Bubble size distribution influences the separation efficiency of dissolved air flotation (DAF) while it is mainly affected by the coalescence phenomenon. In this study

both experiments and numerical simulation are performed to study the bubble coalescence in the contact zone of a DAF tank. By comparing the simulation results with experimental measurement data

the numerical simulation method based on population balance model is established to investigate the bubble coalescence behavior in the contact zone (c-zone). Luo aggregation model

Free molecular aggregation model and Turbulent aggregation model are adopted to predict the bubble size distribution (BSD)

respectively. The results indicate that Turbulent aggregation model reproduces the BSD accurately and is more suitable for bubble coalescence simulation in the c-zone. During the flotation process

the bubble coalescence mainly takes place at the lower part of the c-zone and the bubble diameter does not change at the upper part of the c-zone. Besides

the addition of bubbles enhances the diffusion of flow with eddy occurring at the upper part and horizontal flow from the wall to the center at the lower part of the c-zone.