MIGRATION OF PARTICLES UNDERGOING PRESSURE-DRIVEN FLOW IN A CIRCULAR CONDUIT

This study focuses on the demixing of neutrally buoyant suspensions of spheres during slow, pressure driven Bows in circular conduits. Distr ibutions of the solid fraction of particles, phi, and the suspension v elocity, v, are measured at different lengths from a static in-line mi xer. Experiments were conducted over a range of volume average solids fractions, phi(bulk) (0.10 less than or equal to phi less than or equa l to 0.50), and at two different ratios of the particle radius, a, to the radius of the circular conduit, R (a/R = 0.0256 and a/R = 0.0625). At phi(bulk) greater than or equal to 0.20, the particles rapidly mig rate to the low-shear-rate region in the center of the conduit. This m igration results in a blunting of the v profile, relative to the parab olic profile observed in homogeneous Newtonian fluids. For the flow ge ometry with the smaller ratio of a/R, the phi profile builds to a shar p maximum or cusp in the center. Particle structures are observed in t he experiments with the higher a/R. The entrance lengths for the devel opment of the phi and v fields, L-phi and L-v, respectively, are stron g functions of a/R and phi(bulk). L-phi and L-v rapidly decrease as ph i and a/R increase. Over the range of our data, the v profiles are obs erved to develop more rapidly than the phi profiles. The experimental results are compared with fully developed flow predictions from the sh ear-induced migration (SIM) model and the suspension balance (SE) mode l. At the smaller a/R, the SIM model more accurately predicts the expe rimental results. At larger a/R, some qualitative features of the expe rimental results are better predicted by the SE model, however, neithe r model provides good quantitative predictions,especially at low phi(b ulk). (C) 1997 The Society of Rheology.