A 2-D numerical study of contact angle effect on transient falling-film flow over horizontal tubes

The coverage of the liquid film over the horizontal tubes, particularly the wetting ratio, is important for heat transfer mechanism of gravity-driven evaporators and absorbers. A 2D, two-phase CFD model was developed to examine the falling-film hydrodynamics and transient flow mechanism for Reynolds numbers ranging from 100-500, contact angles spanning from 0°/30°/60°/90°, and inter tube spacing of 10 mm. The VOF method is used in this article to capture the gas-liquid 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 was worth noting that each contact angle must have a minimum Reynolds number in order to keep the surface completely wet. The research also found that for higher Reynolds numbers, the influence of contact angle on wettability of tube wall can be ignored. The liquid propagation time required to wet the tube surface increases as the contact angle value increases for the same Reynolds number. Fouling over the tubes can be aided by the formation of air voids near the lower stagnation zone.