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

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 (delta) delta ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of delta 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), H ), tube diameter (d), d ), Reynolds number (Re), Re ), inlet temperature ( T inlet ), and mass fraction of R32 in the liquid-phase mixture ( M R32_liquid ) on delta were analyzed under sheet flow. The results show that an increase in T inlet , H , and d is favorable for increasing the average delta (delta ave), delta ave ), whereas an increase in M R32_liquid and Re leads to a reduction in delta ave . Besides, the decreasing rate of the delta ave decreases gradually as H and d increase. When H is 4 mm, the local delta ( delta local ) first decreases and then increases as the circumferential angle (Phi) Phi ) increases. When H ranges from 6 mm to 13 mm, as Phi increases, delta local first increases slightly, then decreases, and finally increases significantly near the wake region. The delta local of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the delta local of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum delta local occurs at Phi between 130 degrees degrees and 140 degrees, degrees , and with an increase in H and d , Phi corresponding to the minimum delta local decreases.