Influence of gas flow rate on the structure of trailing vortices of a rushton turbine: PIV measurements and CFD simulations

Trailing vortices behind rotating impeller blades play crucial role in determining gas accumulation behind them. The gas accumulation behind blades affects the pumping and power dissipation capacity of the impeller and thus significantly affects the performance of gas-liquid stirred reactors. Understanding fluid dynamic characteristics of these trailing vortices and capacity to computationally simulate these vortices is, therefore, essential for reliable design and scale-up of stirred reactors. In this paper, we have used particle image velocimetry (PIV) technique and CFD model based on computational snapshot approach for systematically studying influence of gas flow rate on structure of trailing vortices behind blades of a Rushton turbine. PIV measurements were carried out in a standard, fully baffled stirred vessel (H/T = 1) with a flat bottom. Vessel diameter was 0.4 m. A six bladed standard Rushton turbine was placed at one third of liquid height with a ring sparger. Four baffles of 1/10 T width were placed at equal spacing. Tap water was used as a medium in the vessel. Measurements were carried out at five different gas flow rates to vary the dimensionless gas flow number in the range of 0.01 to 0.06. Both, angle resolved and angle averaged flow fields near the impeller blades were measured. The structure of trailing vortices in presence of gas was studied in detail. A Eulerian-Eulerian, two fluid model was used to simulate dispersed gas-liquid flow in stirred vessel. A computational snapshot approach was used to simulate impeller rotation. The computational model was implemented using the commercial CFD code, FLUENT (of Fluent Inc., USA) with the help of user defined subroutines. The computational model was used to simulate flow in stirred vessel operating under conditions used in the experiments. The results of this study will have important implications for extending the applicability of CFD models for simulating multiphase stirred reactors.