MPa, and Pinj = 30 MPa, as shown in Figure 3.17.
Figure 3.18 (a) shows the fuel adhesion thickness along x = 0 line. The horizontal axis isfuel adhesion thickness, and the vertical y axis is from -5 to 30 mm. For Ra2.5, x0 increases from
-5 to -2.5 mm and remains almost constant from -2.5 to 20 mm; it finally decreases with large
fluctuations. For Ra7.7, x0 increases from -5 to -2.5 mm and remains almost constant from -2.5
to 10 mm; it finally decreases with considerably larger fluctuations than those of Ra2.5. At theconstant value stage, x0 of Ra7.7 is larger than that of Ra2.5. The fluctuation of Ra7.7 issignificantly remarkable than that of Ra2.5. This indicates that the fuel adhesion is more uniform
for the smaller roughness of the wall.
Figure 3.18 (b) shows the fuel adhesion thickness along different y lines. The horizontal x
axis is from -15 to 15 mm, and the vertical axis is the fuel adhesion thickness. For Ra2.5, y0, y10,
and y20 increase from -15 to -5 mm, then remains almost constant from -5 to 5mm, and finallydecreases. At the constant value stage, y0 are similar to y10, but smaller than y20. The
fluctuation of y0 is similar to that of y10, but smaller than that of y20. This indicates that, forRa2.5, fuel adhesion thickness downstream is larger, and the uniformity of fuel adhesion
thickness downstream is considerably inferior to that in the upstream and midstream.
For Ra7.7, the tendency is similar to Ra2.5, except that the value at the constant stage isslightly different. At the constant stage, y0 of Ra7.7 is larger than that of Ra2.5, whereas y10 and
y20 of Ra7.7 is almost the same as those of Ra2.5. This suggests that, the fuel adhesion thicknessof upstream is thicker for larger roughness of the wall. For y0, y10, and y20, the fluctuations ofRa7.7 are significantly remarkable than those of Ra2.5. This indicates that the fuel adhesion ismore uniform for smaller roughness of the wall.

84

3.3 EFFECT OF IMPINGEMENT DISTANCE
3.3.1 Experimental Conditions
The reduction of particulate number (PN) emissions is the major concern of a gasoline DI engine
due to the introduction of PN standards in Euro 6 emission regulations. During the catalystwarm-up operation, the retarded injection timing is used to increase the exhaust gas temperature.
Although multiple-injection strategy is used to reduce the wall wetting, the late injection near top
dead center (TDC) resulted in an increase in PN emissions. To clarify the spray-wall interactions
under the aforementioned engine conditions, the equivalent experimental conditions in theconstant volume chamber were determined in Table 3.3.
Table 3.3 Experimental conditions
Injection Conditions
Fuel

TolueneInjection Mass (Minj)

3.0 mgInjection Pressure (Pinj)

10, 20, 30 MPa

Injector Type

Mini-Sac, Single-HoleNozzle Hole Diameter (d)0.135 mm

Injection Duration (td)

2.4, 1.65, 1.36 ms

Ambient Conditions
Ambient Gas

NitrogenPressure (Pamb)

0.5 MPa

Temperature (Tamb)

300 K

Density (ρamb)

5.95 kg/m3

Impingement Conditions
Impingement Plate

Quartz Glass

Impingement Distance (Dimp)

28, 40 mm

Impingement Angle (θimp)

45°

Surface Roughness

Ra7.5 μm

85

The fuel tested in this study was toluene, as a surrogate fuel for gasoline. The injection mass
was kept constant at 3.0 mg equivalent to the total injection quantity of the multi-hole injector
divided by hole number. The injection pressures changed from 10 to 30 MPa considering thephenomena, resulting in different injection durations of 2.4, 1.65, and 1.35 ms. The ambientdensity of non-evaporating conditions is kept the same as that of in-cylinder conditions. A minisac injector with a single hole (0.135 mm) was used. To determine the effect of impingement
distance on fuel adhesion, it ranged between 28 and 40 mm from the nozzle exit to the wall along
the spray axis. The impingement angle was 45 deg from the spray axis to the flat wall. The
surface roughness of the new piston used in gasoline engine is approximately Ra1.0, but it may
increase up to Ra10.0 or more due to deposit accumulation (Luo et al., 2017). Therefore, a circle
plate made of quartz glass with surface roughness of Ra7.5 μm was placed under the injector as aflat wall, representing a used piston in the engine. As shown in Figure 3, the diameter of the plate
was 50 mm and its thickness was 2 mm. The coordinate system was defined, and the intersection
point o of the spray axis and the wall was defined as the impingement point.

3.3.2 Spray Impingement Characteristics
Figure 3.19 shows Rs under three conditions (Pinj = 10, 20, and 30 MPa). The horizontal axis is
the time after start of injection, and Rs is in the vertical axis. Rs of Dimp = 28 mm is larger thanthat of Dimp = 40 mm under all conditions. The ambient pressure (Pamb = 0.5 MPa) results instrong interaction between the fuel and nitrogen, decelerating the droplets before impingement.
During the spray propagation at the shorter impingement distance, the fuel with higher Weber
number diffuses around after impingement on the wall. As a result, Rs of Dimp = 28 mm is larger
than that of Dimp = 40 mm. Moreover, Rs increases with an increase in the injection pressure. It
can be expected that the increased injection pressure enhances the kinetic energy of fuel.

86

Figure 3.19 Impinging spray radius, Rs (Front view)
Figure 3.20 shows Hv under three conditions (Pinj = 10, 20, and 30 MPa). The horizontal
axis is the time after start of injection, and Hv is in the vertical axis. Hv of Dimp = 28 mm is larger
than that of Dimp = 40 mm under all conditions. Owing to its shorter impingement distance, thefuel with higher Weber number and momentum impinges on the wall, leading to a decreasing
number of droplets depositing on the wall, and an increasing number of splashing droplets (Baiet al., 1995). Therefore, Hv of Dimp = 28 mm is larger than that of Dimp = 40 mm. In contrast to
the different injection pressures, Hv increases more rapidly with an increase in injection pressure.
It can be expected that the increased injection pressure enhances the Weber number and theinitial kinetic energy of the fuel spray, resulting in faster vortex generation.

87

Figure 3.20 Impinging vortex height, Hv (Front view)
Figure 3.21 presents S, and the impingement distance is shown by the broken line. Three
conditions (Pinj = 10, 20, and 30 MPa) were investigated. For all conditions, the spraydevelopment can be divided into two stages. It increases almost linearly before impingement,
and the gradient of S decreases after spray impingement (Luo et al., 2017). The drag force fromthe wall and ambient gas can be regarded as the main reasons for this phenomenon. It isnoteworthy that before impingement, S is almost the same under different impingement distances.
However, after impingement, S of Dimp = 40 mm is slightly larger than that of Dimp = 28 mm
under all conditions. One possible reason is that, after impingement, the fuel disperses andspreads in all directions, with the result that the interaction between the droplets and air is muchstronger than before. Moreover, friction from the wall can be regarded as another reason. As aresult, S increases with the increased impingement distance. Additionally, under different
88injection pressures, the S of 30 MPa is larger than that of 10 MPa, and the impingement time of
30 MPa is shorter than that of 10 MPa owing to higher Weber number of the spray.

Figure 3.21 Spray tip penetration, S (Side view)
Figure 3.22 presents Hi under three conditions (Pinj = 10, 20, and 30 MPa). The horizontal
axis is the time after start of injection, and Hi is in the vertical axis. It is clear that Hi of Dimp = 28mm is larger than that of Dimp = 40 mm under all conditions. This phenomenon can be attributed
to the different momentum of the droplets impinging on the wall. The droplets of Dimp = 28 mm
have higher Weber number when impinging on the wall, and many droplets splash around after
impingement, resulting in larger Hi compared with that of Dimp = 40 mm. In contrast to different

89conditions, Hi increases and the gradient of Hi becomes larger owing to the effect of theenhanced kinetic energy of fuel.

Figure 3.22 Impinging spray radius, Rs (Front view)903.3.3 Fuel Adhesion Characteristics

Figure 3.23 Fuel adhesion evolution (Bottom view)
During the spray, some scattered light from the floating droplets above the impingement region.
In order to eliminate the stray light error, all results are after the end of injection (EOI) (Luo etal., 2017).
91

Figure 3.23 shows the evolution of the adhered fuel under different conditions. The fueladhesions on the wall at 5, 10, 20, and 40 ms ASOI are shown. The pseudocolor represents theadhered fuel thickness, varying from 0 to 2.5 μm, and the impingement point is shown by thecross symbol. Under each condition, all cases at different times show similar structures, and thefuel adhesion areas are almost symmetric. It is evident that the wetted area increases underhigher injection pressure, and the better atomization of a high injection pressure should be
responsible for this. More importantly, when Dimp = 28 mm, the fuel adhesion becomes a little
thicker at the upstream, whereas when Dimp = 40 mm, the thicker region moves downstream.
There may be two reasons for this. One is that there are different impact regimes for droplets
impinging on the wall: “stick,” “spread,” and “splash” (Bai et al., 1995). The increased
impingement distance decelerates the droplets owing to the ambient pressure (Pamb = 0.5 MPa).
Thus, after impingement on the wall, some droplets may change their behavior from “splash” to“spread,” or even to “stick”, resulting in thicker fuel adhesion of Dimp = 40 mm. The second
reason may be that even though some droplets splash off the wall, the droplets with low velocity
easily drop back on the wall, causing the thicker fuel adhesion to move down.

Figure 3.24 Fuel adhesion mass

92

Figure 3.25 Fuel adhesion area
The fuel adhesion mass and area are depicted in Figures 3.24 and 3.25. The adhesion mass
ratio is defined as the ratio of adhesion mass to total injection mass. The fuel adhesion mass andarea increase with time even after the end of injection because there are still some droplets
dropping on the wall. An increase in injection pressure under a certain ambient pressure
increases the fuel adhesion mass and area owing to better atomization. Furthermore, both fueladhesion mass and area increase at a large impingement distance. A decrease in the number ofsplashing droplets tends to be responsible, which agrees well with the Mie scattering results.
Additionally, the bigger spray width and better atomization should be other reasons for this. Parket al. (2004) have already proven that the spray width increases as the spray flows downstream,
and the SMD decreases at a large impingement distance. Therefore, the wider spray and better
atomization of Dimp = 40 mm is formed before impingement, resulting in bigger adhesion areaand mass on the wall. The same tendency of the wider fuel adhesion at Dimp = 40 mm, can also
be observed in the comparison between Figures 3.23 (b) and (c).
Figure 3.26 shows the adhesion mass ratio under different conditions at 40 ms ASOI. For
Dimp = 28 mm, the adhesion mass ratio increases from 2.9% to 3.7% with an increase in injection
pressure. However, by increasing the Dimp to 40 mm, the ratio increases from 7.8% to 8.6% atincreased injection pressures, and thus, the ratios of Dimp = 40 mm are more than twice at Dimp =28 mm.
93

Figure 3.26 Adhesion mass ratio (40 ms ASOI)

Figure 3.27 Probability of thickness with time (Pinj = 30 MPa, Dimp = 28 mm)
To further investigate the adhesion thickness, the probability of thickness was determined.
As shown in Figure 3.27, the horizontal axis is the fuel adhesion thickness, and the vertical axis
is the probability of mass. Figure 3.27 describes the probability of thickness with different times
at Pinj = 30 MPa and Dimp = 28 mm, and the average values of three shots were presented. It isreported that the peak value of the curves decreases, and the curve becomes a little wider with94

time, which indicates that the uniformity of adhesion thickness becomes worse with time. And
the same observation can be derived from Figure 3.23 (b). One possible explanation could bethat some rebounding and splashing droplets fall on the wall. 

Figure 3.28 Probability of fuel adhesion thickness (40 ms ASOI)
Figure 3.28 illustrates the effects of impingement distance and injection pressure on
adhesion thickness, and the average values of three shots were presented. First, the results of Dimp
= 28 mm are examined. Only one peak value exists and the increased injection pressure causes
the curve to shift to the left, leading to thinner fuel adhesion. It can be argued that high injection
pressure improves the atomization and dispersion of droplets, resulting in thinner fuel adhesion,and the same conclusion can be drawn from the comparison between Figures 3.23 (a) and (b).
Secondly, there are two peak values with Dimp = 40 mm, which indicates that an increase in
impingement distance deteriorates the uniformity of adhesion thickness. The transition of “splash”
to “spread” and “stick” phenomenon could be a possible explanation for this. The sameobservation can also be noted from the comparison of Figures 3.23 (b) and (c). It is interesting tofind that the maximum thickness with Dimp = 40 mm is approximately 2.2 μm, and it is larger
than that with Dimp = 28 mm. The main possible reason is that the number of splashing droplets
decreases owing to low velocity at a large impingement distance.

