Mixing enhancement technique for laminar flow in stirred tank

doi:10.11949/j.issn.0438-1157.20180552

Abstract

Abstract:

Mixing technology has a wide range of applications in chemical production, pharmaceutical and environmental protection. The mixing efficiency and flow field distribution are important indicators for measuring the quality of the mixing. However, in the laminar agitated flow field, the periodic disturbance of the agitating paddle creates an annular dynamic flow field resulting in the existence of a mixed isolation zone in the agitation tank. The existence of the isolation zone is the main obstacle to achieving efficient mixing. In this study, a planar laser induced fluorescence (PLIF) technology was employed to visualize the dynamic structure of mixing flow field. The post-image processing has been done with MATLAB and the custom-made functions was designed to evaluate the mixing efficiency. The effects of Reynolds number and the geometrical parameter of the stirred tank on the flow features were studied. The results show that the non-mixing zone always appears regardless of the mixing time, and it will not be broken down as the increase of Re in laminar mixing region. Different baffles are designed to destroy the symmetry of the flow field and thus substantial is increased the mixing efficiency. The results show that non-mixed regions of annular flow field can be almost completely removed if the baffle (ring baffles and cuboid baffles) structures are embedded in the specific position, and the mixing efficiency increased from 65% to 97% in 200 s.

Key words: laminar flow, mixing, measurement, planar laser induced fluorescence, mixing enhancement, image processing

摘要：

机械搅拌作为一种重要的混合技术，在化工生产、环保安全、生物制药等领域有着广泛的应用。搅拌效率和流场的分布是衡量搅拌质量的重要指标。然而，在层流搅拌流场中，搅拌桨周期性的扰动产生了环形动力流场导致搅拌槽内普遍存在混合隔离区，隔离区的存在是实现高效混合的主要障碍。本实验基于平面激光诱导荧光技术实现了二维瞬态液相混合流场结构的可视化，研究了Reynolds数和搅拌槽的几何参数对流场结构时空变化规律的影响。基于MATLAB软件对实验图像进行后处理得到非混合区域面积覆盖率，定量计算出流体的混合效率。结果表明，层流搅拌中搅拌槽内叶轮上方和下方出现对称的隔离区域，并且不会随着Reynolds数的增加而消失。通过在搅拌槽内部特定位置（侵入隔离流场几何中心）设置几何挡板结构，破坏隔离区域环形流场的对称性和封闭性，引发混沌混合从而提升混合质量。设置在特定位置的长方体挡板和扇形圆环挡板分别使搅拌槽内流体混合效率在200s内从65%提升至95%和97%。该研究可为高效层流搅拌混合器的设计提供实验数据与理论指导。

关键词: 层流, 混合, 测量, 平面激光诱导荧光, 混合增强, 图像处理

CLC Number:

TQ027.2

LIU Hailong, CAO Yu, DING Xuechong, MAO Baodong, WANG Yuerou, WANG Junfeng. Mixing enhancement technique for laminar flow in stirred tank[J]. CIESC Journal, 2018, 69(12): 5042-5048.

刘海龙, 曹宇, 丁学翀, 毛宝东, 王悦柔, 王军锋. 搅拌槽内液相层流荧光可视化及高效混合技术[J]. 化工学报, 2018, 69(12): 5042-5048.

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