Three-dimensional Numerical Simulation Of The Two-phase Flow In Rectanangular Sedimentation Tanks Using CFD

In this paper, the data of Imam’s experiment is used to validate the Mixture two-phase flow model and the RNG k-ε turbulence model applicated in rectangular sedimentation tank. The validated mathematical model is applied to the rectangular sedimentation tank designed, and the three-dimensional numerical simulation of two-phase flow in the rectangular sedimentation tank is performed by the computational fluid dynamics software Fluent. The simulation is carried out after changing one of the sedimentation tank parameters, including the sludge hopper, the bottom slope, the location of inlet, the submerged depth of inlet and out baffles, the location of inlet and out baffles. The flow pattern and distribution of suspended solids is obtained, ways to improve the settling effect of rectangular sedimentation tanks is presented as well, which can provide a reference for the design of rectangular sedimentation tanks in practical engineering.It is found that velocity near the inlet and outlet baffles changes greatly, velocity in other regions where are mainly horizontal flow is less. In the tank there are mainly four circumfluence areas:behind the inlet baffle, over the sludge hopper, under the outlet and at the bottom of the tank. Sinking velocity formed afer the fluid encounter the inlet baffle, on the one hand help the Precipitation of suspended solids, on the other hand may take the suspended solids out of the sludge hopper if it is close to the sludge hopper which is not conducive to the precipitation of suspended solids.When changing the sludge hopper and bottom slope, effective precipitation volume will change smaly. It is found that with the same volume of single bucket, the removal rate of suspended solids of one and tow sludge hoppers differs little, and the removal effect is better. When the volume of single bucket changes and the whole effective precipitation volume is invariable, the settling effect using a sludge bucket is slightly better than using two sludge hoppers. So it is proposed that the number of sludge hoppers are not more than tow.When the depth of tank is invariable, the more bottom slope the less effective depth and the easier to take out the suspension solids at the bottom of the tank,which cause the removal of suspended solids become worse. When discharging sludge using hydrostatic pressure, the bottom slope can be set to0.01. While using mechanical dredge, it can be directly set to flat.Without changing other conditions but changing the inlet location, the submerged depth of inlet baffle and its location respectively, after simulation it is found that to obtain a greater settling effect requiring the velocity near the inlet not be too great, the possible smallest circumfluence areas, having appropriate sinking velocity to avoid taking out the suspended solids in the sludge hopper. After analysing and optimizing the inlet, the optimal structural parameters for inlet are moving down the inlet1meter, setting the submergence depth of inlet baffle0.5meters and its distance from the inlet1meter.Without changing other conditions but varying the submerged depth of outlet baffle and its location respectively, after simulation it is found that to obtain a greater settling effect requiring uniform outflow, possible smallest velocity at the bottom and avoiding the suspended solids at the bottom of the sedimentation tank being rolled up. Simulation results show that when without setting outlet baffle the removal effect of suspended solids is best. When designing it is in need of a outlet baffle to retain the scruff, so after optimization the proposed outlet structure parameters are setting the submerged depth of outlet baffle0.3meters and its distance from the outlet0.5meters.