气浮接触区气泡聚并行为的数值模拟

Numerical simulation of bubble coalescence behavior in contact zone of dissolved air flotation tank

在气浮接触区内,聚并会导致气泡直径增大,对分离效果产生影响。采用相群平衡模型对接触区气泡聚并行为进行数值模拟,研究了气泡聚并发生的原因及来液流量、回流流量对气泡聚并的影响。首先分别应用Schiller-Naumann、Grace和Tomiyama 3种曳力系数模型进行模拟,所得气泡直径均与实验值吻合,无明显差异,选定Schiller-Naumann曳力系数模型对气浮中两相流动进行模拟。通过对模拟结果进行分析,表明回流入口周围上下行流过渡区域存在较大速度梯度,是导致气泡聚并的关键因素。最后研究了来液流量和回流流量对接触区气泡尺寸的影响,接触区上部气泡直径随回流流量增大而明显增大,原因在于增大回流流量使得过渡区域速度梯度升高,气泡聚并频率提高;而来液流量对气泡尺寸基本无影响。

Bubble coalescence can make the increase of bubble size in the contact zone of the dissolved air flotation tank, which will finally influence the separation efficiency. Population balance model was employed to study bubble coalescence behavior in the contact zone. With this numerical method, the bubble coalescence mechanism and effect of mainstream and recycle flow rates on coalescence phenomenon were studied. At first, Schiller-Naumann, Grace and Tomiyama drag coefficient models were included in the simulation process, respectively. The results showed that the bubble sizes simulated with these three models were similar with good accordance with the experimental data. Schiller-Naumann was adopted in the following simulation as its computational cost was lower than Grace and Tomiyama model. By comparing the bubble size distribution with velocity profile, high velocity gradient was detected at the upstream and downstream flow transition region around the recycle flow inlet. The high velocity gradient was proved to be the key factor causing bubble coalescence. At last, the influence of mainstream and recycle flow rates on bubble size was investigated. Bubble size in the contact zone increased with the increase of recycle flow rate because the velocity gradient at upstream and downstream transition region increased, which can enhance the bubble coalescence. On the contrary, mainstream flow rate showed no effect on bubble size distribution in the contact zone.