Condensation heat transfer of carbon dioxide inside horizontal smooth and microfin tubes at low temperatures

This paper presents heat transfer data for the condensation of CO₂ at low temperatures in horizontal smooth and microfin tubes. The test tubes included a 3.48 mm inner diameter smooth tube and a 3.51 mm melt-down diameter microfin tube. The test was performed over a mass flux range of 200-800 kg/m₂ s and at saturation temperatures of -25°C and -15°C, respectively. The effect of various parameters-diameter, mass flux, vapor quality, and temperature difference between inner wall and refrigerant-on heat transfer coefficient and enhancement factor is analyzed. The data are compared with several correlations. The existing correlations for the smooth tube mostly overpredicted the heat transfer coefficients of the present study, which is possibly resulted from the characteristics of carbon dioxide as a "high pressure refrigerant." For the microfin tubes, due to the complexity and variety of fin geometry and flow mechanisms in microfin tubes, most of the correlations for the microfin tube were not applicable for the experimental data of the present study. The average enhancement factors and penalty factors evidenced that it was not always true that the internally finned geometry guaranteed the superior in-tube condensation performance of the microfin tube in refrigeration and air-conditioning systems.