| Low temperature environment of -40 C ?120 ?C has been used in a wide range of fields such as energy, military project, space technology, medicine, biology, life science , and so on. When refrigeration in this temperature range is realized by using single stage refrigerator with mixed refrigerant, not only the structure of the refrigerator is simplified, thus the operation reliability of the refrigerator is increased, but also the refrigerator can operate with a higher thermodynamic efficiency than that of conventional ones. Up to now, research on the refrigerator in this temperature range has still been very little. The aim of this study is to compare the thermodynamic characteristics of different J-T refrigerator cycles with mixed refrigerant in - 40 "C ~ -120"C temperature range and to find out the refrigerator type with high thermodynamic efficiency and high reliability, as well as the corresponding optimum composition of the refrigerant mixture.An optimizing model of J-T refrigerator cycle with mixed refrigerant is proposed in this paper. Discussions are made on the objective function, constrains and designing variables in the optimizing model, based on three practical and feasible optimizing principles suggested by this thesis. Optimization for different refrigeration cycles has been conducted.The analyses of the characteristics of different types of refrigeration cycles using mixed refrigerant revealed the underlying thermodynamic relationship between them. The conclusion is that if an appropriate refrigerant mixture is used, the LHR cycle can have a relatively higher thermodynamic efficiency than that of basic cycle, in case their pressure ratios are the same. The thermodynamic efficiency can be further improvedwhen the ACR cycle is used.An optimizing method with constant pressure ratio is initially applied to the mixed refrigerant cycle. This method greatly simplifies the calculation of the cycle, and is helpful in numerical optimization of various cycles, especially in the complex cycles. In addition, this method helps shed light on the influence of the pressure variable on the features of cycles as well as the nature of various cycles.Based on a steady heat transfer model with constant specific-heat fluids, a thermodynamic optimizing analysis, driven by two heat transfer forces, is conducted on the important parts of the cycles ?the heat exchangers. The result of the analysis has significant implications for the analysis of real heat exchange processes of heat exchangers in the refrigerator.The numerical optimization and the detailed exergy analysis of the cycles approved the conclusions rendered in the analysis of the cycle's features. We also obtained the optimum compositions of the mixture refrigerant for the rectification cycle under the pressure ratio of 8, at the temperature levels of -60癈, -80癈, -100*C, and -120癈 respectively.The method of equivalent entropy generation is initially used in this study. Two new notions, equivalent entropy generation per heat load and equivalent heat load per power consumption, are proposed and used, which provides a powerful tool for a thorough analysis of the irreversible loss in an heat exchanger.An experimental apparatus is specially designed and amounted for the purpose of comparing the characteristics of cycles of LHR, ACR_I, ACR_R. At the temperature level of -60 癈, verious experiments are conducted on the cycles of LHR, ACR_I, ACR_R, with two component mixed refrigerants. The results are consistent with that of the numerical optimization.This study puts emphasis on the optimization of composition in the experiments at the temperature level of -100癈. At this level, the COP of the cycle reaches 7.4%. The thermodynamic perfect degree of the cycle is 5.3%. When the same composition is used, at temperature levels of -120癈, -80 癈 and -60 ?C, the thermodynamic perfect degree of the cycle are 2.8%, 6.6% and 6.5% respectively.
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