The general performance characteristics of an n-stage combined refrigeration system affected by multi-irreversibilities

A universal irreversible combined refrigeration cycle model, which is of general significance for both physics and engineering, is established and used to investigate the optimal performance of an n-stage combined refrigeration system affected by the irreversibility of heat transfer across finite-temperature differences, the heat leak loss between the external heat reservoirs, and the internal dissipation of the working fluid. The coefficient of performance is taken as an objective function for optimization. Some reasonable equivalent parameters are introduced so that the related calculations are simplified. A fundamental optimum relation is derived. The general performance characteristic curves of the system are obtained. The maximum coefficient of performance and the corresponding parameters are calculated. The optimal combined conditions between two adjacent cycles in the system, i.e. the optimal distribution of the heat-transfer areas of the heat exchangers in the system and the optimal ratios of the temperatures of the working fluids in the heat-transfer processes, are given. Several special cases are discussed, so that the main conclusions in the relative literature are included in this paper. The results obtained here are quite general and useful. Consequently, the optimal performance of an arbitrary-stage irreversible, endoreversible, or reversible combined refrigeration system can be directly derived for specific choices of some performance parameters.

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