Comparative investigation of agitator flows using three-dimensional CFD simulations

Abstract

The flows generated by agitators, as used in activated sludge tanks of municipal wastewater treatment plants, were investigated by applying three-dimensional computational fluid dynamics. The studies are focused on flow characteristics of four typical submersible agitators and their mixing properties. The simulations were carried out with the established software ANSYS-Fluent [9] for a developed study box.
Massless particles are added at the inlet of the study box to evaluate the mixing of the water body. The quality of the mixing is analysed by using particle flux density.
Three of the investigated agitators produce a strongly focussed jet that determines the main flow direction but makes mixing of the water body considerably more difficult. One agitator acts different, because the jet collapses immediately on the downstream side of the propeller and turbulence occurs, too. In this case mixing is much better.
There is a conflict between the directed flow, the mixing and the energy input, as not all objectives can be met simultaneously.