95

Figure 3.29 Fuel adhesion thickness distribution (40 ms ASOI)
Further investigation was carried out to clarify the effect of impingement distances on the
distribution of fuel adhesion thickness (shown in Figure 3.29). The fuel adhesion was divided by
y = 0, 10, and 20mm lines that represent the upstream, midstream, and downstream of the fueladhesion. When Dimp = 28 mm, the fuel adhesion thickness decreases from upstream todownstream, and the uniformity of thickness improves from upstream to downstream. However,when Dimp = 40 mm, the fuel adhesion thickness increases from upstream to midstream, and thendecreases from midstream to downstream. The changed regime of the droplets, from “splash” to“spread” with a large impingement distance is a possible explanation for that distribution pattern.
Furthermore, when Dimp = 40 mm, the thickness uniformity on upstream is similar to that
occurring on midstream, but both uniformities are worse than that located on downstream. In
contrast to other impingement distances, the fuel adhesion on upstream and midstream becomes
thicker with a large impingement distance, and the uniformity of thickness deteriorates with alarge impingement distance. On the contrary, the fuel adhesion on downstream shows a slightchange.

96

3.4 SUMMARY
The characteristics of the fuel spray and fuel adhesion at different pressures, wall roughnesses
and impingement distances were investigated experimentally. The spray tip penetration andimpinging spray height were acquired, the fuel adhesion evolution was analyzed, and thethickness distribution of fuel adhesion thickness and along different lines were discussed. The
conclusion and summary are listed as follows:
1. According to the above discussion, the fuel adhesion mass and area increase with time evenafter the end of injection. Both ambient and injection pressure favor the atomization of spray.
The ambient pressure improves the uniformity of the fuel adhesion, while the injection pressure
has little effect on it.
2. The tip of fuel adhesion is shorter than the spray tip because fuel droplets splash above thewall after impingement. Fuel droplets still exist in the air above the wall even after the end of
injection. An increase in wall roughness decreases the spray tip penetration and increases theimpinging spray height.
3. An increase in wall roughness increases the mass, area and maximum thickness of the fueladhesion, and it deteriorates the uniformity of the fuel adhesion. The fuel adhesion becomesslightly thicker in downstream for Ra2.5, whereas the thicker area moves upstream for Ra7.7.
4. With a large impingement distance under ambient condition, the velocity of droplets decreases
significantly, resulting in more droplets adhering to the wall instead of splashing out of the wall.
As a result, Rs, Hv, and Hi decrease with an increase in impingement distance. However, after
impingement, owing to the stronger drag force by the ambient gas and friction from the wall, S
increases with the increase in impingement distance.
5. At a large impingement distance, the lower velocity, bigger spray width, and better
atomization are the main reasons for increased fuel adhesion on the wall after impingement.
Under the large impingement distance condition, more fuel adheres on midstream and thethickness uniformity of fuel adhesion becomes worse. Moreover, the maximum thickness of fueladhesion increases with a large impingement distance.

97

CHAPTER 4 FUEL ADEHISON FORMATION UNDEREVAPORATION CONDITIONS
4.1 EFFECT OF AMBIENT TEMPERATURE
A series of extensive studies have been conducted on fuel adhesion characteristics under nonevaporation conditions in chapter 3. However, the air temperature in a real engine during
operation is quite high, especially when the engine is under high load. Until now, the formationof fuel adhesion on the wall is still ambiguous and requires more research. In order to enhance
our understanding, the formation of fuel adhesion and the mechanisms behind it will be
investigated under evaporation conditions in this chapter.

4.1.1 Experimental Conditions
The injector parameters and impingement conditions are listed in Table 4.1. A mini-sac injector
with a nozzle hole diameter of 0.155 mm was used. The nozzle was a conventional straight-hole
type without a counterbore, and the length-to-diameter (L/D) ratio was 4.2. The impingement
angle was 45 deg, and the impingement distance of 22 mm from the nozzle exit to the point of
impingement along the spray axis was the same as the distance near top dead center (TDC). The
surface roughness of the new piston used in gasoline engine is approximately Ra (arithmeticalmean deviation of the profile) of 1.0 μm, but it may increase up to Ra10.0 μm or more due todeposit accumulation [Luo et al., 2017; Maligne et al., 2011]. Therefore, the flat wall was madeof quartz glass (Sigma Koki, DFSQ1-50CO2) with a surface roughness Ra7.0 μm, measured by aportable high-performance surface roughness and waviness measuring instrument (Kosaka
Laboratory Ltd., SE300) with a resolution of 0.0064 μm to represent the rough surface of thepiston. As shown in Figure 2.3, the impingement plate with a diameter of 50 mm and a thicknessof 2 mm was placed under the injector. The point of intersection (o) of the nozzle center axis andthe wall was defined as the impingement point. The direction of spray after impingement is the
positive y axis, while the positive z axis is parallel to the injector, and the x axis is defined as theperpendicular direction to the yz plane, pointing out of the figure.

98

Table 4.1 Injector parameters and impingement conditions
Injector Parameters
Injector Type

Mini-Sac

Hole Number

1

Hole Type

Straight-Hole without Counterbore

L/D Ratio

4.2

Nozzle Hole Diameter (mm)0.155

Impingement Conditions
Impingement Plate

Quartz Glass

Impingement Distance (mm)22

Impingement Angle (°)

45

Surface Roughness (μm)

Ra 7.0

Table 4.2 Test conditions
Non-evaporation conditions
Test Fuel

Evaporation Conditions

TolueneFuel Temperature (K)Injection Mass (mg)298
4

Ambient Gas

NitrogenInjection Pressure (MPa)
Injection Duration (ms)

1.7

Ambient Density (kg/m3)

5.95

Ambient Temperature (K)Ambient Pressure (MPa)

30

298
0.5

433
0.73

The test conditions are listed in Table 4.1. Toluene was employed as a substitute for gasoline.
The fuel temperature (before injection) was regulated by a cooling system to maintain it at room
temperature. The injection mass was 4.0 mg with a pressure of 30 MPa and a duration of 1.7 ms.
This experiment was conducted in a constant high-pressure chamber filled with nitrogen gas.
Temperature was set to 298 and 433 K in order to determine the effect of ambient temperature on
the experiment. Also, the ambient density was kept constant at 5.95 kg/m3 by adjusting theambient pressure at 0.5 and 0.73 MPa, accordingly. One thing worth noting that the saturated99

temperatures (Tsat) at 0.5 and 0.73 MPa are 450 and 472 K, which can be seen that Tamb ˘ Tsatunder both conditions.

4.1.2 Spray Impingement Characteristics
Fuel adhesion during injection has been shown in Figure 4.1, where the impingement point isdenoted by the cross symbol. The spray above the impingement wall was well illuminated. It was
hard to separate the fuel adhesion from the impinging spray after processing the image by RIM,
which was the reason why the fuel adhesion mass and area results were shown after the end of
injection (EOI) in the next section to eliminate the stray light error. Furthermore, the fueladhesion on the wall was divided into regions I and II as shown in Figure 4.1, with more
description to be provided in the following section.

Ⅰ
Sprayillumination

Ⅱ

Figure 4.1 Fuel adhesion during injection duration (Tamb = 433 K, 1.5 ms ASOI)
4.1.2.1 Spray impingement
To discuss the spray characteristics under different ambient temperatures, the images of spray
and adhered fuel at 1.5 ms and 4.0 ms after start of injection (ASOI) under Tamb = 298 K arepresented in Figure 4.2 (a). The sprays are displayed as binary images acquired from the Miescattering experiment, and the fuel adhesions are shown in pseudocolor images obtained from the
RIM experiment. The varying adhesion thickness from 0 to 2.0 μm was represented by thepseudocolor images with the impingement point denoted by the cross symbol. During injection
the fuel adhesion is invalid (left figure) due to the illumination of the droplets above the wall, as
explained above. After injection (right figure), the fuel adhesion is complete and thus available.
100After impingement, the impinging spray height at 4.0 ms ASOI is a little larger than that at 1.5
ms ASOI as more droplets get accumulated at the downstream. More importantly, the fueladhesion at impingement region (region I) has greater thickness than that at the other region(region II) and the thickness decreases along y axis, although some thick adhesion can be seen inregion II. This is due to different mechanisms in the formation of fuel adhesion, which will be
explained in more detail in the following section.
Binary Image (Mie Scattering)

15

Binary Image (Mie Scattering)15

1.5 ms10

4.0 ms ASOI

10

5
Wall
Surface

Wall
Surface0

Fuel Adhesion Image (RIM)

Fuel Adhesion Image (RIM)

5
0

2.0
1.6

Ċ

ĉ

Ċ

yĉ

1.2

o

x

0.8
0.4
0
Thickness (μm)(a) Tamb=298 K
Binary Image (Mie Scattering)15

1.5 ms10

Wall
Surface

Binary Image (Mie Scattering)15

4.0 ms ASOI

10

Wall
Surface

5
0

0

2.0

Fuel Adhesion Image (RIM)

Fuel Adhesion Image (RIM)

5

1.6
Ċ

ĉ

Ċ

1.2

ĉ

0.8
0.4
0
Thickness (μm)(b) Tamb=433 K
Figure 4.2 Spray impingement and fuel adhesion during and after injection
101

The images of spray and adhered fuel at 1.5 ms and 4.0 ms after start of injection (ASOI)
under Tamb = 433 K can be seen in Figure 4.2 (b). Under condition of high ambient temperature,
the impinging spray height is shorter than that under Tamb = 298 K, owing to the evaporation offuel droplets. This phenomenon is more obvious after injection (at 4.0 ms AOSI). Moreover, thespray area is smaller in contrast to that observed under Tamb = 298 K, which can be also
explained by the evaporation of droplets during spray. In this case, the fuel adhesion area issmaller when compared to that in Figure 4.2 (a). The possible reasons for it may be summarized
as follows: during spray, some droplets evaporate before impingement; and even when some fueladheres on the wall, it evaporates rapidly under high ambient temperature. By comparing Figure
4.2 (a) and (b), it is evident that evaporation is more significant at the periphery of the fueladhesion than that observed in region I.
4.1.2.2 Spray tip penetration and impinging spray height
Injection Duration

50

Penetration Height
298 K
433 K

40
30

40
30

Impingement Distance
20

20

10

10

0

0
0

0.5

1.0
1.5
2.0
2.5
3.0
3.5
Time after Start of Injection (ms)4.0

Impinging Spray Height (mm)Spray Tip Penetration (mm)

50

4.5

Figure 4.3 Spray tip penetration and impinging spray heightThe spray tip penetration S and impinging spray height Hi can be seen in Figure 4.3. The
horizontal axis represents time after start of fuel injection, with the left vertical axis being S, andthe right vertical axis being Hi. The results under Tamb = 298 K and 433 K are plotted using solidand open data. The broken line represents the impingement distance. It can be seen that at thestart of injection, S under Tamb = 433 K is shorter than that under Tamb = 298 K. The evaporationof the atomized droplets is believed to be responsible for this. Furthermore, this tendency
102

increases slightly after impingement, which can be attributed to the fact that impingement
contributes towards the breakup and atomization of droplets (Pei et al., 2017; Luo et al., 2018).
Since smaller droplets evaporate under Tamb = 433 K, Hi under Tamb = 433 K is always shorter
than that under Tamb = 298 K. It is interesting to find that after EOI, a large difference can beobserved between different temperatures. Even after EOI, the incoming fuel droplets are stillobserved above the wall after impingement as seen in the binary images shown in Figures 4.2.
However, under Tamb = 433 K, the incoming droplets are not sufficient owing to the evaporationof droplets during propagation. Additionally, the droplets above the wall evaporate quickly. The
same conclusion can also be obtained by comparing the images between Figures 4.2 (a) and (b).

