Numerical Simulation Of Bubble Coalescence And Flow Structure In Rectangular Dissolved Air Flotation Tank

As a highly efficient separation technology,Dissolved air flotation(DAF)is widely used in wastewater treatment,especially in the treatment of oily wastewater,which using micro-bubbles adhere to the solid impurities in the water to purify the sewage.In order to optimize the flow structure of DAF,improve the separation efficiency and determine the scope of the process parameters,this paper gives a research on bubble coalescence behavior and flow structure in a DAF Tank by using numerical simulation software FLUENT,which could provide data support and theoretical basis for improving the separation performance of the DAF tank.Firstly,the bubble coalescence behavior of the DAF tank in contact zone was studied by using Euler-Euler model and PBM model,the results show that: bubble coalescence occurs mainly in the bottom of the contact zone in DAF tank,which gradually weakened with height increasing.The greater the initial bubble diameter is,the less the number of bubbles get with the same air concentration,so that the collision probability decreases and bubble coalescence weakened.The greater air concentration at inlet increase the number of bubbles and collision probability,so bubble coalescence effect is enhanced.With greater hydraulic surface loading and recycle rate,the fluid flow rate increases,flow structure become disordered and the air concentration is bigger in contact zone,so the probability of collision between bubbles become higher,bubble coalescence is more obvious.Secondly,the effects of different structures,physical properties and operating parameters on the flow structure were studied based on the PBM model,the results show that:(1)the flow structure in contact zone is characterized by a turbulent lower region and a plug-flow higher region,an increased height of the shaft wall enhanced the higher plug-flow region.The cases with 910~1110mm shaft wall generate a larger plug-flow region and the flow structure is more stable;the case with 240 mm contact zone length forms a distinct plug-flow region.The separation zone is characterized by a stratified flow structure higher region and a plug-flow lower region.The cases with 810~1010mm shaft wall form two distinct stratified flow.The inclination of the shaft wall does not seem to have a significant influence on the flow structure.(2)A distinct stratified flow becomes clearer with the increase of the initial bubble diameter,and the horizontal flow constantly moves up,air concentration gradually reduces,when the bubble diameter is 50~60?m,the stratified flow is obvious,and the separation performance is better.When the air concentration at the nozzle is increased,the stratified flow gradually becomes clearer and constantly moves up,air concentration gradually increases,the case with 0.055(air/water)air concentration generate a better flow structure.(3)With the hydraulic surface loading increases,the thickness of the horizontal flow increases constantly,air concentration gradually increases,nearly uniform distribution eventually,the case with 11m/h hydraulic surface loading forms the most obvious stratified flow.The effect of recycle rate on the flow structure is similar to that of the air concentration at the nozzle,the case with 10% recycle rate generate a better flow structure and higher separation performance.