组合桨搅拌槽内混合过程的实验研究及大涡模拟

Experimental study and large eddy simulation of mixing in a multiple impeller stirred tank

采用实验研究和数值模拟相结合的方法对直径为0.19 m的三层组合桨(HEDT+2WHU)搅拌反应器(直径0.48 m)内的混合过程进行了研究。实验采用褪色法和光功率计相结合的方式,考察了7个不同监测点对混合效果的响应情况,并利用高速相机记录了示踪剂在反应器内的浓度分布随时间的变化。数值模拟采用LES模型对反应器内的混合特性进行研究,并与标准k-ε模型的模拟结果和实验数据进行对比。结果表明示踪剂从液面加入后,依次到达中层桨上方、顶层桨和中层桨之间以及底层桨下方的3个循环子域,在每个子域中,示踪剂先进行轴向扩散再沿径向和切向扩散;中层桨位置处测得的混合时间最短,并分别向液面和槽底依次增大;LES预测的示踪剂浓度分布与实验结果吻合,而标准k-ε模型预测的示踪剂浓度分布不准确;数值模型预测的混合时间在轴向的分布与实验吻合,数值偏大,标准k-ε模型的预测偏差为35%,LES预测的偏差降低到14%。

The mixing progress in a stirred tank of diameter 0.48m agitated by a triple-impeller（HEDT＋2WHU） of diameter 0.19m has been experimentally investigated and numerically simulated by large eddy simulation（LES）.In the experiments,the mixing times at seven positions in the tank were measured by the chemical decolorization method using an optical power meter.The concentration distribution in the tank was captured by a high speed camera.In simulations,the mixing characteristics were simulated by LES and compared with experimental data and the results from the k-ε model.The results showed that after a tracer had been added to the liquid surface,it dispersed in cycles above the upper impeller,between the upper two impellers,and below the bottom impeller in turn.In each cycle,the tracer first dispersed axially,and then dispersed radially and tangentially.The mixing time was smallest near the middle impeller and larger at higher or lower positions.The concentration distribution estimated by the k-ε model was not accurate,while the results of LES were in agreement with the experimental data.The trends in mixing times obtained by computational fluid dynamics（CFD） were consistent with the experimental variation but the simulated magnitudes were larger.The maximum deviation between the experimental and calculated values of the mixing times was reduced from 35% for the k-ε model to 14% for LES.