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ILASS Americas, 25th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013
Effects of Spray Characteristics of Emulsified Diesel on Soot Emissions in a Constant Vol-
ume Chamber
Ming Huo1, Han Wu
2,1, Nan Zhou
3,1, Karthik Nithyanandan
1, Chia-fon F. Lee
*1.4
1Department of Mechanical Science and Engineering
University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
2School of Automobile, Chang’an University, Xi’an, Shaanxi, 710064, China
3State Key Laboratory of Automotive Simulation and Control,
Jilin University, Changchun 130025, China
4Center for Combustion Energy and State Key Laboratory of Automotive Safety and Energy
Tsinghua University, Beijing, 100086, China
Abstract
Emulsions of diesel and water are a potential solution to meet the increasingly stringent emission regulations as they
are able to simultaneously reduce both nitrogen oxide (NOx) emissions and particulate matter (PM) from diesel en-
gines. The PM reduction capability is often associated with the unique atomization characteristic known as micro-
explosion. In this work, emulsified diesel, with the amount of water ranging between 10% and 20% by volume, was
injected and combusted in an optical constant volume chamber. With controlled combustion of an acetylene mixture
prior to fuel injection, the chamber is able to provide high ambient temperature and pressure to mimic the real en-
gine operation. In this study, ambient temperatures ranging from 800K to 1200K were investigated. Mie scattering
images, at 15000 fps, were first taken to record the evolution of the spray using a Phantom 7.1 high speed CCD
camera coupled with a copper vapor laser as the light source. The spray images revealed longer liquid penetration
and wider cone angle at the beginning stage of the injection event for emulsified fuel, supporting the occurrence of
micro-explosion. The integrated broadband luminosity suggested that emulsified fuel coupled with low temperature
combustion may optimize the emission control strategy.
*Corresponding author: [email protected]
ILASS Americas, 25th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013
Introduction
The simultaneous reduction of nitrogen oxides
(NOx) and particulate matter (PM) emissions without
after-treatment devices remain a challenge for compres-
sion ignition engines. Advanced combustion concepts
such as homogeneous charge compression ignition
(HCCI), reactivity controlled compression ignition
(RCCI), low temperature combustion (LTC) have been
developed in recent years to lower the NOx and PM
emissions. These technologies are generally associated
with engine modifications to some extent: highly ad-
vanced injection/port fuel injection for HCCI, heavy
EGR introduction for LTC, dual fuel utilization for
RCCI, etc. In this respect, water emulsified fuel re-
mains an attractive solution to reduce the NOx and PM
in exhaust gas simultaneously since it can be introduced
into the current fleet of diesel engines with no major
modifications on the engine. [1]
Numerous studies have been conducted to evaluate
the engine performance by replacing diesel with emul-
sified fuel. [2-10] Hironori et al. [2] investigated the
feasibility of using water-in-oil type emulsified fuels in
small DI diesel engines and reported that lower fuel
consumption and NOx emission can be achieved by the
control of injection timing for W/O emulsified fuels
with water content ratio less than 20%. Hountalas et al.
[3] showed comparative evaluation of EGR, intake wa-
ter injection and W/O emulsion as NOx reduction tech-
niques for a heavy duty diesel engine. Watanabe and
Tamura [4] introduced mono-dispersion emulsified fuel
produced by their own original way using multi-porous
glass and reported effectiveness of mono-dispersed
emulsified fuel on simultaneous reductions of NOx, PM
and fuel consumption under slow engine speed condi-
tion of 1000 RPM. Masatoshi et al. [5] studied the in-
fluence of fuel injection timing and water content on
diesel engine performance and concluded that influence
of the water is mainly attributed to the micro-explosion
feature of emulsified fuel because the water presented
in the combustion chamber by direct water injection
method has almost no influence on engine performance
except lowered NOx emission. The impact of cooling
loss reductions on engine performance with emulsified
fuel operation was investigated by Yasufumi [6] and it
was determined that the reduced cooling loss improved
indicated thermal efficiency. In comparison, optical
studies using laser diagnostic methods for the emulsi-
fied fuels are rarely found in the literature [7-10]. Raul
et al. [7] reported the spray and combustion characteris-
tics of water-in-diesel emulsions and micro-emulsions
in an optical chamber. Longer droplet penetration was
reported because of the lower volatility of the water.
