Evaluation Analysis of CO₂–Specific Correlations of Flow Boiling Heat Transfer Coefficients

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.