Gas-driven thin liquid films: effect of interfacial shear on the film waviness and convective heat transfer
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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.