HomeMy WebLinkAboutAppendix F- Effect of Cavitating Flow on the Flow and Fuel AEffect of Cavitating Flow on the Flow and Fuel Atomization
Characteristics of Biodiesel and Diesel Fuels
Su Han Park,
†Hyun Kyu Suh,
†and Chang Sik Lee*
,‡
Graduate School of Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, and
Department of Mechanical Engineering, Hanyang UniVeristy, 17 Haengdang-dong, Sungdong-gu,
Seoul 133-791, Korea
ReceiVed June 12, 2007. ReVised Manuscript ReceiVed October 15, 2007
The aim of this work is to investigate the effect of cavitation on the fuel flow and atomization characteristics
of the biodiesel fuel. To study these characteristics of biodiesel as an alternative fuel, two different nozzles
which have different length-to-width ratios were utilized in this experiment. The visualization system visualized
the internal and external flow for investigating the formation and development of cavitation inside the orifice,
and the internal flow characteristics were analyzed quantitatively using dimensionless numbers such as the
Reynolds number, Weber number, cavitation number, and discharge coefficient. Moreover, the droplet measuring
system was installed to study the effect of the formed cavitation on the fuel atomization such as the mean
droplet size and the axial and radial mean velocity. On the basis of the results of the cavitating experiment, it
was revealed that the mean droplet size of biodiesel is larger than that of diesel fuel. The droplet size became
small when it formed the cavitation inside the orifice, and in the case of the high L/W ratio nozzle, the droplet
size is also smaller than that of the low L/W ratio nozzle. From these results, it can be concluded that cavitation
promotes the atomization of fuels at the nozzle exit. Also, it can be concluded from the results of flow
characteristics that the cavitation formed along the nozzle orifice wall because of the change in the flow direction
and the flow velocity near the wall due to the geometry of the orifice inlet.
1. Introduction
Nowadays, a common-rail injector is used for the injection
strategy and the precise control of injection quantity in the high
speed direct injection diesel engine. This diesel injection system
realizes a highly pressurized and minimized nozzle hole size in
order to improve the combustion and emission characteristics.
These conditions, such as the high pressure and the reduction
of nozzle hole, provide surroundings for the occurrence of
cavitation. Therefore, from the viewpoint of the precise control
of the injection and the promotion of the engine performance,
it is necessary to study the formation and the development
process of the cavitation. In the occurrence and growth of
cavitation by flow characteristics, the nozzle geometry and fuel
properties such as density, viscosity, and surface tension are
the main factor of the liquid atomization. In general, cavitation
is considered to be the transition of a fluid from liquid to vapor
due to the low pressure, provoked at the inlet of the nozzle
orifice and caused by strong direction changes in cross section.
1
The cavitation generated inside the nozzle orifice promotes the
atomization of the liquid and the breakup of the issuing jet at
the nozzle exit, which can be used as a means for the reduction
of energy for the disintegration of fuel droplets in diesel engines.
Research about cavitation has progressed actively by many
researchers. Desantes et al.
2 and Payri et al.
3 studied the effect
of cavitation on the injection velocity in the nozzle exit vicinity,
the measurement of the injection rate, and the momentum flux
using three nozzles with different geometry, both experimentally
and numerically. They reported that the increase in exit velocity
with the appearance of cavitation seems to be caused by the
variation of the characteristic of fuel density at the exit of the
nozzle due to this cavitation. They also studied the relationship
among the formation of the cavitation, the spray penetration,
and the flame shape through the spray and flame visualization.
In their study, the generation of the cavitation was affected by
the shape and dimension of the nozzle orifice. Gavaises et al.
4
explained that cavitation is formed not only at the hole entrance
due to the local pressure drop caused by nozzle inlet geometry
but also at the sac volume of the nozzle tip inside the multihole
injector for large diesel engines. In addition, through the CFD
calculation, they revealed that the needle lift, cavitation number,
and Reynolds number affected the formation and growth of the
cavitation. Arcoumanis et al.
