(22e) Radial Mixing in Granular Materials in Rotating Cylinders

Particle self-diffusion has a significant effect on granular mixing. Rock-cuttings, sand, cement, tablets, grain, coal, biomass etc are an example of granular materials. These materials are transported from one place to another using belts, conveyors, drilling fluids or pneumatically. Flow induced mixing is important in handling of these materials. Rotating cylinder is commonly used as a prototype to study the granular mixing. We study the radial mixing using same size but different colors of glass beads in a quasi-2D cylinder rotated in the continuous flow regime. In this regime a shallow surface layer of particles flows steadily while the rest of the material rotates as a solid body. A qualitative study of the mixing is carried out using digital photography. Radial number fraction profiles of the tracer particles are obtained by bulk sampling. Since mixing occurs only in the flowing layer, hence mixing is considered in terms of ``passes'' defined as the number of times the entire material in the bed flows through the layer. Results indicate that the mixing per ``pass'' decreases with increasing rotational speed, and increases with increasing particles size. The mixed state captured by digital photography and the measured radial concentration profiles are well described by a convective diffusion model, using diffusivity as a fitting parameter. Mixing in transverse direction is found faster than azimuthal direction. One dimensional diffusion equation is used and a good agreement is obtained.