Influence of the Static Contact Angle on the Liquid Film Coverage for Falling-Film Systems

The coverage of the liquid film over the horizontal tubes, particularly the wetting ratio, is important for gravity-driven evaporators and absorbers to achieve a better heat transfer mechanism. A 2D, two-phase CFD model was developed to examine the falling-film hydrodynamics and transient film coverage for Reynolds numbers ranging from 100 to 500, static contact angles spanning from 0°/30°/60°/90°, and tube-to-tube distance of 10 mm. The VOF method is used in this article to capture the liquid–gas interface. The findings showed that the complete spreading of the liquid film is difficult at low Reynolds numbers and high contact angles. The formation of dry regions on the tube wall as a result of insufficient liquid supply, liquid film breakage, and liquid film shrinkage. Furthermore, as the Reynolds number increases and contact angle decrease, the wetting ratio over the tube surface increases. It is worth noting that each contact angle must have a minimum Reynolds number in order to keep the surface completely wet. The research also revealed that for higher Reynolds numbers, the influence of contact angle on wettability of tube wall can be ignored. For the same Reynolds number, the liquid propagation time required to wet the tube surface increases as the contact angle value increases. Graphical Abstract