5,6 introduced the breakup model
as a similar approach based on the assumption that the breakup
is dominated by the bubble behavior at the outside of the nozzle.
In their study, a diesel injector was compared with a scaled-up
diesel injector and revealed that the Reynolds number (Re) and
cavitation number (K) are the dominant factors influencing the
pattern of cavitating flow. Sou et al.
7 visualized the generation
and growth progress of the cavitation in the transparent acrylic
* Corresponding author. Phone:+82-2-2220-0427. Fax:+82-2-2281-
5286. E-mail: cslee@hanyang.ac.kr.
†Graduate School of Hanyang University.
‡Department of Mechanical Engineering, Hanyang Univeristy.
(1) Lefebvre, A. H. Published by Taylor & Francis, 1989, ISBN
0-891116-603-3.
(2) Desantes, J. M.; Arregle, J.; Lopez, J. J.; Hermens, S.SAE Tech.
Pap. Ser.2005, 2005-01-2120.
(3) Payri, F.; Arregle, J.; Hermens, S.SAE Tech. Pap. Ser.2006, 2006-
01-1391.
(4) Gavaises, M.; Andriotis, A.SAE Tech. Pap. Ser.2006, 2006-01-
1114.
(5) Arcoumanis, C.; Gavaises, M.Atomization Sprays 1998,8,3.
(6) Arcoumanis, C.; Badami, M.; Flora, H.; Gavaise, M.SAE Tech. Pap.
Ser.2000, 2000-01-1249.
(7) Sou, A.; Lihan, M. M.; Hosokawa, S.; Tomiyama, A.Proc. 10th
ICLASS 2006, ICLASS 06-043.
Energy & Fuels 2008,22,605–613 605
10.1021/ef7003305 CCC: $40.75 2008 American Chemical Society
Published on Web 12/07/2007
resin 2D nozzle. They studied the effect of cavitation number
(K) and Reynolds number (Re) on the cavitating flow under
various flow rate conditions by applying the laser doppler
velocimetry (LDV) system. From their study, the pattern of
cavitating flow was divided into four steps: no cavitation and
wavy jet, developing cavitation and wavy jet, super cavitation
and spray, and hydraulic flip and flipping jet. Roth et al.
8 carried
out the CFD calculation of various nozzle shapes in order to
investigate the effect of the nozzle shape on the internal flow
characteristics of a diesel injector nozzle. They reported that
increasing the orifice inlet radius leads to increasing the mean
exit velocity and discharge coefficient near the orifice wall, while
the region of the cavitation decreases. Soteriou et al.
9 applied a
LDV system inside an enlarged plain orifice nozzle under
noncavitating conditions. They also observed small bubbles in
the downstream direction at the inlet rim and concluded that
turbulence within the cavitating flow is a major factor for
promoting atomization. Daikoku et al.
10 also investigated the
effect of the nozzle length-to-diameter ratio or width on
the liquid breakup in the 2D nozzle. They reported that when
the length-to-width (L/W) ratio is low, the atomization process
is affected by the generation and disappearance of cavitation.
Further, the liquid is ejected as a sufficiently turbulent form,
which promotes the atomization. Extensive experimental and
numerical studies on nozzle cavitation have been carried out
by Lee et al.
11 and Sarre et al.
12 However, the majority of
previous studies provide an understanding of the formation and
influence of the cavitation. In addition, most of these studies
used water for the working fluid, which does not reflect the
properties of fuel.
In the viewpoint of the environment and the energy situation,
the use of biodiesel fuel is under consideration because it can
be used without modification of the fuel supply system in a
diesel engine, and it is already using in many countries. Further,
it can be expected to improve the emission characteristics and
to increase the thermal efficiency by the entire combustion
because the cetane number of biodiesel fuel is higher than that
of diesel fuel.
