Flow boiling heat transfer characteristics of carbon dioxide in a horizontal tube

The flow boiling heat transfer coefficient of CO2 (R-744) in a horizontal tube was investigated experimentally. The test results showed the boiling heat transfer of CO2 has great effect on more nucleate boiling than convective boiling. The boiling heat transfer coefficients of CO2 are highly dependent on the vapor quality, heat flux and saturation temperature. The boiling heat transfer coefficient of CO2 is about 87.2% and 93% higher than that of R-22 and R-134a. The boiling pressure drop of CO2 increases with increasing mass flux, and decreases with increasing saturation temperature. The pressure drop of CO2 is much lower than that of R-22. The boiling pressure drop of CO2 is about 10–15% of R-22. The lower pressure drop of CO2 is attributed to its unique thermal properties. In comparison with test results and existing correlations, the best fit of the present experimental data for heat transfer is obtained with Jung et al. correlation, and pressure drop is obtained with Choi et al. correlation. But most of the existing correlations failed to predict the boiling heat transfer coefficient and pressure drop of CO2.