Experiments on Condensation Heat Transfer of Low GWP Refrigerants Outside Horizontal Tubes

Abstract

In this study, the flow pattern of condensate liquid film and condensation heat transfer characteristics of low GWP refrigerants outside a horizontal smooth tube and a three–dimensional microscopic–grooved tube were experimentally verified. As test refrigerants, R1233zd(E) and R1224yd(Z), which are considered as substitutes for R245fa, were used. The experiments were performed at refrigerant saturation temperatures of 40 and 30°C, and the cooling water flow rate and inlet temperature were varied stepwise. The following conclusions are obtained. (1) The condensate flooding angle of a microscopic–grooved tube can be generally predicted by Honda’s equation. (2) The three–dimensional microscopic–grooved tube has a larger amount of condensate liquid, and transition from the droplet mode to the coexistence droplet–column mode, column mode, at the same wall subcooling degree ΔT than the smooth tube. (3) The experimental values of R245fa, R1233zd(E) and R1224yd(Z) outside the smooth tube can be ± 10% predicted by Nusselt’s equation. (4) The heat transfer enhancement ratio of the microscopic–grooved tube was highest 4.9–6.2 times for R245fa, next 3.9–4.9 times for R1233zd(E), and 3.5–4.0 times for R1224yd(Z).