4.1.3 Fuel Adhesion Characteristics
4.1.3.1 Evolution of Fuel Adhesion
To illustrate the effect of air temperature on the evolution of fuel adhesion, graphs showing thesame at 3, 5, 10, 20, 30 and 50 ms ASOI are arranged in Figure 4.4. The cross symbol represents
the impingement point, and the pseudocolor represents the fuel adhesion thickness varying from0 to 2.0 μm.
-15 -10 -5

05

10 15 (mm)-50Thickness (μm)
2.0

5
10
15

1.6

20
25
(mm)

1.2

3 ms

10 ms

5 ms

0.8

0.4

0

20 ms

30 ms

50 ms ASOI(a) Tamb = 298 K
103

-15 -10 -5

05

10 15 (mm)-50Thickness (μm)
2.0

5
10

1.6

15
20
25
(mm)

3 ms

1.2

5 ms

10 ms
0.8

0.4

0

20 ms

30 ms

50 ms ASOI(b) Tamb = 433 K
Figure 4.4 Evolution of fuel adhesion on the wall
In Figure 4.4 (a), all the given situations at different times show a similar fuel adhesion
structure, except for the thickness. A larger volume of fuel accumulates in the impingement
region when compared to the bottom. It is worth noting that the thickness of the fuel adhesion
increases with time in both regions due to the secondary breakup of fuel droplets re-deposited on
the wall (Luo et al., 2017). Additionally, the adhesion is observed to be much thinner in the
periphery of the adhesion. Compared with Figure 4.4 (b), it is evident that the high airtemperature accelerates the evaporation of fuel adhesion, resulting in less fuel being depositingon the wall. At the bottom, the fuel adhesion is discontinuous and quite thin. With time, the thin
fuel adhesion at the periphery evaporates fast and only the fuel in the impingement region is left
on the wall. Hence, it can be concluded that the evaporation of fuel impinged on the wall occurs
from the region of low adhesion thickness to high adhesion thickness because of the strong heattransfer (Maligne et al., 2011). Another observation that should be noted is that undertemperature of 433 K, the fuel adhesion becomes thicker with time only in the region of
impingement region, which is different from condition of 298 K. Additionally, the fuel adhesion104remaining on the wall at 50 ms ASOI is similar to the adhesion above 1 μm at 3, 5, 10, 20 and 30
ms ASOI on the wall. These phenomena will be discussed in detail in the next section on themechanism of fuel adhesion of. On comparing Figure 4.2 (a) with (b), it can be observed that theuniformity of adhesion thickness at 433 K is much better than that at 298 K, particularly when
the comparison is made at 50 ms ASOI recording time. This can also be due to the evaporationof thin fuel adhesion under 433 K.
4.1.3.2 Fuel adhesion mass and area
Figure 4.5 shows the fuel adhesion mass and area, with time after start of injection as thehorizontal axis is, fuel adhesion mass and area as the left vertical axis and right vertical axes,

0.7

700

0.6

600

0.5

500

0.4

Mass

400

Area

298 K
433 K

0.3

300

0.2

200

0.1

100

0

0

5

10

15
20
25
30
35
40
Time after Start of Injection (ms)45

50Fuel Adhesion Area (mm2)Fuel Adhesion Mass (mg)respectively. The results under Tamb = 298 K and 433 K are plotted using solid and open data.

0

Figure 4.5 Fuel adhesion mass and area on the wall
It reveals that under Tamb = 298 K, the fuel adhesion area initially increases with time (from0 to 20 ms ASOI) owing to some splashing droplets deposited on the periphery of the adhesion.
However, then the fuel adhesion area remains almost constant (from 20 to 50 ms ASOI). This isdue to the droplets being re-deposited on the original region, thus leading to only an increase inmass but not area. The same observation can also be derived from Figure 4.4 (a). Under Tamb =433 K, the area of fuel adhesion initially decreases sharply with time (from 0 to 20 ms ASOI)
and then decreases only slightly (from 20 to 50 ms ASOI). This suggests that the fuel adhesion105

evaporates from the thin region (at the bottom), leading to sharp decrease. Nevertheless, from 20
to 50 ms ASOI, only the thick adhesion is left behind.
The fuel adhesion mass increase slightly (from 20 to 50 ms ASOI) under Tamb = 298 K as
there are still some droplets falling to the wall. But it keeps constant (from 20 to 50 ms ASOI)
under Tamb = 433 K, owing to the competition between the re-depositing droplets andevaporation of the fuel adhesion on the wall.
4.1.3.3 Fuel adhesion thickness
Figure 4.6 shows the average and maximum thickness of fuel adhesion on the wall. Time after
start of injection is the horizontal, and the vertical axis is the thickness of fuel adhesion. The
results under Tamb = 298 K and 433 K are plotted using solid and open data. Both average andmaximum thickness are seen to increase with time, and the maximum thickness is larger than the
average thickness under the same conditions. An interesting observation is that the average
thickness under Tamb = 298 K is smaller than that under Tamb = 433 K. One possible explanation
for this could be that the fuel on the wall evaporates under Tamb = 433 K, and the decrease in theratio of fuel adhesion area is faster than that of decrease in fuel adhesion mass. The sameconclusion can also be drawn from the graphs at 50 ms ASOI in Figure 4.4: under Tamb = 298 K,
even though the maximum thickness increases to 2.0 μm, there is much thinner fuel adhesion
than 0.8 μm; under Tamb = 433 K, almost all fuel adhesion thickness is larger than 1.0 μm.
2.0

Thickness of Fuel Adhesion (μm)

1.8
1.6
1.4
1.2
1.0
Average Maximum

0.8
0.6

298 K
433 K
0

5

10

15

20

25

30

35

40

45

50

Time after Start of Injection (ms)Figure 4.6 Average and maximum thickness of fuel adhesion on the wall
106

The mass of the adhesion thickness was determined via further study. As shown in Figure
4.7, the horizontal axis is the fuel adhesion thickness and the vertical axis is fuel adhesion mass.
The adhesion mass is based on the value of each pixel as shown in the equation below:

M h

¦ hi
f

(4.1)

i 0

where the sum of fuel adhesion mass in the thickness fraction between h r 0.5'h and h isdefined as M h , and Δh is 0.05 μm.
Figure 4.7 describes the fuel adhesion mass varying with thickness at different times, andthe results under Tamb = 298 K and 433 K plotted using solid and open data. It was reported thatthe peak value of the curve increases and moves to the right due to the re-depositing droplets.
These tendencies are similar under different temperatures, although some differences in shapes
are observed. Under Tamb = 433 K, the mass of the fuel adhesion at thickness from 0.05–0.8 μm
is almost 0 when compared to that under Tamb = 298 K. It has also been demonstrated that air
with high temperature contributes more towards evaporation of thin fuel adhesion and promotesa more uniform thickness of fuel adhesion, thus resulting in a narrow shape at 433 K, which
agrees well with the results in Figure 4.4 (b).
0.05
298 K 433 K
5 ms
20 ms
50 ms

Fuel Adhesion Mass (mg)

0.04

0.03

0.02

0.01

0
0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Fuel Adhesion Thickness (μm)

Figure 4.7 Fuel adhesion mass for different thicknesses

107This section seeks to clarify the effect of high ambient temperature on the thickness of fueladhesion along different lines, which were well defined to express them concisely. As shown in
Figure 4.8, the coordinate system is established and the origin o is defined as the impingement
point. As the fuel adhesion is almost symmetric along the x axis, its distribution has only been
discussed along different y lines. The fuel adhesion was divided using y = −2.5, 2.5, 7.5, and 12.5mm at the vertical axis, and the thickness along these lines was defined as H−2.5, H2.5, H7.5, andH12.5, respectively. The values of H−2.5, H2.5, H7.5, and H12.5 at 5 and 50 ms ASOI under Tamb =298 K and 433 K are depicted in Figure 4.9, respectively. The horizontal axis is from −15 to 15
mm, and the vertical axis denotes fuel adhesion thickness.
The results at 5 ms ASOI are shown in Figure 4.9 (a). The thickness under Tamb = 298 K islarger than that under Tamb = 433 K due to non-evaporation. In all cases, thickness initially
increases, then remains constant, and finally decreases, with the exception of H12.5, where it is
discontinuous and variable. At the constant value stage, H−2.5 and H2.5 at 298 K are almost thesame as those at 433 K, but H7.5 at 298 K is slightly larger than that at 433 K. It is particularly
significant that the constant value stages of both 298 K and 433 K are similar, which is defined
as region I and the downstream part whose value is variable is defined as region II. The boundary
between regions I and II is drawn by detecting the constant value in thickness using our in-housecode in the MATLAB software, and the results are shown in Figure 4.8 (a), from which it can beobserved that the region I of 298 K is quite similar to that of 433 K, which agrees well with theobservation in Figure 4.4 (a).
A similar result was observed in Figure 4.9 (b) regardless of the behavior of H−2.5, because
at 50 ms ASOI, H−2.5 at 433 K evaporates for a quite long time, resulting in its sharp reduction.
The regions I and II are shown in Figure 4.8 (b) after applying the same criterion to them. It mustalso be noted that region I is similar at 5 ms and 50 ms ASOI as well, and the same observation
can be seen in Figure 4.4 (b).

108

-15 -10 -5

0

5

-5

H-2.5

0

o

5

ĉ

y

x

1.6

H2.5

ĉ

H7.5

10

1.2

H12.5

15

0.8

Ċ

Ċ

Tamb = 298 K

Tamb = 433 K

20
25
(mm)

Thickness (μm)
2.0

10 15 (mm)0.40(a) 5 ms ASOI

-15 -10 -5
-10

0

5

-5

H-2.5

0

H2.5

5

ĉ

20
25
(mm)

1.6

ĉ

H7.5
H12.5

10
15

Thickness (μm)
2.0

10 15 (mm)

0.8

Ċ

Ċ
Tamb = 298 K

Tamb = 433 K(b) 50 ms ASOI

Figure 4.8 Fuel adhesion under different ambient temperatures

109

1.2

0.4
0

Fuel Adhesion Thickness (μm)

2.0
1.5
1.0
0.5
0

298 K
433 K
H-2.5

2.0
1.5
1.0
0.5
0

298 K
433 K
H2.5

2.0
1.5
1.0
0.5
0

298 K
433 K
H7.5

2.0
1.5
1.0
0.5
0

298 K
433 K
H12.5
-15

-10

-5

0

5

10

15

Fuel Adhesion Thickness (μm)(a) 5 ms ASOI
2.0
1.5
1.0
0.5
0

298 K
433 K
H-2.5

2.0
1.5
1.0
0.5
0

298 K
433 K
H2.5

2.0
1.5
1.0
0.5
0

298 K
433 K
H7.5

2.0
1.5
1.0
0.5
0

298 K
433 K
H12.5
-15

-10

-5

0

5

10

15

(b) 50 ms ASOI
Figure 4.9 Fuel adhesion thickness along different lines
In order to check the fuel adhesion mass in region I, the variation of mass with thickness at
different times is shown in Figure 4.10. The plotted curves of 433 K appear to move to the leftwhen compared to those of 298 K due to evaporation, while maintaining similar shapes, which
also demonstrates the same mechanism in formation. Moreover, the curve shifts to the right with110

time as some droplets are re-deposited on the wall, and the same observation can also be seen in
Figure 4.7.
0.020
298 K 433 K
5 ms
20 ms
50 ms

Fuel Adhesion Mass (mg)

0.015

0.010

0.005

0
0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Fuel Adhesion Thickness (μm)

Figure 4.10 Fuel adhesion mass for different thicknesses in region Ⅰ
In summary, as mentioned above, the fuel adhesion on the wall can be divided into regions I
and II in order to study the different mechanisms of fuel adhesion formation as shown in Figure
4.11.
Region I: Primary impingement region. The direct spray impinges on the wall, causing
some fuel to stick to it to form the adhesion.
Region II: Secondary impingement region. After impingement on the wall, most of the fuelsplashes off it. The splashing droplets are re-deposited on the wall because of coalescence and airforce to form the adhesion.

