In Situ Optical Particle Sizing Technique

Abstract

This paper discusses the application of an in situ optical counter to the measurement of liquid fuel droplets and solid coal particles under combustion conditions. Mie theory computations are used to determine an optimal near-forward light-scattering geometry for sizing both spherical transparent particles and irregularly shaped light-absorbing particles in the 5-80 /im range. Results are presented for a burning methanol spray and for reacting coal particles. I. Introduction T HERE is considerable interest in the measurement of particle size distributions in two-phase flows. Specific examples of energy-conversion devices in which such measurements are desirable include liquid spray and pulverized fuel combustors, and particulate cleanup devices such as electrostatic precipitators. While the particulate characteristics such as mean diameter, size distribution, mass loading, and material properties may vary in such systems, the sizes of interest generally fall in the submicron to 100 jim range, with number concentrations up to 107 cm~ 3 for submicron-sized particulates. The available measurement techniques1'2 also vary widely in type and capability. Several, including microscopy, cascade impactors, Coulter counters, mobility analyzers, and commercial optical counters, require a sample to be extracted from the flow. This poses problems related to obtaining a representative sample, especially for the larger-sized and volatile materials in hot, high-velocity flows. Also, many sampling methods are cumbersome and slow in operation. For these reasons, optical techniques are of special interest since they are capable of making in situ measurements with continuous and rapid readout. Moreover, by using lasers, they are adaptable to high-temperat ure systems having high thermal radiation background. Optical techniques all depend on Mie scattering3'4 and can be broadly divided into imaging and nonimaging types. The former, including flash photography5 and holography,6'7 are limited in practice to sizes 19 the theory and initial measurements of a new in situ counter technique utilizing the last approach have been described. The present paper briefly reviews the basis of the measurement technique described in Refs. 18 and 19, and then discusses the instrument requirements necessary for measurements of relatively large (5-80 /ion) particles with variable light-absorbing properties. Light-scattering computations using the Mie theory are presented for several nearforward light-collection geometries, and optimal conditions are defined. Based on these results and previous corn