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Experimental Investigation on Low Pressure Gas Jet
Characteristics by Tracer-based PLIF Technique
Jingzhou Yu*, Harri Hillamo, Teemu Sarjovaara, Tuomo Hulkkonen, Ossi Kaario and Martti Larmi
Aalto University School of Science and Technology
Puumiehenkuja 5A
Espoo
Finland
firstname.lastname@tkk.fi
*corresponding author
ABSTRACT
Gas fuels such as natural gas and hydrogen are regarded as promising alternative fuels to
resolve the problems of energy shortage and environmental pollution for conventional
internal combustion engine. The better understanding of the effects of gas jet on
concentration distribution and mixing process is helpful for combustion and emission
optimization. The motivation of this work is to gain further insight into the characteristics
of low pressure gas jet. An experimental gas jet investigation has been successfully
conducted using tracer-based planar laser-induced fluorescence (PLIF) technique. Vapor
acetone is used as a gas fuel tracer. A signal synchronization system is used to obtain the
jet at an arbitrary injection delay time. Fluorescence signal which could be detected by a
high-resolution intensified charge coupled device (ICCD) results from the excitation of
acetone with a frequency-quadrupled Nd:YAG laser (266nm). A series of single shot
images captured in different intervals is used to study the time evolution of the spatial
distribution and gas concentration fields and fuel-air mixing processes. The effect of gas
injection pressure is studied to characterize the mixture formation process and the
macroscopic structure. The research results are used to support our ongoing low pressure
direct injection natural gas and diesel pilot ignition engine project.
Keywords: gas fuel jet, mixing process, planar laser-induced fluorescence (PLIF)
1. INTRODUCTION
With concerning energy crisis and environment protection, a higher efficiency internal
combustion engine with lower emissions is crucial for engine manufacturers. However,
conventional internal combustion engines have been nearly fully developed by using
variety of modern high-tech methods. Further improvement for internal combustion
engine might be limited by their structure and working principles, as well as the cost
efficiency. Currently, gas fuels such as natural gas and hydrogen are regarded as
promising alternative fuels to resolve both the problems of energy shortage and
environmental pollution, and there is a great effort in industry and engine research centres
to make these gas fuels viable for engines. In particular, natural gas engines have been
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attracted by many engine research scientists, since not only it is the cleanest fossil fuel,
but also there are huge natural gas reserves in the world [1]. In some countries, like
Germany, the low natural gas taxation rates make this fuel an increasingly interesting
alternative for drivers. Although natural gas for intake manifold injection has been
successfully used in passenger cars and city buses in the past decade, natural gas for
direct-injection compression ignition (DICI) engines are considered to be the final target
due to high volumetric efficiency, the high thermal efficiency and low emissions.
However, there are still some inherent difficulties to obstruct the improvement of DICI
gas engines. For example, the mixture formation process for gas-fueled engines differs
considerably from that of liquid-fueled compression ignition (CI) engines and spark
ignition (SI) engines. In the past decades, most of previous studies focused on liquid fuel
spray and atomization and got a great achievement. Based on the previous research
results, it is well known the injection strategy and injector structure play an important role
for the liquid fuel spray, and the mixing process of liquid fuel spray is largely dependent
upon the droplets atomization and evaporation. In contrast with liquid fuel spray, gas jet
doesn’t have liquid sheets and droplets at all, but the gaseous fuel-air mixture formation
is less efficient due to the low momentum of the jet [2]. On the other hand, the structure
of gas injector should be fully different from the conventional liquid fuel injector due to
the tremendous difference between gas and liquid on their physical properties.
Actually, numerous studies of high pressure gas jet have been conducted using different
optical diagnostic techniques in the past. For example, methane jet characteristics at
injection pressure 180bar in a direct-injection natural gas engine (in-cylinder pressure
20bar, fixed piston) was visualized and analyzed using planar laser-induced fluorescence
(PLIF) technique of by Rubas et al.[3]. Tsujimura et al.[4] investigated the effects of jet
developing process (injection pressure 60-120bar, shadowgraph imaging system) and
thermodynamic states of the ambient gas on auto-ignition delays of hydrogen jets in a
constant chamber (ambient pressure 5-15bar). Their work suggests that the ambient gas
temperature and nozzle hole diameter are significantly effective parameters. Moreover,
their conclusion supposes that the mixture formation process is capable of improving the
auto-ignition and combustion of hydrogen jets. Bruneaux [5] used PLIF technique to
study the mixture formation of natural gas jet (injection pressure 110-150bar, chamber
pressure 13-27bar) in a direct injection diesel like conditions constant volume chamber.
