Falling film evaporation of ammonia on smooth and Low-Fin tube arrays

• Falling film evaporation heat transfer of ammonia on tube arrays was investigated. • Falling film evaporation showed 4.4 times higher heat transfer than flooded evaporation. • Relevant empirical correlations for falling film evaporation of ammonia are proposed. The heat transfer performance of a falling film evaporator using ammonia was thoroughly investigated by directly comparing it with submerged natural convection/pool boiling, focusing on the effect of tube position in a vertical tube arrangement. Falling film evaporation of ammonia in a vertical tube array was examined at saturation temperatures ranging from 10 to 30 ℃, comparing it with submerged natural convection/pool boiling under low heat flux conditions (0.5–18.2 kW/m 2 ). Five stainless steel test tubes having an outer diameter of 15.87 mm were arranged vertically, with one submerged in liquid ammonia and the others exposed to falling film evaporation. Experimental results show that heat transfer coefficients (HTCs) in the falling film evaporation are up to 4.4 times higher than those in the submerged natural convection/pool boiling. The uppermost tube exhibited the lowest HTC among the film-evaporated tubes due to the formation of locally thick liquid films. Empirical correlations were developed to predict the HTC as a function of Reynolds number, Prandtl number, reduced pressure, and dimensionless heat flux, with a mean absolute percentage error of less than 11.1 %. The proposed correlations are applicable to the tested range of 10–30 °C and low heat flux (0.5–18.2 kW/m 2 ). Enhanced low-fin tubes exhibited 10–30 % improved thermal performance over smooth tubes, particularly at higher saturation temperatures, though the performance gain was limited at low temperatures due to hindered liquid spreading by the fin structures. The findings highlight the critical role of tube arrangement, film flow characteristics, and enhanced surfaces in optimizing ammonia falling film evaporators for efficient and environmentally friendly heat pump applications.