Particle segregation and flow in slurries: dependence on the interstitial fluid viscosity and angular velocity

Abstract

A granular binary mixture of particles differing in size in a rotating drum will tend to segregate. The mechanism by which this happens occurs in the ‘flowing layer’ of the drum. As the particles flow through the flowing layer, the smaller particles are more likely to fall through holes created by the moving particles, so they segregate towards the bottom of the flow. Thus, the larger particles are more likely to be pushed to the top of the flowing layer. When the particles ‘freeze,’ or come out of the flowing layer, the particles on the bottom of the flowing layer segregate towards the middle of the drum, and the particles on the top to the outside of the drum. Because the flowing layer is curved, the particles at the bottom of the layer freeze sooner (closer to the center) than the particles at the top. This is called radial segregation forms a ‘half-moon’ pattern. When the drum is filled to about half full (51– 61% full [1]) something more interesting happens. The segregation pattern first forms the half-moon pattern, then forms stripes. These stripes form because small differences in volume fraction lead to different flowing layer velocities, and therefore, to stripes [3]. By the geometric argument proposed by Hill et. al, the stripe width is proportional to the depth of the flowing layer. It is known that a higher viscosity does increase the depth of the flowing layer [2], although this seems to be empirically observed. In this project, the effects of the viscosity interstitial fluid were investigated and speed of the drum rotation on the wavelength and width of the stripes were investigated. This has important applications in industry and chemical engineering.