An optical study of mixture preparation with different nozzle designs (injection pressure
80-116bar) in an optically accessible hydrogen fueled engine was investigated using PLIF
technique by Salazar et al.[6].
On the other hand, high pressure injection needs high performance gas injector (no
lubrication) and high pressure of gas needs too much energy consumption from
compressor. From this point of view, low pressure gas jet should be preferred for future
gas combustion engine. The purpose of this work is to gain further insight into the
characteristics of low pressure gas jet. The remaining paper begins with a description of
the experimental apparatus and optical arrangements. A brief outline of tracer-based PLIF
technique is given. The results and discussion are then presented showing the
characteristics of gas jet and gas-air mixing process with varying injection pressure.
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2. Experimental Procedures and methods
2.1
Experimental set-up
The general schematic of the gas jet PLIF measurement system is shown in figure 1. It
mainly includes three parts: gas jet system, tracer seeding system and PLIF
instrumentation. For all of the tests performed the gas being injected is N2 rather than
natural gas for safety reasons.
In order to add tracer to gas, a pressure vessel which is approximately 2/3 full of liquid
acetone is used as an acetone seeder. The gas from the high pressure gas bottle is split
into a main flow and a bypass flow. The bypass flow is directed through the acetone
seeder with bubbling. There are two ways to adjust the concentration of acetone in the
gas flow. Firstly, it can be easily adjusted by the two needle valves on the main flow pipe
and the bypass flow pipe. Secondly, it is determined by the partial pressure of acetone in
the seeded gas. So the acetone temperature could be adjusted flexibly by the water to
change the acetone pressure in the pressure vessel. To avoid acetone backflow, a check
valve is installed upstream of gas supply pipe near the pressure vessel.
In order to weaken the fluctuation of jet pressure and keep the constant injection rate
during an injection, an accumulator is connected with the injector. The injection pressure
can be controlled and adjusted by a pressure regulator near the high-pressure bottle. The
gas injector was installed on the bottom of the spray chamber. The maximum gas jet
pressure is 7bar, and the pressure and temperature in the chamber are the room
conditions. In this experiment, the concentration of acetone is enough without hot water
outside of the acetone pressure vessel.
Figure 1: Schematic diagram of experimental setup for tracer-based PLIF imaging system
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2.2
Gas fuel Injector selection
In this study, a commercial Bosch NGI2 natural gas injector was used, as shown in figure
2. It is a solenoid-valve injector, and its maximum injection pressure was 7bar (absolute
pressure). It is driven by a standard injector driver, and the supply voltage is 14V with
dead time 0.45ms. Figure 3 shows the injection rate with different injection pressure
using NGI2 natural gas injector. The test medium is nitrogen (N2) at 23℃.
Volume Flow Rate (ml/min)
2,3
2,2
Flow Rate
3200
2,1
2,0
1,9
2800
1,8
1,7
2400
1,6
1,5
3
4
5
6
Average Flow Velocity of Exit /(m/s)
Figure 2: Bosch NGI2 natural gas injector
7
Injection Pressure (Bar)
Figure 3: Flow rate of BoschNGI2 natural gas injector
2.3
General Principle of PLIF
Laser-induced fluorescence (LIF) which has received considerable attention is a widely
used non-intrusive optical diagnostic tool for making temporally and spatially resolved
measurement of the concentration as well as temperature distribution of combustion
species for many years. It can detect flame radical and pollutant species at the ppm or
even sub-ppm level [7]. In particular, LIF is attractive for use in high speed flows due to
its excellent temporal and spatial resolution. The physical principle of LIF is quite simple.
It can be understood as a laser light absorption process followed by light emission
(fluorescence). In LIF measurement system, a laser is tuned to a wavelength matching
that of some absorption line of the atom or molecule of interest. That species is then
elevated to an electronically excited state, from which it fluoresces, and this fluorescent
emission is then detected [8]. The resulting fluorescence signals could be imaged with
charge-coupled device (CCD) or intensified charge-coupled device (ICCD) cameras. The
concentration (mole fraction) and temperature fields can be derived from calibrated LIF
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images. Generally, there is a monotonic relationship between the tracer concentration and
the fluorescence intensity. Higher intensity indicates higher concentration. Detail
explanations of the fluorescence mechanism are given in many references [7-12].
