Numerical Studies on the Flow of Coal Water Slurries with a Yield Stress in Channel Bends

Abstract

Improving the efficiency of transport of coal water slurries (CWSs) and determining pipe wear both necessitate accurate predictions of flow characteristics in pipelines with complex geometries. At the bends of the channels, the flow is significantly influenced by the bend curvature, flow rate, and the rheological properties of the slurries that are viscoplastic. Herein, we numerically simulated the flow of CWS in curved channels with different curvature ratios, at different flow rates, and using different rheological models, respectively. The results showed that, due to the yield stress on the cross-stream slices, the velocity profiles showed an unyielded plug. The plug deflects outwards in most circumstances, except at the bend core in the highly curved channel, and, at the same time, at the lower conveying rate, which is due to the fact that the larger inner-wall-pointed pressure gradient has to be balanced by large velocities at the inner bend and, hence, the centrifugal effects are weakened at the lower conveying rate. Interestingly, the larger curvature, together with a higher conveying rate, induces a kidney-shaped velocity field at the bend exit, with two separated up and down velocity maximum zones, due to the larger wall shear stresses at the top and bottom than occur in the other cases. The bend brings in a secondary flow consisting of the following: an inward transverse flow at the bend entrance; two Dean swirls in symmetry in the vertical direction at the slices of the bend core and bend exit; and decayed swirls near the outlet. As the curvature ratio increases, the location of the strongest swirls switches from the bend core to the bend exit, since the flow in the highly curved channel requires a longer distance to fully develop the vortices. Decrease in the yield stress and decrease in the consistency index induce a shrinkage of the plug and enhance the streamwise flow and, thus, decrease the cross-stream secondary flow, especially in the channel with the larger curvature.