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A simulation of flow field and tracer homogenization was performed using the commercial CFD software FLUENT 6.1. The aim is to investigate the potential of CFD software to predict concentration distribution of added tracer in cylindrical vessels. The calculated results ± dimensionless velocity profiles, power and pumping numbers, dimensionless concentration curves, and mixing times ± were compared with experiments in stirred vessels. In Part I, the study was performed for vessels agitated by one or two impellers on a centric shaft. Two different impellers were used ± a 6-bladed 45 pitched blade turbine and a standard Rushton turbine. The standard k-e turbulence model and multiple reference frames method were used for the simulations. The influence of the grid type was also investigated; three types of grid ± a structured, unstructured and a special user-defined grid ± were studied.

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The Volume of Fluid method together with the Large Eddy Simulation technique for turbulence flow was used for CFD prediction of liquid flow and free liquid surface motion in a pilot plant cylindrical flat-bottomed stirred tank with an inner diameter of 1 m filled with tap water at room temperature. A pitched-blade (45 • ) down pumping impeller (diameter of 0.33 m) with six blades was used for stirring. The impeller speed was 180 rpm. For simulation of the impeller motion, the sliding mesh method was adopted. The CFD simulations of the flow field predicted a highly complex, multi-dimensional dynamic system exhibiting unstable, pseudo-stationary behaviour. The main advantage of this CFD simulation is the prediction of the velocity flow patterns and the free surface elevation/depression in the whole system at one time. The results for the location and dimensions of the free liquid surface fluctuation correspond well with an experimental investigation by a conductivity method.La méthode de volume de fluide avec la technique Large Eddy Simulation pour le flux de turbulence aété utilisée pour prédire la DFN du flux de liquide et du mouvement de la surface liquide libre dans la cuveà agitation cylindriqueà fond plat d'une usine pilote avec un diamètre interne de 1 m remplie d'eau du robinetà température ambiante. Un impulseurà pompage descendantà pales inclinées (45 • ) (diamètre of 0,33 m) avec six pales aété utilisé pour l'agitation. La vitesse de l'impulseurétait de 180 tr/min. Pour la simulation du mouvement de l'impulseur, la méthode des maillages glissants aété adoptée. Les simulations de la DFN du champ du flux ont prédit un système dynamique multidimensionnel très complexe qui affiche un comportement pseudo-immobile et instable. Le principal avantage de cette simulation de la DFN est la prédiction des modèles de la vitesse d'écoulement et l'élévation ou la dépression de la surface libre dans tout le système en même temps. Les résultats de l'emplacement et des dimensions de la fluctuation de la surface liquide libre correspondent bienà l'analyse expérimentale par une méthode de conductivité.

This article deals with CFD simulations of flow inside stirred vessels equipped with three and four radial or axial impellers mounted on the same shaft. A comparison was made between simulated data and experiments for one-and two-impeller systems and was presented in Part I [1]. The effect of the lowest impeller off-bottom clearance, number of impellers used, and impeller type on the tracer distribution was studied. The simulations were mainly focused on the grid size and type and the analysis of the concentration curves in each impeller section. The predicted velocity fields, power and pumping numbers, concentration curves, and mixing times were validated with experimental data. The simulation results show the significant influence of the grid density on the velocity profiles and power and pumping numbers in contrast to the low impact on the concentration curves. A better prediction of the concentration curves was reached when radial impellers were used; the mixing times were generally over-predicted.