Numerical modelling of velocity field and phase distribution in dense monodisperse solid-liquid suspensions under different regimes of agitation: CFD and PEPT experiments

Turbulent suspensions of monodisperse coarse glass particles of 1 and 3 mm diameter in water were numerically simulated at their 'just-suspended' speed Njs and at speeds above it up to 2Njs, in a vessel agitated by a down-pumping pitched-blade turbine. The solid concentration was in the range 5.2-20 wt%. The numerical results are compared to detailed 3-D distributions of the three local phase velocity components and solid concentration obtained by an accurate technique of positron emission particle tracking (PEPT). The predictions of flow number and mean velocity profiles for both phases are generally excellent both at Njs and above it. Predictions of the spatial solids distribution are good except near the base of the vessel and underneath the agitator where they are largely overestimated, however, they improve significantly with increasing solid concentration. At Njs and above it, the liquid velocity field is not significantly affected by the presence of solids up to ∼10 wt%. At higher concentrations, however, some significant reductions in liquid velocity occur near the impeller and along the wall of the vessel. At Njs, there are wide variations in the spatial distribution of the inter-phase slip velocity. The largest total slip velocities are of considerable magnitude, on the order of ∼0.10utip. Increasing the agitation speed up to 2Njs, reduces the normalised slip velocities significantly. Results also indicate that there is no impact on the distributions of turbulent kinetic energy and Kolmogorov length scale. The eddy dissipation rate, however, is increasingly suppressed as solid concentration increases at Njs.