Experimental comparison of the heat transfer of carbon dioxide under subcritical and supercritical pressures

The carbon dioxide (CO₂) trans-critical cycle is widely used in refrigerating systems, air conditioning, and heat pumps. Heat transfer behaviors at subcritical and supercritical pressures play essential roles in trans-critical thermohydraulic systems. In past decades, boiling heat transfer at subcritical pressure and ‘single-phase' convective heat transfer at supercritical pressure have been broadly discussed; however, evaluation of the heat transfer difference of CO₂ between supercritical and subcritical pressures remains lacking. In this study, experimental investigations are conducted to explore the heat transfer characteristics of the vertical flow of CO₂ in a wide pressure range (6.2 MPa–8.0 MPa) within a circular tube (inner diameter = 5 mm). The range of operating conditions includes a mass flux (G) between 0 and 190 kg/m²s and a heat flux (q) of up to 105 kW/m². Parametric effects (e.g., heat flux, inlet temperature, and mass flux) within these experimental conditions are also analyzed. Measurement results show that the inner-wall temperature at subcritical pressure is higher than that at supercritical pressure when a boiling crisis occurs. Closer to the critical pressure, a boiling crisis occurs more easily at lower heat fluxes, but the corresponding temperature peak declines. Higher temperatures are measured farther from critical pressure, indicating that overheating induced by boiling heat transfer at subcritical pressure is far more significant than that governed by the deterioration of convective heat transfer at supercritical pressure. Additionally, a qualitative analysis of physical mechanisms governing observed heat transfer differences is performed, confirming variations in fluid thermophysical properties and a bubble-like flow pattern.