111

Fuel spray

Air velocity

Droplet velocity
at the tip of spray

Splashing
droplets

Main spray
Vortex

Incident
droplets

Re-depositing
droplets

Impinging wall
RegionĊ: Secondary
impingement region

Regionĉ: Primary
impingement region(a) During injection

Air velocity

Re-depositing
droplets

Main spray

Impinging wall
RegionĊ: Secondary
impingement region

Regionĉ: Primary
impingement region(b) After injection
Figure 4.11 Mechanisms of fuel adhesion formation in different regions112

During injection: When the fuel is injected out of the nozzle, it breaks up into ligaments
and large droplets. Through the fuel flow along the spray axis, the ligaments and large droplets
break up into small ones due to the interaction between the liquid fuel and air. Under Tamb = 298
K, after initial impingement on the wall, some of the fuel on the wall forms region I and otherfuel droplets splash off the wall. Nevertheless, the splashing droplets may collide and coalescewith others, resulting in a change in velocity and leading to it being re-deposited on the wall toform region II, which agrees with the observations made in Figure 4.2 (a). However, under hightemperature condition, the fuel in region II is less owing not only to the decrease in impinging
droplets but also the evaporation of the adhered fuel. The same phenomena can be seen in Figs.
4.1 and 4.2 (b).
After injection: Although there is no fuel injected out of the nozzle, some fuel droplets arestill in the air or above the wall. Under Tamb = 298 K, as the spray moves along the wall, the air
velocity changes. Then the air pushes the splashing droplets to re-deposit in regions I and II,
which agrees well with the observations in Figure 4.4 (a). However, under Tamb = 433 K, thesplashed droplets evaporate fast, resulting in few droplets re-depositing in region II. However, as
region I is formed via direct spray impingement, it is affected less by the high ambient
temperature, with only the periphery adhesion evaporating, resulting in fuel adhesion in region I
being almost the same as that under room temperature. The same phenomena can be observed in
Figs. 4.2 (b) and 4.4 (b).
As summarized above, the fuel adhesion in region I is formed by the direct spray and it
shows little change under different ambient temperature conditions. While the fuel adhesion inregion II is formed by the re- depositing droplets, and it is sensitive to the ambient temperature
because some splashing droplets evaporate before re-depositing on the wall and some can evenre-deposit on the wall to form adhesion, ultimately it evaporates more easily owing to the strong
heat transfer. These are the reasons why the fuel adhesion in region I under temperatures of Tamb
= 298 K and 433 K is similar, but varies greatly in region II, as shown in Figure 4.8.

113

4.2 EFFECT OF NOZZLE HOLE DIAMETER
Some investigations have been conducted into the spray characteristics resulting different holediameters. The effect on the spray tip penetration, spray cone angle and spray volume werechecked by many researchers. However, the effect of nozzle hole diameter on the fuel adhesion
has been seldom reported. In this part, the adhered fuel is tested by different injectors of holediameter under both non-evaporation and evaporation conditions. Experiments were performed
in a constant high-pressure chamber. Specifically, all the results were time-resolved to better
understand the impinging spray of a gasoline engine.

4.2.1 Experimental Conditions
In order to investigate the effect of nozzle hole diameter on fuel adhesion. Two types of injector(D13 and D15) were tested, as shown in Figure 4.12. The detailed parameters are listed in Table
4.3. Two mini-sac injectors with a nozzle hole diameter of 0.135 and 0.155 mm were used. The
nozzle was a conventional straight-hole type without a counterbore, and the nozzle hole length
was kept the same value at 0.65 mm, resulting that the different the length-to-diameter (L/D)
ratios are 4.8 and 4.2. The impingement angle was 45° and the impingement distance from thenozzle exit to the point of impingement at the flat wall along the spray axis was 22 mm. The flatwall was made of quartz glass (Sigma Koki, DFSQ1-50CO2) with a surface roughness Ra(arithmetical mean deviation of the profile) of 7.0 μm, measured using a portable highperformance surface roughness and waviness measuring instrument (Kosaka Laboratory Ltd.,
SE300) with a resolution of 0.0064 μm.
The test conditions are listed in Table 4.4. Toluene was employed as a substitute forgasoline. The fuel temperature (before injection) was regulated by a cooling system to maintainit at room temperature. The injection mass by different injectors were 3.0 and 4.0 mg with apressure of 20 MPa and durations of 1.65 and 2.1 ms, respectively. This experiment was
conducted in a constant high-pressure chamber filled with nitrogen gas. Temperature was set to298 and 433 K in order to determine the effect of ambient temperature on the experiment. Also,
the ambient density was kept constant at 5.95 kg/m3 by adjusting the ambient pressure at 0.5 and0.73 MPa, accordingly. One thing worth noting that the saturated temperatures (Tsat) at 0.5 and0.74 MPa are 450 and 472 K, which can be seen that Tamb ˘ Tsat under both conditions.

114

Nozzle Needle

Nozzle Needle

0.
d=
5

5

13

15

0.
d=

Sac Holemmmm

Sac Hole(a) D13(b) D15

Figure 4.12 Specification of the injectors with different nozzle hole diameters
Table 4.3 Injector parameters and impingement conditions
Injector Parameters
Injector Type

Mini-Sac

Hole Number

1

Hole Type

Straight-Hole without Counterbore

Nozzle Hole Length (mm)0.65

L/D Ratio

4.8

4.2

Nozzle Hole Diameter (mm)

0.135

0.155

Impingement Conditions
Impingement Plate

Quartz Glass

Impingement Distance (mm)22

Impingement Angle (°)

45

Surface Roughness (μm)

Ra 7.0115

Table 4.4 Injection and ambient conditions
Injection Conditions
Test Fuel

TolueneFuel Temperature (K)Injection Pressure(MPa)
Injection Mass (mg)

293

3

4

Injection Duration (ms)

1.65

2.1

20

Ambient Conditions
Non-evaporation
conditions
Ambient Gas
Ambient Density(kg/m3)
Ambient Temperature(K)
Ambient Pressure(MPa)

Evaporation Conditions
Nitrogen
5.95

298

433

0.5

0.74

4.2.2 Fuel Adhesion Characteristics
Figure 4.13 shows the evolution of the adhered fuel of different injectors under non-evaporation
condition. The fuel adhesions on the wall at 5, 20, and 50 ms ASOI are shown. The pseudocolor
represents the adhered fuel thickness, varying from 0 to 2.0 μm, and the impingement point isshown by the cross symbol. Under each condition, all cases at different times show similar
structures, and the fuel adhesion areas are almost symmetric. It is evident that the fuel adhesion
is formed at 5 ms, although some differences can be observed at 20 and 50 ms: due to the redeposition droplets on the wall, the wet area and thickness increase with time. It is interesting tofind that the adhesion area of D15 is larger than that of D13. Two reasons should be responsible
for this. One is that the injection mass of D15 is larger. Another is that both Zhang et al. (2017)
and Algayyim et al. (2018) reported the spray angle increase at the increased hole diameter ofnozzle, which leads to the larger fuel adhesion on the wall after impingement. More importantly,
the thickness of fuel adhesion on the wall of D13 is around 1.0 μm, which is smaller than that of
D15. As we known, the small hole favors the atomization of spray, at a result, the droplet
116

diameter decreases with small hole diameter. Then the Re decreases and Oh increases, thus, after
impingement on the wall, some droplets may change their behavior from “deposition” to“splashing”, resulting in thinner fuel adhesion of D13. Moreover, the fuel adhesion after 5 ms
AOSI of D13 changes less compared to that of D15, and the better atomization may be one
possible reason for this.
-15 -10 -5

05

Thickness (μm)
2.0

10 15 (mm)

-5

1.6

0
5

1.2

10
0.8

15
20

0.4

25
(mm)

20 ms

5 ms

50 ms ASOI

0(a) D13-15 -10 -5

05

Thickness (μm)
2.0

10 15 (mm)

-5

1.6

0
5

1.2

10
0.8

15
20

0.4

25
(mm)

5 ms

20 ms

50 ms ASOI

0(b) D15
Figure 4.13 Fuel adhesion evolution on the wall injected by the injectors with different nozzle
hole diameters (Tamb = 298 K)
Figure 4.14 illustrates the evolution of the adhered fuel of different injectors underevaporation condition. The fuel adhesions on the wall at 5, 20, and 50 ms ASOI are shown. The
pseudocolor represents the adhered fuel thickness, varying from 0 to 2.0 μm, and theimpingement point is shown by the cross symbol. Compared to Figure 4.13, fuel adhesion at allcases become smaller due the droplets evaporation. Owing to the better atomization of D13, the117SMD decreases, which promotes the atomization of liquid phase spray and enhances theresistance of the liquid phase spray penetration, leading to shorter liquid fuel spray penetration,
resulting that the adhesion decreases more sharply. Under evaporation condition, small droplets
evaporate fast during spray propagation before impingement, leading to shorter liquid phasespray penetration, which agrees well with the observation of Du et al., 2017. It should be notedthat the thick fuel region moves from upstream to downstream in contrast to non-evaporation
condition. This is attributed to the droplets evaporation before impingement. Furthermore, thefuel adhesion evaporates from the thin part to the thick one, which has been discussed in detail inpart 4.1.
-15 -10 -5

05

Thickness (μm)
2.0

10 15 (mm)

-5

1.6

0
5

1.2

10
0.8

15
20

0.4

25
(mm)

20 ms

5 ms

50 ms ASOI

0(a) D13-15 -10 -5

05

Thickness (μm)
2.0

10 15 (mm)

-5

1.6

0
5

1.2

10
0.8

15
20

0.4

25
(mm)

5 ms

20 ms

50 ms ASOI

0(b) D15
Figure 4.14 Fuel adhesion evolution on the wall injected by the injectors with different nozzle
hole diameters (Tamb = 433 K)

118

Fuel Adheison Mass / Injection Mass × 100 (%)

14

Injection Duration
12
10

298 K 433 K
D13

8

D15

6
4
2
0

0

5

10

15

20

25

30

35

40

45

50

Time after Start of Injection (ms)Figure 4.15 Fuel adhesion ratio under different conditions
600

Fuel Adhesion Area (mm2)

550
500

298 K 433 K

450

D13

400

D15

350
300
250
200
150
100
50
0

Injection Duration
0

5

10

15

20

25

30

35

40

45

50

Time after Start of Injection (ms)Figure 4.16 Fuel adhesion area under different conditions
The fuel adhesion mass ratios are depicted in Figures 4.15. The adhesion mass ratio isdefined as the ratio of adhesion mass to total injection mass. The mass ratio increases with timeeven after the end of injection because there are still some droplets dropping on the wall. An
increase in hole diameter under both non-evaporation and evaporation conditions increases thefuel adhesion mass ratio owing to poor atomization. Moreover, Du et al. [2018] reported that theliquid penetration of the spray by small hole diameter is shorted under both non-evaporation and119evaporation conditions, resulting in less fuel adhering on the wall, which should be anotherreason for the less ratio of D13. Furthermore, the adhesion mass ratio under 298 K increases
significantly from 5 to 15 ms ASOI compared to that under 433 K. The re-depositing droplets on
the wall tends to be responsible, which agrees well with the observation results in Figure 4.13.
And from 15 to 50 ms ASOI, the fuel mass under 298 K increases slightly due to the redeposition, however, it keeps almost constant under 433 K owing to the competition between redeposition and fuel evaporation.
The fuel adhesion areas are depicted in Figures 4.16. The adhesion area increases with timeeven after the end of injection under 298 K because there are still some droplets dropping on thewall. An increase in hole diameter under both non-evaporation and evaporation conditions
increases the fuel adhesion area owing to poor atomization. The same tendency can also be
observed in the comparison between Figures 4.13 and 4.14. Furthermore, the adhesion area
decreases slightly under 433 K owing to the evaporation.
Figure 4.17 shows the maximum thickness of fuel adhesion on the wall. Time after start of
injection is the horizontal, and the vertical axis is the thickness of fuel adhesion. The results
under 298 K and 433 K are plotted using solid and open data. The maximum thickness is seen toincrease with time, and the results of 298 K is larger than that under 433 K. The fuel evaporation
should be responsible for this. It should be noted that the maximum thickness of D13 is smaller
than that of D15 due to the better atomization, and the same conclusion can be found in Figure
4.13 and 4.14.
Figure 4.18 shows the average thickness of fuel adhesion on the wall. Time after start of
injection is the horizontal, and the vertical axis is the thickness of fuel adhesion. The results
under 298 K and 433 K are plotted using solid and open data. An interesting thing is that the
average thickness under 298 K is smaller than that under 433 K. One possible explanation forthis could be that the fuel on the wall evaporates under 433 K, and the decrease in the ratio offuel adhesion area is faster than that of decrease in fuel adhesion mass. And the average
thickness of D13 is smaller than that of D15 due to the better atomization.