The presence of “glowing spots” was observed in the
flames of the emulsified fuel which was attributed to
micro explosion.
In general, the following effects on engine perfor-
mance are expected with the application of emulsified
fuel: the adiabatic flame temperature is lowered with
the presence of water due to water evaporation heat ,
thus NOx emission decreases. Carbon monoxide usual-
ly declines due to more H2O available for the water gas
shift reaction. Soot is also reduced, explained by a
number of reasons: micro explosion of emulsified fuel
enhances atomization and evaporation of the spray in-
jected in the chamber, providing better fuel-air mixing;
the ignition delay is usually extended providing more
mixing time thus shift the combustion mode towards
premixed combustion; the OH release from water dis-
sociation also helps oxidizing the soot. [1]
In spite of the intensive studies on droplet micro-
explosion [11-16], the micro-explosion phenomena are
not expected to be visually observed under high pres-
sure conditions like engine combustion. The challenge
results from the fact that the water nuclei formation and
bubble growth in an emulsified fuel droplet actually
takes time before they can “break out” the entrapping
fuel. This time scale is believed to be longer than the
primary breakup time scale and comparable to the sec-
ondary breakup time scale. Therefore in theory, the
micro-explosion is expected to take place downstream
of the spray jet, where fine droplets may already exist
due to the combined effect of a number of spray physics
such as secondary breakup, evaporation, coalescence,
wall impingement etc. As a consequence, it is extreme-
ly difficult to identify the droplets produced by micro-
explosion.
The primary motivation of this study is to acquire a
more comprehensive understanding of the spray and
combustion characteristics of emulsified diesel under
different ambient temperature conditions using experi-
mental methods and to evaluate the impact of spray
features on soot emission. Emulsified diesel with the
amount of water ranged between 10% and 20% by vol-
ume was injected and combusted in an optical constant
volume chamber. High speeding imaging of both MIE
scattering and broadband luminosity were acquired to
illustrate time resolved data. On the other hand, because
of the current limitations in our understanding of the
spray and jets of the emulsified fuel, the modeling of
the emulsified fuel spray is still very limited. CFD
spray sub-models are often calibrated with measure-
ments of spray penetration and spreading angle in a
well-defined ambient environment. In this respective,
this study also serves as the CFD model calibration in
the future work.
Experiment Setup and Procedure
Apparatus
A constant volume chamber with a bore of 110 mm
and a height of 65 mm is used in this study. The cham-
ber can imitate the spray and combustion process of a
diesel engine, allowing a maximum operating pressure
of 18 MPa. The chamber has an open end on the top
with a fused silica window installed opposite to the
injector allowing optical access (See Fig 1.). The win-
dow, sealed with an energized spring seal, has dimen-
sion of 130 mm in diameter and 60 mm in thickness,
with a high UV transmittance down to 190 nm. A Cat-
erpillar hydraulic-actuated electronic-controlled unit
injector (HEUI) is mounted at the bottom of the cham-
ber. The injector is VCO type, with orifice diameter of
0.145 mm. The injection pressure and duration was kept
at 700 bars and 3.5 ms respectively through the tests.
The cylinder wall is heated to 380 K before the experi-
ment to mimic the wall temperature of a diesel engine
as well as to prevent water condensation on the optical
windows. A quartz pressure transducer (Model: Kistler
6121), embedded in the chamber wall in conjunction
with a dual mode differential charge amplifier, is re-
sponsible for recording the in-cylinder pressure. The
apparent heat release rate can be calculated from this
using the first law of thermodynamics [17]. The high
temperature and pressure environment is created by
burning a mixture of acetylene, air, and N2 using spark
ignition before the injection is triggered. The details of
the experiment procedure can be found in [18].
