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

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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.