Research On The Effect Of The Additive And The Enhanced-tube Gas Cooler On CO₂Trans-critical Cycle Performance

Now the energy shortage and environment pollution are both top concerns of theworld. The CO₂transcritical cycle has great potentials in energy conservation and thenatural refrigerant-CO₂is superior to other conventional refrigerants except that theperformance is poorer. This paper is mainly aimed at improving the performance ofthe CO₂transcritical cycle and enhancing its competitiveness with conventionalrefrigeration and heat pump systems.The author has analysed the status of research at home and abroad. In this paper,the impact of the additive and the enhanced tube gas cooler on cycle performance isresearched by theoretical and experimental methods.The additive mixed into CO₂would influence the phase changing in two ways.First the additive can disturb the expansion process, boost the vapor nucleation siteand reduce the delay time. Second it can enlarge the distance among CO₂moleculesand decrease therir molecule forces. Theoretically, it betters the performance of theexpander. But as for the heat pump system and other components, it is unclear howthe additive act on them. Experiments have been done respectively on the CO₂transcritical cycle heat pump system with a throttling valve and with an expanderunder the environmental condition. The working fluid has been mixed with differentmass fraction of nitrogen. It is found that the nitrogen has negtive effect on the heattransfer performance of gas cooler and evaporator. Besides, the nitrogen will increasethe work consumption in the compressor. So it is improper to mix the CO₂withnitrogen in a throttling valve system. As for the expander system, it is sure that thenitrogen will increase the output work in the expander, but its influence on othercomponents made it quite undesirable.The heat tansfer in gas coolers is relevant to the performance of CO₂transcriticalcycle and the water outlet temperature is an important indicator to the heat pumpsystem. Firstly, the heat transfer performance of the enhanced tube gas cooler is testedby changing the mass flux and the inlet temperature of cooling water, as well as thehigh pressure. The test results show that proper working condition is needed topromote the performance and water outlet temperature of the heat pump system.Second, the enhanced gas cooler was simulated with three selected heattransfer corelations by means of distributed parameters and adding enhancing factorson smooth tubes. The simulation parameters are set as: the mass flux of cooling waterat0.3t/h and the inlet temperature at18℃. Different mass flux of refrigerants andhigh pressure are simulated and simulation results indicate about6%average errorcompared to the experimental data. At last the gas cooler is optimized under theselected working condition, where the high pressure is8.5Mpa, the mass flux andinlet temperature of cooling water is0.3t/h and18℃, and the mass flux of refrigerantis240kg/h. It is found that the outlet temperature will be up to60℃when the tubelenth is longger than11m.