AN EXPERIMENTAL-STUDY OF VISCOUS RESUSPENSION IN A PRESSURE-DRIVEN PLANE CHANNEL FLOW

Resuspension is a process by which an initially settled layer of heavy particles in contact with a clear fluid above it is set into motion by a laminar shear flow. Experiments were performed in a fully-developed Hagen-Poiseuille stratified channel flow with a clear fluid overlying a suspension, in order to measure the pressure drop and the particle velocity at the suspension-clear fluid interface as functions of the well-mixed particle volume fraction phi(s) and a Shields number kappa which is a measure of the relative importance of viscous forces to those of gravity. It was found that, for a fixed feed concentration, the measured pressure drop coefficient K decreased abruptly at kappa similar to 0.006 and attained values which were significantly lower than those predicted theoretically. At the same time interfacial waves were observed which eventually became very strong. A further increase in kappa led to wave destruction and the appearance of clouds of detached particles moving relatively rapidly above the original interface. In this range the pressure drop coefficient increased and reached a value almost independent of kappa. The intensity of wave breaking then lessened but remained significant. The measured particle velocity at the interface showed good agreement with the theory for small values of kappa. At larger values, however, the observed particle velocity at the interface was up to several times larger than that predicted due to the existence of a detached particle layer that moved very rapidly. Finally, an additional flow instability was observed, a ripple type of instability, when the bottom of the channel was covered by a monolayer of particles.