120

Maximum Thickness of Fuel Adhesion (μm)

2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6

298 K 433 K
D13

0.4
0.2
0

D15

Injection Duration
0

5

10

15

20

25

30

35

40

45

50

Time after Start of Injection (ms)Figure 4.17 Maximum Thickness of Fuel adhesion under different conditions
Average Thickness of Fuel Adhesion (μm)

1.4
1.2
1.0
0.8
0.6
0.4

298 K 433 K
D13

0.2
0

D15

Injection Duration
0

5

10

15

20

25

30

35

40

45

50

Time after Start of Injection (ms)Figure 4.18 Average Thickness of Fuel adhesion under different conditions

121

17.5

298 K 433K

Probability of Mass (%)

15.0

D13
D15

12.5
10.0
7.5
5.0
2.5
0
0

0.2

0.4

0.6
0.8
1.0 1.2
1.4
Fuel Adhesion Thickness (μm)

1.6

1.8

2.0

1.6

1.8

2.0

(a) 5 ms ASOI
17.5

298 K 433K

Probability of Mass (%)

15.0

D13

12.5

D15

10.0
7.5
5.0
2.5
0
0

0.2

0.4

0.6
0.8
1.0 1.2
1.4
Fuel Adhesion Thickness (μm)(b) 50 ms ASOI
Figure 4.19 Probability of mass along thickness
To further investigate the adhesion thickness, the probability of thickness was determined. As
shown in Figure 4.19, the horizontal axis is the fuel adhesion thickness, and the vertical axis is
the probability of mass. The probability of mass is based on the value of each pixel, and theprobability must satisfy the normalization conditions as equation (4.1)
122

Figure 4.19 (a) describes the probability of fuel adhesion mass varying with thickness at 5ms, and the results under 298 K and 433 K plotted using solid and open data. It reveals that thepeak value of the curve increases by increased the ambient temperature. It has been demonstrated
that air with high temperature contributes more towards evaporation of thin fuel adhesion,
resulting in peak value increasing. Moreover, the curve become a little narrow compared to that
under 298 K, indicating that the uniformity of the fuel adhesion is improved. Furthermore, it isclear that the curve of D15 move to right compared to D13. The poor atomization and more fueladhesion on the wall should be responsible for it. For the probabilities at 50 ms ASOI in Figure
4.19 (b), the curves under 298 K move to right, because that some re-depositing droplets on thewall to increase the adhesion thickness, which agrees well with the results in Figure 4.13 and4.17. However, under 433 K, owing to the evaporation, the curves become narrow, and peakvalues increase, leading to better adhesion uniformity.
Further investigation was conducted to clarify the effect of hole diameter on fuel adhesion
thickness along different lines. In Figure 4.20, the coordinate system is defined, and the origin ois determined as the impingement point. The positive y axis is along the direction of fuel splash
after impingement, and positive x axis is perpendicular to the y axis, pointing to the left. Because
fuel adhesion is almost symmetric about x = 0 axis, we divided fuel film by y = 0, 5 and 10mmlines that represent the upstream, midstream, and downstream of the fuel adhesion.
To express concisely, the fuel adhesion thickness y = 0, 5, and 10 lines were defined as y0,y5, and y10, respectively. In the following discussion, the terms of y0, y5 and y10 are used torepresent the fuel adheison thickness along these different lines. The experimental condition
discussed in this section is at 5 ms ASOI, as shown in Figure 4.20.

123

-15 -10 -5

05

Thickness (μm)
2.0

10 15 (mm)

-5
0

o
x

y

5
10

y0

1.6

y5

1.2

y10
0.8

15
20

0.4

25
(mm)

D13

D15

0

(a) 298 K-15 -10 -5

05

Thickness (μm)
2.0

10 15 (mm)

-5
0
5

o
x

y

10

y0

1.6

y5

1.2

y10
0.8

15

0.4

20
25
(mm)

D15

D13

0

(b) 433 K
Figure 4.20 Definition of fuel adhesion thickness along different lines (5 ms ASOI)

124

2.0
1.5
1.0
0.5
0

D13
D15
y0

2.0
1.5
1.0
0.5
0

D13
D15
y5

2.0
1.5
1.0
0.5
0

D13
D15
y10
-15

-10

-5

0

5

10

15

(a) 298 K
2.0
1.5
1.0
0.5
0

D13
D15
y0

2.0
1.5
1.0
0.5
0

D13
D15
y5

2.0
1.5
1.0
0.5
0

D13
D15
y10
-15

-10

-5

0

5

10

15

(b) 433 K
Figure 4.21 Fuel adhesion thickness distribution along different lines (5 ms ASOI)
Figure 4.21 (a) shows the fuel adhesion thickness distribution under 298 K. For D13, thefuel adhesion thickness and uniformity of thickness decreases from upstream to downstream.
However, for D15, y0 and y5 are almost the same, and they are bigger than y10. Moreover, theuniformity of y0 and y5 is better than y10. In contrast to different hole diameter, the fuel adhesion125becomes thicker with a large hole diameter, and the uniformity of thickness improves with alarge hole diameter. This is attributed to the larger spray angle of D15, more fuel adheres on thewall uniformly. Under 433 K, shown in Figure 4.21 (b), the thickness decreases from upstream
to downstream for both D13 and D15. And the uniformity become worse from upstream todownstream. Owing to the fuel evaporation, thickness is smaller than that under 298 K. In
contrast to different hole diameter, the fuel adhesion becomes thicker with a large hole diameter,
and the uniformity of thickness improves with a large hole diameter.

4.3 SUMMARY
It is widely known that the fuel rich zones in engines are the major contributors towards sootemissions, which is why they must be avoided as much as possible. Unfortunately, it is difficult
to predict the occurrence of fuel adhesion, as the mechanism behind it is still not clear. The aimof this chapter was to investigate the fuel adhesion formation under high temperature conditions
to understand its mechanism. For this, toluene was injected via a single-hole injector on the wall.
Mie scattering and RIM methods were applied to observe the impinging spray structure and fueladhesion on the wall. The spray tip penetration and impinging spray height were compared, andthe evolution of fuel adhesion along with its mass, area, and thickness were studied. The main
conclusions are summarized as follows:
1. High ambient temperature has a significant impact on the impinging spray and fuel adhesionon the walls of the engine. A high ambient temperature favors the evaporation of fuel, thus
decreasing the penetration of the spray tip and the height of the impinging spray. Moreover, it
decreases the mass, area, and maximum thickness of the fuel adhesion owing to the evaporation
effect. Also, is increases the average thickness with the decrease in ratio of fuel adhesion area
faster than that of fuel adhesion mass.
2. The fuel adhesion continues to increase even after the end of injection, because of droplets
being re-deposited on the wall. However, under Tamb = 433 K, the adhered fuel evaporates fromthe periphery to the impingement region even where there are still some droplets being redeposited on the wall, but the increased adhesion can only be observed in region I. By comparing
the evolution of fuel adhesion under different ambient temperatures, it can be concluded thathigh ambient temperatures lead to more uniform fuel adhesion on the wall.

126

3. The distribution of adhesion mass and thickness was studied in regions I and II. Similar curves
were observed in region I under different ambient temperatures, whereas there was great
variation in region II. Based on these results, different mechanisms were described for theformation of fuel adhesion during and after injection. The primary spray impinging on the wall
contributed to the formation of fuel adhesion in region I, known as the primary impingement
region. The splashing droplets impinging next on the wall contributed to the formation of fueladhesion in region II, known as the secondary impingement region. Furthermore, high ambient
temperatures exert more influence on the fuel adhesion formation in region II when compared toregion I.
4. Decreasing the nozzle hole diameter can reduce the fuel adhesion mass, area, average
thickness and maximum thickness owing to the better atomization. However, the large holediameter improves the uniformity of the thickness.

127

CHAPTER 5 MICROSCOPIC BEHAVIORS OF IMPINGINGSPRAY
It is well known that the microscopic characteristics of fuel spray are very important on theatomization and mixture formation. The droplet size and number distributions reveal the qualityof spray and atomization, which affects the subsequent combustion and emissions characteristics.
In this study, the droplet size was measured with particle image analysis (PIA) technique.
Microscopic characteristics of impinging spray were obtained using ultra-high speed imaging.
By changing the injection and ambient pressures, the influences of breakup and coalescence on
the droplet behavior were investigated. Before impingement, the region near the center of sprayhas the larger droplets size and lower number density than the edge, which suggests that thespray breakup and atomization was poor in the center region. After impingement, the droplet size
decreases along the distance from the wall under low ambient pressure. However, large droplets
size appears in the region far from the wall under high ambient pressure, indicating the existenceof coalescence effect on the droplets.

5.1 EXPERIMENTAL CONDITIONSThe experimental conditions were given in Table 5.1. Two injection pressures of 10 and 20 MPa,and two ambient pressures of 0.1 and 0.5 MPa, equivalent to the ambient gas densities of 1.19
and 5.95 kg/m3, were investigated. This experiment was conducted in a constant volume
chamber filled with nitrogen gas at room temperature. A mini-sac injector with a nozzle holediameter of 0.135 mm was used. The nozzle is a conventional straight-hole type without
counterbore. The length-to-diameter (L/D) ratio was 4.8. Toluene was employed as a surrogate
fuel for gasoline. The injection mass was kept constant at 3.0 mg under different conditions,
resulting in different injection pulse widths of 2.4 and 1.65 ms at different injection pressures.
The impingement angle was 45° and the impingement distance was 22 mm from the nozzle exit
to the impingement point of the flat wall along the spray axis. The flat wall was made of quartz
glass (Sigma Koki, DFSQ1-50CO2) with a surface roughness of Ra7.7 μm. Moreover, thesurface roughness was measured by a portable high-performance surface roughness andwaviness measuring instrument (Kosaka Laboratory Ltd., SE300) with a resolution of 0.0064 μm.

128

Table 5.1 Experimental conditions.
Injector Conditions
Injector Type

Mini-Sac, Single-Hole

Hole Type

Straight-Hole without Counterbore

L/D Ratio

4.8

Nozzle Hole Diameter (d)0.135 mm

Injection Conditions
Fuel
Injection Mass (Minj)

Toluene
3.0 mgInjection Pressure (Pinj)

10, 20 MPa

Injection Duration (td)

2.4, 1.65 ms
Ambient Conditions

Ambient Gas

NitrogenAmbient Pressure (Pamb)

0.1, 0.5 MPa

Ambient Temperature (Tamb)

300 K
1.19, 5.95 kg/m3

Ambient Density (ρamb)

Impingement Conditions
Impingement Plate

Quartz Glass

Impingement Distance (Lw)

22 mm

Impingement Angle (θimp)

45°

Surface Roughness

Ra7.7

5.2 MIEROSCOPIC MORPHOLOGY OF IMPINGEMENT ON THE WALL
5.2.1 Before Impingement
Figure 5.1 shows the spray morphology along the spray axis. Four conditions (Pinj = 10 MPa,
Pamb = 0.1 MPa; Pinj = 10 MPa, Pamb = 0.5 MPa; Pinj = 20 MPa, Pamb = 0.1 MPa; Pinj = 20 MPa,
Pamb = 0.5 MPa) were investigated.

129

0

0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

Droplets

03.2(mm) 0

0.4

0.4

0.8

0.8

1.2

1.2

1.6

1.6

2.0

2.0

2.4
(mm)
0

Ligament(a) Pinj = 10 MPa, Pamb = 0.1 MPa
0.4

0.8

1.2

1.6

2.0

2.4

2.8

0

2.4
(mm)
0
3.2
(mm) 0

0.4

0.4

0.8

0.8

1.2

1.2

1.6

1.6

2.0

Ligament

2.4(mm)(c) Pinj = 20 MPa, Pamb = 0.1 MPa

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2
(mm)

Ligament(b) Pinj = 10 MPa, Pamb = 0.5 MPa
0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2
(mm)

2.0
2.4
(mm)

(d) Pinj = 20 MPa, Pamb = 0.5 MPa

Figure 5.1 Spray morphology at the position of before impingement.
Generally, when the fuel injected out of the nozzle hole, they start to break-up into liquid
ligaments and small droplets (Baumgarten et al., 2006). It is clear that the fuel jet developed withheterogeneity in droplet concentration: the core region had high concentration, while theperiphery had low one. It can also be found that the spray flow is not symmetric, and that maybe
due to the special structure of the needle hole. Shown in Figure 5.1 (a), some relatively large
droplets and ligaments are observed at downstream of the spray axis, whereas most of tiny
droplets are at upstream. Compared with Figure 5.1 (b), it is evident that with an increase ofambient pressure, droplets become smaller, and droplets density are quite higher, ligaments
become shorter and smaller, which suggests that an increase in ambient pressure promotes liquid
fuel breakup process, resulting in smaller droplets. The much stronger drag force should be
responsible for smaller droplets. An increased ambient pressure increases the gas density in thechamber, resulting stronger interaction between fuel liquid and nitrogen gas. Therefore, droplets
and ligaments tend to break up easily. Moreover, the spray width is a little larger than that of130

Pamb = 0.1 MPa, which is consistent with previous studies (Guan et al., 2015; Feng et al., 2016).
Comparing with Figure 5.1 (a) and (c), it can be seen that droplets and ligaments become smaller
under high injection pressure. The enhanced kinetic energy leads to better dispersion andatomization should be the reason for it. Additionally, under high injection and ambient pressure,there are many tiny droplets, making the droplets too dense to detect the ligaments, shown in
Figure 5.1 (d).

