A two-phase model for particle local equilibrium applied to air classification of powders

Abstract

A mathematical model has been devised to study air classification of fine powders. The model assumes that the particles have short relaxation times in the carrier gas. Hence, the particles are regarded locally as stationary, settling particles. The particles are allowed to exchange momentum with the fluid and they can alter the turbulent structure of the carrier gas. The present work considers only steady operation. A version of a continuous ACUCUT air classifier has been studied. The turbulent flow pattern in the classifier is analysed, and grade efficiency curves are predicted. The results show that increased particle loading in the classifier can improve its performance, by damping the turbulence. However, high loading may change the macroscopic flow pattern due to momentum coupling between the phases. The analysis also suggests that vortex shedding may occur in the classifier. Such unsteady flows may impair classification and should be avoided. The present model can be used to modify the classifier design to obtain steady flow.