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Study of Submicron Particle Size Distributions by Laser Doppler Measurement of Brownian Motion.

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Annual rept. 1 Sep 83-30 Sep 84,

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Few nonintrusive techniques are available for particle measurements in the submicron size range 0.1 um diameter, yet measurement of these particles is basic to an understanding of important processes in combustion, such as soot formation and oxidation. The objective of the present research is the development and application of a technique for measurement of individual submicron particles in a gas stream. The approach is to measure the inertial relaxation time of individual particles in Brownian motion, by statistical analysis of the time-resolved 100 MHz heterodyne signal obtained in an interferometer system resembling a more conventional laser Doppler velocimeter. Progress to date has included the development of an optical system and fast data acquisition system for conducting the experimental studies. A detailed Monte Carlo numerical simulation of the Brownian motion of isolated particles has been developed and applied to the calculation of the time dependent signals expected in the experiment. These simulated signals have been analyzed for correlations between the particle size and statistical properties of the signal. The mean time between signal extrema has been found to be a proportional to the square root of the product of the particle relaxation time, a quantity which is proportional to the particle diameter, and the data sampling interval. Continuing research on this program will be initially focused on validation of the experimental approach, by statistical analysis of signals obtained with the interferometer for particles of known size. Following the demonstration of this approach, the technique will be applied to the study of the size evolution of combustion particulates.

Subject Categories:

  • Statistics and Probability
  • Test Facilities, Equipment and Methods
  • Combustion and Ignition

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