Mixing of viscous liquids and large concentrations of spherical particles in stirred tanks

Despite the importance of laminar solid-liquid mixing operations in industry for the production of various goods such as pastes, greases, paints, pharmaceuticals and food products, a large proportion of the reported results in this field has been obtained in the turbulent regime. The intrinsic multiscale nature of solid-liquid flows in stirred tanks and the complexity arising from both the geometry of the impeller and the rheology of such suspensions, particularly in the case of high solids contents, create complex flow patterns within the tank, which may drastically affect both the yield and the quality of the desired product. Furthermore, the estimation of key mixing parameters such as the impeller torque, the just-suspension speed (Njs) and the cloud height remains unclear in all flow regimes. In particular, the Zwietering correlation has several limitations that may lead to poor predictions of Njs. Therefore, more modeling and experimental work is needed in order to shed light on these issues. A variety of models have been developed to simulate solid-liquid flows. These include the classical Eulerian-Eulerian (or two-fluid) model and the combination of the Discrete Element Method (DEM) for the particles and CFD methods for the liquid (CFD-DEM). Despite their relative advantages and drawbacks, which will be briefly discussed during this talk, we believe that the CFD-DEM approach can be both efficient and accurate for the simulation of such flows. The objective of this work is then to introduce a multiscale CFD-DEM model, applicable to concentrated suspensions, wherein the particles are not resolved explicitly on the CFD grid but taken into account through the so-called volume-averaged Navier-Stokes equations. This entails projecting the particles onto the grid so that the solid-liquid momentum transfer term appearing in these equations, such as the drag force exerted by the particles on the liquid, can be locally evaluated. After describing the proposed model in detail, simulation results obtained with it for the laminar mixing of concentrated solid-liquid suspensions in stirred tanks will be presented