EFFECT OF TURBULENT DISPERSION ON THE SOLID CONCENTRATION DISTRIBUTION IN STIRRED TANKS

The prediction of the concentration distribution of solid particles in turbulent fluids is a challenge for Computational Fluid Dynamics (CFD) models, particularly when the particle volume fraction exceeds 10-3, that is often considered as the boundary value between dilute and dense suspensions (Balachander and Eaton, 2010). Solid-liquid systems in industrial equipment of complex geometry and large scale are mostly simulated in the realm of Eulerian-Eulerian two-fluid models, which development and validation have advanced significantly in the past years. Nevertheless, the influence of the dispersed phase on the continuous phase fluid dynamics (i.e. two-way coupling) and the interactions between particles (i.e. four-way coupling) are very tough to predict accurately by closure models. Recently, in addition to the drag force correlations, that have doubtless a significant impact on the solid distribution (e.g. Tamburini et al., 2013), inter-particle collision has received attention in the simulation of high solid loading stirred tanks (e.g. Wadnerkar et al., 2016; Xie and Luo, 2018). Comparatively less efforts have been devoted to the effect of the solids on the liquid phase flow field, although the liquid turbulent characteristics are important in the current closure models for the turbulent dispersion due to the solid volume fraction fluctuations. In this work, glass particles of different sizes in a stirred tank with water at impeller speed below and above the just-suspended condition are considered. The solid suspension and the solid concentration distribution in the tank are discussed considering experimental particle concentration profiles collected by Electrical Resistance Tomography. The model adopted for the turbulent fluctuations of the solid volume fraction, whose formulation depends on the equations averaging procedure, significantly affects the simulated solid distribution, particularly at incomplete solid suspension conditions. As can be observed in Figure 1, with the most widespread model formulations, the contribution of the solid volume fraction fluctuations is included not only in the turbulent regions of the stirred tank, but it is artificially introduced also in the almost motionless region, where the turbulent viscosity should be equal to zero. It is apparent that model refinements are required to prevent the turbulent dispersion force to unphysically suspend the solid phase in the regions of settled solids or almost stagnant liquid.