Imaging particle migration with electrical impedance tomography: an investigation into the behavior and modeling of suspension flows
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, there is little data for particle migration when buoyancy effects are important. An accurate and efficient electrical impedance tomography (EIT) imaging technique is developed to non-invasively measure the distribution of positively and negatively buoyant particles in low Reynolds number pressure-driven flow in a pipe. vii Additionally, a bimodal suspension of heavy particles in a low Reynolds number pressure-driven pipe flow is investigated. The effects for a range of buoyancy numbers were probed by varying the flow rate. In all of the experiments, a significant fraction of the particle phase is 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 center of the pipe increases with increasing magnitude of the buoyancy number. The bimodal suspension displayed an adverse density gradient for low buoyancy numbers. A scaling analysis is introduced to predict the formation of an unstable particle distribution. Furthermore, observations of the phase distribution at several positions downstream of the inlet indicate that suspension flows of buoyant particles become fully developed earlier than that observed for neutrally buoyant particles, with higher buoyancy numbers becoming fully developed more rapidly. A scaling for the prediction of the fully developed length is presented that matches experimental observations reasonably well. The experimental observations of the particle distributions were favorably compared to the predictions of an isotropic suspension balance model.