Particle migration in pipe flow of dense suspensions

Migration of particles in dense suspensions can have a significant effect on the friction coefficient in pipe flow: particles clustering in the core can reduce the friction factor by 20-30% compared to a well-mixed particle suspension, due to the reduced effective viscosity near the pipe wall. While the entrance effects of singe-phase flows are well-established, relatively little is known about the development of the velocity and concentration profiles of dense suspension flows (volume fractions of 0.05-0.45). We study the migration of neutrally-buoyant particles in a pipe flow, with a focus on intermediate bulk Reynolds numbers (100-5000), starting from a nearly uniform concentration profile. We use ultrasound imaging to capture the evolution of the suspension at various downstream locations. We leverage the ease at which these measurements can be done at arbitrary locations along the pipe. The images are used to estimate the local velocity profile (`ultrasound imaging velocimetry'). They also shed light on the development of the concentration profile, even though the local image intensity does not readily translate to concentration profiles. Nevertheless, the 'convergence' towards an equilibrium intensity profile is used to determine the migration speed. We will investigate the role of volume fraction and Reynolds number on the migration, in order to understand the scaling laws involved.