Flow visualization by PLIF technique and numerical modeling of mixing enhancement in stirred tank under electric fields

doi:10.16085/j.issn.1000-6613.2021-1398

Abstract

Abstract:

In laminar mixing, the periodic perturbation of the impeller causes the enclosed and isolated annular flow field in the stirred tank. The isolated flow field seriously obstructs the effective exchange between the stirring medium and reduces the stirring efficiency. Therefore, an applied electric field was proposed in this paper to enhance the laminar mixing efficiency, and the electrohydrodynamic effect was employed to induce the polarization force of the mixed fluid. This effect changed the symmetrical structure of the flow field, and eliminated the isolated mixing regions. The real-time visualization of the flow field structure in the stirred tank was realized by using Planner laser induced fluorescence (PLIF) technology. The isolated flow region was identified and the percentage of the unmixed area was calculated in post-image processing by custom-made functions. The results showed that the mixing efficiency could be gradually increased to 98% under the electric field intensity of 1.5kV/cm. In addition, the simulation platform based on finite element method and concentration diffusion model was established. Through the simulation analysis, it was found that the secondary eddy appeared once the applied electric field intensity upon to 0.5kV/cm. The secondary eddy interacted with the radial mixing flow which diminished the isolated flow region. Under the condition of constant electric field strength, periodic electric field can further improve the mixing efficiency. Periodic electric field can make the mixing efficiency reach more than 98%, when electric field was at 1kV/cm.

Key words: mixing, planar laser induced fluorescence(PLIF), computational fluid dynamics (CFD), electro-hydro dynamics (EHD)

摘要：

在层流搅拌中，搅拌桨的周期性扰动使搅拌槽内出现封闭、孤立的环状隔离流场。隔离流场严重阻碍了搅拌介质之间的有效交换，降低了搅拌效率。本文提出外加电场以强化层流搅拌的方案，利用电流体动力学效应改变流场的对称结构，消除混合死区。试验采用平面激光诱导荧光（planner laser induced fluorescence, PLIF）技术实现了搅拌槽内流场结构的实时可视化，并通过自编程程序识别并计算出非混合区域面积百分比。结果表明，随着电场强度的增大，混合效率逐渐提升，当电场强度为1.5kV/cm时混合效率可达98%。研究建立了基于有限元法及浓度扩散模型的混合搅拌模拟平台，探究搅拌槽内部流场结构时空演变规律。通过模拟分析发现，当外加平行板电场强度达到0.5kV/cm以上时，搅拌槽内部出现明显的二次涡流。二次涡流的出现与径向混合相互作用从而不断削弱隔离流场。在电场强度不变的条件下，外加周期性电场可以进一步提高搅拌效率，电场强度1kV/cm条件下的外加周期性电场可以使搅拌效率提升至98%以上。

关键词: 混合搅拌, 平面激光诱导技术, 计算流体力学, 电流体动力学

CLC Number:

TQ027

SUN Jingchen, LIU Hailong, WANG Junfeng, HE Fachao. Flow visualization by PLIF technique and numerical modeling of mixing enhancement in stirred tank under electric fields[J]. Chemical Industry and Engineering Progress, 2021, 40(12): 6547-6556.

孙靖晨, 刘海龙, 王军锋, 何发超. 电场强化层流搅拌的荧光可视化试验及模拟分析[J]. 化工进展, 2021, 40(12): 6547-6556.

Figures/Tables 10

（Re=5.3）

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