Formulation and validation of a computational model for a dilute biomass slurry undergoing rotational mixing

In this work we focus on development and validation of a computational fluid dynamics model of a dilute biomass slurry, which is a highly viscous particle-laden fluid for which settling effects can dominate. We model the biomass slurry as a generalized Newtonian fluid that includes biomass-concentration-dependent viscosity and solids transport due to settling and shear. We solve the model with the Nek5000 spectral-finite-element code in a simple vane mixer and calculate the torque as a function of various rotation rates. We compare simulation results against those from experiments for a suspension of 5% wt alpha-cellulose in water. Excellent agreement is demonstrated in the well-mixed regime, and the model satisfactorily captures the transition from the well-mixed regime (at higher rotation rates) to the settled regime (at low rotation rates), but is unable to capture the yield-stress behavior evident in experiments as the rotation rate goes to zero.