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 c ontraction in pipe diameter, through a section of smaller diameter pip e, and out of an abrupt expansion back to the original pipe size. Susp ensions of 50% by volume of particles in a Newtonian liquid were force d to flow by a plunger moving at a constant, slow velocity. Two sizes (100 and 675 mu m diameter) of suspended spheres were studied. Conditi ons were such that buoyant, inertial, Brownian, and surface forces cou ld be assumed to be negligibly small. Little change in particle concen tration was seen in the region of the contraction until the plunger wa s 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 partic les. Over time, particles were partially swept out of the region immed iately downstream of the expansion joint. Although Reynolds numbers ba sed on average suspension properties were identical in the two suspens ions, the velocity fields in the expansion region differed, showing th at demixing may markedly influence the downstream flow field in system s with complex geometry. (C) 1997 The Society of Rheology.