5.2.2 After Impingement
Figure 5.2 shows the spray morphology after impingement. The same four conditions were
examined. The solid line represents the surface of the wall.
0

0.4

0

0.8

1.2

1.6

2.0

2.4

2.8

Droplets

03.2(mm) 0

0.4

0.4

0.8

0.8

1.2

1.2

1.6

1.6

2.0

Surface of the wall

2.4
(mm)
0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

0

0
3.2
(mm) 0

0.4

0.4

0.8

0.8

1.2

1.2

1.6

1.6

2.0

Surface of the wall

2.4
(mm)

(c) Pinj = 20 MPa, Pamb = 0.1 MPa

0.8

1.2

1.6

2.0
2.4
(mm)

(a) Pinj = 10 MPa, Pamb = 0.1 MPa

0.4

2.0
2.4
(mm)

2.0

2.4

2.8

3.2
(mm)

Surface of the wall(b) Pinj = 10 MPa, Pamb = 0.5 MPa
0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2
(mm)

Surface of the wall(d) Pinj = 20 MPa, Pamb = 0.5 MPa

Figure 5.2 Spray morphology at the position of after impingement.
It is interesting to find that only small droplets can be observed in the figures which can be
attributed to the impingement favors the process of ligaments breakup into droplets. The regions131

of near the wall are very dense as many droplets impinge on the wall and deposit on the wall or
splash off wall. The droplets far away from the wall are clear and easy to be distinguished. Sometendencies can also be noticed. Droplets appears smaller with an increase of injection andambient pressure.
With these observations, it becomes clear that rapid breakup and atomization happens underhigh injection and ambient pressure condition. However, there is another coalescence effect ofhigh ambient pressure, which will be discussed in the following section. Additionally, wall
impingement makes the ligaments disappeared.

5.3 DROPLET SIZE DISTRIBUTION
5.3.1 Before Impingement
Droplets size distribution before impingement is shown in Figure 5.3. Figure (a) describes thesmall size (0-50 μm) distribution under four conditions (Pinj = 10 MPa, Pamb = 0.1 MPa; Pinj = 10
MPa, Pamb = 0.5 MPa; Pinj = 20 MPa, Pamb = 0.1 MPa; Pinj = 20 MPa, Pamb = 0.5 MPa). It isreported that curve moves towards to left and peak value of the curve increase with an increaseof injection and ambient pressure, indicating better breakup. More importantly, the peak value ofhigh injection pressure condition (Pinj = 20 MPa, Pamb = 0.1 MPa) is larger than that of highambient pressure condition (Pinj = 10 MPa, Pamb = 0.5 MPa). One possible explanation for this
could be that there are different mechanism effects of high injection and ambient pressure on thedroplets size. Droplets with more momentum under high injection pressure entrain withsurrounding gas more easily, facilitating the breakup of fuel jet. However, the elevated ambientpressure leads to strong drag force between air and droplets that shears droplets into small ones.
As a result, the inner effect of droplets is more significant than the outer one on breakup
phenomenon, especially for small size droplets. Moreover, high ambient pressure may also affectthe coalescence of the droplets, which will be discussed in the following sections.

132

(a) Small size distribution.

(b) Large size distribution.
Figure 5.3 Droplet size distribution before impingement.
Figure 5.3 (b) shows the large size (50-100 μm) distribution under the same four conditions.
The probability of large size droplet decreases with an increase in injection and ambient pressure.
It should be noted that the probability of high injection pressure condition (Pinj = 20 MPa, Pamb =0.1 MPa) is larger than that of high ambient pressure condition (Pinj = 10 MPa, Pamb = 0.5 MPa).
This can be argued that the elevated ambient pressure has a more positive effect on the breakup133

of large size droplets because of air drag force, and the same conclusion can also be observed
from Figure 5.1 (b) and (c). Generally, SMD is used to evaluate atomization, defined as follows:
SMD

¦N d
¦N d
i

i

3
i
2
i

(5.1)

As the number of larger size droplet increases under Pinj = 20 MPa, Pamb = 0.1 MPa, SMD islarger than that under Pinj = 10 MPa, Pamb = 0.5 MPa.

5.3.2 After Impingement
Droplets size distribution after impingement is shown in Figure 5.4. Figure (a) provides the small
size (0-50 μm) distribution under the four conditions. It can be found that curve moves towards
to left and peak value of the curve increase in contrast to before impingement, which indicatesthe impingement facilitates the breakup of small size droplets. One important thing is that thepeak value of higher injection pressure condition (Pinj = 20 MPa, Pamb = 0.1 MPa) is larger thanthat of higher ambient pressure condition (Pinj = 10 MPa, Pamb = 0.5 MPa), and even larger thanthat of higher injection and ambient pressure condition (Pinj = 20 MPa, Pamb = 0.5 MPa). It can be
expected that some small droplets coalesce under higher ambient pressure. After impingement on
the wall, droplets splash off the wall. Because of the changed velocity direction, some dropletscollide with each other, then they appear to coalesce under high ambient pressure. In addition,
the vortex after impingement also helps to enhance the coalescence behavior. It is interesting thatafter impingement, SMD under higher ambient condition (Pinj = 10 MPa, Pamb = 0.5 MPa) islarger than high injection pressure condition (Pinj = 20 MPa, Pamb = 0.1 MPa), and even larger
than low injection and ambient pressure condition (Pinj = 10 MPa, Pamb = 0.1 MPa). Highambient pressure decelerates the droplets, and the coalescence phenomenon after impingementby the help of vortex appears much stronger, especially on droplets of low velocity.
Figure 5.4 (b) provides the large size (50-100 μm) distribution. The probability of large size
droplet decreases sharply compared with before impingement, which manifests the impingement
also has a positive effect of breakup. Furthermore, the probability under Pinj = 10 MPa, Pamb =0.5 MPa is higher than that of Pinj = 10 MPa, Pamb = 0.1 MPa to some extent, which can also
explain the reason SMD under Pinj = 10 MPa, Pamb = 0.5 MPa is larger than that of Pinj = 10 MPa,
Pamb = 0.1 MPa.
134

16
14

Pinj - Pamb
10 MPa-0.1 MPa
10 MPa-0.5 MPa
20 MPa-0.1 MPa
20 MPa-0.5 MPa

Probability (%)

12
10
8
6
4

D32
DD
3232 D32

2
0

0

10

20
30
Droplet diameter (μm)40(a) Small size distribution.

(b) Large size distribution.
Figure 5.4 Droplet size distribution after impingement.

135

50

Figure 5.5 SMD before and after impingement.
In order to clarify the atomization characteristic clearly, SMD before and after impingement
were depicted in Figure 5.5. Before impingement, SMD under Pinj = 10 MPa, Pamb = 0.1 MPa isused as reference condition, and both injection and ambient pressure favor the atomization as
SMD decreases compared to the reference condition. Further, high ambient pressure has a morepositive effect on atomization due to low probability of larger size of droplets. After
impingement, SMD under low ambient pressure drops dramatically but SMD under high ambientpressure increases to some extent. More violent coalescence effect on droplets after impingement
under high ambient pressure should be the reason for that.

5.4 DROPLET NUMBER DISTRIBUTION
For a better understanding the effect of breakup and coalescence on droplets, spatial distributionof droplets size was analyzed.

5.4.1 Before Impingement
Figure 5.6 shows the definition of radial distance from the axis of spray before impingement.
The dashed dotted line represents the axis of the spray. Some regions were divided by every 0.4
mm apart from the spray axis. Droplets number and SMD were examined at each region.

136

D

C

B

0.4 m)
m
0.8 ce (an
1.2 distl
1.6 adia
R

f
so

A

O

ay
pr

-A

s

i

Ax

-B

0

-C
-D

Before impingement
Figure 5.6 Definition of radial distance from the axis of spray before impingement.

40
Pinj - Pamb

Probability of droplets number (%)

35

10 MPa-0.1 MPa
10 MPa-0.5 MPa
20 MPa-0.1 MPa
20 MPa-0.5 MPa

30
25
20
15
10
5
0

-D -C
-1.6

-B -A

O

A

C

D

-1.2 -0.8 -0.4
0
0.4
0.8
1.2
Radial distance from the center of spray (mm)1.6

BFigure 5.7 Probability of droplet number along the radial distance before impingement.
Figure 5.7 shows the probability of droplets number along radial distance. All cases show
similar “M” type curves except for Pinj = 20 MPa, Pamb = 0.5 MPa condition. As the spray isquite dense under high injection and ambient pressure condition, it is difficult to detect the
droplets, therefore the probability in the center region is “0”. There are some ligaments and big
137drops near the axis of the spray, called the core of spray. Apart from the axis, the number ofdroplets increases as better breakup and dispersion. Around the periphery, only some tiny
droplets can be detected.
50

Pinj - Pamb
10 MPa-0.1 MPa
10 MPa-0.5 MPa
20 MPa-0.1 MPa
20 MPa-0.5 MPa

45

SMD (μm)

40
35
30
25
20
15

-D -C
-1.6

-B -A

O

A

C

D

-1.2
-0.8
-0.4
0
0.4
0.8
1.2
Radial distance from the center of spray (mm)1.6

BFigure 5.8 SMD distribution along the radial distance before impingement.
Shown in Figure 5.8, it is SMD distribution along the radial distance. It can be found allcases show similar “ġ” type curves, although some differences are observed. The main reason
for it may be summarized as follows: the droplets on the edge of the spray are affected by shear
force between the air and spray envelope, consequently, these droplets may break up into small
ones during they progress into the surrounding gas. It can be also concluded that both injection
and ambient pressure facilitates the atomization of droplets due to SMD decreasing, comparedwith square, circle, and up triangle data. This indicates that the spatial distribution of droplets
also can provide a good estimation of breakup and atomization process, which agrees well withthe observation results in Figure 5.1. One thing should be noted, under Pamb = 0.5 MPa, SMD at
the periphery is a litter larger and then decreases along the radial distance to the axis to someextent, which suggests the evidence that ambient pressure has the effect of coalescence on small
droplets around periphery during the spray. With an increase of ambient pressure, firstly the138

enhanced air drag force promotes the breakup of droplets significantly. Secondly, as theincreased ambient pressure slows droplets down dramatically, when some droplets collide, they
seem easy to coalesce with each other. Additionally, the coalescence effect is more obvious on
the droplets of lower velocity, compared with SMD under Pamb = 0.1 MPa. This coalescenceeffect of ambient pressure was also reported by some previous literatures (Payri et al., 1996;
Lacoste et al., 2003).

5.4.2 After Impingement
The definition of vertical distance from the wall after impingement is shown in Figure 5.9. As
the near-wall region is difficult to analyze, the region within 0.6 mm from the wall is considered
unavailable. Data from the region farther than 0.6 mm from the wall, which has almost triple thearea of the unavailable region can be obtained for analysis and comparison. Starting from a
distance of 0.6 mm from the wall, the available area is subdivided into four regions labeled I–IV
in Fig. 13, with the inter-region dividing lines being lines of equal distance from the wall at 0.4
mm intervals. The droplet number and SMD were examined in each region.
Figure 5.10 provides the probability of droplets number along vertical distance. All cases
under different conditions show similar “>” type curves. Firstly, the number of droplets increases

Vertical distance (mm)along the vertical distance, and then decreases at the periphery.

