Concentrated suspension flows through an abrupt axisymmetric expansion

This work provides a detailed set of data systematically obtained over a range of flow conditions to further the study of suspension flows through complex geometries. In particular, this study focuses on the concentration distributions, velocity flow fields, and pressure drops for suspensions of bulk particle volume fractions ranging from 0.2 to 0.5 through a 1:4 axisymmetric contraction and/or expansion. Nuclear magnetic resonance (NMR) imaging was used to investigate the effects of particle size, bulk particle volume fraction, flow Reynolds number, and inlet length on the suspension behavior in this system. The concentration and velocity data obtained from NMR imaging were then supplemented with pressure drop measurements across the narrow tube. Concentration distributions and velocity streamlines were also compared with a current continuum model of suspension flow behavior. It is our intention that the data provided here will provide benchmarks for future modeling work.; Results indicate that inlet concentration profiles formed in the upstream, narrow tube greatly influence behavior downstream, more so than direct inter-particle collisions in the abrupt expansion. In all cases, the lengths of recirculating regions are greater for suspensions than for Newtonian fluids at equivalent Reynolds numbers and increase with bulk particle volume fraction. The size of the recirculation regions is also sensitive to inlet conditions, which influence the ratio of tube center to wall local viscosity values. The viscosity ratio dictates the size of the recirculation regions, and as more migration occurs in the inlet, a larger local viscosity ratio and longer recirculation lengths are observed. As the particle volume fraction increases, the normalized pressure drop rises in a similar manner to the area of the recirculation regions in the corner of the expansion. The pressure measurements along with NMR imaging results suggest that the viscosity ratio between the center and wall regions of the suspension, recirculation region size, and pressure drop are all related in contraction-expansion flows of suspensions. The continuum model qualitatively captures the major features of the observed concentration distributions and flow fields, but there is not yet quantitative agreement, partly due to differences in inlet conditions and geometry between the computational and experimental systems.