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

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.