Characterization of DIdiesel Sprays Payri 96
Characterization of DIdiesel Sprays Payri 96
Characterization of DIdiesel Sprays Payri 96
SAE TECHNICAL
PAPER SERIES 960774
SAE
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I N T E R N A T I O N A L February 26-29, 1996
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SF6 Air
Dynamic viscosity 1.5*10-5Ns/m2 1.8*10-5 Ns/m2
(3000K)
Density at atm. 6.2 kg/M3 1. 17 kg/M3
Conditions
Density in engine - 15 - 35 kg/m3
Conditions
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PRELIMINARY TESTS ON THE INFLUENCE Figure 3 - Tip penetration with different gas densities.
OF THE DENSITY ON THE SPRAY
PENETRATION
with air and SF6 and modifying the gas density. Figure 4
shows the same results but with modified axes, being now
the horizontal axis t0.5 while the vertical one is S*ρg0-25. All
the four curves are almost superimposed with a linear
behaviour and different slopes for the first injection step and
the main injection, showing a good agreement with
previous reported data [2][4][12][13] and theoretical
considerations [4] [11] according to which:
The first one with a lower tip velocity corresponds where K is only function of the injection parameters.
to the first injection step with low injection rate. At 0.4 - 0.5
ms after injection beginning, a sudden increase in the tip INJECTION CONDITIONS
velocity can be observed due to the main injection that
overtakes the fuel injected in the first step. The plotted lines After the preliminary tests, a new injection system
were obtained with air and SF6 at a density of 5.5 kg/m3. was installed in the test rig whose main components are
Both curves are nearly super-imposed showing that the summarized in table 3. A double spring injector with open
similarity hypothesis previously discussed are correct. hole nozzle was used. In conventional DI injectors, the
Figure 3 shows results for the same injection conditions injection rate is mainly controlled by the injection pressure
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for a given nozzle bore. Double spring injectors for two step drop across the needle seat, that becomes comparatively
injection behave in a different manner and the injection rate important for low needle lifts. For these reasons, the fuel
is controlled by the needle position. The global effective injection process in double spring injectors can be very
injection section is determined by two serial restrictions, the different when modifying the operating conditions of the
area between the needle cone and its seat in the injector pump and even very different along a single injection when
housing (variable) and the overall nozzle section in multi- the two step injection principle is under operation.
hole injectors (constant).
A B C
r.p.m. (pump) 400 800 500
load Low full fill
pump Bosch VE
nozzle Bosch DSLA-PV - 5 * φ 0.23 mm
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Figures 6, 7 and 8 show the needle lift, the values of Ta2ρ1/ ρg are lower than 1. A nearly constant spray
injection rate given by the Bosch method and the pressure opening angle was measured for very different injection
drop across the injection nozzle for the three operating conditions, and a reduction of the spray angle proportional
conditions labeled A, B and C. Condition A corresponds to to ρg0.4 was observed, as explained later. It is also note
idle in real engine operation and only the first spring is worthy that the spray opening starts immediately after the
compressed with an injection pressure of the order of 2 injection nozzle exit. From the previous observations it can
MPa. Condition B corresponds to a fall load condition with be concluded that in all three operating conditions the spray
almost all the injection process occurring at high needle is completely developed. The atomization process is very
lifts, with injection pressures of the order of 20 Mpa. fast at the exit of the injector nozzle due to the strong
Finally, test condition C is a two step injection process with aerodynamical interaction with the high density gas, even
a first step similar to condition A and a main injection for low pressure drops across the nozzle (case A). Liquid
similar to condition B. core outside the injector is very short, typically of the order
of the nozzle diameter [19], with short break-up lengths [1].
Phenomena associated with the primary atomization process
and father spray development seem to be similar for the
three cases tested despite the different injection conditions.
DISCUSSION OF RESULTS
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injection overtakes the fuel injected during the first step, Compared with the initial injection velocity, the
2.25 ms after injection beginning. velocity profile at 10 mm from the injector shows an abrupt
During the first 2.25 ms, tip penetration of test velocity increase up to 35 m/s, a peak velocity of about 40
case C is only a little higher due to a higher pressure m/s at 0.1 ms and a slow decrease arriving to the gas
injection. After this instant, tip velocity of test case C is convection velocity at 0.8 ms. The velocity versus time
higher than for test case B, in spite of the lower injection profile suffers a clear deformation with a concentration of
pressure. This observation suggest that, for Diesel spray high velocity droplets just after the spray tip. The leading
conditions, the fuel droplets at the spray tip have a different droplets are suddenly decelerated due to the aerodynamical
behaviour those in the main spray, due to different drag of the nearly stagnant air. Droplets behind those in the
aerodynamic interactions with the surrounding gas. spray tip interact with entrained air traveling at nearly
These phenomena, together with the non-steady character of droplet velocity, so the aerodynamic interaction is different.
the injection conditions, make Diesel sprays highly When these droplets arrive to the front of the spray,
unsteady, influencing penetration and atomization of fuel as different phenomena may occur. On the one hand, an
explained later. overtaking and acceleration of the previous spray tip; this
means that the fuel concentration of the leading edge of the
spray increases when moving away from the injector. On
the other hand, a "pushing" of the previously decelerated
droplets occurs which generates a vortex in the leading edge
of the spray. These displaced droplets are further entrained
together with air by the spray behind its tip [22]. Both
assumptions, which are not mutually exclusive, lead to the
conclusion that high droplet concentration in the leading
edge of the spray must exist when it moves away from the
injector. Atomization phenomena are controlled by the
droplet Weber number (Wed) and so they are very sensitive
to the difference between droplet velocity and surrounding
gas velocity. Coalescence phenomena are mainly controlled
by the droplet concentration. As discussed before, these two
processes are unsteady and variable along the spray due to
different conditions found by the traveling droplets or fuel
Figure 10 - Tip penetration vs time. portions. The balance between atomization and coalescence
provides the droplet characteristic diameter.
DROP VELOCITIES - Figure 11 shows for test case A the
fuel velocity at the nozzle exit calculated from the injection DROP SIZE - PDA measurements become more difficult
rate together with the averaged drop velocity in the spray with dense spray. In our experiments it was not possible to
axis at a distance of 10 mm from the injector. Time equal to take confident measurements at distances from the injector
zero corresponds to injection start for the first curve and much lower than 10 mm. Validation rates between 20% and
spray arrival to the measuring point for the second one. 60% were obtained, depending on the distance and the
injection conditions. With these validation rates, a bias
towards larger droplet sizes is expected. The effect of this
bias will be larger on the average diameter than on the
Sauter mean diameter. Figure 12 shows as an example the
time evolution of the droplet velocities and Sauter mean
diameter (SMD) for test condition A at a distance of 20 min
from the injector nozzle, on the spray axis.
The velocity plot is qualitatively coincident with
that of figure 11, with lower averaged velocities, as
expected. In what concerns the SMD, the smallest values of
22 gm are obtained in the tip of the spray. After that, an
increase of the SMD has been measured up to a value of 30
mm which remains constant for all the spray trailing edge.
This behaviour is qualitatively similar to that found in
references [17][20][23], and differs from measurements
taken at atmospheric density (for instance [18] ) where the
Figure 11 - Nozzle exit velocity and droplet velocities on SMD in the leading edge is very similar to the SMD in the
the spray axis, at 10 mm from the injector (test case A). trailing edge, or even higher.
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