Rotational flows and mixing enhancement

这项研究旨在研究圆柱几何结构中的旋转诱导流动，为流动转变和不稳定性的类型、性质和发生提供新的见解，并为更好地了解相关设备中的混合机理提供新的信息，从而发展和改进混合模型。混合是日常生活中经常遇到的一种物理现象。最简单的例子之一就是用勺子搅动咖啡杯来搅拌牛奶。在利用两个或多个相或反应物混合的化学和生化工业中，可以发现许多应用。从这个角度来看，可以区分几种以不同方式实现混合的反应器设计：振荡流混合器(OFM)、静态混合器、搅拌容器和摇瓶。后两种类型的反应器是旋转诱导混合反应器，其流动是通过搅拌叶轮或通过托盘沿圆周轨道移动保持圆柱形容器而产生的。这两种类型的反应器之间的相似之处更加明显，因为摇瓶通常是在生物工艺开发的早期阶段使用的小型混合器，在所开发的过程在大规模工业搅拌槽中实施之前。然而，尽管搅拌槽反应器的流体力学已经得到了广泛的研究，但对摇瓶中发生的流动模式、流动转变和不稳定性知之甚少。这种缺乏知识的后果是双重的：(A)在摇瓶中开发的过程不能被完全描述，破坏了它的重现性；(B)它的操作条件不能与工业、放大的反应器(通常是搅拌槽)中的操作条件适当地关联起来，这些反应器的大量数据可以在文献中获得。考虑到流动几何的不同，某些类型的不稳定性和不稳定在少数关于摇瓶中的流体力学的论文中被记录下来，回想一下那些在具有旋转端壁的圆柱形容器中遇到的情况，这些数据可以在文献中获得。这两个系统(即圆柱形摇瓶和圆柱形容器)之间的相似之处还表现在这样一个事实上，即摇瓶的运动可以被视为圆柱体绕其轴线的旋转运动和圆柱体本身围绕偏心轴的逆旋转轨道运动的叠加。第一种类型的运动实际上与具有旋转端壁的圆柱形容器非常相似。因此，本研究方案在圆柱形容器中由三种不同类型的旋转运动引起的流动中进行了三组实验。一方面，这种方法与在相同几何形状中研究的三种不同流动之间的类比，应该提供了一种从一般角度更好地理解流动不稳定和过渡发生的原始方法，并将能够评估决定混合的物理机制。另一方面，本研究的目的是从流体力学的角度确定相关参数，以促进搅拌釜式反应器和摇瓶之间的放大/缩小过程。

The proposed research study aims at investigating rotationally induced flows in cylindrical geometries to bring new insight on the type, nature and occurrence of flow transitions and instabilities as well as to offer novel information to better understand mixing mechanisms in related equipment and therefore develop and improve mixing modelling.Mixing is a physical phenomenon that is often encountered in everyday life. One of the simplest example is that of a spoon stirring a coffee cup to blend milk. Numerous applications can be found in the chemical and biochemical industries that utilise mixing of two or more phases or reactants.From this point of view several reactor designs can be distinguished that attain mixing in different ways: oscillatory flow mixers (OFM), static mixers, stirred vessels and shaking flasks. The last two types of reactors are rotationally induced mixing reactors for which a flow is generated by a stirring impeller or by the movement of the tray holding a cylindrical container along a circumferential orbit. The analogy between these two types of reactors is even more evident when considering that shaking flasks are often small scale mixers employed in the early stage of bioprocess development, before the developed process is implemented in a large scale industrial stirred tank.However, while the fluid mechanics of stirred tank reactors have been extensively studied, little is known about the flow patterns, flow transitions and instabilities taking place in shaken flasks. The consequences of this lack of knowledge are twofold: (a) a process developed in the shaken flasks cannot be fully characterised, undermining its reproducibility; (b) its operating conditions cannot be properly correlated to those present in industrial, scaled up reactors, usually stirred tanks, for which a vast amount of data is available in literature.Taking into account the differences in the flow geometry, certain types of instability and unsteadiness that are documented in the few papers dealing with fluid mechanics in shaken flasks, recall those that are encountered in a cylindrical container with a rotating endwall for which data is available in literature. The similarity between the two systems (i.e. shaking flask of cylindrical shape, and cylindrical container) is also indicated by the fact that the movement of the shaken flask can be seen as the superimposition of the rotating movement of a cylinder around its axis and a counter rotating orbital movement of the cylinder itself around an eccentric axis. The first type of movement is in fact very similar to that of a cylindrical container with a rotating endwall.Therefore the present research proposal has been formulated in three sets of experiments that will be carried in flows induced by three different types of rotational movements in a cylindrical container.On the one hand this methodology, with an analogy between three different flows investigated in the same geometry, should provide an original way to understand better the occurrences of flow instability and transitions from a general point of view, and will enable to assess the physical mechanisms determining mixing. On the other hand the present investigation aims at determining relevant parameters based on fluid mechanics aspect that will facilitate the process of scale up/down between stirred tank reactors and shaken flasks.