Nuclear magnetic resonance imaging of particle migration in suspensions undergoing extrusion

Nuclear magnetic resonance imaging was used to measure fluid velocity and fluid fraction in suspensions flowing into an abrupt four-to-one contraction in pipe diameter, through a section of smaller diameter pipe, and out of an abrupt expansion back to the original pipe size. Suspensions of 50% by volume of particles in a Newtonian liquid were forced to flow by a plunger moving at a constant, slow velocity. Two sizes (100 and 675 mu m diameter) of suspended spheres were studied. Conditions were such that buoyant, inertial, Brownian, and surface forces could be assumed to be negligibly small. Little change in particle concentration was seen in the region of the contraction until the plunger was within about one pipe diameter of the contraction. The particles in the small diameter section of pipe migrated toward the pipe axis, the region of lowest shear rate. Particle concentration varied downstream of the pipe expansion, especially in a suspension of the lar er particles. Over time, particles were partially swept out of the region immediately downstream of the expansion joint. Although Reynolds numbers based on average suspension properties were identical in the two suspensions, the velocity fields in the expansion region differed, showing that demixing may markedly influence the downstream flow field in systems with complex geometry. (C) 1997 The Society of Rheology.