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

Abstract

Horizontal-tube falling-film evaporator is regarded as an effective alternative to full-liquid evaporator because of its higher heat transfer coefficient and lower refrigerant charge. In the process of falling-film evaporation, the thin liquid film thickness (δ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of δ can effectively prevent the reduction in heat transfer efficiency caused by local wall dryout. In this study, non-azeotropic refrigerant R32/R134a was taken as the research object. By incorporating the multi-component phase change model and contact angle model into the governing equations, a two-dimensional numerical model of falling-film evaporation outside a horizontal circular tube was established. The effects of the spray height (H), tube diameter (d), Reynolds number (Re), inlet temperature (Tinlet), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on δ were analyzed under sheet flow. The results show that an increase in Tinlet, H, and d is favorable for increasing the average δ (δave), whereas an increase in MR32_liquid and Re leads to a reduction in δave. Besides, the decreasing rate of the δave decreases gradually as H and d increase. When H is 4 mm, the local δ (δlocal) first decreases and then increases as the circumferential angle (Φ) increases. When H ranges from 6 mm to 13 mm, as Φ increases, δlocal first increases slightly, then decreases, and finally increases significantly near the wake region. The δlocal of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the δlocal of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum δlocal occurs at Φ between 130° and 140°, and with an increase in H and d, Φ corresponding to the minimum δlocal decreases.