Numerical simulation of heat transfer characteristics of falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube

Abstract

In response to the restrictions of the international community on the utilization of refrigerants with high GWP, the replacement of high-GWP refrigerants with non-azeotropic refrigerants and reduction in refrigerant charge have gradually become important research topics in academia. In this study, based on the VOF method, a 2D model of a sheet flow pattern was developed by considering the thermophysical properties and multicomponent phase change of a mixture. The falling-film evaporation of a non-azeotropic refrigerant composed of R32 and R134a outside a horizontal tube was numerically simulated. The effects of the spray height (H), tube diameter (d), inlet temperature (Tinlet), Reynolds number (Re), heat flux (q), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on the heat transfer coefficient (HTC) were studied. The results indicate that with the increase of circumferential angle (Φ), the local HTC exhibits a trend of decreasing, increasing, decreasing, and increasing in sequence. The largest contribution to the increase in the average HTC comes from the impact and fully developed regions. An increase in H, Tinlet, and MR32_liquid is beneficial to the expansion of the fully developed region, whereas a decrease in d leads to expansion of the fully developed region. An increase in Re does not affect the extent of the fully developed region. The increase in H, Re, Tinlet, and MR32_liquid increases the average HTC, but the effect of Tinlet is not significant. Under the conditions of d = 24 mm, H = 13 mm, Re = 4000, MR32_liquid = 0.5, and q = 10 kW/m2, as the inlet temperature increases from 243.15 K to 283.15 K, the average HTC increases by 2.5%. An increase in q does not affect the average HTC, whereas an increase in d does not improve the average HTC.