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when the droplet sizes, calculated from the phase
difference detected in different positions, were the
same or their differences were within 10%, we
regarded them as the spherical droplets. There-
fore, it is found that there are ligaments, non-
spherical droplets or large-sized droplets that
exceed the measurement range of the PDPA sys-
tem used near the central part of the spray. Mean-
while, after the central part of the spray passes
through the measurement point, the detecting fre-
quency of the droplets increases prominently.
From these time-resolved evolutions of the dro-
plet size, the structure of the diesel spray can be
divided into the three main parts, as depicted in
Fig. 3 : (a) the leading edge affected by the surro-
unding air and composed of small droplets; (b)
the central part surrounded by the leading edge
and mixing flow region, and scarcely affected by
the surrounding air ; (c) the trailing edge formed
after the central part passed.
Figure 4(a) shows the results for the 5-hole
nozzle. Slightly large-sized droplets are detected
at the leading edge and central part of the spray,
and the detecting frequency of the droplets is low.
Numerous droplets over D >40 ,am are measured
in the beginning region of the trailing edge of the
spray. After 1.51 ms from SOl, the size of most
droplets shows below 25 ,am, but large-sized
droplets are detected irregularly. The AMD is
approximately 25 ,am during the injection period,
but it maintains the value of about 10,am after
1.5 ms from SOl. The trend of the SMD variation
is quietly similar to that of the AMD, and the
difference between the AMD and the SMD is not
so large. It means that the droplet distribution is
quietly uniform because the SMD, which is
defined as the ratio of volume (d a) to surface area
(d2), has the weighting factor to the large-sized
droplets, so that the difference between the AMD
and the SMD decreases when the droplet size
shows the quietly uniform distribution.
Figure 4(b) shows the results for the single-
hole nozzle. The trend of the droplet diameter
distribution with time from SOl is similar to that
for the 5-hole nozzle, but large-sized droplets are