Purpose

This paper aims to compare the performance of flow pulsation versus flow stirring in the context of mixing of a passive scalar at moderate Reynolds numbers in confined flows. This comparison has been undertaken in two limits: diffusion can be neglected as compared to convection (very large Peclet) and diffusion and convection effects are comparable. The comparison was performed both in terms of global parameters: pumping power and mixing efficiency and local flow topology.

Design/methodology/approach

The study has been addressed by setting up a common conceptual three-dimensional problem that consisted of the mixing of two parallel streams in a square section channel past a square section prism. Stirring and pulsation frequencies and amplitudes were changed and combined at an inlet Reynolds number of 200. The numerical model was solved using a finite volume formulation by adapting a series of open-source OpenFOAM computational fluid dynamic (CFD) libraries. For cases with flow pulsation, the icoFoam solver for laminar incompressible transient flows was used. For cases with stirring, the icoDyMFoam solver, which uses the arbitrary Lagrangian–Eulerian method for the description of the moving dynamical mesh, was used to model the prism motion. At the local flow topology level, a new method was proposed to analyze mixing. Time evolution of folding and wrinkling of sheets made up of virtual particles that travel along streak lines was quantified by generating lower rank projections of the sheets onto the spaces spanned by the main eigenvectors of an appropriate space-temporal data decomposition.

Findings

In the limit when convection is dominant, the results showed the superior performance of stirring versus flow pulsation both in terms of mixing and required pumping power. In the cases with finite Peclet, the mixing parameters by stirring and flow pulsation were comparable, but pulsation required larger pumping power than stirring. For some precise synchronization of stirring and pulsation, the mixing parameter reached its maximum, although at the expense of higher pumping power. At the local flow topology level, the new method proposed to quantify mixing has been found to correlate well with the global mixing parameter.

Originality/value

A new systematic comparative study of two methods, stirring and pulsation, to achieve mixing of passive scalars in the mini scale for confined flows has been presented. The main value, apart from the conclusions, is that both methods have been tested against the same flow configuration, which allows for a self-consistent comparison. Of particular interest is the fact that it has been found that accurate synchronization of both methods yields mixing parameters higher than those associated to both methods taken separately. This suggests that it is possible to synchronize mixing methods of a different nature to achieve optimum designs. The new theoretical method that has been proposed to understand the mixing performance at the local level has shown promising results, and it is the intention of the authors to test its validity in a broader range of flow parameters. All these findings could be taken as potential guidelines for the design of mixing processes in the mini scale in the process industry.

中文翻译：

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通过脉动和动态搅拌的三维流动混合的比较：应用于不同温度下平行流的混合

目的

本文旨在比较在受限流中以中等雷诺数混合被动标量的情况下，流动脉动与流动搅拌的性能。这种比较在两个限制条件下进行：与对流（非常大的 Peclet）相比，扩散可以忽略不计，并且扩散和对流效应具有可比性。比较是在全局参数方面进行的：泵送功率和混合效率以及局部流动拓扑。

设计/方法/方法

该研究已通过设置一个常见的概念性三维问题得到解决，该问题包括在方形截面通道中通过方形截面棱镜混合两条平行流。在入口雷诺数为 200 时改变和组合搅拌和脉动频率和振幅。通过采用一系列开源 OpenFOAM 计算流体动力学 (CFD) 库，使用有限体积公式求解数值模型。对于有流动脉动的情况，使用 icoFoam 求解层流不可压缩瞬态流动。对于有搅拌的情况，icoDyMFoam 求解器使用任意拉格朗日-欧拉方法来描述移动动力学网格，用于对棱镜运动进行建模。在局部流拓扑级别，提出了一种分析混合的新方法。

发现

在对流占主导地位的极限内，结果表明，在混合和所需泵送功率方面，搅拌与流动脉动相比具有优越的性能。在有限 Peclet 的情况下，搅拌和流动脉动的混合参数相当，但脉动需要比搅拌更大的泵送功率。对于搅拌和脉动的一些精确同步，混合参数达到其最大值，尽管以更高的泵送功率为代价。在局部流拓扑级别，已发现提出的量化混合的新方法与全局混合参数有很好的相关性。

原创性/价值

对两种方法（搅拌和脉动）进行了新的系统比较研究，以在有限流的微型尺度中实现被动标量的混合。除了结论之外，主要价值是两种方法都针对相同的流配置进行了测试，这允许进行自洽的比较。特别令人感兴趣的是，已经发现两种方法的精确同步产生的混合参数高于与分别采用的两种方法相关联的参数。这表明可以同步不同性质的混合方法以实现最佳设计。为了解局部水平的混合性能而提出的新理论方法已显示出有希望的结果，作者打算在更广泛的流量参数范围内测试其有效性。所有这些发现都可以作为过程工业中小规模混合过程设计的潜在指导方针。