Granular Motion in a Rotary Kiln: the Transition from Avalanching to Rolling

Abstract

We report measurements of flow transitions, from avalanching to rolling, for granular material in rotary kilns. In the avalanching mode, the surface slips periodically; in the intervals between avalanches, all particles rotate with the kiln. In the rolling mode, the surface particles slide down continuously; the material underneath the surface rotates with the kiln. Our measurements give Froude numbers (Rω2/g) for transitions, which are significantly different for sand and TiO2 powder. For the avalanching mode, we measured cycle times and deduced t12, the avalanche time; t12 was also measured directly by video photography. For kilns of diameters 0.2-0.5 m, both methods give t12, of order 1-2 sec and it appears to be proportional to √l, l being the chord length of the granular bed, the maximum distance of fall for avalanche material. Simple theory, assuming the avalanche particles slide down a frictional surface, gives fair estimates of t12 and may be a basis for predicting avalanche-to-rolling transitions in large industrial kilns.