Enhancement of Flow Boiling and Condensation Heat Transfer to Alternative Refrigerant Mixtures for HCFC-22 in Tubes

对替代工质R-410A和R-407C在水平微翅片管和人字形管内的流动进行了对流沸腾和冷凝换热实验，结果表明：一般情况下，R-410A的换热可以看作是纯制冷剂的换热，但R-407C的换热系数随着质量流量的增加而降低，这是由于R-407C在低质量和低流量下的传质阻力较小而导致的。与光滑管相比，两种管的强化换热效果都很好，在低流量下，微翅片管的强化效果更好，而在高流量时，人字形管的强化效果更好。通过对两种管内空气-水两相流动行为的观察，解释了强化换热的机理：在两种混合物的冷凝过程中，换热系数随质量流量的增加而增大，但热流密度对传热系数的影响不大。与微翅片管相比，在低质量、低流量条件下，人字形管内冷凝换热的强化程度更大。针对二元和三元混合制冷剂在水平光滑管内的冷凝换热，提出了一种预测局部冷凝换热系数的方法，并提出了基于流动观测的微翅片管内气膜冷凝换热的预测方法，在两种管内，R-407C的压降一般比R-410A的高20~30%，对应于蒸汽密度的差异。人字形管内的压力降最大比微翅片管内的大50%。

Experiments were made on convective boiling and condensation heat transfer for the flow of alternative refrigerant mixtures of R-410A and R-407C for HCFC-22 inside horizontal micro-fin and herringbone tubes, and following results were obtained.In general, the heat transfer for R-410A can he regarded as a pure refrigerant heat transfer, but the heat transfer coefficient for R-407C reduces due to a mass transfer resistance at low quality and low flow rate in both the tubes.In boiling, the heat transfer coefficient for both the mixtures increases with increasing mass flow rate. With both the tubes, the heat transfer is greatly enhanced over a smooth tube ; enhancement becomes larger in the micro-fin tube at low flow rate but in the herringbone tube at high flow rate. Observations of air-water two-phase flow behavior in both the tubes were made to explain the mechanism of the heat transfer enhancement.In condensation of both the mixtures, the heat transfer coefficient increases with increasing mass flow rate, but there is little effect of heat flux. The enhancement of condensation heat transfer in the herringbone tube becomes larger at low quality and low flow rate compared to the micro-fin tube. A method for predicting the local condensation heat transfer coefficient was developed for the condensation of binary and ternary refrigerant mixtures inside a horizontal smooth tube, and a prediction method was also presented for film condensation heat transfer in the micro-fin tube based on a flow observation.In both the tubes, a pressure drop for R-407C is generally 20 to 30 % higher than that for R-410A corresponding to the difference in the vapor density. The pressure drop in the herringbone tube was a maximum of 50% larger than that in the micro-fin tube.