Usually, the laser light sheet technique that the laser beam is expanded in one direction
with a group of cylindrical lenses is applied in LIF measurement system for observation
of the instantaneous two-dimension of concentration and temperature of the flow field.
This kind of extended LIF technique is called planar laser-induced fluorescence (PLIF).
PLIF has been commonly used for remote detection and non-intrusive imaging of the
gaseous fuel concentration and combustion temperature by many engine researchers in
last twenty years.
2.4
Tracer Selection
Nearly all aliphatic hydrocarbons, such as Methane (the main component of natural gas)
and propane, are transparent within the spectral range of interest, and there is no
fluorescence signal emission when the laser goes through them [11]. Moreover, most
laser pump systems are only capable of producing discrete wavelengths, thus limiting the
choice of excitable molecules [13]. For example, the Quanta-Ray Lab-Series Pulsed
Nd:YAG Laser is commonly used as a excited light source in PLIF measurement system,
but it is a pulsed oscillator-only system that has four kinds of output wavelengths (1064,
532, 355 and 266nm). Therefore, it is of great importance that an appropriate
fluorescence tracer should be selected for indirect visualization of the non-fluorescence
fuel concentration distribution. For these reasons, the method widely used for measuring
the fuel concentration distribution in the past few decades is the so-called Tracer-based
PLIF.
Existing studies [9, 12, 14-16] indicate that acetone (CH3COCH3) seems to be an ideal
tracer for PLIF concentration measurements in gaseous flows. Acetone nearly meets all
above requirements as a fluorescence tracer. It has good signal levels, low toxicity, a high
saturation vapor pressure (24kPa at 293K). Moreover, acetone has not only a broad
absorption band stretching from 225nm to 320nm with a peak between 270nm and
280nm, but also a broad fluorescence emission, ranging from 350nm to 550nm with a
peak near 445nm and 480nm. Therefore, it allows straightforward experimental
implementation of acetone PLIF measurement. Taking into account the above reasons,
acetone as a tracer is doped into the gas flow in this experiment. However, acetone should
be evaporated before mixing with the gas fuel, because it is liquid state under room
conditions. The acetone seeding system will be explained in the following subsections. It
is important to note that PLIF signal is strongly dependent on temperature, therefore our
measurements will be done at constant room temperature to ensure relatively
homogeneous temperature field.
2.5
PLIF optical measurement system
In this work, the gas jet concentration field measurements were performed using a PLIF
system by LaVision. The laser source for this experiment is a Spectra-Physics Lab-170
pulsed Nd:YAG laser. The source is able to produce high energy (850mJ/pulse) pulses
high repetition rate (10Hz) but short duration (8~10ns) at fundamental wavelength
1064nm. In order to excite the tracer acetone, the fourth harmonic generator (FHG) of the
laser which can generate 90mJ pulse energy and 8ns duration and 10Hz repletion rate at
wavelength 266nm has been used. The laser pulse is of sufficiently short duration to
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provide essentially instantaneous images of the fuel distribution. Moreover, in order to
measure two-dimension gas jet, the laser beam is transformed into a collimated thin light
sheet by using a combination of two spherical lenses and one cylindrical divergence lens.
The quality of the laser beam and laser sheet is very important for LIF measurement
results. In this experiment, the high quality of the beam and sheet was achieved, as shown
in figure 4. The diameter of beam is 8mm and the thickness of sheet is about 0.25mm.
Then the light sheet passes through the fuel jet axis of symmetry. Fluorescence signal of
acetone is collected by a charge-coupled device (CCD) camera (12bit, 1376×1040
pixel2, pixel size 6.45µm, and 10frames/second). To increase the fluorescence signals
level, a gated intensifier ( ∆t = 100ns ) with a Nikon 94mm f/4.1 lens is associated with
the CCD camera. Moreover, a UV filter was mounted in front of the image intensifier to
simultaneously eliminate laser beam light and Mie scattering light so that only
fluorescence could be captured. The location between ICCD camera and laser light sheet
is at 90 degree also shown in figure 1.
