Sedimentation of particles from a convecting fluid

Abstract

THE sedimentation of particles from a convecting fluid is a process of much interest to engineers, fluid dynamicists, geologists and metallurgists. For example, the process plays a fundamental role in controlling the settling behaviour of phenocrysts in magma chambers, crystals and impurities in metallic castings and carbon microparticles in combustion chambers. Previous studies of particle settling in convecting systems^1–5 have assumed that the concentration of particles is so small that their presence does not modify the convective motion. Here we present experimental results which show that convective motion due to heating from below can be affected by the presence of particles and is controlled by the bulk density of a particulate suspension, which is a function of both temperature and particle concentration. Above a certain concentration, the particle distribution initially stabilizes the bulk density gradient during sedimentation, and convection is confined to a sedimenting layer beneath a clear layer. Eventually, however, othe destabilizing thermal gradient exceeds the stabilizing effect of the particles, and a sudden overturn of the whole system results. Our experimental results agree with a simple physical model.