(65b) Radial Segregation of Binary-Sized Mixture of Ellipsoids in a Rotating Drum

Rotating drums are widely used in industry for mixing, milling, coating and drying process. Segregation is inevitable when using rotating drum to mix granules differing in size, density or shape. It is well accepted that percolation effect dominates the size segregation process. The fine particles could penetrate through the voids between coarse ones, then accumulate in the core of the bed. Though size segregation in the rotating drum has received comprehensive studies over the past decades, most of the studies concentrate on spherical particles. In fact, particle shape could also affect size segregation in terms of equilibrium extent of segregation or segregation rate. There have been a few studies in investigating the shape effects on size segregation, for example, the impact of blockiness on the formation of steak patterns for size segregation of super-quadric particles [1]. However, the understanding of particle shape induced size segregation is still not well addressed in the literature.

In this work, discrete element method is employed to study the radial segregation of binary-sized mixtures of ellipsoidal particles in a rotating drum. The segregation patterns for both spheres and ellipsoids are presented. The effects of rotating speed and aspect ratio of ellipsoids on the extent of segregation in the equilibrium state are investigated. The results show that at 15 rpm, a flat bed surface is observed, indicating the bed is in the rolling regime. However, cascading regime has appeared at the same rotation speed for ellipsoids, characterised by an S-shaped surface. As the revolution increases, the segregation pattern becomes increasingly obvious. Coarse particles tend to segregate to the periphery of the bed, whereas fine ones are trapped in the central area. It is noted that percolation mechanism, which accounts for size segregation of spherical particles, could be applied to ellipsoids as well. However, the deviation of aspect ratio from unity, i.e. elongated or platy shape-type, lowers the extent of segregation at the steady state. It could be conjectured that percolation effect is mitigated when aspect ratio varies. Further attempts are made to investigate the effect of rotation speed on the equilibrium extent of segregation. It is found that in rolling or cascading regime, increasing rotating speed could reduce the extent of segregation for both spheres and ellipsoids. This is because more fine particles could mix into the flowing layer other than percolate through the voids between coarse ones at a higher rotation speed.

Reference

[1] G.G. Pereira, S. Pucilowski, K. Liffman, P.W. Cleary, Streak patterns in binary granular media in a rotating drum, Appl Math Model, 35 (2011) 1638-1646.