Granular axial band formation in rotating tumblers: a discrete element method study

The onset mechanism for band formation of a granular mixture in long rotating tumblers is still largely unresolved. We study this issue for axial segregation of binary mixtures having different size particles, using discrete element method simulations. Endwalls initiate axial segregation via an axial flow due to friction. The non-uniform distribution of axial velocity in the flow together with simultaneous radial segregation via percolation results in the axial flow rate of the two types of particles differing in the upstream and downstream portions of the flowing layer. Thus, small particles are driven further from the endwalls, while large particles accumulate at the endwalls. Once this occurs, a cascading mechanism begins so that other bands form due to the gradient in particle concentration near the endwalls. A small axial flow between segregated bands of small and large particles persists even after the bands are fully developed.