13–16 In a diesel engine, the fuel properties of
biodiesel influence the spray characteristics and combustion
performance. In this point of view, the effect of fuel properties
such as viscosity and surface tension on the biodiesel fuel
atomization was conducted by Ejim et al.
17 Also, the investiga-
tion on the formation of cavitation and atomization of biodiesel
fuel is a necessity because spray characteristics and structure
were affected by different properties of biodiesel fuel compared
to diesel fuel.
The aim of this paper is to investigate the effect of cavitation
on the flow and atomization characteristics of biodiesel fuel in
visualized and enlarged nozzles. Moreover, the experiment was
performed to analyze the influence of the different nozzle length-
to-width (L/W) ratios and the formation process of the cavitation
inside the nozzle orifice and its effects on the external flow
pattern of biodiesel and diesel in the vicinity of the 2D nozzle
exit by using dimensionless numbers such as the Reynolds
number (Re), Weber number (We), cavitation number (K), and
discharge coefficient (Cd). To investigate the effect of cavitation
on the fuel atomization, the droplet measuring system was used
to study the mean droplet size, the axial mean velocity, and the
radial mean velocity.
2. Experimental Apparatus and Procedures
2.1. Flow Visualization and Spray Measuring System.In this
study, the experimental apparatus, as shown in Figure 1, was
designed to investigate the flow characteristics and visualized the
formation of cavitation inside the nozzle orifice and its effects on
the external flow characteristics of diesel and biodiesel fuel. The
experimental setup consisted of the fuel supply system and the flow
visualization system. Test fuels were filtered in order to remove
the impurities and were pressurized by nitrogen gas. At the same
time, the instant flow rate and pressure were measured at various
injection pressures by a flow meter (A109LMA, GPI) and pressure
gauge. When diesel and biodiesel fuel passed through the nozzle
orifice, the internal and external flow images were visualized with
the high resolution ICCD (intensified charge couple device) camera
(Dicam PRO, The Cooke Corp.) with a spot lamp as a light source.
The injected test fuel was recirculated through the circulation pump
(PW-200M, WILO) to the fuel tank.
Two different types of nozzles were used to investigate the effect
of the L/W ratio on the internal flow characteristics and the
formation of cavitation at the orifice, as shown in Figure 2. Detailed
specification and reference about nozzles was shown in Table 1.
The fuel droplet measuring system (PDPA, phase Doppler
particle analyzer) was installed for the measurement of the droplet
mean diameter (SMD, Sauter mean diameter), the axial mean
velocity, and the radial mean velocity of injected fuels in the nozzle.
Considering the measuring accuracy and the signal intensity of the
signal analyzer, the power of the Ar-ion laser was set at 0.7 W as
a light source of the PDPA system. Moreover, the droplet measuring
system consisted of a transmitter, a receiver, and a signal analyzer.
2.2. Experimental Procedure.In order to examine the internal
and external flow characteristics and the effect of cavitation on the
atomization characteristics of biodiesel and diesel fuel in the nozzle,
the experiment was performed by using two nozzles, a flow
visualization system, and a fuel droplet measuring system, as
illustrated in Figures 1 and 2. Test transparent nozzles were made
from the acrylic acid resin. The main raw material of it is a “methyl
methacrylate”, and it has a transmissivity of 98% and a reflexibility
of 1.49. Diesel and biodiesel fuel derived from soy bean oil was
used for the test fuel in this study. The fuel properties of the test
fuel are listed in Table 2.
(8) Roth, H.; Gavaises, M.; Arcoumanis, C.SAE Tech. Pap. Ser.2002,
2002-01-0214.
(9) Soteriou, C.; Andrews, R.; Simth, M.SAE Tech. Pap. Ser.1999,
1999-01-1486.
(10) Daikoku, M.; Furudate, H.; Inamura, T.Proc. 9th ICLASS 2003,
Paper No. ICLASS 12-7.
(11) Lee, J. W.; Min, K. D.Trans. KSME 2006,30–6, 553–559.