Figure 1. Schematic of the test chamber
Diagnostics
High speed imaging for both spray and combus-
tion studies was carried out using a non-intensified high
speed digital camera (Phantom V7.1) located above the
optical chamber. The camera is coupled with a Nikkor
UV lens with 105 mm focal length. For the spray stud-
ies, the light source is supplied by a copper vapor laser
(Oxford Lasers LS20-50) which can be externally con-
trolled to run up to a maximum frequency of 50 kHz
with pulse duration of 25 ns. The high-speed camera
and the copper-vapor laser were synchronized to 15,037
frames per second with an exposure time of 3 µs to
produce time resolved measurement at a spatial resolu-
tion of 512×256 pixels. The peak wavelength of the
copper vapor laser beam is at 510 nm, thus a 510 nm
narrow band pass filter (10 nm FWHM) is fitted in front
of the lens to block the signals of other wavelengths
emitted from the burning spray. The liquid spray is cap-
tured because of the difference in refractive index be-
tween the fuel and ambient gas. The continuous-wave
laser beam was expanded to completely illuminate the
liquid spray. This “volume-illumination” method, rather
than a laser sheet, was utilized to ensure that all drop-
lets spreading from the nozzle were illuminated to iden-
tify the maximum axial and radial distances of any liq-
uid-phase fuel. The input beam was directed at a slight
angle to avoid interference with the camera.
The broadband natural flame luminosity imaging
setup was very similar to the spray studies except that
both the laser beam as well as the band-pass filter was
removed. Two different camera configurations were
used as shown in Fig.2. In the first configuration (the
same configuration as in MIE scattering, thus exactly
the same field of view), the camera resolution is
512×216 with a speed of 15037 fps. A relative larger
camera aperture size of f/22 was chosen such that
stronger signal at lift-off region can be captured; in the
second configuration, the resolution is set to 640×480
with speed of 8082 fps. The minimum aperture size of
f/32 on the lens was used to ensure no pixel saturation
occurred downstream of the image. Quantitative analy-
sis such as spatial integrated broadband luminosity as
well as soot lift-off length will be based on this configu-
ration.
Figure 2. Broadband luminosity images a)
512×216 resolution, 15037 fps, camera aperture f/22, b)
640×480 resolution, 8082 fps, camera aperture f/32, the
images are from two different spray events
Test Fuel
Ultra low sulfur diesel with minimum cetane index
of 40, 90% distillation point between 293 oC to 332
oC
and viscosity around 3 cst was used as a base fuel in
emulsified diesel in current study. Span 80 and Tween
80 were used as surfactants to create O/W/O type emul-
sion. The detailed fuel preparation procedure can be
found in [9]. Two kinds of emulsified diesel with water
content 10% and 20% by volume were tested. The pure
diesel and two water emulsified diesel will be referred
to as D100, W10 and W20 in the rest of the text. All the
prepared fuels were found to be stable for at least two
weeks before separation was observed.
Image processing
The image processing procedure for the continuous
liquid penetration length and cone angle will be detailed
in the following section. The raw images obtained from
each complete injection sequence were first corrected
by the first three images of the respective sequence
which was taken right before the fuel injection. The
histogram equalization was then performed to enhance
the contrast of each image and minimize the effect of
the illumination intensity variation due to the ambient
temperature difference and light degradation from case
to case. It is also found that this procedure eliminates
the bulk noise of the background which later makes
easier the determination of both the liquid penetration
and cone angle. The liquid penetration length can be
defined as the distance between the injector tip and the
first pixel above a preset threshold along the jet center-
line. It is also worthwhile to mention that the algorithm
searches the aforementioned pixel from the spray tip,
thus it is referred as “continuous” liquid penetration. As
a matter of fact, droplets and ligaments were also ob-
served at the tip of the spray. At this point, we have not
fully developed an algorithm that can completely define
the droplets and ligaments boundary. As a simple way
to account for these breakup events, the penetration is
artificially decided by 3×3 pixel arrays whose values
are all above the threshold instead of just “one” pixel
such that the possibility of detecting false penetration
tip can be minimized. Once the liquid penetration was
determined, the cone angle can be measured by finding
the farthest 3×3 pixel array above the same preset
threshold perpendicular to the jet centerline (at 2/3 of
the penetration length) in a similar fashion as the liquid
penetration determination. All the quantitative analyses
were averaged over at least five shots for a statistical
base.
