The kinematics of non-cohesive, sphero-cylindrical particles in a low-speed, vertical axis mixer

Abstract

A discrete element method model is used to examine the velocity, solid fraction, and particle orientation fields of non-cohesive, sphero-cylindrical particles agitated in a vertical axis mixer for a range of particle aspect ratios and bed depths. The model is validated against experimental measurements of the rotating shaft torque. The particle trajectories within the bed are similar to those that have been reported previously for spheres, with a vortex circulating in the direction opposite of the blade rotation on horizontal planes of the bed. Increasing the particle aspect ratio generally decreases the particle velocities relative to the blade, implying reduced mixing. The solid fraction is largest just upstream of the blades and toward the base of the container for sphero-cylindrical particles. The smallest solid fractions are located in the wake region and at the bed's free surface. These results are generally consistent with those for spherical particles, although for spheres the solid fraction is smaller upstream of the blades near the container base. In general, larger particle aspect ratios decrease the overall bed solid fraction as well as the solid fraction uniformity. Particles with an aspect ratio larger than one have major axes that are offset between 10° and 20° from the flow streamlines. The degree of alignment between particles increases near boundary regions. In addition to the strong correlation between the particle principal orientation and velocity vectors, regions of larger velocity gradient magnitude result in smaller solid fractions and smaller degrees of three-dimensional alignment between particles.