Design of Atomizers and Burners for Coal-Water Slurry Combustion Page: 4 of 7
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respectively. The strain rate of the filament will be determined from the experimental values of UL
and -2 d/d. Any discrepancy between the experimental values of LIL and -2 d/d, will indicate that the
volume of the filament during the stretching process is not exactly constant. The diameter and mass
of the droplet will be measured using a digital image analyzer.
Summary of Technical Progress
Development of a Laser Attenuation Technique for Measurement of Ligament Diameter:
This reporting period was dedicated to the development of the laser attenuation technique (see
Fig. 2). Measuring the ligament diameter relies on the principle of light attenuation. A 15 mW
Helium Neon laser beam is spread into a laser sheet by using a cylindrical lens. The laser sheet is
passed through the filament before being captured by the photodiode. The amount of light being
attenuated by the filament is directly proportional to the diameter of the filament. After passing
through the filament, the laser sheet is directed through additional optics and onto a photodiode. As
such, we can correlate the diode output to the ligament diameter.
During the experiments it will be insured that the diameters measured are within the
homogeneous part of the filament. The use of a laser sheet instead of a cylindrical beam has the
purpose of generating a uniform light intensity (at least within the central portion of the sheet)
before interaction of the laser with the filament. If the laser intensity is not spatially uniform, any
swaying of the liquid filament will result in erroneous diameter measurements. The edges of the
laser sheet, where the intensity is not spatially uniform, is blocked by using pinholes. A number of
pinhole sizes (i.e., diameter) are used and calibration curves are generated for each pinhole by
using liquid jets of various diameter. Initially, the experiment relied on the attenuation of a laser
beam, unaltered by the cylindrical lens. This method produces ambiguity, however, due to the
Gaussian nature of the intensity profile intrinsic to any laser. Such a distribution would indicate a
non-linear relation between the intensity of light which strikes the diode and the diameter of the
ligament. This unneeded complexity is effectively reduced by the use of the cylindrical lens.
Specifically, the lens will stretch the profile of the laser to expose its central, most intense portion.
This stretched central region is effectively constant in intensity, and thus will produce a linear
relation between the diode signal and the ligament diameter. In addition to a more linear
relationship, the laser sheet will help to prevent the error caused by 'swaying' ligaments. When the
droplet falls, the trailing ligament becomes very small. As a result, the ligament is influenced by air
drafts and can easily sway away from the path of a single laser beam. With the optics, however,
the ligament cannot sway out of the range.
Calibration of the system involved a magnifying CCD camera and an array of fine bore glass
nozzles. Using the nozzles, we were able to produce water jets varying in size from approximately
120 to 2000 microns. Once the exact diameter for each stream was determined with the magnifying
camera, we could find the corresponding output voltage from the photodiode by placing the jet in
the path of the laser sheet. With each jet, we obtained a different voltage and thus we could
construct the curve relating the diode voltage to the ligament diameter. The collecting optics were
contained within a protective housing fitted with a 3 mm aperture. Such an aperture was sufficient
small to block the non-linear portion of the laser sheet, yet large enough to allow the proper
measurement of the largest diameter ligaments. In addition, it was necessary to eliminate the
influence of fluctuating laser intensity. This was done by normalizing the output voltages
associated with each jet with voltage values recorded with no ligament present. Figures 3 and 4
show the raw and normalized output voltages, respectively. Figures 3 and 4 show the calibration
curves using a 3 mm diameter pinhole. Observe that the measured signal is proportional to the jet
diameter; the signal decreases with increasing diameter.References/Publications: None
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Chigier, N. & Goldberg, P. Design of Atomizers and Burners for Coal-Water Slurry Combustion, report, December 31, 1995; [Pittsburgh, Pennsylvania]. (https://digital.library.unt.edu/ark:/67531/metadc670172/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.