Particle migration in tube flow of suspensions

In this research, we investigated the migration of particles in the tube flow of suspension for a wide range of particle loading (φ0) and particle Reynolds number (Rep), using a magnetic resonance imaging (MRI) technique. The suspension consisted of nearly monodisperse polymethylmethacrylate spheres in a density matched Newtonian fluid. The volume fraction of the solid was 0.06–0.40. Both the velocity and the concentration distributions were measured under fully developed conditions. It has been found that, when φ0 was small (⩽0.1) and Rep was not small (≈0.2), the particles moved toward the position at a distance of 0.5–0.6 R (tube radius) from the tube axis and the velocity profile was parabolic. When φ0=0.4, particles always moved toward the center of the tube and the velocity profile was blunted. The degree of blunting was larger for smaller Rep. Between these two limiting cases, the particle migration was dependent on Rep. When Rep is small the particles move toward the tube axis regardless of φ0. When φ0 is 0.2–0.3 and Rep≈0.2, particles are concentrated both at the center and at the middle of the tube axis and tube wall. The velocity profile keeps the parabolic form unless the particles are concentrated regardless of Rep. Apparent wall slip is not observed except for the case of φ0=0.40. It is suggested that, when the particle Reynolds number is larger than 0.1, the inertial effect cannot be neglected regardless of the average particle concentration.