Particle migration in tube flow of suspensions

In this research, we investigated the migration of particles in the tube fl ow of suspension for a wide range of particle loading (phi(0)) and particle Reynolds number (Re-p), using a magnetic resonance imaging (MRI) technique . The suspension consisted of nearly monodisperse polymethylmethacrylate sp heres in a density matched Newtonian fluid. The volume fraction of the soli d was 0.06-0.40. Both the velocity and the concentration distributions were measured under fully developed conditions. It has been found that, when ph i(0) was small (less than or equal to 0.1) and Re-p was not small ( > appro ximate to 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 parab olic. When phi(0) = 0.4, particles always moved toward the center of the tu be and the velocity profile was blunted. The degree of blunting was larger for smaller Re-p. Between these two limiting cases, the particle migration was dependent on Re-p. When Re-p is small the particles move toward the tub e axis regardless of phi(0). When phi(0) is 0.2-0.3 and Re-p > approximate to 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 Re-p. Apparent wall sli p is not observed except for the case of phi(0) = 0.40. It is suggested tha t, when the particle Reynolds number is larger than 0.1, the inertial effec t cannot be neglected regardless of the average particle concentration. (C) 1999 The Society of Rheology. [S0148-6055(99)00905-0].