Computational investigation of particulate two-phase flows.

Abstract

Numerical simulations of particles dispersed in simple flows (a linear shear flow and a pressure-driven flow) were performed by solving the full Navier-Stokes equations in two- and three-dimensions. A front tracking/finite difference method was used to solve the governing equations for both the dispersed and the continuous phases. The particles were assumed to be deformable, and the surface tension force was included. Simulations of particles randomly distributed in a shear flow showed that the flow reaches a stationary state that depends on the Reynolds and Weber number. Both dilute and concentrated suspensions were studied and the salient features of the suspension in these limits were investigated. In concentrated suspensions two different types of behaviors were observed. Rigid particles at high volume fractions accumulated at the center of the channel, while deformable particles formed dense layers close to the walls. A weak shear thickening was found for suspensions of heavy drops at finite Reynolds numbers. This contrasts with the creeping flow limit where suspensions have a shear thinning behavior. Simulations for the pressure-driven flow of particles in a channel showed that particles migrate to the middle of the channel and form clusters that move as a plug inside the flow. As a result of the lateral migration, particle free layers, that experience a high shear rate are formed close to walls of the channel. For these conditions, the effective viscosity of the suspension strongly depends on the volume fraction. Simulations with polydisperse systems showed particle size segregation in channel flows. Small particles form layers in the regions close to the walls, whereas large particles concentrate at the center of the channel. The fluidization of layers of droplets in a shear flow was studied by examining the effect of the Reynolds and Weber numbers on the transient and steady state behavior of the flow. It was found that particles suspend both faster and to a higher level as the Reynolds number is increased. More deformable particles suspend more slowly than rigid particles, although the steady state suspension height does not depend strongly on the Weber number.