Abstract
In the present study, the characteristics of film condensation were investigated in tube bundles with R718 as a refrigerant. The experiments were conducted at the saturated temperature of 36.5℃ and the cooling water was heated through tube bundles from 30 to 35℃. In addition, three types of a circular tubes, namely, Bare, Corrugate, and Endcross tubes, were used to compare the characteristics of film condensation according to the structure of the tube surface. From the result, Endcross tube showed the highest condensation heat transfer coefficient, but Bare tube showed the lowest condensation heat transfer coefficient. It seemed that this was due to the increase in the surface area for Corrugate and Endcross tubes in comparison to Bare tube. In addition, due to the combined effect of the subcooling at the tube surface and the inundation in tube bundles, condensation heat transfer coefficient tended to decrease and then increased again as the sequence of the tube increased. Finally, an abrupt increase of condensation heat transfer was showed at a specific sequence of the tube despite the presence of inundation effect. It was postulated that the falling droplets having a large momentum caused the liquid film on the surface of the lower tube to move and break. As a result, the impact of falling droplets might enhance heat transfer under certain circumstances.
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