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    Gas-driven thin liquid films: effect of interfacial shear on the film waviness and convective heat transfer
    (Elsevier France-Editions Scientifiques Medicales Elsevier, 2019) Budaklı, Mete; Gambaryan-Roisman, Tatiana; Stephan, Peter
    This study is aimed at experimental investigation of hydrodynamics and convective heat transfer in gravity and gas-driven thin liquid wall films. The liquid film has been annularly applied on a vertically aligned heated tube mounted in a flow channel. In the arranged two-phase flow domain, both the liquid film flow and co-current air flow were thermally and hydrodynamically developing. The Reynolds numbers of liquid and gas flows have been varied between 80 - 800 and 10(4) - 10(5), respectively. The wall heat flux was kept constant at 15 W/cm(2). In order to elucidate the effect of the film waviness on the convective heat transfer between the heated wall and the liquid film, the wave frequencies and standard deviations of film thickness have been evaluated by applying high-speed shadowgraphy technique. The wall temperature distribution in streamwise direction has been measured. The experimentally obtained average Nusselt numbers have been compared with the wave frequencies and standard deviations of film thickness. Up to a gas Reynolds number of 4.10(4) and for liquid Reynolds numbers between 80 and 800, the convective heat transfer preliminary depends on the liquid Reynolds number rather than the Reynolds number of the gas flow. For this range, the average Nusselt numbers from the gas-driven film experiments are close to those for falling film flows. At the gas Reynolds numbers starting from 7.10(4), significant heat transfer enhancement with the gas flow has been registered over the full range of liquid Reynolds number.
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    Investigation of flow patterns and heat transfer in gas-driven thin liquid films
    (International Symposium on Turbulence and Shear Flow Phenomena, TSFP, 2019) Budaklı, Mete; Gambaryan-Roisman, Tatiana; Stephan, Peter
    Gas-driven liquid films are a promising candidate for heat up and evaporation of liquids in an efficient way. Many industrial fields benefit of this type of treatment, because of an intensive heat transfer, such as upcoming modern combustion chambers, reboilers, condensers or cooling applications, while gas-driven films represent a complex interaction of two fluids. The state of the film flow and the complex transport mechanisms owing to the interaction at the liquid-gas interface can have significant influences on the overall heat transfer performance, film stability, rupture and wetting process. One advantage of gas-driven liquid films is the stability against film rupture, since close to the film rupture conditions the shear force at the liquid-gas interface guides the liquid along the flow direction and promotes the wetting of the solid wall. In this study, gas-driven thin liquid film flows on unstructured and structured heated walls are investigated experimentally. The effect of different inlet temperatures of both the fluids on the heat transfer rate and the flow pattern is determined. Another aim of the present study is to give insight to the complex flow mechanisms and wetting behaviour of gas-driven thin liquid films on undulated surfaces fabricated as microgrooves. The gas and liquid Reynolds number are varied between 0 to 84000 and 80 to 1900, respectively. Measurements on wall temperature distribution are performed and Nusselt numbers are calculated. By using a high-speed infrared camera, the flow pattern of the liquid is recorded for different experimental parameter configurations revealing the wetting behaviour, flow patterns and rupture of the thin liquid layer. The measured results disclose that using shear at the interface of a thin liquid governs the film stability and heat transfer. An increase of interfacial shear leads to a highly wavy film flow, which is characterized mostly by 3-dimensional surface structure (see Figures below). Another major finding is that at small liquid mass flow rates, the gas flow does not have significant influence on the heat transfer enhancement. The dominance of the gas flow comes more into account at elevated film flow rates. © 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved.