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

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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.