Modeling of Microbubble Flow and Coalescence Behavior in the Contact Zone of a Dissolved Air Flotation Tank Using a Computational Fluid Dynamics–Population Balance Model

Computational fluid dynamics coupled with a population balance model was implemented to resolve microbubble flow and coalescence behavior in the contact zone of a dissolved air flotation tank. Influences of two important interphase forces and coalescence models were examined. The results indicated that the lift force has negligible influence on gas holdup, while incorporating appropriate turbulent dispersion force model can substantially improve the predicted gas holdup and make it consistent with experimental data. The population balance model considering both the eddy-capture and velocity gradient mechanisms could accurately predict the bubble diameter and its size distribution. The relative difference between the simulated and the measured mean bubble diameter is lower than 10.2%. It was also found that bubble coalescence mainly happens below the nozzle inlet and around the nozzle inlet pipe, and the bubble coalescence was enhanced by the flow direction transition and nonuniform gas holdup distribution in these two regions, respectively.