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，但在粘弹性流体的使用中存在着深远的知识空白。由于缺乏对物理关系、实验数据的基本理解以及数学模型的复杂性，弹性流动性质对搅拌过程的影响在科学和工业中仍然经常被忽视。该项目旨在通过建立实验数据库和开发缺少的数值工具（CFD）来改变这一点。实验研究在搅拌槽中进行，搅拌槽具有六叶盘和斜叶涡轮，搅拌器与槽直径比不同。它们分别代表径向和轴向泵送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被确定。这是通过使用模型流体，其（粘弹性）的流变学是专门调整和测量与旋转和拉伸viscometers.By使用激光-光学测量方法（粒子图像测速仪和平面激光诱导荧光），空间和时间分辨的流场和混合信息将被确定为第一次上述搅拌几何形状的粘弹性流体。通过这些方法，可以将流场反转、弹性流动区的形成及其对功率特性的影响等特征流动现象可视化和量化，实验结果也为数值分析提供了验证依据。基于有限体积求解器在初步工作中开发的差分，粘弹性本构模型的流场，混合时间和搅拌容器中的功率输入的计算的适用性进行了研究。初步的模拟结果证明了多模式模型（Giesekus，N-Thien-坦纳）首次定性再现上述流动现象的能力。虽然非常好的定量协议，特别是当流场发生逆转，拉伸粘度仍然需要考虑更准确的结果有关的大小和形状的弹性流动隔间。最后，模型的比较，使规则的推导模型是最适合于模拟搅拌槽，这取决于粘弹性流动特性和操作条件以及精度和计算efforts.By相关的混合特性从这些调查获得的几何和操作参数，放大标准将被评估或开发，这是有选择地通过实验验证。

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.