(438a) A CFD-DEM Model for Viscous Solid-Liquid Mixing in Stirred Tanks

Viscous solid-liquid mixing plays a key role in the production of a large variety of consumer goods such as pastes, paints, cosmetics, pharmaceuticals and food products. Despite this industrial relevance, the majority of reported results in solid-liquid mixing has been geared towards the turbulent regime, and little is known about the laminar and transitional regimes. In particular, it remains unclear how the rheology of a suspension, the particle interactions and the presence of a complex rotating geometry impact the flow patterns as well as critical parameters such as the impeller torque, the just-suspension speed (Njs) and mixing indices. To shed light on these issues, numerical and experimental work is essential.

A variety of models have been developed to simulate solid-liquid flows. These include the Eulerian-Eulerian model, and the combination of the Discrete element method (DEM) for the particles and CFD methods for the liquid phase (CFD-DEM). While it possesses a huge potential due to its formulation, notably as regards its natural capacity to reproduce the maximal packing fraction of solid particles, the ability of the CFD-DEM approach to accurately model viscous solid-liquid flow in complex geometries has yet to be assessed, and the method has not been validated experimentally in the field of mixing.

In the present work, we extend the so-called CFDEM framework, which combines OpenFOAM and LIGGGHTS, to study viscous solid-liquid mixing. First, the governing equations for the liquid and the solid phases are presented along with a two-way coupling strategy. The model is next applied to the study of solid-liquid mixing in a stirred tank equipped with a pitched-blade turbine (PBT). In particular, experimental visualization of flow patterns is used to assess the accuracy of the proposed model. Finally, the model is validated quantitatively by comparing the fraction of suspended solid measured as a function of the impeller rotational speed.