Chapter 2 - Turbulence generation by different types of impellers

Abstract

This chapter focuses on turbulence generation by three different types of impellers, a Rushton turbine, a 4/45 ° pitched blade impeller, and a hydrofoil impeller, using a refractive index matched, automated LDA-technique. The peak values of turbulence kinetic energy are associated with the presence of trailing vortices. It is found that the extent and intensity of trailing vortices is determined by the type of impeller. The intensity of the trailing vortices can be determined by the vorticity, which is very intense within and low outside the vortex regions. The trailing vortices contain the highest turbulence kinetic energy values, which are k/ = 0.158 for the Rushton turbine, k/ = 0.078 for the 4/45° pitched blade impeller, and k/ = 0.028 for the hydrofoil impeller. In addition, it is found that the distribution of turbulence kinetic energy in the discharge flow depends significantly on the type of impeller. While high values of turbulence kinetic energy are present along the entire discharge profile of the Rushton turbine, the discharge flow of the hydrofoil impeller exhibits only two peaks at the inner and outer edges of the blade, where vortices are generated. The data obtained contributes to a better understanding of the mixing performance of different types of impellers. In addition, the chapter emphasizes that the validation of numerical simulations of the flows in stirred tank reactors with computational fluid dynamics (CFD) requires more detailed information on the flow field generated by different types of impellers than is available to date.