With growing environmental concerns of global warming and ozone depletion, the environmentally benign natural refrigerant carbon dioxide has attracted considerable attention. Due to its critical temperature is very low (31.1 °C), the use of a transcritical carbon dioxide refrigeration cycle is proposed, in which the heat rejection process takes place in the supercritical region (about 7.4-12Mpa), while the absorbing heat process occurs under the subcritical condition. The important characteristic of transcritical cycle is that the operating pressure is very high. The thermophysical properties like specific heat, thermal conductivity, density, viscosity and surface tension, etc exhibit extremely rapid variations with the change of temperature near the critical region. Moreover, although the absorbing heat process remains below the subcritical region as in the conventional vapor compression cycle, the evaporating pressure is quite high (2.0-6.0Mpa), which is about 7-8 times the pressures for conventional refrigerants. The physical and transport properties are very unique in this process. Thus the conventional heat transfer correlations are not suitable for CO2 and some new correlations have been developed. In a word, the thermophysical properties of carbon dioxide in transcritical cycle are very special. Consequently, comprehensive understanding of heat transfer characteristics in transcritical cycle should be done in order to design high efficiency gas cooler and evaporator.
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