Environmental Engineering Reference
In-Depth Information
type, and fuel on combustion performance can be delineated from the emissions
measurements.
Direct photographs of
fl
flame were taken by a digital camera to obtain a quali-
tative understanding of the
flame characteristics. These photographs are reproduced
in black and white in Fig.
4
for both injector types using the diesel fuel. For each
case, images are shown for atomizing air (AA)
fl
fl
flow rates varying from 10 to 25 %
of the total air
flame indicative of
the fuel droplets burning in the diffusion mode for low AA rate, i.e., AA = 10 %. At
higher AA
fl
ow rate. Images revealed a yellow
orange, sooty
fl
-
flames showed a distinctive blue color typical of
premixed combustion, suggesting that the fuel droplets prevaporize and premix
with air before combustion takes place. A comparison of images for the two
injectors revealed that the FB injector requires a smaller AA
fl
ow rates, however, the
fl
fl
ow rate to produce
the premixed blue
flame. The transition from diffusion to premixed mode of
combustion occurred between AA = 10 and 15 % for the FB injector and at
AA = 20 % for the AB injector. The images in Fig.
4
make a clear distinction
between the two injectors; the FB injector produces cleaner
fl
fl
ames as compared to
the AB injector. Figure
5
quantitatively supports these
findings in terms of the
emissions measurements at the combustor exit plane. CO and NO
X
emissions
decrease with increasing AA
5 times lower for the
FB injector as compared to the AB injector, evidently because of the smaller fuel
droplets produced by the FB injector.
fl
flow rate, and they are always 3
-
5 Spray Characterization
Figure
6
visually illustrates the differences in water sprays formed by AB and FB
injectors for identical
flow inlet condition with atomizing air-to-liquid ratio by mass
or ALR = 3.5. These images represent a
fl
7mm
immediately downstream of the injector exit albeit with different spatial and tem-
poral resolutions to highlight the key features; higher exposure time was necessary
to acquire the image for the FB injector since it produced smaller droplets, which
limits the scattered light for visual imaging. Figure
6
reveals that the AB injector
produces the typical liquid core in the near
field of view (FOV) of 7 mm
×
field while the FB injector forms a
ne
spray, illustrating the signi
cantly improved atomization capability of the latter.
The
fineness of the FB injector spray offers the opportunity to employ advanced
fl
flow diagnostics techniques to understand the liquid breakup process in the near
field. Thus, we have employed a high-speed camera (Photron FASTCAM SA5)
mounted with 100-mm focal-length lens, a 2x extender and 130 mm length of
extension tubes to acquire visual images for rather small FOV with dimensions of
2.3 mm
×
1.4 mm at the injector exit. For acquisition rate of 100,000 frames per
second (fps), the image renders spatial resolution of 7.16
µ
m per pixel, to distin-
guish individual though stationary and/or large droplets. Background illumination
is provided by a very high-intensity white light source (Energetiq LDLS) produced
by laser-driven plasma to minimize the blurring caused by the droplet motion
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