CFD modeling of non-Newtonian fluid mixing accounting for transient changes in local solids concentration – application to an agitated pulp stock chest

Abstract

The mixing of solid-liquid systems exhibiting non-Newtonian flow behaviour is studied using CFD. It is shown how transient variations in the local solids concentration can be included by adding a scalar transport equation for concentration and introducing a rheological model that is dependent on the local concentration. As an example, the transient behaviour of an agitated pulp stock chest subjected to variations in inlet pulp concentration is treated. In the chests, the mixing of the pulp suspension is important for decreasing the amplitude of the variations in the outlet flow from the chests, and this mixing is highly dependent on the local fibre concentration in the chest. The pulp suspension was modelled as a Bingham fluid, with a yield stress related to the local fibre concentration. It is shown that the change in the local fibre concentration affects the cavern formation, thus also affecting the mixing efficiency of the tank. The flow pattern obtained in the transient simulation, including the fibre concentration variation, indicates that there are areas that will remain unchanged over time. It is also shown that the yield stress level strongly affects the efficiency of the chest. Results of CFD model simulations agree fairly well with in-line fibre concentration measurements of the inlet and outlet flow from an industrial chest.