Figure 4: The profile of laser beam and light sheet
2.6
Signal synchronization system
Signal synchronization system plays an important role in the whole experimental work. In
order to capture the gas jet images, the injection triggering and the signals of laser as well
as imaging triggering must be synchronized. In this experiment study, programmable
timing unit (PTU) which is controlled by LaVision software is used to synchronize these
signals. Figure 5 shows the timing sequence of triggering pulses of synchronization
system developed by this study. The triggering delay for gas injector ∆T1, the triggering
delay for laser ∆T2 and the triggering delay for imaging ∆T3 as well as CCD exposure
duration ∆T8 and injection duration ∆T9 can be easily adjusted from LaVision software.
The triggering delay ∆T4, ∆T5 and ∆T6 are intrinsic delays of the ICCD system which
are determined by the travelling time of the signal pulse through the components. ∆T7 is
the cathode gate duration of intensifier. It can be set by the image intensifier control unit.
The cathode gate of intensifier should be decreased to about 100ns to minimize the
effects of ambient light. It is worth mentioning that the since a camera is combined with
an intensifier, the effective exposure time of CCD camera is determined by the cathode
gate of intensifier. The gate should be always inside the camera exposure time. Moreover,
in order to capture the fluorescence images by CCD camera, the signals for laser should
be triggered inside the cathode gate. This can be adjusted by setting ∆T2 and ∆T3. In
addition, the injector signals can be delayed by the PTU so the fluorescence images can
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be captured at another arbitrary timing of developing spray. In this experiment, the
injection duration time ∆T9 was fixed at 3.0ms.
Figure 5: Timing sequence of triggering pulses and data acquisition cycle of ICCD
camera
3. Results and Discussion
Detailed knowledge and information of in-cylinder mixing process and concentration
distribution are crucial for the combustion efficiency and low pollutant emissions of the
modern combustion engines. Based on the tracer PLIF method, PLIF images of gas jet
were obtained successfully. This section will give some detailed information of the
characteristics of low pressure gas jet according to experimental PLIF images.
3.1
Initial stage pattern of gas jet
The time evolution of the transient gas jet could be investigated by capturing a sequence
of images using different gas jet delay times ∆T1 (as show in figure 5). Figure 6 shows a
series of single shot images of initial stage of gas jet at different injection timing with
injection pressure 7bar. Due to intrinsic delay (the travelling time of the signal pulse
through the components) of the whole experiment system and the camera image area is
slightly higher than the injector nozzle tip, the first PLIF image could be achieved only
after start of injection (ASOI) trigger at 0.9ms. In other words, the time at the left-top
corner on each single shot image is an injection delay time. The qualitative fuel
concentration distribution is indicated by the intensity of fluorescence signal. Higher
intensity means higher gas fuel concentration. It is worth mentioned that the experimental
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parameters, such as laser pulse energy, cathode gate of intensifier, intensifier gain and
acetone tracer concentration, are constant during the experiment.
As shown in figure 6, it is obvious that the low pressure gas jet is not a laminar flow but
an intensely turbulent flow. The large-scale motion could be also noticed. At the initial
stage of single shot gas jet (from ASOI trigger 0.9ms~1.7ms), the distribution of fuel
concentration fields presents quite steep at the sides and the tip of jet. It means that the
gas jet mixing efficiency is poor. One reason is that high flow rate leads to high
momentum of the gas jet at the initial stage. So the jet could not be easily influenced by
the surrounding air. In other words, the jet is strong enough to overcome the resistance of
gas medium. Another reason is too short mixing time between fuel and surrounding air at
the initial stage. As time elapses, although the penetration of the jet is increasing, the
accumulation of the fuel results in a higher concentration distribution.
In addition, it can be seen that the high fluctuation region occurs at the tip of jet at the
initial stage of jet. It indicates that the jet tip could be easily influenced by the
surrounding air, although the jet has more momentum at the initial stage. To our
knowledge, this phenomenon is quite different with liquid fuel spray. For high pressure
liquid fuel spray, it nearly can’t be affected by the surrounding air at the initial stage,
because its high flow rate leads to high momentum. For low pressure liquid spray, it can’t
be also affected by the surrounding air because there is a long intact liquid core with high
momentum at the start of the injection.
