Heat rejection from R744 flow under uniform temperature cooling in a horizontal smooth tube around the critical point

Heat rejection from CO₂ flow near the critical point, where commercial refrigerators spend most of their operating hours, was investigated in this study. Experimental results on the heat transfer coefficient and pressure drop of mass flux from 100 to 240 kg m^-2 s^-1 at pressures from 5 to 7.5 MPa in a horizontal smooth tube of 6.1 mm inner diameter are provided and compared with correlations. The heat rejection process below critical pressure was categorized into superheat, two-phase, and subcool zones in which the bulk-mean temperature was superheated, saturated, and subcooled, respectively. The results indicated that the heat transfer coefficient in superheat zone is significantly higher than correlations proposed for single-phase turbulent flow, and the condensation was identified from the tube wall temperature below saturation temperature. This superheat zone accounts for a significant portion of the heat rejected in the subcritical cycle and affects on condenser sizing.