(12) Sarre, C. K.; Kong, S. C.; Reitz, R. D.SAE Tech. Pap. Ser.1999,
1999-01-0912.
(13) Yoon, S. H.; Park, S. W.; Kim, D. S.; Kwon, S. I.; Lee, C. S.Proc.
ICEF 2005, 2005–1258.
(14) Lee, C. S.; Park, S. W.; Kwon, S. I.Energy Fuels 2005, 2201–
2208.
(15) Suh, H. K.; Park, S. W.; Kwon, S. I.; Lee, C. S.Trans. KSAE 2004,
12–6, 23–29.
(16) Zhang, Yu.; Boehman, A. L.Energy Fuels 2007,21, 2003–2012.
(17) Ejim, C. E.; Fleck, B. A.; Amirfazli, A.Fuel 2007,86, 1534–1544.
Figure 1.Schematic of the flow visualization system.
606 Energy & Fuels, Vol. 22, No. 1, 2008 Park et al
This work was carried out to investigate the effect of the nozzle
L/W ratio on the internal and external cavitating flow characteristics.
It was carried out using various enlarged nozzles, as classified in
Figure 2. Figure 2 shows nozzles of the rectangular orifice inlet
shape with 1.5 and 3.0 of the L/W ratio. In the figure, a dotted line
indicates the interested visualization region.
To investigate the atomization characteristics of biodiesel fuel
under various injection conditions, the measuring points were
selected at 10 mm intervals from 40 to 150 mm according to the
axial direction and at 2 mm intervals to radial direction, as shown
in Figure 4, assuming that the spray was axial symmetric. At each
point, the approximately 20 000 droplets were captured, and droplets
in the range from 2 to 80 µm were averaged and analyzed. The
measurement of the SMD and axial velocity of biodiesel and diesel
fuel droplets was conducted at the representative injection pressure
for turbulent flow, growth of cavitation, and hydraulic flip by using
nozzles R and L. The flow visualization and droplet measuring
experimental conditions are listed in Table 3.
In this investigation, the flow characteristics of cavitating flow
were analyzed in terms of following a nondimensional number such
as the Reynolds number,Re )FVW/µ, and the cavitation number,
K )2(Pb -Pv)/FV2 .InRe and K,F is the fuel density,V is the
injected velocity of the fuel droplets, and Pb and Pv indicate the
ambient pressure and vapor pressure, respectively. Additionally,
W means the representative length of a nozzle orifice width.
The injection flow rate, the injection pressure, and the fuel
properties were analyzed and compared according to the nozzle
length-to-width ratio. Moreover, the discharge coefficient (Cd)
expresses all of the losses in the nozzle, and it should be considered
because it is an important factor for analyzing the cavitating flow
of the injector nozzle and for designing the nozzle. This factor is
the ratio of the ideal flow rate to the actual flow rate. The ideal
flow rate was derived by the continuous equation and Bernoulli’s
equation, and the discharge coefficient can be expressed as the
following equation.
Cd )
Qact
Qideal
)
Qact√1 -2
A2
2
F ∆P +2g∆Z
where the subscript 2 is for downstream and ∆Z is the position
difference between upstream and downstream,is the contraction
ratio of the nozzle cross section (Anozzle) and the orifice cross section
Figure 2.Test nozzles and visualization region.
Figure 3.Schematic of the droplet measuring system.
Table 1. Specifications of the Droplet Measuring System
light source Ar-ion laser
wavelength 514.5 nm, 488 nm
laser beam diameter 1.4 mm
beam expander ratio 0.5
focal length 250 mm for transmitter
250 mm for receiver
collection angle 30°
Table 2. Test Fuel Properties
fuel diesel biodiesel (soy bean oil)
density (kg/m
3)830 880
surface tension (N/m) 0.026 0.028
dynamic viscosity (Ns/m
2)0.00223 0.00389
vapor pressure (MPa),0.1 ,0.1
Figure 4.Measurement points of the droplet measuring experiment.
