Particle Sizing by Quantitative Acoustic Emission

Abstract

Abstract Two methods of particle sizing using an acoustic method are presented. The technique relies on the detection and measurement of elastic waves arising from impact of the particles with a small target plate. The acoustic impact signal, as measured from a high fidelity piezoelectric transducer, is characterised by the first wave arrival (compression wave) amplitude and risetime. A theoretical approach, based upon Hertzian impact theory and elastic wave propagation theory, is developed and used to determine the relationship between the impact dynamics of the particle and the acoustic signal. The first method of particle sizing relies upon amplitude measurements alone but requires accurate system calibration and full knowledge of the particle impact velocity, incident angle and coefficient of restitution. The velocity measurements were determined using a laser Doppler technique. The second approach for particle sizing incorporates risetime measurement and only depends very weakly upon the compression wave amplitude, thus minimizing the need for accurate absolute calibration. Knowledge of the impact velocity, incident angle, and coefficient of restitution were not required. Spherical particles of glass in the size range 35 to 140 μm diameter were dropped onto an aluminium target plate at an incident velocity of 8.1 m/s and angle to the surface normal of 0°, 40°, and 61°. The two acoustic methods were used to size the particles, and the results were compared to the particle size distributions obtained from video microscope measurements. The first sizing method undersized the smaller particles by 20% to 30% and oversized the larger ones by 5% to 10%. However, the second approach gave much smaller errors and consistently undersized all sizes of particle by just a few percent. This undersizing was understood in terms of some plasticity in the target that occurred during particle impact.