A Discrete Element Method Study of Monodisperse Mixing of Ellipsoidal Particles in a Rotating Drum

Abstract

Rotating drums are widely used in the industry for mixing, milling, coating, and drying processes. In the past decades, mixing of granular materials in rotating drums has been extensively investigated, but most of the studies are based on spherical particles. Particle shape has a significant effect on the flow behavior and thus mixing performance, but studies and understanding are still limited. In this work, discrete element method is employed to study the radial mixing of ellipsoids in a rotating drum. The effects of rotation speed and aspect ratio of ellipsoids on mixing quality and rate are investigated, and the underlying mechanisms are further developed. The results show that mixing index increases rapidly over time for both spheres and ellipsoids. Particles with various shapes of ellipsoids can reach well-mixed states after sufficient revolutions. The increase in rotation speed decreases the mixing rate for both spheres and ellipsoids. Generally, ellipsoids exhibit a higher mixing rate when the rotational speed ranges between 25 and 40 rpm. At 40 rpm, as the aspect ratio of ellipsoids deviates from 1.0, the mixing rate increases significantly and achieves the maximum when the aspect ratio is 0.75 or 1.5. A further increase in particle non-sphericity incurs a decrease in mixing rate. The increase in the deviation of aspect ratio from 1.0 contributes to stronger “slipping” of particles relative to the drum wall. Compared with ellipsoids, spheres have stronger diffusive mixing and weaker convective mixing in rolling or cascading regimes.