Table 3. Experimental Conditions
(a) Flow Visualization
fuel injection pressure (MPa) 0.13-0.45
test nozzles nozzle R, nozzle L
ambient temperature (K) 293
ambient pressure (MPa) 0.1
(b) The Droplet Measuring System
nozzle R (L/W )1.5) nozzle L (L/W )3.0)
fuel diesel biodiesel diesel biodiesel
turbulent flow 0.16 MPa 0.16 MPa 0.20 MPa 0.20 MPa
growth of cavitation 0.30 MPa 0.30 MPa 0.35 MPa 0.35 MPa
hydraulic flip 0.42 MPa 0.42 MPa 0.42 MPa 0.45 MPa
Biodiesel Flow and Fuel Atomization Characteristics Energy & Fuels, Vol. 22, No. 1, 2008 607
(Aorifice), and ∆P is the difference of the ambient pressure and the
vapor pressure.
3. Results and Discussion
3.1. Flow Characteristics of Diesel and Biodiesel Fuels.
In the present study, the experiment was conducted to investigate
the internal and external flow characteristics and to visualize
the formation and development of cavitation of biodiesel and
diesel fuel using nozzle R. Also, an experiment using nozzles
R and L was carried out to analyze the flow characteristics and
the formation of cavitation of biodiesel on the effect of the
nozzle L/W ratio.
Figure 5 shows the cavitating flow patterns of diesel in nozzle
R and biodiesel in nozzle L and nozzle R at various injection
pressure and flow rates. The patterns can be divided into four
regions: turbulent flow, beginning point of cavitation, growth
of cavitation, and hydraulic flip.
12 The beginning point of
cavitation is the point where the cavitation bubble occurs
Figure 5.Comparison of the visualization images at various nozzle types.
608 Energy & Fuels, Vol. 22, No. 1, 2008 Park et al
initially, and the hydraulic flip is the phenomenon occurring
when the fuel passes through the nozzle orifice without
cavitation reattachment to the wall of the formed cavitation.
The region before the beginning point of cavitation is called
turbulent flow, and the interval from the beginning point of
cavitation to hydraulic flip is called growth of cavitation. As
shown in parts a and b of Figure 5, the cavitation was formed
at an injection pressure of 0.20 MPa and an injection flow rate
of 9.7 L/min in the case of diesel fuel, and it formed at an
injection pressure of 0.20 MPa and an injection flow rate of
9.35 L/min in the case of biodiesel fuel. When the injection
pressure increased, the cavitating flows of diesel and biodiesel
fuels had a similar pattern. However, the injection flow rate of
biodiesel fuel was a little lower than that of diesel fuel. Figure
5c illustrates the flow characteristics of biodiesel fuel at the
nozzle with a L/W ratio of 3.0. The cavitation formed at an
injection pressure of 0.25 MPa and an injection flow rate of
11.3 L/min, and the hydraulic flip began at an injection pressure
of 0.45 MPa and an injection flow rate of 16.7 L/min. For nozzle
L (L/W )3.0), the injection pressure and flow rate for the
formation of cavitation increased in contrast to nozzle R (L/W
)1.5). It can be conjectured that the friction loss between the
wall and the fuel increased with the increase of pressure and
flow rate. Figure 6 shows the comparison of the injection
pressure and flow rate in two nozzles.
On the basis of the results of Figure 5, Figure 7 shows the
injection flow rate as the unit of the volume per minutes when
the injection pressure increases. In this figure, biodiesel fuel
has a slightly lower injection flow rate than diesel fuel at all
over the injection pressure. However, as the unit of the mass,
biodiesel fuel has a little higher injection flow rate than diesel
fuel because the liquid with a higher density has a lower volume
at the same mass quantity, as shown in Figure 8.
