CFD Numerical Simulation Of Gas-Liquid Flow In Aeration Tank

Water is a sort of natural resource that can't be replaced.It is also one of the important supports for the sustainable development of the society and economy.As a result of the rapid development of urbanization,industrialization and explosive population growth,it has brought huge pressures on environment.The available fresh water recourse is reducing day by day.So this problem is paid more attention to by industrial and academic circles and is solved by increasing the operation efficiency of activated sludge process and lowering the cost.Activated sludge process is constituted by aeration tank,sedimentation tank,sludge circulation,removal system of excess sludge.The aeration tank is the main structures in wastewater treatment of activated sludge process.So the properties of aeration tank directly influence wastewater treatment efficiency.Many domestic &foreign experts in water treatment study biological reaction,purification mechanism and operation management of aeration tank by experiments in comparative water quality.Because of engineering devices complexity and limited experiment condition,some problem solving is impossible in the lab.But to base on mathematical calculation and numerical simulation of CFD and mainframe computer makes it possible to solve the problems on engineering scaling up.Numerical simulation of aeration tank belongs to gas-water two-phase flow category.It is common material flow and has been widely applied to engineering and environmental field.But so far,people are unaware of two-phase interaction, mass transfer and heat-transfer mechanism.The problems must be solved in many engineering field.In this paper,three-dimensional numerical simulations of gas-liquid flow in partial and central aeration tank with two-fluid approach were conducted.The gas velocities were 0.05m/s, 0.20m/s,0.70m/s respectively.A standardκ-εturbulence model was used to describe liquid phase turbulence in the simulations,which only adopted the drag force for interphase momentum transfer.Gas hold-up distribution and flow pattern in central aeration tank are compared with experimental and computational results in the literature.The results show two-fluid model and a standardκ-εturbulence model are applicable to the simulation of aeration tank.The liquid velocities and gas holdup distribution in the different radial positions were obtained in the simulations.In two reactors,with the increase of gas velocity,the gas holdup and liquid circulation rate increased.As we know,the more sufficiently gas and liquid mix,the better the aeration effect will be.The difference of entering air mode directly influence gas hold up and flow pattern.So the gas velocity and the difference of entering air mode are important hydrodynamic parameters for design and improvement of reactors.In two reactors,at first bubbles ascended vertically as straight line and then the line begin to swing around the central shaft.In central gas aerated tank,gas mainly exists in the center of the reactor.With the increase of gas velocity,the gas distribution becomes broader.(In smaller velocity(0.05m/s),gas only exists in the center of the reactor and hardly diffuses to two walls),and the swing of bubble group is increasing.It leads to the wider gas distribution.The liquid flow field in the liquid vector diagram is as follows:liquid of the central tank ascends,and the both sides descend,so there is liquid circulation in the both sides.It has the same results as the flow pattern which D.Pfleger (1999) and Luo Wei study.In partial gas aerated tank,gas mainly exists in the right of the reactor.(bubble flow field).In the bubble flow field,with the increase of height gas hold up is smaller.But in the right of bubble flow field,with the increase of height,gas hold up is larger.Increasing the gas velocity makes gas hold up a good distribution,vortex structures generation.So two-phase in the flow field achieves good mixture effects.From the angle of the computer,this paper analyzes that the different gas velocities play an important roles in gas distribution and flow pattern.It may lay the foundation for optimization design of aeration tank.It finds that it has remarkable difference between central gas aerated tank and partial gas aerated tank.In central gas aerated tank,gas mainly exists in the center of the reactor,hardly in both sides.In partial gas aerated tank,gas mainly exists in the right of the reactor.And its gas hold up distribution uniformity is better than central gas aerated tank.So from the point of dissolved oxygen,partial gas aerated tank is better than central gas aerated tank,but from the point of energy, is worse than central gas aerated tank.