Influence of drag and turbulence modelling on CFD predictions of solid liquid suspensions in stirred vessels

Suspensions of solid particles into liquids within industrial stirred tanks are frequently carried out at an impeller speed lower than the minimum required for complete suspension conditions. This choice allows power savings which usually overcome the drawback of a smaller particle-liquid interfacial area. Despite this attractive economical perspective, only limited attention has been paid so far to the modelling of the partial suspension regime. In the present work two different baffled tanks stirred by Rushton turbines were simulated by employing the Eulerian-Eulerian Multi Fluid Model (MFM) along with either the Sliding Grid algorithm (transient simulations) or the Multiple Reference Frame technique (steady state simulations). In particular, a comparison of alternative modelling approaches for inter-phase drag force and turbulence closure is presented. The results are evaluated against a number of experimental data concerning sediment features (amount and shape) and local axial profiles of solids concentration, with emphasis on the partial suspension regime. Results show that some of the approaches commonly adopted to account for dense particle effects or turbulent fluctuations of the volumetric fractions may actually lead to substantial discrepancies from the experimental data. Conversely simpler models which do not include such additional effects give the best overall predictions in the whole range of partial to complete suspension conditions.