Migration of buoyant particles in low-Reynolds-number pressure-driven flows

Abstract

Neutrally buoyant particles in low-Reynolds-number pressure-driven suspension flows migrate from regions of high to low shear, and this migration is a strong function of the local concentration. When the particle density differs from that of the suspending fluid, buoyancy forces also affect particle migration. It is the ratio between the buoyancy and viscous forces, as quantified by a dimensionless buoyancy number, which determines the phase distribution of the suspension once the flow is fully developed. Although several experiments have verified shear-induced particle migration in neutrally buoyant suspensions, data for particle migration when buoyancy effects are important are scarce. Electrical impedance tomography (EIT) is used here to non-invasively measure particle concentration across a pipe arising from the low-Reynolds-number flow of heavy conducting particles and light non-conducting particles in a viscous suspending fluid. A range of buoyancy numbers was investigated by varying the flow rate. In all of the experiments, a significant fraction of the particle phase was observed to migrate towards the top or bottom of the pipe, depending on the relative density of the particles. The amount of migration away from the centre of the pipe increased with increasing magnitude of the buoyancy number. Furthermore, observations of the phase distribution at several positions downstream of the inlet indicate that these suspension flows become fully developed earlier than that observed for neutrally buoyant particles. A scaling analysis for the prediction of the fully developed length is presented, which predicts shorter lengths for higher buoyancy numbers and is consistent with experimental observations. The experimental data were compared to an isotropic suspension balance model, and it was found that the particle phase distributions predicted by this model agree fairly well with the experimental observations.