辐流式二沉池两相流数值模拟优化研究

Optimized simulation of the two-phase flow of a secondary circular sedimentation tank

用计算流体力学的方法对城市污水处理厂二沉池进行了优化设计。利用改进的RNGκ-ε模型和多相流混合(Mixture)模型,在沉淀池外部结构不变的情况下,对辐流式二沉池中部无挡板和有挡板两种情况下的质量浓度场、颗粒沉降速度和沉淀效率进行了数值模拟。在此基础上加入稀疏颗粒,采用离散相模型(DPM)对悬浮物颗粒在沉淀池中的运动轨迹进行了追踪模拟。结果表明,沉淀池中固相颗粒的行为和分布受湍流和漩涡的影响,显示出与理想沉淀理论不同的轨迹。水流进入沉淀池后形成异重流迅速向下流动,在池内形成一个大的回流区。在池半径中部安装竖直挡板能有效增加沉淀池后半部分的沉速,有利于颗粒物的沉降,能够将沉淀池中悬浮物去除率从76.9%提高到91.2%,使出水中悬浮颗粒质量浓度降低约62%。

The present paper is engaged in an optimized simulation of two-phase circulation test for a secondary circular sedimentation tank so as to investigate and raise the efiqciency of the baffle installation and the particle-settling performance of the said tank at the mid-ra- dius. For our research purpose, we have adopted the computational fluid dynamic （CFD） methods to optimize the design of the sedimen- tation tank in a municipal sewage-processing plant （SPP）. Our quali- tative simulation experiments were awarded with a remarkable sewage- processing efficiency, as the experiment load has been dropping to a much lower level. The installation we used in our experiments was a round-shaped, centro-feeding secondary sedimentation tank with a flat bottom and a suction-lift sludge-removing device. Considering the in- variant external configuration of the tank, we have also adopted an improved RNG turbulence model and a multi-phased mixture model to simulate the particle concentration distribution, enabling us to speed up the particle settling velocity and increase the particle-removing ef- ficiency from the stream in a two-way secondary sedimentation tank, one without a baffle and the other with it. At the same time, the set- tling trace of the suspended particles in the tank can be kept calculat- ing in a Discrete Particle Model （DPM）. The results of our experi- ments indicate that the particle performance and distribution were in- fluenced by the turbulent current and eddies formed at the entry zone. As a result, the simulated sedimentation trajectory of the suspended particles turns to be different from the ideal ones as described in the theory of the optimal sedimentation tanks. What is more, when the current reaching the tank, a gravity flow would come into being and forms a big recirculation zone. At this moment, an additional baffle may come into being as the mid-radius was extended from the floor upwards to the mid-depth to increase the particle settling velocities, thus enhancing the particle settling by leading them to the bottom of the sedimentation tank at higher speed. It has been found that the overall particle-removing efficiency can be made as high as 91.2% as compared to that of 76.9% on the condition with no baffle, which may cause the reduction of the effluent particle concentration of ap- proximately 62 %.