2.0

Č

1.6

ċ

1.2

Ċ

0.8

ĉ

0.4
SurfaceSu
ce of thehe wallll0After impingement
Figure 5.9 Definition of vertical distance from the wall after impingement.
139

Vertical distance from the wall (mm)

2.2

Ⅳ

2.0

Pinj - Pamb

1.8

10 MPa-0.1 MPa
10 MPa-0.5 MPa
20 MPa-0.1 MPa
20 MPa-0.5 MPa

1.6
1.4

Ⅲ
Ⅱ

1.2
1.0

Ⅰ

0.8
0.6

0

5

10

15

20

25

30

35

40

Probability of droplets number (%)
Figure 5.10 Probability of droplet number along the vertical distance after impingement.

Vertical distance from the wall (mm)

2.2
2.0

Pinj - Pamb

Ⅳ

10 MPa-0.1 MPa
10 MPa-0.5 MPa
20 MPa-0.1 MPa
20 MPa-0.5 MPa

1.8
1.6

Ⅲ

1.4
1.2

Ⅱ

1.0

Ⅰ

0.8
0.6

16

18

20

22

24
SMD (μm)

26

28

30

32

Figure 5.11 Probability of droplet number along the vertical distance after impingement.

140

Regarding droplets size after impingement, results depicted in Figure 5.11. It illustrates that
impingement on the wall make droplets smaller, compared with Figure 5.8. It is worth notingthat SMD decreases from the near wall region to periphery under Pamb = 0.1 MPa, which can be
attribute to better breakup at periphery. However, under Pamb = 0.5 MPa, reversal phenomenon
can be observed that SMD increase from the near wall region to periphery. This result can also
demonstrate that there is coalescence effect on droplets under high ambient pressure. And thecoalescence effect is more obvious on the droplets of lower velocity, in contrast to before
impingement. As mentioned above, during impinging spray, air drag force decelerates droplets,
then the droplets appear to coalesce by the help of vortex and ambient pressure more easily. Thisis also the reason for slight increase in SMD under high ambient pressure compared between
before and after impingement in Figure 5.5.
In summary, Figure 5.12 reveals the schematic of coalescence and breakup model before
and after impingement.
Breakup model: where the droplets break up into small one by the interaction of air, or
droplets collide with a moderate velocity, and then break up into smaller ones.
Coalescence model: where the droplets collide with a quite slower velocity, and thencoalesce into big ones.

(b) Coalescence model(a) Breakup model

Figure 5.12 Schematic of breakup and coalescence models.

141

Ambient pressure

Breakup

BreakupCoalescence

Coalescence

Breakup / Coalescence
Velocity

Figure 5.13 Overview of transition conditions at fixed velocity and ambient pressure.
Figure 5.13 illustrates the transition conditions at fixed velocity and ambient pressure
conditions. Generally, both breakup and coalescence phenomena exit during the spray. When theambient pressure is increased, droplets with lower velocity become collide with each other easily
and coalesce into larger ones at the periphery of the spray. However, that effect becomes weaker
when the velocity of droplets is high. Because the droplets with high momentum can struggle out
after collision, resulting in small droplets again. After impingement, as the vortex changing thedirection of droplets, droplets are easy to collide with each other, resulting more obvious
phenomenon of coalescence, especially on droplets with low velocity.

5.5 COMPARISON OF FUEL ADHESION FORMATION WITH SINGLE
DROPLET ANALYSISThe Ohnesorge number (Oh) is a dimensionless number that relates the viscous forces toinertial and surface tension forces (McKinley et al., 2011).

Oh

μ
ρσd0

(5.2)

Weber number (We) and Reynolds number (Re) are also used governing droplet impact(Schulz et al., 2016).

142

We

ρd 0 v 2
σ

(5.3)

Re

ρvd0
μ

(5.4)

In order to evaluate the influence on fuel adhesion mass correctly, the Re, We and Oh were
calculated in Figure 5.14 and 5.15 respectively. The spray droplets under ambient pressure 0.1
and 1.0MPa are potted in black and white respectively. Additionally, three exemplary splashing
limitations for a wetted wall are shown in Figure 5.14, which are taken from Vander et al.,Kuhnk et al., and Kalantari (2004; 2006; 2007). Actually, these three-limitation equations are
similar to each other, deposition and splashing regions are located on left and right of the lines.
When ambient pressure is 1.0 MPa, the adhered fuel adhesion mass is almost larger than that 0.1
MPa. Although there is some difference, the spray-wall impingement is more complicatedbecause of the effect of vortex. That effect is not obvious when the size of droplet is big(Pamb=1.0 MPa and Pinj=10 MPa. The different size of droplet can explain the fuel adhesion masschanging. In Figure 5.15, although researchers seldom used the We-Oh to separate the spray-wall
impingement behavior, it can be make sense that left and right part represent the deposition andsplashing region respectively.

100
Vander-Wal
KuhnkeKalantariOh

Pamb
0.1 MPa
10-1

1.0 MPa
Deposition

Splashing

Fuel adhesion 0.507
mass (mg)
10-2
101

0.310

0.331 0.587

102

103
Re

0.280
0.340

104

Figure 5.14 Re to Oh under different conditions

143

105

100

Pamb
0.1 MPaOh

1.0 Mpa
10-1

Deposition

Splashing

Fuel adhesion
mass (mg)0.331 0.507

10-2
101

0.310
0.280

0.587

102

0.340

103
We

105

104

Figure 5.15 We to Oh under different conditions

Fuel Adhesion Mass (mg)

0.65
0.60
0.55
0.50

Pamb
0.1 MPa

30 MPa

1.0 MPa

Criteria Obtained byDroplet Experiment

20 MPa

0.45
0.40
0.35

Deposition

Splashing
10 MPa10 MPa0.30
0.25 0
10101

K

20 MPa30 MPa57.7 102

Oh  Re1.25 We 0.5  Re0.25

103

Figure 5.16 Relationship between fuel adhesion mass and K
To delineate the so-called impact regimes from each other, researcher used thedimensionless K number (K) as the splashing threshold parameter (Mundo et al., 1995; Moreira
et al., 2010).

K Oh  Re1.25 We0.5  Re0.25

(5.5)

144

The empirically found K is integrated in most spray-wall impingement models used in CFD.
And Mundo et al. (1995) also found the value of K (57.7) to separate the deposition and splash,which can be seen in Figure 5.16. The spray droplets under ambient pressure 0.1 and 1.0 MPa
are potted in black and white respectively. When K is smaller than 57.7, most droplets deposit on
the wall, otherwise most droplets splash off the wall, resulting left part fuel adhesion mass islarger than right part. The excluded different point (Pamb=1.0 MPa and Pinj=10 MPa) is the sameas Re and We number, and the reason has been explained above.

5.6 SUMMARY
In this chapter, an experimental study was conducted to investigate the microscopic
characteristics of impinging spray under gasoline conditions. PIA was applied to observe themicroscopic spray structure, and droplets were detected by refined criteria. The droplet size
distributions and SMD were calculated and compared under various injection and ambientpressures. The main conclusions are drawn as follows:(1) Before impingement, large droplets and ligaments can be observed. Both injection andambient pressures increase the droplet density by promoting the breakup process. However,under high injection and ambient pressures, the spray droplets are dense, and it is difficult toobserve the ligaments and large droplets. Moreover, after impingement, the near-wall region
becomes very dense as many droplets accumulate, whereas the droplets far away from the wall
are clear and easily observed.
(2) Before impingement, injection pressure has a more positive effect on the distribution of small
droplets, and ambient pressure favors the breakup of large droplets. Furthermore, after
impingement, the probability of small droplets under Pinj = 20 MPa, Pamb = 0.1 MPa is higher
than that under Pinj = 20 MPa, Pamb = 0.5 MPa. And the probability of large droplets under Pinj =10 MPa, Pamb = 0.5 MPa is higher than that under Pinj = 10 MPa, Pamb = 0.1 MPa, which
demonstrates the coalescence effect of ambient pressure.
(3) Before impingement, the probability distribution of droplet number along the radial distancefrom the spray axis shows “M”-type curve. And the SMD distribution along the radial distanceshows “ġ”-type curve. After impingement, the probability distribution of droplet number along
the vertical distance shows similar “>”-type curve. The SMD decreases from the near-wall145

region to the periphery under Pamb = 0.1 MPa, whereas a reversal phenomenon can be observed
under Pamb = 0.5 MPa.
(4) Both the breakup and coalescence behaviors of droplets occur during the impinging spray. At
low velocity, the coalescence phenomenon is more obvious at the periphery of spray under highambient pressure. However, it weakens at increased droplet velocity.
(5) Key factor governing the spray-wall impingement behavior were found. The adhered fuelmass increase with smaller Re, We, and K numbers

146

CHAPTER 6 CONCLUSIONSIn the current study, the impinging spray on the wall by single-hole gasoline injector under nonevaporation and evaporation conditions were investigated experimentally. The high-speed video
observation method (Mie scattering) and Refractive Index Matching (RIM) were implemented
under different engine dynamic operation to visualize the spray and fuel adhesion, respectively.
The characteristics of spray morphology such as spray tip penetration, impinging spray height
were compared during the injection process and the evolution of fuel adhesion such as mass, area
and thickness were analyzed after the injection processes in detail. The effect of injection
pressure (10, 20, 30 MPa) and ambient pressure (0.1, 0.5, 1.0 MPa) were paid attention to firstly,
and then the surface roughness of the impinging wall (Ra 2.5, Ra7.7), and impingement distances(28 and 40 mm) were selected to acquire a better understanding about the impinging spray under
non-evaporation conditions. Furthermore, the effect of ambient temperatures (298 and 433 K) on
fuel adhesion was considered, along with the effect of hole diameter (0.135 and 0.155 mm).
Moreover, droplets behaviors before and after impingement were discussed deeply by Particle
Image Analysis (PIA) equipment. Additionally, the relationship between the fuel adhesion on thewall and the corresponding spray behaviors was investigated under the same conditions of theexperiments as well. The main conclusions are summarized in this chapter.

6.1 FINDINGS OF THE CURRENT STUDY
The motivation and significance of this study were introduced, and a review of the previous
research in this field was conducted to give more background information of the current study in
Chapter1. After that, the experimental approaches applied in this research were introduced invery detail in Chapter 2. The discussion and analysis about the results were in chapter 3-5. The
main findings are classified as following.

6.1.1 Non-evaporation Spray Evolution and Fuel Adhesion Formation
The characteristics of fuel spray and adhesion under different pressures, wall roughnesses andimpingement distances were investigated experimentally under non-evaporation condition inchapter 3. The values of Rs, Hv, S, and Hi were acquired, and the fuel adhesion evolution was

147

analyzed. Furthermore, the probabilities of adhesion and thickness distribution were discussed.
The major conclusions are as follows:x

The fuel adhesion tip is shorter than the spray tip because fuel droplets splash above the wallafter impingement. Fuel droplets still exist in the air above the wall even after the end of
injection. Owing to better atomization with high injection pressure, the fuel adhesion on thewall increases. An increase in ambient pressure improves the uniformity of the fuel adhesion.

x

An increase in wall roughness decreases the spray tip penetration and increases theimpinging spray height. The fuel adhesion mass and area increase with time even after theend of injection. The fuel adhesion becomes slightly thicker in downstream for Ra2.5,
whereas the thicker area moves upstream for Ra7.7. An increase in wall roughness increases
the mass and area of the fuel adhesion. The ambient pressure decreases the differences
between the two types of walls, whereas the injection pressure increases the differences. A
higher roughness results in a higher maximum thickness of the fuel adhesion. An increase inroughness deteriorates the uniformity of the fuel adhesion. Wall roughness affects the fueladhesion thickness along different lines. For x0, the adhesion thickness for Ra7.7 is higher
than that for Ra2.5 at the constant value stage, and the thickness uniformity for Ra7.7 isconsiderably inferior to that for Ra2.5. For y lines, the adhesion thickness for Ra7.7 is higher
than that for Ra2.5 in the upstream region, and the thickness uniformity for Ra7.7 is poor
compared to that for Ra2.5.

xWith a large impingement distance under ambient condition, the velocity of droplets
decreases significantly, resulting in more droplets adhering to the wall instead of splashingout of the wall. As a result, Rs, Hv, and Hi decrease with an increase in impingement distance.
However, after impingement, owing to the stronger drag force by the ambient gas andfriction from the wall, S increases with the increase in impingement distance. Both the highinjection pressure and large impingement distance increase the fuel adhesion mass and area,
but the mechanisms are different. Owing to better atomization with high injection pressure,the fuel adhesion on the wall increases. At a large impingement distance, the lower velocity,
bigger spray width, and better atomization are the main reasons for increased fuel adhesionon the wall after impingement. Under the large impingement distance condition, more fueladheres on midstream and the thickness uniformity of fuel adhesion becomes worse.
148

Moreover, the maximum thickness of fuel adhesion increases with a large impingement
distance.