Figure 9a shows the relationship between the injection
pressure and cavitation number (K) for biodiesel fuel at two
nozzle types. Cavitation number (K) means the ratio of the
Figure 6.Comparison of the injection pressure and the flow rate in
nozzle R and nozzle L.
Figure 7.Injection flow rate at various nozzle types and fuels.
Figure 8.Injection flow rate of diesel and biodiesel fuel (nozzle R).
Figure 9.Cavitation number and Reynolds number along the injection
pressure.
Biodiesel Flow and Fuel Atomization Characteristics Energy & Fuels, Vol. 22, No. 1, 2008 609
dynamic pressure and the static pressure. Theoretically, a
cavitation occurs when the cavitation number is below 1.0 in
this investigation. As the cavitation number decreased, the
cavitation intensity was stronger. Also, the cavitation number
was in inverse proportion to the square of injection velocity.
As shown in Figure 9a, the injection pressure for the formation
of cavitation increases, the hydraulic flip was also occurred at
the higher injection pressure as the change from nozzle R to
nozzle L. In these results, the condition of the cavitation
inception was much affected by the nozzle length-to-width ratio.
Figure 9b shows the change of the Reynolds number according
to the injection pressure. Biodiesel with a high viscosity has a
lower range of values and a lower value than that of diesel fuel
as the Reynolds number is the ratio between the inertia force
and the viscous force. The Reynolds number was used as the
measure of the turbulent flow; accordingly, it is conjectured that
the flow irregularity of biodiesel is lower than that of diesel
fuel due to the high viscosity.
Figure 10 shows the change of the discharge coefficient when
the cavitation number increases. As shown in Figure 10a, the
discharge coefficient of both fuels increases a little after the
occurrence of the cavitation. However, it immediately decreases
after the transition to the hydraulic flip for both fuels. The
discharge coefficient of diesel fuel is a little higher than that of
biodiesel fuel. It was also affected by the density and viscosity
of fuels. Figure 10b shows a comparison of the discharge
coefficient between two types of nozzles using biodiesel fuel.
The Cd value of nozzle L is lower than that of nozzle R in most
of the test range due to the long flow friction region. However,
after the formation of cavitation, the Cd value of nozzle L is
higher than that of nozzle R. It is explained that the ruptured
energy of cavitation and the momentum were much stored
through the long flow region compared with nozzle R.
Figure 11 shows the classification of cavitating flow patterns
for the Weber number between diesel and biodiesel fuels in
nozzle R. Classification of the cavitating flow pattern was
Figure 10.Discharge coefficient for cavitation number.
Figure 11.Classification of cavitating flow patterns for the Weber
number and Reynolds number.
Figure 12.Mean droplet size distribution along the axial distance.
610 Energy & Fuels, Vol. 22, No. 1, 2008 Park et al
divided by the Weber number, such as turbulent flow (We <
34 000), growth of cavitation (34 000 <We <75000), and
hydraulic flip (We >75 000). As shown in Figure 11, the Weber
number of biodiesel fuel is larger than that of diesel fuel by
about 2.6 times at the same Reynolds number. This is the reason
why the ratios between the surface tension and the density of
diesel and biodiesel fuels are almost the same [(F/σ)diesel )
1.03(F/σ)biodiesel]; the velocity of biodiesel fuel is faster than that
of diesel fuel by about 1.6 times. On the other hand, the
Reynolds number of diesel fuel is larger than that of biodiesel
fuel by about 1.6 times at the same Weber number because the
ratio between the viscosity coefficient and the density of diesel
is larger than that of biodiesel.
3.2.Atomization Characteristics of Diesel and Biodiesel
Fuel.Fuel atomization as the concept of the extension of the
surface area is an important factor in the design of diesel engines
in terms of thermal efficiency and emission performance. In
the present work, the experiment using the droplet measuring
system was carried out to investigate the atomization charac-
teristics of biodiesel fuel in nozzle R and L, such as the mean
droplet size distribution, the axial mean velocity, and the radial
mean velocity.
