Mixing of fluids with viscoelastic flow behaviour in stirred tanks

虽然经典混合领域已经在公开文献中进行了大量的研究，但粘弹性流体的使用存在着深远的知识空白。由于缺乏对物理关系和实验数据的基本理解，以及由于数学模型复杂，弹性流动特性对搅拌过程的影响在科学和工业中仍然经常被忽视。拟议的项目旨在通过创建一个实验数据库和开发缺失的数值工具（CFD）来改变这种状况。在不同搅拌直径比的六叶盘式和斜叶式搅拌槽中进行了实验研究。它们分别代表径向泵和轴向泵搅拌器的标准几何形状。有了这些，流场、功率输入、混合时间以及几何和流变参数之间的依赖关系就确定了。这是通过使用模型流体来实现的，模型流体的（粘弹性）流变性是用旋转和拉伸粘度计专门调整和测量的。利用激光光学测量方法（粒子图像测速法和平面激光诱导荧光法），首次获得粘弹性流体中上述搅拌几何形状的时空分辨流场和混合信息。通过这些方法，流场逆转、弹性流室的形成及其对功率特性的影响等特征流动现象将被可视化和量化。实验结果也为数值分析提供了验证依据。基于前期工作中开发的有限体积求解器，将研究微分粘弹性本构模型对搅拌容器内流场、搅拌次数和功率输入计算的适用性。初步的模拟结果首次证明了多模态模型（Giesekus, Phan-Thien-Tanner）对上述流动现象的定性再现能力。虽然在流场发生逆转时，可以获得非常好的定量一致性，但为了获得关于弹性流室大小和形状的更准确结果，仍然需要考虑拉伸粘度。最后，通过对模型的比较，可以推导出最适合搅拌槽模拟的规则，该规则取决于粘弹性流动特性和操作条件，以及精度和计算工作量。通过将从这些研究中获得的混合特性与几何和操作参数相关联，将评估或制定放大标准，并通过实验选择性地验证。

Although the field of classical mixing has been the subject of considerable investigation in the open literature, far-reaching gaps of knowledge exist in the use of viscoelastic fluids. Due to the lack of a fundamental understanding of physical relationships, experimental data and because of mathematically complex models, the effects of elastic flow properties on stirring processes are still often neglected in science and industry. The proposed project aims to change this by creating an experimental database and by developing missing numerical tools (CFD).The experimental investigations are carried out in stirred tanks with six-bladed disk and pitched blade turbines with varying stirrer to tank diameter ratios. They represent standard geometries of radial and axial pumping stirrers, respectively. With these, dependencies between flow fields, power inputs, mixing times as well as geometric and rheological parameters are determined. This is achieved by the use of model fluids, whose (viscoelastic) rheology is specifically adjusted and measured with rotational and extensional viscometers.By using laser-optical measuring methods (particle image velocimetry and planar laser-induced fluorescence), spatially and temporally resolved flow field and mixing information will be determined for the first time for the above-mentioned stirring geometries in viscoelastic fluids. With these methods, characteristic flow phenomena such as flow field reversals, the formation of elastic flow compartments and their influence on power characteristics will be visualized and quantified.The experimental results also serve as validation basis for the numerical analysis. Based on finite volume solvers developed in preliminary works, the suitability of differential, viscoelastic constitutive models for the calculation of flow fields, mixing times and power inputs in stirred vessels will be investigated. Preliminary simulation results prove the ability of multimode models (Giesekus, Phan-Thien-Tanner) to reproduce the above-mentioned flow phenomena qualitatively for the first time. While very good quantitative agreement is achieved especially when flow field reversals occur, extensional viscosities still need to be considered for more accurate results concerning size and shape of elastic flow compartments. Finally, a comparison of the models enables the derivation of rules which model is best suited for the simulation of stirred tanks, depending on viscoelastic flow properties and operating conditions as well as on accuracy and calculation effort.By correlating mixing characteristics obtained from these investigations with geometric and operating parameters, scale-up criteria will be assessed or developed, which are selectively verified by experiments.