CFD Prediction of Flow Near the Agitator in Stirred Tank

The mixing processes depend on the magnitude and local distribution of turbulence that is generated by the stirrer. The trailing vortex system generated near the impeller blades, in particular, has been identified as the major flow mechanism responsible for mixing and dispersion in stirred vessels. Modeling the three-dimensional, transient motion of an impeller using the sliding mesh approach was the most rigorous and fully predictive analysis method of the field of stirred tank. The objective of this paper is to study the flow field near the impeller using this method. Numerical simulations of flow induced by a Rushton turbine were conducted using sliding mesh method with three kinds of density grid. The trailing vortices around the blade were resolved. Prediction results of different density grid were compared with experimental data. It was found that with high-density grid the trailing vortices achieved by the simulation were in good agreement with the experimental data, but damped quickly. The predicted radial and tangential velocity distributions around the blade tip were in good agreement with the experimental results. The grid density had great influence on the maximal radial and tangential velocity. Even with high-density grid, the turbulence kinetic energy was under-predicted severely.