Near-Nozzle Diesel Spray Imaging
Using Visible Light
T.E. Briggs & P. V. Farrell
532 nm
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The area of interest is the burgundy zone at the left. The
spray is beginning to break up and shed drops that will
provide the initial combustion field. The rest of the
combustion process is determined by what happens here.
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The diagnostic method is relatively simple. Two closely-spaced
wavelengths are used to back-illuminate the spray. The light beams
are separated spectrally and imaged side-by-side on a high speed
camera. The scattering in each image is identical, so it may be
removed by dividing one image by the other. The remaining
information is due to the absorption of the light by the dye in the fuel.
This signal is directly related to the mass of fuel that was along the
path the light traveled. Once the mass is known, the volume fraction of
the liquid fuel in the spray may be determined as well.
GM
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Diesel combustion systems must continue to evolve to
meet future emissions regulations - aftertreatment will not
be sufficient to meet the requirements
Improving combustion requires understanding the spray
physics well enough to design injection systems that will
enable the required combustion performance
It is not sufficient to know the downstream spray
conditions such as drop sizes and mass fractions - we
must know how the spray is behaving from the injector
onward
Strong light scattering in the dense spray near the injector
has prevented typical spray measurement techniques
from providing information on the spray structure or
conditions
X-ray imaging has been shown to be immune from the
problems of scattering and has given mass and volume
fraction data on the near-injector spray. The technique is
expensive and only provides averaged results, however.
By using two visible wavelengths of light and an absorbing
dye in the fuel, sufficient redundant information about the
light scattering can be obtained to remove the scattering
signal from a spray image, yielding a purely absorptionbased imaging technique.
This provides equivalent results to the x-ray imaging
method, but at a significant cost reduction and with
instantaneous imaging capability.
Collaborative Research Lab
488 nm
23mm
100s ASI
This is what the camera sees. Approximately 20 frames are captured
for each injection event. The images are separated and normalized
using Matlab. They are then processed to generate an absorption
image and a mass image of the spray at each timestep.
100s ASI
23 mm
100s ASI
X-ray (mass in g)
Beam diameter
Optical (mass in g)
Here is a comparison of the final mass image from the visible technique (on the
right) with a corresponding image of the same spray using the x-ray technique
(on the left). Note that the visible image captures much more detail of the spray
due to the instantaneous nature of the measurement. Averaging temporally as
the x-ray method does leads to lower-mass regions not being adequately
imaged - the fuel is in a different location each injection, so the averages tend
to give a zero result. Our current efforts are to obtain quantitative agreement
with the x-ray measurement and to add volume fraction calculations to the
processing code.
UW