Investigation on flow condensation of refrigerant in annulus of smooth and enhanced tube-in-tube heat exchanger

Abstract

An experimental investigation of R22 and R410A condensation outside a horizontal smooth tube, a herringbone tube and a newly developed 3D enhanced heat transfer (1EHT) dimple tube has been conducted. The herringbone tube has a fin root diameter of 11.43 mm, a helical angle of 21.3 °, 48 fins with a fin height of 0.262 mm and an apex angle of 36 °; the 1EHT tube has an inner diameter of 11.5 mm with a dimple enhancement; while the smooth tube has an inner diameter of 11.43 mm; and all the tubes have an outer diameter of 12.7 mm. Experiments were performed for a constant saturation temperature of 45°C; with a constant inlet vapor quality of 0.8 and a constant outlet vapor quality of 0.1; for a mass flux ranging from 5 kg/(m2 s) to 250 kg/(m2 s). In addition, annular side condensation experiments were performed using an outer shell tube with outer diameters of 17 mm and 25.4 mm. Heat transfer performance varied with mass flux. At a low mass flux the enhanced dimple tube had the smallest heat transfer coefficient; while at higher values of mass flux, the smooth tube had the smallest heat transfer coefficient. Finally, the effect of average vapor quality on the heat transfer coefficient was also investigated. Characteristic analysis was performed in order to account for the various phenomena found in this series of experiments. Annular side heat transfer performance combined with pressure drop measurements reveal that the herringbone tube generally had a better heat transfer performance than the other tubes, and can be a good choice for use in annular side condensation applications.