Giuseppina Montante

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

Abstract 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.

Eulerian-Eulerian Simulations of Segregating Binary Gas-Solid Fluidized Beds

Abstract This work is aimed at investigating a bidisperse gas-solid fluidized bed by Computational Fluid Dynamics simulations. The selected simulation approach is based on the Eulerian unsteady equations of motion for each phase coupled with a model based on the granular kinetic theory. The effect of grid size and time step on axial segregation of two different binary solid mixtures is investigated. The analysis of the simulation results, which strengthen previous findings with new data, confirms that firm conclusions on the capacity of the CFD model to predict the segregation dynamics in fluidized beds can be drawn only after careful verification of numerical uncertainties at the operating conditions under investigation. The relationship between the average absolute error, which is evaluated by comparison of the simulation results with literature experimental data, and a global parameter, which takes into account both the operating conditions and the spatial and temporal resolutions, is shown.