Aerodynamic particle size measurement by laser--Doppler velocimetry. Publication number 343

Abstract

A method of measuring the aerodynamic diameter of aerosol particles was investigated. The method consists of accelerating particles in a coverging nozzle and measuring their velocities near the exit of th nozzle with a laser--Doppler velocimeter. The experimental studies utilized a test nozzle with a converging angle of approximately 15/sup 0/ and an exit diameter of about .1 cm. The pressure drop across the nozzle was varied from 2.54 to 276 cm of H/sub 2/O, and particle velocity was observed to vary from approximately 0.5 to 1.0 times the gas velocity at the exit of the nozzle. A theoretical analysis utilized boundary layer theory to predict the velocity of the gas in the nozzle, and then the equations of particle motion were integrated to give the theoretical particle velocities. These values agreed with the experimental values to within a few percent. The effects of nozzle geometry, flow rate, particle density, and particle size were studied using the results of calculations made with dimensionless equations. The velocity of a particle in a given nozzle and flow depends upon the aerodynamic diameter of the particle and the particle density. The geometry and flow can be chosen to minimize the effect of particle density. Assuming that the density of particles in the atmosphere ranges from 1 g/cm/sup 3/ to 3 g/cm/sup 3/, the aerodynamic diameter of particles can be measured with an uncertainty of +- 10% in the size range from .5 ..mu..m to 10 ..mu..m.