Three-Dimensional Numerical Simulation In Rectangular Secondary Sedimentation Tanks Based On CFD

The design of secondary sedimentation tanks was based on the hypothesis of ideal sedimentation tanks, so it would depend greatly on empiricism, subjectivity and arbitrariness. It is necessary to study the flow field and the distribution of suspended solid concentration in the sedimentation tanks, and then the size and sedimentary efficiency of the tank could be determined more accurately. They would provide useful references for the design and operation management of the sedimentation tank.Firstly, based on the present research all over the world, a three-dimensional Euler two-phase flow model is built. The solid phase is treated as pseudo fluid in the model, which is coupled with particular high Reynolds number form of the k -εmodel. And the response function is introduced into the turbulence model of solid phase. The influence of interaction between liquid and solid, that is to say, the influence of interphase drag force, lifting force perpendicular to the relative velocity, virtual mass force, as well as the volume fraction of suspended solids and density difference effects between liquid and solid, has been considered. Secondly, a three-dimensional quasi-single phase model is built in the secondary sedimentation tank. Then the distributions of flow and suspended solids in different dimensions of the tanks under the specific inlet condition are simulated.The PISO algorithm of the finite volume method is used to compute the three-dimensional mathematics model. Good agreement is obtained between present CFD predictions and published experimental data of Imam. The simulated results are as follows:(1) The circumfluence areas are under the inlet and over the sludge hopper. The distribution of velocity is not uniform. The current along z-axis in the upper part of tank flows direct to the sidewalls from the center plane and reverses in the bottom, and the velocity value of the center plane is relatively small. The high-speed regions are mainly near the upper part of the tank and in the front of sludge hopper, which are in the front of the tank.(2) Higher concentration area is near the baffles of inlet. In the central region of the tank, due to flowing toward the surface of the tank, the concentration of suspended solid near the bottom is lower than the top. A large amount of suspended solid settles in the sludge hopper.(3) The turbulent kinetic energy of flow is high in the influent zone, while in the settling zone relatively low, and it increases near the outlet.(4) Under the specific inlet condition, the length and location of baffle have some effect on the flow field and concentration field in the sedimentation tank, while the dimension of suspension granule may not affect on the flow field, but influences the distribution of suspended solid in the tank.(5) Under the specific inlet condition, the function between the removal efficiency of suspended solid and the length, width and height of the tank is gained by simulating the distributions of flow and suspended solid in different dimensions of the tanks.