【作者】 张仙平；

【导师】 沈恒根； 范晓伟；

【作者基本信息】 东华大学 ， 供热、供燃气、通风及空调工程， 2011， 博士

【摘要】 HCFC22是目前国内热泵系统中最常用的工质,但HCFC22对环境有危害(ODP=0.05,GWP=1810)。随着环保要求的提高,开始采用替代工质HFC134a、HFC410A和HFC407C等。这些替代工质的ODP为零,但GWP还比较高,国内外在不断探寻更环保的替代工质。自然工质R744(C02)因此重新得到启用,发达国家已进入实用阶段,我国尚未实现商品化。当前使用的R744热泵存在着一些问题,如放热侧压力较高。本课题以R744为基本组分,混入适量其他工质,可望降低系统放热侧压力,并改善热泵循环性能。根据文献调研,目前对R744混合工质的研究主要集中在(自)复叠式低温制冷和空调系统,R744混合工质用于热泵系统的研究还处在起步阶段,针对这类热泵系统更加深入的理论与试验研究亟待开展。在课题组对R744单一工质热泵系统研究的基础上,围绕R744混合工质第二组分工质的优选与R744混合工质在热泵工况下的系统性能,本课题主要做了以下研究：(1)针对特定供热温度的热泵系统,根据环境性能、安全性能、降低运行压力性能、温度滑移性能等方面的优劣,对适合与R744混合的第二组分工质进行筛选,得出备选组分工质及适合采用的循环方式。基于传热窄点温差,建立混合工质热泵循环的计算模型,对不同R744混合工质亚临界、跨临界循环的工作特性进行计算和分析,综合考虑制热COPh、最优压力、容积制热量、排气温度、易燃易爆性等因素,得出R744和R290为最优的制冷工质组合。(2)通过对不同配比R744/R290自然混合工质的环境性能、安全性能、温度滑移性能、热力学性能及热泵循环性能的分析,得到合适的质量配比范围为80/20～100/0,并分析过热度和回热器的效果以使热泵系统具有较高的COPh。(3)设计并建立跨临界循环热泵试验装置,将R744和R290按不同质量配比混合后进行试验研究,探索循环性能随组分工质配比、制冷工质充注量的变化规律,得出不同配比的最佳充注量。最终得到R744/R290混合工质的最优质量配比为95/5,在此最优配比条件下,最优放热侧压力降低,系统COPh提高。试验结果验证了理论分析选择R744/R290自然混合工质的可行性。所得到的试验结果,为R744/R290热泵装置进一步优化和完善提供有益的参考。(4)针对优选配比95/5的R744/R290混合工质,分别采用理论模拟和试验研究的方法对最优压力的影响因子进行研究。首先建立基本循环热力学模型,分析影响R744/R290混合工质最优放热侧压力的因素,并对模拟结果进行回归分析,得到最优压力和显著影响因子之间的函数关系式,然后通过试验研究验证了气冷器出口制冷工质温度是最优压力的最显著影响因子。更多还原

【Abstract】 Nowadays, HCFC22 is the mainly used refrigerant in China for heat pump systems, despite the fact it is harmful to the environment (ODP=0.05, GWP=1810). With the stricter environmental protection requirements imposed, alternatives, such as HFC 134a, HFC410A and HFC407C, have been gradually taking its place. These alternatives have the zero ODP; however, they still belong to high GWP gases, which leads to the never-ending researches at home and abroad for more environmentally friendly replacements. Therefore, the natural working fluid R744 (or CO2) has regained a new life in heat pump applications, and in fact it has already been commercialized in some developed countries, but still in research and development phase in China’s mainland. The reason is that there are still some problems with the currently used R744 heat pump, such as the high heat rejection pressure. In this research, in order to decrease the heat rejection pressure, and increase the system efficiency as well, R744 as the basic component is mixed with other working fluid in an appropriate amount.According to the literature on R744 mixtures, the current studies have been mainly aimed at the cascade or autocascade refrigeration system for low temperature and the cooling performance for air conditioning system. The investigations on the heating perfonnance of heat pump using R744 mixtures as refrigerants, especially for experimental studies, are still scarce and at the initial stage in the published literature. More theoretical and experimental researches on R744 mixtures should be carried out. Based on the researches carried out by our team on the heat pump system using single component R744 as a refrigerant, this project focuses on a second component working fluid selection, heat pump system characteristics using R744 mixtures. And this thesis focuses on the following elements:(1) For heat pump with specific temperature for heating, the second component fluid which is suitable to mix with R744 is selected according to the fluids’ environmental performance, safety performance, reduced heat rejection pressure and temperature glide properties. The suitable component candidates and the suitable cycles are given. Based on the pinch point of heat transfer, the numerical models of heat pump cycle using R744-based mixture are developed. For different R744 mixture candidates, the operation characteristics in subcritical cycle and transcritical cycle are calculated and discussed. Based on a comprehensive consideration of COPh, optimum heat rejection pressure, volumetric heating capacity, discharge temperature, flammability and explosive property, the binary mixture R744/R290 is determined as the best working fluid for specific heat pump application.(2) For R744/R290 with different mass fraction, it discusses respectively how the R744 varies with the mass fraction of R744 based mixture’s performance, safety performance, temperature glide property, physico-chemical properties and heat pump cycle performance. And the optimum mass fraction range of 80/20～100/0 for R744/R290 is obtained, and the influences of superheating degree and internal heat exchanger are analyzed to achieve a high COPh.(3) The experimental testrig is designed and set up for the transcritical cycle for heat pump system. The experimental researches with different R744 mass fraction are carried out, which conduct a study on the variations of heat pump performance, component’s mass fraction and working fluid charge. The optimum working fluid charges for different mass fraction are given, and the optimum mass fraction of R744/R290 is found to be 95/5, with which the optimum heat rejection pressure is decreased while COPh of the system is increased. The experimental results validated the R744/R290 natural mixture proposed in theory. The experimental results provide useful reference on the optimization and improvement of R744/R290 heat pump testrig.(4) For R744/R290 with optimum mass fraction of 95/5, the influences of operation parameters on the optimum heat rejection pressure are discussed by the theoretical simulation and experimental research. The thermodynamic model for the basic cycle is proposed to study the property which will influence on the optimum heat rejection pressure. A regression analysis to the simulation results is made and then the correlation of the optimal heat rejection pressures in terms of main influencing parameters is developed. Then through experimental researches, the working fluid temperature at the exit of gas cooler is validated to be the main impact factor for the optimum heat rejection pressure.更多还原