Examining the effect of hydraulic turbulence in a rapid mixer on turbidity removal with CFD simulation and PIV analysis

Abstract

In order to investigate the effect of mixer shape and mixing intensity on hydraulic turbulence and velocity field in a rapid mixer, and relate the results of the investigation to the performance of the rapid mixer with respect to coagulant dispersion and turbidity removal, this study conducted wet tests, CFD simulation and PIV analysis, using three different shapes of jar: a circular jar with squared baffles, a circular jar without baffles and a Hudson jar. From the results of the wet tests, it was observed that the performance of rapid mixing in the circular jar without baffles was better than in the other shapes of jar. Also, the shape of jar is found to be a factor affecting the performance of the rapid mixer and ultimately the efficiency of coagulation. The results of CFD simulation and PIV analysis confirmed this by showing that, since it forms moderate turbulence throughout the jar and minimizes localized dead zones, the circular jar without baffles produced the best mixing conditions among the jars. From all these results, this study concludes that turbulent fluid conditions in a rapid mixer, including distribution of turbulence and formation of dead zones, are important factors in determining performance of the rapid mixer. Furthermore, it is suggested that mixing intensity and mixer shape are determined considering those fluid conditions.