Figure 6: Time evolution single shot images of initial stage of gas jet (Pinj=7bar)
3.2
Development stage pattern of gas jet
Figure 7 shows single shot images of full development stage of gas jet at injection
pressure 7bar. In contrast with the initial stage, the development stage of jet not only
shows the large-scale structure, but also displays that the air-entrainment phenomenon
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occurs at the sides of jet. The air-entrainment phenomenon indicates the jet is more easily
affected by the ambient air. It can be seen the high fluctuation regions occur at the tip of
jet and in the center of the jet. According to figure 7, the concentration distribution in the
center of jet is increased slightly from ASOI trigger 1.8ms to 2.2ms. However, the
concentration in the center of jet is decreased a little bit after ASOI trigger 2.4ms. In
addition, the center of the jet has been broken to several high concentration parts. The
cause would be the decreased momentum of jet as time goes by, so the jet is easily
influenced by the turbulence and the entrainment air.
It worth to be noted that the concentration distribution near the injector nozzle is quite
low in the last image (ASOI trigger 3.9ms) from figure 7. This image shows the pattern
of the end of jet. In this experiment the injection duration is 3.0ms, and the intrinsic delay
of the injection system is about 0.8ms. So the injection signal should be off at ASOI
trigger 3.9ms. As we know that the injector can’t be fully closed at once when the
injection signal is off. Therefore, the gas could still flow from the injector after the
injection trigger is off.
Figure 7: Time evolution single shot images of development stage of gas jet (Pinj=7bar)
In general, the concentration distribution of jet at the full development stage is more even
than that at the initial stage. Although the large-scale motion which entrains air into the
jet mainly occurs at the early jet stage, there is not enough time for fuel-air mixing with
each other. At fully developed stage, the small-scale motion plays an important role in the
jet, so there are more opportunities for fuel-air to molecularly mix at Kolmogorov scale,
especially at the sides and the tip of the jet.
3.3
Structure and mixing process of gas jet
Figure 8 shows the comparison of selected instantaneous images of well development jet
and the corresponding average image of fuel concentration distribution at injection
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pressure7.0bar and at ASOI trigger 3.0ms. Figure 8 (a), (b) and (c) show the variation of
single shot-to-shot image under same experimental conditions. Although the jet
penetration of each single shot image is in the same range, the jet pattern and local
concentration fields are significantly different from each other. The border of each single
shot image is also quite irregular, especially far away from the injector. The variation of
injection-to-injection and intensely turbulent flow of gas jet would be the major reasons.
It might be another reason that the low momentum of gas jet could be easily and
randomly disturbed by surrounding air. This type of behaviors is also seen in liquid
sprays.
Figure 8(d) displays the corresponding average image based on 30 single shot images. It
can be seen that the structure and concentration field between instantaneous image and
average image are definitely different. The fuel concentration distribution field of the
corresponding average image shows a progressive decay from the center to the border,
but the instantaneous jet concentration profiles are characterized by sharp decay both in
the center and at the sides.
(a) Single shot
(b) Single shot
(c) Single shot
(d) Average
Figure 8: Comparison the instantaneous images and average image of fuel concentration,
(a)~(c): single shot images; (d) corresponding average image based on 30 single shot
images (ASOI trigger 3.0ms, Pinj=7bar)
The air-entrainment which is caused by the shear induced turbulence and momentum
exchange between fuel and air is the dominant factor for gas jet mixing process. The
main mechanism for entrainment is through engulfment of ambient air along the
upstream edge of the large-scale structures [17]. Subsequent mixing occurs as the
interface formed between the entrained ambient air and the jet gas is stretched, and both
jet and entrained air come into contact at ever smaller scales until the fluid from the two
streams becomes molecularly mixed at the Kolmogorov scale [17]. Figure 9(a) displays
the conceptual profile for the entrainment and mixing process of axisymmetric gas jet. It
shows that the axisymmetric gas jet has a well organized and large-scale structure. A
rather clear single shot PLIF image of jet is shown in figure 9(b). The air-entrainment
area (red arrow as shown in figure 9(b)) in the experiment image further verifies the
proposed concentration field and mixing process as shown in figure 9(a). The gas jet has
been broken into several high concentration parts shown in figure 9(b). Probably
turbulence phenomenon is the main reason. This behavior is common in liquid fuel spray.
Figure 9(c) is the velocity field of diesel spray measured by particle image velocimetry
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(PIV) technique [18]. It indicates that the liquid fuel spray can also be strongly influenced
by the turbulence, especially at the stage of development.
