Influence of structured surfaces and a counter-current gas flow on inverted falling liquid films during the appearance of Rayleigh-Taylor instabilities

液膜流动是一种在广泛的技术应用中常见的流体动力学现象，需要充分了解才能对工业过程进行建模、设计和控制。因此，本文主要研究正倾斜和垂直衬底上的液膜流动，在这些衬底上，结构化表面和逆流气流对流体动力学行为有显著影响。然而，倒置衬底底部的液膜流动过去很少被研究，但在技术应用中也出现了这种情况。此外，还没有关于结构表面和逆流气流对倒置液膜流动的影响的研究。由于瑞利-泰勒不稳定性(RTI)在倒置液膜中的出现，液滴的形成和脱落是可能的，这导致了与正倾斜和垂直衬底上的液膜完全不同的流体动力学行为。这个研究项目的初步调查已经表明，与现有的光滑表面的理论关联相比，结构化表面对RTI外观的影响很大。因此，本研究项目的目的是阐明结构表面和逆流气流对RTI出现过程中倒置液膜流动的流体动力学行为的影响。因此，将分析特征流体动力学参数，如作为不同RTI情形转换判据的临界倾角、液滴脱离流量以及2D波和3D棱锥体结构表面上的液膜厚度。因此，光诱导荧光法(LIF)作为一种光学非侵入性测量方法将被用于测量液膜厚度。此外，根据不同的流体性质以及动力学和几何参数，将建立数学关联式来描述在有和没有逆流气流的结构表面上的RTI和液滴从液膜流动中分离的现象。该项目的结果可用于开发倒置降膜技术应用的设计概念，并进一步为验证气膜流动的数值模拟奠定基础。

Liquid film flow is a common fluid dynamic phenomenon in a wide range of technical applications that needs to be fully understood to be able to model, design and control industrial processes. Liquid film flow has been therefore mainly investigated in case of positive inclined and vertical substrates at which structured surfaces and a counter-current gas flow have a significant influence on the fluid dynamic behavior. However, liquid film flow at the underside of inverted substrates has been rarely investigated in the past but is also occurring in technical applications. Furthermore, no research has been undertaken regarding the influence of structured surfaces and a counter-current gas flow on inverted liquid film flows. Due to the appearance of Rayleigh-Taylor Instabilities (RTI) in inverted liquid films, droplet formation and droplet detachment is possible which leads to a fundamentally different fluid dynamic behavior compared to liquid films on positive inclined and vertical substrates. Preliminary investigations for this research project already show a significant influence of structured surfaces on the appearance of RTI compared to existing theoretical correlations for smooth surfaces. The goal of this research project is therefore to clarify the influence of structured surfaces and a counter-current gas flow on the fluid dynamic behavior of inverted liquid film flows during the appearance of RTI. Hence, characteristic fluid dynamic parameters will be analyzed such as the critical inclination angle as criterion for the transition between the different RTI-cases, the droplet detachment flow rate and liquid film thicknesses on 2D-wave and 3D-pyramide structured surfaces. Therefore, Light Induced Fluoresence (LIF) as an optical noninvasive measurement method will be used to measure liquid film thicknesses. Furthermore, mathematical correlations will be established to describe the appearance of RTI and droplet detachment from the liquid film flow on structured surfaces with and without a counter-current gas flow depending on various fluid properties as well as kinetic and geometry parameters. The results of this project can be used to develop design concepts of technical applications with inverted falling films and are further a basis for validations of numerical simulations of film flows.