Results and Discussion
Spray characteristics
The sequence of spray images without histogram
correction for W10 and D100 at ambient temperature of
1000 K are shown in Fig.3. Although we focused on
only one plume from the injector, all six plumes were
actually more or less presented in the image. Since both
sets of images were taken from an individual injection
event, plume-to-plume variation was expected. The
images were “reversed” for better presentation of the
downstream portion of the spray jet, which means that
the “black” region is the area that was actually illumi-
nated while the “white” region is actually dark in the
original raw image. By comparing the spray images of
the two fuels, there are apparently some similarities and
differences. In the first shot of the spray image, W10
saw a larger spreading cone angle and “fattened” spray
pattern whereas the spray jet from D100 is narrow and
sharp which may indicate the impact of micro-
explosion. Subsequently, as the jet progresses along the
axial direction, spray tip thickening was observed for
both fuels till around 1.63 ms. Due to the tip thickening,
a sharp gradient existed between the spray and the
background. After 1.7 ms, the abrupt change at the
spray tip with chunk of ligaments detached from the
main jet body can be observed indicating that vigorous
droplet breakup was taking place. The pixel gradient
between the spray and background became less appar-
ent. A blurred area was observed after the droplets and
ligaments region which grew and stretched into the
downstream region; the continuous part of the liquid jet
however did not penetrate further. The signal captured
in the blurred area may come from a number of sources.
The pure white area is most likely caused by the soot
emitted at the flame front extinguishing the laser light,
which can be confirmed from the broadband luminosity
images. The area with coarse texture may be contribut-
ed by the strong soot incandescence that was not com-
pletely blocked by the narrow bandpass filter; the com-
bustion waves left by the premixed burn could also ac-
count for some signals. These blurry areas might also
be attributed to the Rayleigh scattering since the down-
stream region was also illuminated by the laser beam.
Experiments are currently being carried out in the au-
thor’s lab to inject the fuel into inert environment so
that the spray would not burn. This would help to con-
firm if the signal at the downstream region is mainly
from unfiltered flame luminosity or gas scattering.
Figure 4. Shot-to-shot variation on continuous liquid
penetration length
Using the image post processing method, the con-
tinuous liquid penetration length can be acquired. Fig.4
shows data averaged over five individual runs. Shot-to-
shot variation can be clearly seen and the uncertainty
varies with time. During the initial stage of the injec-
tion, a “bump” can be seen on the penetration length
curve and the variation is minimal among individual
runs; once it reaches a peak value, the penetration
length starts to decrease and reaches a quasi-steady
state before the injection event is terminated. It is with-
in this duration that much higher variations were de-
tected. As visualized in Fig.3, the gradient between the
tip of the spray and background became less apparent
due to the vigorous breakup events, which posed a chal-
lenge on the precise determination of the penetration
length. Among all the cases conducted, shot-to-shot
variation is typically around 10% during this period.
Figure 5. Continuous liquid penetration at different
ambient temperature for W10 and D100
Fig.5. illustrates the continuous penetration curve
for W10 and D100 at different ambient temperatures.
The temperature impact on the continuous penetration
length is very obvious as higher temperature yields
lower penetration. It is also of great interest to see that
the “bump” becomes much more remarkable at lower
ambient temperature while at the ambient temperature
of 1200K; the penetration length curve almost remains
a plateau. This feature is closely associated with the
ignition delay at the given condition. Lower ambient
temperature yields much longer ignition delay and
therefore longer penetration, which has been discussed
in a number of studies. [19-20]. Once the jet was ignit-
ed and the diffusion flame is formed, the temperature in
the reaction zone ahead of the spray tip rises quickly
and as a result, the spray evaporation speeds up and the
droplets breakup becomes much more violent, which
essentially swallows back the continuous liquid jet
body. In comparison, the ignition delay was much
shorter at high ambient temperature so that the diffusion
flame quickly formed and developed upon the start of
injection; therefore the penetration length was limited
to a quasi-steady state value throughout the entire injec-
tion event.
Regarding the water impact, the emulsified fuel
generally presented longer penetration at the “bump”
area on the curves due to the lower volatility of the wa-
ter. It is seen that the lower the ambient temperature,
the larger the discrepancy between the two fuels. At
low temperature of 800 K in particular, significant
longer penetration length is observed for W10 indicat-
ing that the impact of water addition is much more pro-
nounced at low ambient temperature environment. It is
also noted that after the initial bump on the curve, the
trend of longer penetration with emulsified fuel no
longer remains as short penetrations were observed for
W10 especially at 800K. One possible explanation is
that the atomization of the emulsified fuel was en-
hanced due to micro-explosion making the continuous
liquid part shorter.
