CFD and experimental studies on the impact of impeller type on flow field and floc size evolution in a stirred tank

The impact of impeller type on floc size evolution during flocculation of polystyrene particles with aluminum sulfate has been investigated by both Computational Fluid Dynamics (CFD) and experimental methods. The Lightning R100, A200 and A310 impellers were respectively examined. The results show the impeller type greatly affects the hydrodynamic environment and floc growth process at a given spatially averaged velocity gradient, G. The R100 impeller, as a radial one, can generate great fluid shear force and thus effectively improve floc structure to eventually form large flocs. In contrast, the A200 and A310 impellers, as axial ones, can create great axial-flow circulation that effectively speeds up floc growth at the early stage of flocculation but would not be conductive to the broken-floc restructuring and result in forming small flocs at the later stage. Therefore, the two types of impellers shall be used as a supplement to each other to improve the quality of final flocs. Moreover, the results have proven that the spatially averaged velocity gradient, G, cannot be the single parameter sufficient to model flow field and flocculation efficiency.