CFD prediction of fluid flow and mixing in stirred tanks: Numerical issues about the RANS simulations

This work is aimed at verifying the effect of numerical issues on the RANS-based predictions of single phase stirred tanks. In particular, the effect of grid size and discretization schemes on global parameters, mean velocity, turbulent dissipation rate and homogenization is considered. Although contradictory results have been reported so far on the capability of RANS methods in fluid mixing, the most widely accepted conclusion is that adequate values are generally to be expected for the predicted mean flow quantities, while much less confidence must be put on the calculated turbulent quantities and related phenomena. The results obtained in this work partially revise this last statement and demonstrate that firm conclusions on the limits of RANS simulations can be drawn only after careful verification of numerical uncertainties. The simulation results are discussed and compared to the literature experimental data and to original passive tracer homogenization curves determined with planar laser induced fluorescence.