CFD simulations of dense solid–liquid suspensions in baffled stirred tanks: Prediction of solid particle distribution

Industrial tanks devoted to the mixing of solid particles into liquids are often operated at an impeller speed N less than Njs (defined as the lowest speed allowing the suspension of all particles): under such conditions the distribution of solid-particles is very far from being homogeneous and very significant concentration gradients exist. The present work is devoted to assessing the capability of Computational Fluid Dynamics (CFD) in predicting the particle distribution throughout the tank. The CFD model proposed by Tamburini et al. (2011a) and successfully applied to the prediction of the sediment amount and shape was adopted here to simulate the particle distribution under partial-to-complete suspension conditions. Both transient (via the Sliding Grid approach) and steady state (via the Multiple Reference Frame approach) RANS simulations were carried out for the case of a flat bottomed baffled tank stirred by a Rushton turbine. Results show that the model can reliably predict the experimental particle distribution at all investigated impeller speeds. Transient simulations were found to predict slightly better the experimental data with respect to steady state simulations. Radial gradients of solids concentration, usually neglected in the literature, where found to be significant in the presence of unsuspended solid particles settled on the vessel bottom (i.e. incomplete suspension conditions).