6.1.2 Evaporating Spray Evolution and Fuel Adhesion Formation
The aim of this study in chapter 4 was to investigate the effect of high temperature on fueladhesion under gasoline engine conditions to understand its mechanism. For this, toluene wasinjected via a single-hole injector on the wall. Mie scattering and RIM methods were applied toobserve the impinging spray structure and fuel adhesion on the wall. The spray tip penetrationand impinging spray height were compared, and the evolution of fuel adhesion along with its
mass, area, and thickness were studied. Finally, the mechanisms of fuel adhesion formation were
established in regions I and II. Moreover, due to the importance in the design of the injector holediameter, especially for the characteristics of atomization, the effect of hole diameter the fueladhesion was checked. RIM experiment was done to measure the fuel adhesion under different
hole diameters of gasoline injector. The main conclusions of this study are summarized asfollows:x

High ambient temperature has a significant impact on the impinging spray and fuel adhesionon the walls of the engine. A high ambient temperature favors the evaporation of fuel, thus
decreasing the spray tip penetration and impinging spray height of liquid phase. Moreover, it
decreases the mass, area, and maximum thickness of the fuel adhesion owing to theevaporation effect. Also, it increases the average thickness owing to the decrease in ratio offuel adhesion area being faster than that of fuel adhesion mass. The fuel adhesion continues
to increase even after the end of injection, because of droplets being re-deposited on the wall.
However, under Tamb = 433 K, the adhered fuel evaporates from the periphery of theimpingement region even where there are still some droplets being re-deposited on the wall,
but the increased adhesion can only be observed in region I. By comparing the evolution offuel adhesion under different ambient temperatures, it can be concluded that high ambient
temperatures lead to more uniform fuel adhesion on the wall.

x

The distributions of adhesion mass were studied in regions I and II. Similar curves wereobserved in region I under different ambient temperatures, whereas there was great variationin region II. Based on these results, different mechanisms were described for the formationof fuel adhesion during and after injection. The primary spray impinging on the wall149

contributed to the formation of fuel adhesion in region I, known as the primary impingement
region. The splashing droplets impinging next on the wall contributed to the formation offuel adhesion in region II, known as the secondary impingement region. Furthermore, highambient temperature exerts more influence on the fuel adhesion formation in region II whencompared to region I.
x

Decreasing the nozzle hole diameter can reduce the fuel adhesion mass, area, average
thickness and maximum thickness owing to the better atomization. However, the large holediameter improves the uniformity of the thickness under both non-evaporation andevaporation conditions.

6.1.3 Microscopic Spray Behaviors
The microscopic characteristics of impinging spray under gasoline conditions were studied inchapter 5. PIA was applied to observe the microscopic spray structure, and droplets weredetected by refined criteria. The droplet size distributions and SMD were calculated andcompared under various injection and ambient pressures. Furthermore, the correlation between
droplets behaviors and fuel adhesion on the wall was discussed in this chapter, too. The main
conclusions of this study are drawn as follows:x

Before impingement, large droplets and ligaments can be observed. Both injection andambient pressures increase the droplet density by promoting the breakup process. However,under high injection and ambient pressures, the spray droplets are dense, and it is difficult toobserve the ligaments and large droplets. Moreover, after impingement, the near-wall region
becomes very dense as many droplets accumulate, whereas the droplets far away from thewall are clear and easily observed. Before impingement, injection pressure has a morepositive effect on the distribution of small droplets, and ambient pressure favors the breakupof large droplets. Furthermore, after impingement, the probability of small droplets under
Pinj = 20 MPa, Pamb = 0.1 MPa is higher than that under Pinj = 20 MPa, Pamb = 0.5 MPa. And
the probability of large droplets under Pinj = 10 MPa, Pamb = 0.5 MPa is higher than that
under Pinj = 10 MPa, Pamb = 0.1 MPa, which demonstrates the coalescence effect of ambientpressure.

x

Before impingement, the probability distribution of droplet number along the radial distancefrom the spray axis shows “M”-type curve. And the SMD distribution along the radial
150

distance shows “ġ”-type curve. After impingement, the probability distribution of droplet
number along the vertical distance shows similar “>”-type curve. The SMD decreases fromthe near-wall region to the periphery under Pamb = 0.1 MPa, whereas a reversal phenomenon
can be observed under Pamb = 0.5 MPa. Both the breakup and coalescence behaviors ofdroplets occur during the impinging spray. At low velocity, the coalescence phenomenon ismore obvious at the periphery of spray under high ambient pressure. However, it weakens atincreased droplet velocity.
x

Key factor governing the spray-wall impingement behavior were found. The adhered fueladhesion mass increase with smaller Re, We, and K numbers. Although combination with thefuel adhesion mass and droplets behavior has been done to find some partial conclusion, thegasoline spray-wall is totally complicated and need more investigation have to be taken to
identify the fuel adhesion formation and impingement process.

6.2 RECOMMENDATIONS FOR FUTURE WORKS
There is much more effort, which can be made in the following study of this field.
For example, the sprays and fuel adhesions in the current study are all injected by thesingle-hole injector, hence the spray evolutions and adhesions by the multiple-hole injector
should also be measured in the further. The application of 2D cavity and even real engine may
release more practical situations.
There are no combustion conditions in the current investigation. The detail mechanism ofimpinging spray combustion characteristics needs to be further explored. As a result, the nozzle
spray combustion processes under the corresponding conditions should be evaluated in detail in
the further.
Although the ambient physical condition implemented in the current study are aiming to
reproduce the real engine operation environment, the turbulent, tumble, and squish flow inside
the combustion chamber all play significant roles in the mixture formation and combustion
process. Therefore, it is recommended to correlate the fuel adhesion formation with combustion
characteristics of impinging spray in optical engine as well.

151

The effect of most basic and important parameters (injection pressure, ambient pressure,ambient temperature, wall roughness and hole diameter) on spray evolution and fuel adhesion areinvestigated in the current experimental study. However, some more crucial factors, such as thewall temperature and injection strategy should also be taken into the consideration when
conduction the fuel spray and fuel adhesion formation study under the practical gasoline
conditions.
The model for impinging spray should be considered in the CFD to increase the accuracy ofsimulation results in the future.

152

ACKNOWLEDGEMENT
I would like to express my sincere gratitude to my supervisor, Professor Keiya NISHIDA,
for his invaluable guidance and constant encouragement throughout the years of my PhD study.
He has been a source of inspiration and motivation for my research. Without his guidance andsupport, this research could not be completed properly.
I am also greatly acknowledged Professor Yukihiko MATSUMURA, Associate Professor
Youichi OGATA and Associate Professor Shuhei INOUE. They gave me so much positive help
on the research, the accumulation of the knowledge and my life, and I have learnt lot from them.
In addition, I would like to express my heartfelt thanks to Dr. Kim for his technical in reviewing
my research.
My great appreciation also goes to other members of our gasoline spray group throughout
the research, Mr. Uchitomi, Mr. Watanabe, and Mr. Ichikawa for their help in setting up andcollecting data of this work. I also show my appreciation to Mazda Motor Corporation, not only
for the finical support, but also technological support. Especially I would like to thank Dr. Zhang
from Mazda for his dedicated supports, insightful comments and guidance in my study.
Then I want greatly thank Mr. Si, Mr. Yang and Mr. Rizal, who came to the group with me
together. We studied together and helped each other for research and living. Moreover, I would
also like to thank all the past and present members of Fluid Engineering and Spray/Simulation
Technology Laboratory, in Department of Mechanical Systems Engineering, Hiroshima
University, for their enthusiastic help to both of my life and study.
Finally, my thanks would go to my beloved family for their loving considerations and great
confidence on me all through these years. Last but not the least, I owe special thanks to my wife,
Dr. Yu, for being supportive and patient throughout the years, for her love and encouragement.

153

LIST OF PUBLICATIONSInternational Journals
1. Hongliang Luo*, Shintaro Uchitomi, Keiya Nishida, Youichi Ogata, Wu Zhang, Tatsuya
Fujikawa. Experimental Investigation on Fuel Film Formation of Spray Impingement on Flat
Walls with Different Surface Roughness. Atomization and Sprays, 27(7): 611–628, 2017.
(SCI, IF=1.235) DOI: 10.1615/AtomizSpr.2017019706
2. Hongliang Luo, Keiya Nishida*, Shintaro Uchitomi, Youichi Ogata, Wu Zhang, Tatsuya
Fujikawa. Microscopic behavior of spray droplets under flat-wall impinging condition. Fuel,219: 467-476,2018. (SCI, IF=4.908) https://doi.org/10.1016/j.fuel.2018.01.059
3. Hongliang Luo*, Keiya Nishida, Shintaro Uchitomi, Youichi Ogata, Wu Zhang, Tatsuya
Fujikawa. Effect of Spray Impinging Distance on Piston Top Fuel Adhesion in Direct
Injection Gasoline Engines. International Journal of Engine Research,2018 (Online First).
(SCI, IF=2.775) https://doi.org/10.1177/1468087418774175
4. Hongliang Luo*, Keiya Nishida, Shintaro Uchitomi, Youichi Ogata, Wu Zhang, Tatsuya
Fujikawa. Effect of Temperature on Fuel Adhesion under Spray-Wall impingement
Condition. Fuel, 234:56-65,2018. (SCI, IF=4.908) https://doi.org/10.1016/j.fuel.2018.07.021

International Conferences
1. Hongliang Luo, Shintaro Uchitomi, Keiya Nishida, Youichi Ogata, Wu Zhang, Tatsuya
Fujikawa. Fuel Adhesion Characteristics of Flat Wall-Impinging Spray under DISI Engine
Conditions. 29th Annual Conference on Liquid Atomization and Spray Systems-Americas(ILASS-Americas 2017), 15-18, May, 2017, Atlanta, GA.
2. Hongliang Luo, Shintaro Uchitomi, Keiya Nishida, Youichi Ogata, Wu Zhang, Tatsuya
Fujikawa. Effect of Impingement Distance on Fuel Adhesion of Hole-Nozzle Spray under
Various Injection Pressures. The 9th International Conference on Modeling and Diagnostics
for Advanced Engine Systems (COMODIA 2017), 25-28, Jul, 2017, Okayama, Japan.
3. Hongliang Luo, Keiya Nishida, Shintaro Uchitomi, Youichi Ogata. Fuel Adhesion
Characteristics of Flat-Wall Impinging Spray under Gasoline Engine Conditions. The
International Conference on Advances in Construction Machinery and Vehicle Engineering(ICACMVE 2017), 14-16, Sep, 2017, Qinhuangdao, China.
154

4. Hongliang Luo, Shintaro Uchitomi, Keiya Nishida, Youichi Ogata. Fuel Adhesion
Characteristics by Spray Impingement on the Flat Wall. The 1st International Symposium on
Fuels and Energy, 10-12, Jul, 2017, Hiroshima, Japan.
5. Hongliang Luo, Keiya Nishida, Youichi Ogata. Characteristics of gasoline fuel adhesion on
the impingement wall. KAUST Research Conference: Combustion in Extreme Conditions,
5-8, Mar, 2018, King Abdullah University of Science and Technology, Sandi Arabia.
6. Shintaro Uchitomi, Hongliang

Luo, Keiya Nishida, Youichi

Ogata. Adhesion

Characteristics of Fuel Spray Impinging on Flat Wall under High-Temperature and HighPressure Ambient Condition. 18th Annual Conference on Liquid Atomization and Spray
Systems-Asia (ILASS–Asia 2017), 18-21, Oct, 2017, Jeju Island, Korea.
7.

Keiichi Katayama, Gnanasekaran KishorKumar, Hongliang Luo, Keiya Nishida, Yoichi
Ogata. Spark-Ignited Combustion Characteristics of Gasoline Spray Injected by Hole-TypeNozzle for Direct Injection Engine. 18th Annual Conference on Liquid Atomization andSpray Systems-Asia (ILASS-Asia 2016), 7-9, Nov, 2016 Chennai, India.

155

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