Based on the concentration distribution, the vortex regions (white circle as shown in
figure 9(b)) could be seen at the sides but near the tip of jet. During the gas injection, the
still ambient air is pushed out by the jet tip, and then the vortex structure is formed by the
interaction between the jet and the ambient air. This kind of vortex structure could also
easily happen in flash boiling spray [19] and swirl injector spray [20]. The more basic
reason for vortex structure should be also seen in liquid sprays or flash boiling sprays tip.
Actually, both the vortex formation and vortex broken are quite useful for fuel-air mixing
and the mixture diffusion.
(a) Air-entrainment
model
(b) Concentration field of gas jet
(PLIF image)
(c) Velocity field of diesel
spray (PIV image)
Figure 9: Concentration field and entrainment pattern of jet, (a) conceptual profile for the
entrainment and mixing process [17]; (b) single shot PLIF image (ASOI trigger 3.0ms,
Pinj=7bar); (c) Velocity field of diesel spray measured by PIV technique [18]
3.4
Comparative study: the influence of jet penetration
Figure 10 shows the effect of injection pressure on the full development of gas jet
structure and concentration distribution. Figure 11 displays the concentration distribution
of the corresponding average image. It can be seen that higher injection pressure leads to
longer penetration in the same injection time delay. The fluctuation at the tip of jet is
more obvious for higher injection pressure. Comparison with the gas jet of higher
injection pressure, the lower injection pressure not only leads short jet tip penetration, but
also results in higher concentration field in the center of jet. The reason is that gas jet
mixing process is mainly controlled by the shear induced turbulence which occurs at the
sides of jet. Higher injection pressure means higher gas flow rate and higher jet
momentum, and then higher mixing efficiency. The increased jet momentum also leads to
higher air-entrainment between gas fuel and air.
Figure 12 shows the temporal variations of jet tip penetration at injection pressure 7bar
and 3bar. The penetration is measured from the nozzle tip to the jet tip based on the
average images. Like liquid fuel spray, the gas jet tip penetration increases as time goes
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by. The injection pressure has a significant effect on the jet penetration. Higher injection
pressure leads to longer jet tip penetration. At the initial jet, the discrepancy of jet tip
penetration is quite small between injection pressure 7bar and 3bar, but this difference is
increased later on.
(a) injection pressure 7bar
(b) injection pressure 3bar
Figure 10: Effect of injection pressure on the instantaneous low pressure gas jet
characteristics (ASOI trigger 3.0ms)
(a) injection pressure 7bar
(b) injection pressure 3bar
Figure 11: Effect of injection pressure on the concentration fields of average images
(ASOI trigger 3.0ms)
50
Pinj 7bar
Jet Penetration/ (mm)
40
Pinj 3bar
30
20
10
0
1,2
1,6
2,0
2,4
2,8
3,2
3,6
4,0
4,4
ASOI Trigger/ (ms)
Figure 12: Temporal variation of jet tip penetration of different injection pressure
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4. Conclusion
The characteristics of low pressure gas jet were investigated experimentally by means of
tracer based planar laser-induced fluorescence (PLIF) technique. Specifically the gas jet
structure and mixing process were analyzed based on both single shot images and average
images. The following conclusions are derived from this study.
1. The large-scale motion is existing in the gas jet. Small-scale motion is main existing
in the development stage of jet. Fuel-air molecular mixing process mainly depends
on the small-scale motion.
2. The fuel concentration distribution of average field shows a progressive decay from
the center to the border, but the instantaneous jet concentration profiles are
characterized by sharp decay in the center and at the sides.
3. Fully developed gas jet is broken into several high concentration parts by turbulence.
4. The gas jet mixing rate is mainly controlled by the shear induced turbulence. High
injection pressure leads to higher jet momentum and turbulence energy, which means
that fuel-air mixing rate could be increased by increasing injection pressure.
5. References
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[9]
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N. Wermuth and V. Sick, Absorption and Fluorescence Data of Acetone, 3Pentanone, Biacetyl, and Toluene at Engine-Specific Combinations of
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M. Tamura, T. Sakurai, and H. Tai, A Study of Crevice Flow in a Gas Engine
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M. C. Thurber, Acetone Laser-Induced Fluorescence for Temperature and
Multiparameter Imaging in Gaseous Flows, Topical Report TSD-120, 1999.
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