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The calculation of two-phase flow boiling heat transfer of R22 in channels is required in a variety of applications, such as chemical process cooling systems, refrigeration, and air conditioning. A number of correlations for flow boiling heat transfer in channels have been proposed. This work evaluates the existing correlations for flow boiling heat transfer coefficient with 1669 experimental data points of flow boiling heat transfer of R22 collected from 18 published papers. The top two correlations for R22 are those of Liu and Winterton (1991) and Fang (2013), with the mean absolute deviation of 32.7% and 32.8%, respectively. More studies should be carried out to develop better ones. Effects of channel dimension and vapor quality on heat transfer are analyzed, and the results provide valuable information for further research in the correlation of two-phase flow boiling heat transfer of R22 in channels.

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Carbon dioxide (CO2 or R744) is an important alternative refrigerant. The two-phase flow boiling heat transfer characteristics of CO2 are quite different from those of conventional refrigerants due to its much higher working pressure. It is necessary to develop CO2-specific correlations of flow boiling heat transfer coefficients, and a number of them have been proposed. There is some literature to evaluate existing CO2-specific correlations. However, either the number of correlations or the number of data points used was limited. Hence, the results were often not consistent, even controversial. This work presents a comparative review of existing CO2-specific correlations of flow boiling heat transfer coefficients. Based on 2872 experimental data points of CO2 flow boiling heat transfer from 10 independent laboratories around the world, nine CO2-specific correlations of flow boiling heat transfer coefficients are analyzed and evaluated, which provides an in-depth understanding of the existing CO2-specific correlations. The Fang [1] correlation performs best, with a mean absolute deviation of 15.5%. More efforts should be made to better understand the mechanism of dryout and channel dimension effects.