LDA measurements and CFD simulations of flow generated by impellers in mechanically agitated reactors

Abstract

The turbulent flow produced by various designs of axial flow impellers in a stirred vessel was measured using a laser Doppler anemometer (LDA). Flat-bottomed cylindrical vessels of diameters 0.3 m and 0.5 m provided with 4 baffles of T/10 width were used as the reactors. The standard two-equation (k-ε ) turbulence model was used to numerically simulate the flow (both 2D and 3D). Three numerical schemes, namely upwind scheme, hybrid scheme and power-law scheme, were used to evaluate the competitiveness of the various schemes. The effects of the initial guess values of the flow variables, the under-relaxation parameters and internal iterations etc. on the rate of convergence were analysed for both 2D and 3D models. The effect of grid size was also studied in both the cases. The 2D and 3D predictions were compared and it was observed that the 2D predictions were not good enough to give the details of the flow. So, the sensitivity of the model parameters on the flow characteristics was investigated thoroughly in the case of 3D models. It was observed that no single set of model parameters could yield even reasonable agreement between the model predictions and the experimental observations throughout the vessel. Therefore, the concept of zonal modelling was introduced for the r-z plane. It was observed that with the introduction of zonal modelling the predicted values of flow variables were in good agreement with experimental data even close to the top surface of the tank. The same could not be obtained with a standard set of parameters and single zone.