Dynamics of concentrated suspensions in two-dimensional channel flow for non-Newtonian slurries

Abstract

The evolution of concentrated particle suspensions in a two dimensional channel flow between parallel plates is considered. We developed a numerical model based on the finite volume method to solve the full unsteady Navier Stokes equations coupled to a constitutive advection-diffusion equation in order to determine shear-induced migration phenomenon in concentrated suspensions of monodisperse and non-colloidal spherical particles in two dimensions. The numerical model predicts the unsteady flow field, particle migration fluxes and gravitational mechanisms responsible of solids transport. The particle migration phenomena was studied in Newtonian and non Newtonian carrier fluids and the code was validated with both Newtonian and non-Newtonian carrier fluids solving the suspension advection-diffusion constitutive equation to determine the volume fraction contribution to the apparent viscosity of the fluid. In the case of a Newtonian carrier fluid the unsteady numerical simulations predicted a particle migration mechanism effectively occurring from high to low shear regions but evolving downstream. A cusped concentration profile was found at the channel centerline accompanied with a wall depletion region where the concentration was below the initial bulk value. The effect of gravity versus particle migration fluxes was studied though the introduction of initial uniform spot-like patches of bulk concentration in particular locations of the channel. For a Newtonian carrier, under the effect of a gravitational flux, the model predicts the formation of a sediment on the channel wall identified by an increase of local concentration similarly to the formation of a bottom bed found in mining slurries. In the case of a non-Newtonian Bingham carrier fluid without gravitational effects, the model predicts symmetrically cusped concentration profiles at the interface between yielded and unyielded regions and a particle depletion region close to each wall, where the particle concentration fall below the bulk concentration.