Flow boiling heat transfer and pressure drop characteristics of CO2 in horizontal tube of 4.57-mm inner diameter

This paper presents the flow boiling heat transfer and pressure drop of CO2 (R744) in a horizontal tube. The test tube had an inner diameter of 4.57 mm, with a 0.89-mm wall thickness and length of 4.2 m. Experiments were carried out for heat fluxes ranging from 10 to 40 kW/m2 K, mass fluxes ranging from 200 to 1000 kg/m2 s, and saturation temperatures ranging from 0 to 40 °C. The boiling heat transfer of CO2 has a greater effect on nucleate boiling than convective boiling and highly depends on the vapor quality, heat flux, and saturation temperature. The boiling heat transfer coefficient of CO2 increases with a decrease in the inner tube diameter. In a comparison to previous boiling heat transfer correlations, the correlation of Cheng et al. showed fairly good agreement with the experimental data for CO2. The boiling pressure drop of CO2 increased with an increase in the mass flux and decreased with an increase in the saturation temperature. None of the empirical correlations from the existing literature were able to predict the pressure drop of CO2 in good agreement with the experimental data. Therefore, it is necessary to develop a reliable and accurate prediction for estimating the boiling pressure drop of CO2.

Research highlights
► The boiling heat transfer of CO2 showed a higher value than those of R134a and R22.
► The boiling heat transfer coefficient of CO2 increased with a decrease in the inner tube diameter.
► The boiling heat transfer of CO2 has a greater effect on nucleate boiling than convective boiling.