Spatial and temporal evolution of floc size distribution in a stirred square tank investigated using PIV and image analysis

Abstract

In this study, non-intrusive measurements were performed in order to determine the spatial and temporal evolution of the floc size distribution in a square 7.3 L tank stirred with an A310 hydro foil impeller. The data was collected in situ using particle image velocimetry. The analysis of the data was done using a connected component labelling technique. It was found that the reproducibility of the system was adequate. The results show that there are large spatial differences in the mean size and the shape of the floc size distribution within the tank. It was found that steady-state was reached when stirrer speed was increased. It was also found that effects of the interaction between flocs and the water surface had substantial influence on the local size distribution. It was determined that the surface was more important for breakage of flocs than the impeller region. Results also showed clear number gradients in the tank. It is clear from this study that any population balance model developed in order to predict floc size distributions should incorporate spatial dependence.