An acoustic method for in-line particle size measurement in flowing bulk solids

Abstract

The propagation velocity of acoustic waves through a packed bed of particles depends on, among other things, the wave frequency and the size of the particles. In this paper, theory is developed to describe the propagation velocity based on consideration of viscous interactions between the gas and solid phases. Experimental measurements in the frequency range 50–10 000Hz, and for particles ranging in mean size from 65 to 6000μm, agree well with the theory, particularly at low frequencies. The propagation velocity also correlates strongly with measurements of the surface mean particle diameter, independently of changes in particle density, and a method is described using this relationship as the basis of an in-line particle size measurement instrument for flowing bulk solid systems. Measurements using different size fractions of sand suggest that changes in mean particle size of less than 1.5 per cent can be resolved using the simple prototype developed for this work.