Film-wise condensation of pure vapour in flattened tubes: A numerical study of the combined influence of aspect ratio and rotation angle

Vapor condensation is a physical phenomenon that finds application in heat removal systems. The traditional design of these systems involves round tubes but experience shows that this geometry is not optimal for heat transfer. Flattened tubes on the other hand, have been found to offer potential for improvement as their geometry increases the condensation surface, which fosters higher heat transfer rates. However, the effects of tube shape (aspect ratio) and orientation (rotation angle) on film-wise condensation dynamics are not fully understood. In this work, we numerically simulate a model of the condensed vapor layer thickness distribution on the flattened tube inner surfaces taking into account bulk and surface forces (gravity, surface tension, shear stress) for a thin layer of liquid. We consider various configurations of aspect ratios (circular, and AR = 2, 4, and 6) and rotation angles (0°, 10°, 20°, 30°, 45°, 60°, 75°, and 90°). Our simulations allow for an improved understanding of how these geometric parameters as well as their interplay, influence the thickness distribution of the condensate film on the tube's inner surface, and facilitate the identification of configurations that maximize heat transfer efficiency. Considering water as a working fluid, results show a possible heat transfer enhancement of up to 74% compared to the round tube geometry for an aspect ratio of 6 and a rotation angle of 90°.