ON FLOW-INDUCED KINETIC DIFFUSION AND ROTARY KILN BED BURDEN HEAT TRANSPORT

Abstract The cross-section of a partially-filled cylindrical kiln rotating on its horizontal axis and processing granular solids produces a shear zone (active layer) at the free surface which grows with the kiln's rotational rate. The active layer, although relatively thin, compared with the rest of the bed burden, drives all physical/chemical reactions. This is because of the high rate of surface renewal which, in turn, promotes heat exchange between the exposed surface and the higher temperature freeboard gas. Unlike packed beds, particulate diffusion induced by the flow of granules, adds a significant component to the overall heat transfer in the bed. Problem formulation and modeling of heat conduction using flow fields derived from experiments suggest that at slow kiln speeds the diffusion effect may not be recognized due to long term duration of particle contacts and hence packed-bed heat conduction models may provide adequate characterization. However, at moderate and high kiln speeds particle colli...

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