Particle Size Measurement by Laser Anemometry

Abstract

At the 15th Aerospace Sciences Meeting the authors described a technique for particle size measurement using laser doppler anemometry. The present paper reports on developments which have taken place since that time. An additional gate photomultiplier has been introduced at right angles to the optical axis in order to select only those particles passing through the central region of the measurement control volume. This development eliminates the dependence of the measurements on position of the particle along the length of the measurement volume. It thus simplifies the analytical method of correcting data for the effect of particles being equally likely to pass through any position in the measurement volume. Particle sizing measurements have been made in sprays of glass particles using the modified laser anemometry system. Measurements in fuel sprays are also reported and compared with the results obtained by a photographic technique. The application of the particle sizing technique to opaque particles is investigated and suitable optical arrangements are suggested. Light scattering characteristics of laser anemometry systems for different optical geometries are calculated to select the optimum optical arrangement for the particle sizing measurements. HE measurement of the sizes, velocities, and concentrations of spherical particles in two-phase flows, particularly in fuel sprays of 5 <£/<300 /*m droplet diameter range, is of importance in determining spray characteristics. Time averaged, rather than spatially averaged, droplet size distributions and size velocity correlations in fuel sprays can be obtained from the simultaneous particle size and velocity measurements. The single-particle-counting forwardscattering laser doppler anemometer (LDA) technique described in the present paper, which is a modified form of the technique described previously,l can be applied to twophase flows for simultaneous particle size and velocity measurements for particle diameters larger than the fringe spacing in the measurement control volume (MCV) of crossed laser beams. There is relatively little published work relating particle sizes to the signals measured by single-partic le-counting LDA systems. Most interest has focused on utilizing the relationship between signal visibility and particle size, e.g., Meyers and Walsh,2 Hong and Jones,3 and Farmer.4'5 For large particle diameters the application of this method is limited by ambiguities which arise when the particle diameter exceeds the fringe spacing and the visibility function is resolution limited for particle diameters smaller than 0.3 /xm.6 Durst and Umhauer7 have used a separate white light source to measure the particle size, whereas an LDA system was used to measure the velocity. The amplitude of the scattered light was related to particle diameter by using the Mie theory. Their system was used for particles of the order of 1 /xm in diameter. Durst and Zare8 have investigated the possibility of measuring the radius of curvature of the partly or totally reflecting or refracting surfaces in the plane of the two crossing laser beams by measuring the fringe separation distances of the scattered light pattern. Chou and Waterson9 have used the ratio between the forward- and back-scattered