Rotational flows and mixing enhancement

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.