Falling film evaporation is one of the most efficient heat transfer methods in heat exchangers and evaporators. Its performance strongly depends on liquid film hydrodynamics and surface distribution. This study investigated the effect of three circumferential fin geometries on hydrodynamic characteristics and thermal performance of a falling film over a horizontal tube using CFD. Simulations were at Re=217 with VOF two-phase model and constant wall temperature. Liquid volume fraction, film thickness, velocity and temperature fields, and local heat transfer coefficient were extracted at various angles and cross-sections along the tube.

Results showed fin geometry plays a decisive role in redistribution, wettability, and thermal resistance. Model F1 gave the best balance between hydrodynamics and heat transfer. Despite 15.4% higher average film thickness than the smooth tube, it enhanced the heat transfer coefficient by 16.4%. Model F2 showed a 14.9% increase in heat transfer. However, its less uniform distribution caused greater local fluctuations. Model F3, due to accumulation and 69% thicker average film, caused a 74.8% reduction in heat transfer.

A key contribution of this study is the simultaneous assessment of the effects of fin geometry on film distribution and thickness, surface wetting, and heat transfer, thereby elucidating the role of film uniformity in thermal performance. The results demonstrate that increasing the finned surface area or even the average film thickness does not necessarily enhance heat transfer. Instead, uniform film redistribution and the prevention of local liquid accumulation play a more significant role in improving the heat transfer coefficient.