Marangoni convection during droplet birth and coalescence

Marangoni效应显著改善了传质，并直接影响局部流体动力学，特别是在液滴的产生和合并过程中。然而，缺乏对局部现象的详细定量了解，因此在工业设计中被忽视。萃取塔中液滴的寿命包括形成、上升或下降和合并。这些阶段是强耦合的，并且取决于流体动力条件和传质。当液滴形成时，质量传递就开始了。在接下来的上升阶段，液滴可能会与其他液滴碰撞，并与它们合并或反弹。聚结概率和上升时间--因此，液滴在装置中的停留时间--都取决于上升速度。此外，聚结行为影响液滴的尺寸分布，而液滴尺寸分布又控制着可用于传质的界面面积。物种的迁移可以引起界面张力梯度，从而在界面处产生额外的剪应力。因此，界面的运动被诱导，产生强烈且不规则的流动模式。这种被称为Marangoni效应的现象导致了动量和质量传递的极强耦合，对这些复杂现象的研究通常是通过研究单个液滴的流体力学和传质特性来进行的。然后使用修正因子来解释诸如Marangoni对流这样的非理想情况。在最新的设备模拟中，这种局域现象通常被完全忽略。在该研究项目中，开发了新的测量系统和方法，用于现场、高分辨率地研究Marangoni对流对质量传输在空间和时间上的影响。然而，液滴形成的最初几秒钟只能用数值方法来捕捉。计划的延长期将用于生成一个数据池，并验证新开发的建模方法，以便为技术单位模拟创建新的核心。目前，在装置设计中没有考虑Marangoni对流。

Marangoni effects significantly improve mass transfer and directly influence local hydrodynamics, especially during droplet birth and coalescence. However, a detailed quantitative understanding of local phenomena is missing and therefore neglected in industrial design.The lifetime of a droplet in an extraction column comprises formation, rise or fall and coalescence. These stages are strongly coupled and depend on the fluid dynamic conditions and mass transfer. Mass transfer set in while the droplet is being formed. During the following rise stage, the droplet may collide with other droplets and either coalesce with them or bounce. Both the coalescence probability and the rise time - and, consequently, the droplet residence time in the apparatus - depend on the rise velocity. Moreover, the coalescence behaviour influences the droplet size distribution, which, in turn, governs the interfacial area available for mass transfer. Species transport can evoke interfacial tension gradients, which lead to additional shear stresses at the interface. Thereby, a movement of the interface is induced, creating strong and irregular flow patterns. This phenomenon, called Marangoni effect, leads to extremely strong coupling of momentum and mass transfer.Investigation of these complex phenomena is often performed via studying of fluid dynamics and mass transfer characteristics of single droplets. Correction factors are then used to account for such non-idealities as is Marangoni convection. In the state-of-the-art of apparatus simulation such local phenomena are usually completely ignored.In the research project, new measuring systems and methods have been developed for an in situ, high resolution experimental investigation of the influence of Marangoni convection on mass transport in space and time. However, the first seconds of droplet formation can only be captured with numerical methods. The planned extension period will be used to generate a data pool and to validate newly developed modelling methods enabling creation of new kernels for technical unit simulation. Currently, Marangoni convection is out of consideration in the apparatus design.