In order to enhance the accuracy of the investigation, the
experiment was conducted under a data rate over 150 Hz in all of
the experimental conditions. Also, the data rate gradually increases
along the axial distance. In the near region of the nozzle exit, the
data rate is so low because the liquid jet was dense. On the other
hand, the valid percent is more than 99.0% in all of the measuring
points. On the basis of these results, the results of the droplet
measuring system can be trusted. Values of the data rate and valid
percent were averaged at the same axial distance.
Figure 12 shows the mean droplet size distribution when the
axial distance increases from 40 to 150 mm. As shown in Figure
12a, the mean droplet size of biodiesel is larger than that of
diesel fuel in the turbulent flow. In the region of growth of
cavitation, the droplet size became small compared with the
region of turbulent flow in both fuels. This is explained by
homogeneous nucleation, one of the cavitation formation
Figure 13.Mean droplet size distribution along the radial of diesel
and biodiesel fuel.
Figure 14.Comparison of the nozzle exit velocity for the injection
pressure.
Biodiesel Flow and Fuel Atomization Characteristics Energy & Fuels, Vol. 22, No. 1, 2008 611
theories.18 The cavitation grew from the energy difference
between the stored energy at the surface of the bubbles by the
surface tension and the work energy by the growth of bubbles.
In the growth process, the maximum growth of cavitation was
ruptured and its energy was diffused.
18 It can be concluded that
cavitation plays a key role in the liquid atomization because its
breakup energy of cavitation affects the fuel atomization. Figure
12b represents the effect of the length-to-width ratio of the
nozzle on the atomization characteristics of biodiesel fuel. After
the occurrence of cavitation, the SMD of both nozzles became
small. As the L/W ratio increases, the SMD became small after
the formation of cavitation, because the ruptured energy
considered the homogeneous nucleation theory was much stored
on the surface of the cavitation bubbles in nozzle L.
Figure 13 shows the size distribution of droplet mean diameter
along the radial distance of diesel and biodiesel fuel. As shown
in Figure 13, the droplet mean diameter at the region of turbulent
flow is larger than that of the region of growth of cavitation in
(18) Brennen, C. E. Published by Oxford university press, 1995, ISBN
0-19-509409-3.
Figure 15.Axial velocity along the axial distance of fuels (nozzle R).
Figure 16.Droplet distribution along the axial velocity (LZ )100 mm,
LR )0 mm).
Figure 17.Comparison of the radial and axial mean velocity distribu-
tions of fuel and nozzle type according to the radial distance.
612 Energy & Fuels, Vol. 22, No. 1, 2008 Park et al
both fuels. From these results, it was concluded that the
occurrence and collapse of cavitation affected the fuel atomi-
zation, like the preceding conclusions about the axial direction.
In addition, the SMD and AMD increase with increasing radial
distance. It was concluded that the larger droplets move to the
outer side of the spray due to its momentum.
When the injection pressure increased, the nozzle exit velocity
increased, as shown in Figure 14. In this figure, the theoretical
velocity was obtained by the Bernoulli equation. The experi-
mental value was calculated from the flow rate and the cross-
sectional area of the nozzle orifice at each injection pressure.
The mean droplet velocity from the PDPA system was
calculated by the means of four points at 40 mm from the nozzle
exit. In the case of diesel fuel, the difference between the
theoretical and experimental value was 15.01% of the minimum
value at 0.22 MPa in the beginning stage of cavitation and 27.5%
of the maximum value of 0.43 MPa in the hydraulic flip region.
