Environmental Engineering Reference
In-Depth Information
bubble explosion or FB atomization occurs mainly between the discharge ori
ce
and injector exit. This process is hereby denoted as the primary breakup by bubble
explosion or FB atomization. Most of the droplets cannot be spatially resolved in
the images because of the blurring caused by the droplet motion in spite of the small
camera exposure time. However, larger droplets on the injector periphery are
spatially and temporarily resolved. In Fig.
7
, the diameter of the droplet A is about
100
µ
m, droplet B is about 21
µ
m, and that of droplet C is about 72
µ
m. These
results clearly demonstrate, for the
first time, that the bubble explosion or primary
breakup occurs mainly within the FB injector and the larger droplets appear only
occasionally at the injector periphery.
A quantitative description of the spray quality is given by radial pro
les of sauter
mean diameter (SMD) measured by the phase Doppler particle analyzer (PDPA) at
different axial locations in the water spray as shown in Fig.
8
for ALR = 2.5
(Simmons et al.
2009
; Simmons and Agrawal
2011
). Figure
8
a shows the SMD
pro
les of the AB and FB atomizers at Y = 1.0 cm, the axial distance from the
atomizer exit. For the FB atomizer, the peak SMD of 13
m at the spray center
decreases with increasing radius until a minimum of about 9
µ
m occurs at radial
distance, r = 4 mm. Thereafter, the SMD increases to reach a value of about 12
μ
m
near the periphery of the spray located at r = 8 mm. An important observation is
that the FB atomizer produces droplets of 9
μ
m or within the narrow range of
4
μ
m. For the AB atomizer, the minimum SMD of 7
μ
m at the center increases
gradually to about 20
-
13
μ
m at the periphery of the spray at r = 12 mm. Thus, the
droplet SMD range is much wider (>15
25
-
μ
m) with the AB atomizer. Clearly, the FB
atomizer offers unique features to distinguish it from the AB atomizer.
Figure
8
b shows the SMD pro
μ
les for the two atomizers at a downstream
location, Y = 2.0 cm. Results show trends similar to those observed at Y = 1.0 cm.
The SMD pro
le of the FB atomizer no longer has a peak at the center indicating
that this region undergoes signi
ow between
Y = 1.0 and 2.0 cm to breakdown the larger droplets. The SMD still varies over a
narrow range of 9 and 13
cant interaction with the surrounding
fl
m while the spray width increases to about r = 10 mm.
The AB atomizer still demonstrates a wide range of SMDs, from around 6 to over
20
μ
m, with the smallest droplets contained in the center of the spray, and the
largest droplets con
μ
ned to the periphery.
Cumulative mass fraction versus droplet diameter distribution plot in Fig.
9
a
shows that toward the edge of the spray, Y = 1.0 cm, r = 0.7 cm, droplets smaller than
30
m comprise 100 % of the liquid mass in the FB spray but only 70 % of the liquid
mass in the AB spray, where the remaining mass is contained in larger (30
µ
m)
droplets. Figure
9
b shows that at the center of the spray at Y = 3.0 cm, a greater
percentage of smaller droplets are produced by the AB atomizer. At Y = 3.0 cm and
r = 0.5 cm, Fig.
9
c shows that droplets smaller than 20
-
75
µ
µ
m contain 95 % of the liquid
mass in the FB spray as compared to 85 % of the liquid mass in the AB spray. These
results demonstrate the superiority of the FB atomizer over AB atomizer, especially
in the outer regions of the spray.
Next, Fig.
10
a shows the radial pro
les of SMD in non-reacting sprays of diesel
and VO produced by the FB injector at the same operating conditions. The diesel
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