u d

as automobile air-conditioning and heat pump systems, the heat rejection temperatures are usually above the critical temperature of carbon dioxide ~31.1°C!, cycles using carbon dioxide as the refrigerant will have to operate in a transcritical cycle. As such, the heat rejection takes place above the critical pressure ~about 74;120 bar! in a so-called gas cooler ~corresponding to the con-denser in the conventional subcritical systems!, while the heat absorption remains below the subcritical region. The high working pressure and favorable heat transfer properties of CO2 enable re-duced tube diameters. A gas cooler made of extruded flat micro-channel tubes of 0.79 mm in diameter has been reported @2#. The use of microchannels in the gas cooler will allow one to effec-tively handle high pressures without excessive wall thickness and material weight. In order to design a high efficiency gas cooler, a thorough understanding of convective heat transfer for supercriti-cal carbon dioxide in cooled tubes is needed. One of the most important characteristics of supercritical fluids near the critical point is that their physical properties exhibit ex-tremely rapid variations with the change of temperature, espe-cially near the pseudocritical point ~the temperature at which the specific heat reaches a peak for a given pressure!. This can be clearly seen from Fig. 1, where the specific heat cp and density r of CO2 at supercritical pressures of 80 and 100 bar (pcr 573.8 bar) are plotted based on the data from the NIST Refrig-Forced convection of supercritical fluids such as water, carbon dioxide, nitrogen, hydrogen, and helium in channels have been extensively studied both experimentally @4–6 # and numerically @7–9#. Many correlations, most of the Dittus-Boelter type Nu 5C RemPrn, have been proposed for supercritical fluids with heat-ing. The reference temperature method and the property ratio method have been employed to incorporate the variable-propert