In the case of biodiesel fuel, the difference between the
theoretical and measured value was 18.7% of the minimum
value at 0.20 MPa and 20.88% of the maximum value at 0.43
MPa. From these results, it can be said that internal and external
flows were affected by the cavitation. The flow velocity of
biodiesel fuel is lower than that of diesel fuel due to the larger
flow resistance from higher fuel viscosity, density, and the
friction between the orifice wall and the fluid. The flow velocity
measured by the droplet measuring system is in agreement with
the experimental value obtained from the flow meter except the
point of hydraulic flip. Parts b and c of Figure 14 show the
effects of different L/W ratios on the change of the axial velocity
at various injection pressures. After 0.25 MPa of injection
pressure to start the cavitation, the flow rate of nozzle L was
larger than that of nozzle R at the same injection pressure due
to the orifice length. This is why the energy for the fluid flow
surpassed the consuming energy for overcoming the friction of
the orifice wall and the fluid flow after an injection pressure of
0.25 MPa. The measurement error in Figure 14 was calculated
as the difference of the theoretical and experimental values. This
error is the energy loss by the friction of the orifice wall as
well as the internal nozzle generated turbulence, cavitation
effects of fluid density, and exit velocity profile.
Figure 15 shows the distribution of axial mean velocity for
nozzle R at the regions of turbulent flow and growth of
cavitation. As shown in Figure 15, the axial mean velocity
increases after the formation of cavitation. This is confirmed in
Figure 16. Figure 16 represents the number of droplets related
to the axial mean velocity. From turbulent flow to hydraulic
flip, the axial velocity of the peak droplets number increases to
the high velocity.
Figure 17 shows a comparison of the radial and axial mean
velocity distributions of fuel and nozzle type according to the
radial distance. As shown in Figure 17, the formation of
cavitation affects the increase of the axial and radial mean
velocities of diesel and biodiesel fuels. Figure 17a illustrates a
comparison of the radial mean velocity of diesel and biodiesel
fuels. Both fuels have nearly the same value and the increase
pattern after the occurrence of cavitation. In Figure 17a and b,
the velocity increases with increasing radial distance, due to
the increase of the droplet momentum by the increase of the
droplet size, as shown in Figure 12. On the other hand, the axial
velocity decreases when the radial distance increases, as
illustrated in Figure 17c.
4. Conclusions
This work was carried out to examine the flow and fuel
atomization characteristics of biodiesel and diesel fuel through
an investigation on the effect of the length-to-width ratio on
the formation of cavitation and the effect of cavitation on the
external flow pattern. The conclusions of this study are
summarized as follows:
The cavitating flow rate of biodiesel fuel was slightly lower
than that of diesel fuel, while the cavitating flow patterns of
biodiesel and diesel fuel which the cavitation formed along the
orifice wall were similar.
When the length-to-width ratio of the nozzle increased from
1.5 to 3.0, a higher pressure by about 25% was needed to obtain
the cavitation. The consuming energy of the higher L/W ratio
nozzle increases for the occurrence of the cavitation in the
nozzle.
The discharge coefficient increases a little after the occurrence
of the cavitation. However, it immediately decreased after the
transition to the hydraulic flip.
In the region of cavitation growth, the droplet size of biodiesel
and diesel fuels became small compared with the region of
turbulent flow in biodiesel and diesel fuel along the axial and
radial directions.
The axial mean velocity increases after the formation of
cavitation. It is proved that the axial velocity at the peak droplets
number increases to the high velocity from turbulent flow to
hydraulic flip.
On the basis of the SMD and velocity measurements, the
cavitation in the nozzle orifice promoted the atomization of fuels
at the nozzle exit. It can be concluded that the energy generated
during the formation, growth, and rupture of cavitation enhances
the energy for the atomization of fuels.
Acknowledgment.This work was supported in part by the CEFV
(Center for Environmentally Friendly Vehicle) of the Eco-STAR
project of the MOE (Ministry of the Environment in Seoul, Republic
of Korea). Also, this work was financially supported by the Ministry
of Education and Human Resources Development (MOE), the
Ministry of Commerce, Industry, and Energy (MOCIE), and the
Ministry of Labor (MOLAB) through the fostering project of
the Laboratory of Excellence. In addition, this study was supported
by the Second Brain Korea 21 Project in 2006.
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