Bubble coalescence can make the increase of bubble size in the contact zone of the dissolved air flotation tank

which will finally influence the separation efficiency. Population balance model was employed to study bubble coalescence behavior in the contact zone. With this numerical method

the bubble coalescence mechanism and effect of mainstream and recycle flow rates on coalescence phenomenon were studied. At first

Schiller-Naumann

Grace and Tomiyama drag coefficient models were included in the simulation process

respectively. The results showed that the bubble sizes simulated with these three models were similar with good accordance with the experimental data. Schiller-Naumann was adopted in the following simulation as its computational cost was lower than Grace and Tomiyama model. By comparing the bubble size distribution with velocity profile

high velocity gradient was detected at the upstream and downstream flow transition region around the recycle flow inlet. The high velocity gradient was proved to be the key factor causing bubble coalescence. At last

the influence of mainstream and recycle flow rates on bubble size was investigated. Bubble size in the contact zone increased with the increase of recycle flow rate because the velocity gradient at upstream and downstream transition region increased

which can enhance the bubble coalescence. On the contrary

mainstream flow rate showed no effect on bubble size distribution in the contact zone.