Modelling Flow Fields in Stirred Tanks using computational Fluid Dynamics

Abstract

A computational study has been carried out to study the capability of various solving techniques such as RANS and the LES to predict the velocity and turbulence fields in stirred tank. In addition, the capability of the solvingtechniques such as the MRF and the SM approach has been shown in comparison with the previously published measurements to determine the most appropriate method for predicting the flow field in stirred tanks. The results is obtained by LES techniqueas it is the best turbulence model to predict the flow field, although it requires very high computational time. The effect of the impeller geometry and number of blades on mixing and power consumption in stirred tanks has been also studied and highlighted in a tank with cross section area of 270*270 mm and 200 mm hieght and using downward pitched blade impeller rotates at the range of 120 to 1080 RPM which correspondes to Reynolds number of 6.26E+05 to 5.63E+06.The impeller diameter is maintained 120 mm with different blade’s height and different angles. The impeller clearance is maintained at one – third, where the impeller clearance is defined as the ratio between the impeller height from the tank bottom surface to the tank height. The tank upper surface is assumed to be a surface with zero shear to avoid air entrapment. All the simulationshave been carried out by using the commercialcode FLUENT14.0. In this regard, it was found that using an impeller clearance ratio of 1/3 produces higher mixing degrees without causing a significant increase in the consumed power. Increasing the impeller height enhances the mixing efficiency, while increasing the impeller speed of rotation enhances the shear number but increases the power consumed.The optimum mixing degree occurs for the pitch blade angles between 45oand 55o for high Reynolds number.