Determination of Heat Transfer Coefficients During In-Tube Gas Cooling of Supercritical Carbon Dioxide

The world-wide agreement to restrict the use of chlorofluorocarbons (CFCs) and hydrofluorocarbons has prompted recent research exploring the possibilities of replacement refrigerants, particularly those occurring naturally in the environment. One such natural refrigerant, carbon dioxide, has been shown for particular applications to have a COP which is competitive with current refrigerants. However, in order to use carbon dioxide, entirely new components must be designed to accommodate the high pressures. In fact, the entire heat rejection process of a carbon dioxide vapor compression cycle may take place above the critical temperature and pressure. Hence, the design of the heat rejecting heat exchanger requires knowledge of carbon dioxide transport properties associated with temperatures and pressures above the critical point. While some correlations for in-tube cooling of supercritical carbon dioxide do exist in the literature, only very recently has there been heat transfer research of carbon dioxide with respect to its role as a refrigerant. An experiment at Herrick Laboratories at Purdue University sponsored by ASHRAE is exploring the nature of in-tube cooling of supercritical carbon dioxide. The test apparatus built exclusively for the experiment is being used to quantify the local heat transfer coefficients as well as pressure drop associated with temperature ranges and refrigerant mass fluxes which correspond to typical refrigerating capacities. In addition, the test stand is being used to investigate the transport properties of supercritical carbon dioxide with typical compressor oils entrained in the refrigerant and with in-tube surface enhancement. INTRODUCTION TO CARBON DIOXIDE Background Regulations prohibiting the use of chlorofluorocarbons ( CFCs) as refrigerants by industrialized nations are in place, and guidelines for phasing out hydrochlorofluorocarbons (HCFCs) as refrigerants in the future have already been agreed upon by nations of the world. This has prompted researchers worldwide to consider the development of new refrigerants and/or the investigation of refrigerants occurring naturally in the environment. However, the development of new fluids to be used as refrigerants may result in the same dilemma the world currently faces with several fluorocarbon based refrigerants: the potential for damage to the earth's environment. There is no possible way of knowing the long-term environmental or health impact of a brand-new refrigerant with absolute certainty. The use of a substance which is found naturally in the biosphere as a refrigerant eliminates the need for such speculation. Carbon dioxide is a substance which not only is naturally occurring in the environment, but has a successful history of use as a refrigerant. Prior to the development of CFCs in the 1930s, carbon dioxide was widely used as a refrigerant in cooling human occupied enclosures. Ironically, carbon dioxide has recently been considered as a replacement for fluorocarbon based refrigerants (which made carbon dioxide obsolete as a refrigerant). Recent research has indicated that although carbon dioxide may not be a superior refrigerant for all air conditioning/refrigerating applications, there are certain situations for which carbon dioxide may be suitable. These cases take advantage of the thermophysical differences of carbon dioxide compared to the HCFCs.