Combustion Characteristics
The apparent heat release rate calculated based on
in-cylinder pressure is shown in Fig. 6. More premixed
burn indicated by the first peak on the heat release
curve is observed with the decrease of the temperature,
which is expected as longer ignition delay at low ambi-
ent temperature provided more time for air-fuel mixing.
Longer ignition delay is also generally expected with
the emulsified fuel mainly due to the evaporation heat
of water. At 800K, the W20 exhibited much retarded
ignition timing compared with W10 and D100 indicat-
ing the impact of water on ignition delay only becomes
apparent beyond a certain mixing percentage. In con-
trast, little difference was perceivable at a higher ambi-
ent temperature of 1000 K. Since the injection duration
was kept the same, the total amount of fuel injected into
the chamber is a constant, leading to lower energy input
for emulsified diesel. Therefore, the total heat release
decreased with the addition of water.
Figure 6. Apparent Heat Release Rate at different am-
bient temperature with injection pressure of 700 bar
The spectral sensitivity of the high speed camera is
around 400 nm to 700 nm, thus although termed
“broadband”, the signal captured should be within this
spectral range. The flame luminosity consists of two
parts; chemiluminescence and soot incandescence. The
latter is much stronger than the former one, thus it is
reasonable to claim that the soot luminosity can be well
represented by the broadband luminosity. The sequence
of the broadband luminosity images for W10 and W20
are illustrated in Fig. 7. Larger aperture size f/22 was
used in this configuration so that slight image saturation
was yielded downstream of the flame. The line-of-sight
flame front and flame liftoff can actually be visualized
under this configuration. In the conceptual diesel com-
bustion model raised by Dec [21], the central region
just downstream of the lift-off is where fuel rich mix-
ture reacts and the premixed flame is hypothesized.
This is also the primary region where soot precursors
started to form. A relatively darker region compared
with the bright soot incandescence was indeed observed
at this location of the lift-off which supports this theory.
Space integrated broadband luminosity at different
temperature for W10 and D100 derived from the picture
in the second configuration are illustrated in Fig 8. In
this configuration the overexposure was by no means
avoided and the entire flame, including flame wall in-
teraction, was included in the field of view. It can be
seen that very limited reduction of luminosity resulted
from the water addition at temperature above 900 K. At
low ambient temperature of 800 K however, the lumi-
nosity is decreased by up to 50% by W10. As explained
above, the broadband luminosity can be regarded as a
good representative of the actual soot emission; the
results indicate that the advantage of using emulsified
fuel to suppress soot formation can be maximized if
coupled with a low temperature combustion strategy.
The reason for this feature can be mainly explained by
the different spray characteristics of W10 at 800 as par-
ticular long initial continuous liquid penetration was
observed which greatly enhanced the air fuel mixing.
Figure 8. Space integrated broadband luminosity at
different temperature for W10 and D100
Conclusion
In this work, the spray and combustion characteris-
tics of emulsified diesel were investigated in a constant
volume chamber. A number of conclusions can be
reached based on the study. At low ambient tempera-
ture, emulsified fuel showed significantly longer liquid
penetration at the beginning stage of the injection indi-
cating that the impact of water is more pronounced at
low ambient temperatures. This finding is also support-
ed by the space integrated broadband luminosity, which
saw the most reduction only at low ambient temperature.
It is thus suggested that emulsified fuel coupled with a
low temperature combustion may optimize the emission
control strategy. The beginning stage of the liquid pen-
etration also saw a larger spreading cone angle and “fat-
tened” spray pattern for emulsified diesel which could
be attributed to micro-explosion. Regarding the ignition
delay, the retarded ignition only becomes apparent at
low ambient temperature and with a water content of 20%
by volume.
Acknowledgements
This material is based upon work supported by the
Nation-al Science Foundation under Grant No.
CBET‐1236786. Any opinions, findings, and conclu-
sions or recommendations ex-pressed in this publication
are those of the author(s) and do not necessarily reflect
the views of the National Science Foundation. The au-
thors are grateful for the support from NSF.
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Figure 3. Sequence of liquid spray evolution at ambient temperature of 1000 K with injection pressure 700bar a)
W10, b) D100