【作者】 刘圣春；

【导师】 马一太；

【作者基本信息】 天津大学 ， 工程热物理， 2006， 博士

【摘要】 随着国际上对自然工质CO2跨临界循环研究的深入,扩展CO2跨临界制冷循环系统的应用领域,提高系统的循环效率,加快其向实用化迈进是当前研究的关键问题。本文通过理论分析,计算机模拟和实验研究相结合,重点对超临界CO2特性以及跨临界循环系统开展了研究。论文结合当量温度法,针对空调系统的运行工况,分析了采用内部热交换器、提高压缩机效率、利用膨胀机代替节流阀、双级压缩以及优化循环方式等措施对系统效率的提高幅度。通过对CO2跨临界双级压缩循环的性能分析,得出了CO2跨临界双级压缩循环的最佳高压压力和最佳中间压力计算关联式,为系统的设计运行提供理论支持。论文从分子间力和分子热运动的角度分析了液体表面张力的成因,分析了CO2亚稳态的特性以及气泡成核机理,分析了超临界流体相变机理,计算了亚稳相变中气泡形成所消耗的表面功,得到了该不可逆损失占理论膨胀功的比例。论文还针对CO2跨临界循环系统采用的润滑油,分析了合成润滑油的相关性能,对比分析得到POE和PAG最适合应用于CO2跨临界循环系统。在压缩机和膨胀机中,泄漏损失是影响效率的主要原因之一。论文总结了各种型式的泄漏模型,重点分析了CO2超临界流体在平板模型中的泄漏,在相同入口压力下,得到了CO2的密度变化最大点的温度与泄漏量达到最大的温度的一致性,分析了润滑油和泄漏通道间隙对泄漏量的影响,得到了润滑油的含量对泄漏的影响较小,而泄漏通道的间隙对泄漏的影响最大。根据分析建立了超临界CO2泄漏实验系统,验证了对泄漏模型的理论分析。根据实验结果,得出了超临界CO2理论泄漏模型,对于设计CO2跨临界循环压缩机和膨胀机具有一定的指导意义。论文针对家用空调使用的管翅式换热器,建立了CO2气体冷却器和蒸发器模型,模拟了CO2气体冷却器和蒸发器管内换热及压降性能随各参数的变化情况。建立了CO2跨临界空气-空气循环系统实验台,并对该系统进行了性能测试。不同工况下的实验研究表明,所采用的换热器的换热性能良好,系统匹配性达到设计要求,在现有的实验条件下,系统COP达到3.0,为CO2跨临界空调系统的实用化提供了实验数据。更多还原

【Abstract】 As more and more international research has been doing on natural working fluid CO2 transcritical cycle, scope enlargement of the CO2 transcritical refrigeration cycle’s application, improvement of cycle performance and the promotion of its practicality all become the key problems of current concerning research. Through theoretical analysis in the thesis, combined with computer simulation and experimental research, research has been done with focus on characteristic of supercritical CO2 and transcritical cycle system.The equivalent temperature method is given in this paper and referring to the working condition of air conditioning system, improvement on system efficiency by employing heat regenerator, improving compressor efficiency, replacing throttling valve with expander ,two stage compression and optimizing cycle types are analyzed in the paper. From the performance analysis on CO2 transcritical two stage compression cycle, the optimum high pressure and the optimum media pressure can be obtained by calculating correlations for providing theoretical support for designed operation of the system.From the respect of molecular forces and molecular heat motion, the cause of formation of liquid surface tension, CO2 metastable characteristics, the bubble nucleation mechanism and phase change mechanism of supercritical fluid are analyzed respectively, the power for metastable phase changer and the percentage of this power in theoretical expansion power are calculated in the paper. By analyzing concerning properties of synthetic lubrication oil employed in CO2 transcritical cycle system, it is concluded that POE and PAG are the best ones for CO2 transcritical cycle system.In compressor and expander, leakage loss is the main factor of influencing their efficiencies. In this paper, various types of leakage model are generalized with focus on leakage of CO2 supercritical fluid in plate model. At the same inlet pressure, the temperature corresponding to the greatest change point of CO2 density accords with the temperature at which leakage mass comes to the greatest value. And by analyzing the influence of lubrication oil and leakage clearance onleakage, it is obtained that the content of lubrication oil has little influence on leakage, while the leakage clearance exert the greatest influence on leakage. By building up the leakage test rig of supercritical CO2, theoretical analysis of leakage model is verified, according to the experimental result, it is gained that the current theoretical models of supercritical fluid and supercritical gas, which has certain guidance for designing high efficiency compressor and expander in CO2 transcritical cycle.Based on the tube fine type heat exchangers in household air conditioners, models of both gas cooler and evaporator are established, and it is simulated that the heat transfer and pressure drop in both heat exchangers under various parameters. The experimental rig of CO2 transcritical air to air cycle system has been set up for performance testing of air conditioning system. Experimental research under various working conditions illustrate that the heat exchangers employed in the system have good performance and system matching meets the need of design requirement. Under current experimental condition, COP of the system can reach 3.0, which offers experimental data for the application of CO2 transcritical air conditioning system.更多还原