Conference Year

2016

Keywords

Zeotropic mixtures, Gas-gas ejector, Autocascade refrigeration cycle, Performance enhancement

Abstract

A novel gas-gas ejector enhanced autocascade refrigeration cycle is proposed in this paper. The novel cycle uses an additional gas-gas ejector and a phase separator to accomplish a secondary composition separation for the zeotropic mixture used in the cycle. This proposed cycle can lift the evaporating pressure and further improve the cycle performance. The performance comparisons between the novel cycle and a basic autocascade refrigeration cycle using the zeotropic mixtures of R23/R134a are conducted by theoretical method. In the evaporator inlet temperature range of -60°C to -40°C, the coefficient of performance (COP) and volumetric cooling capacity of the novel cycle can be improved by up to an average of 25.9% and 32.7% compared to that of the basic cycle, respectively. The effect of some key parameters on the performance of the novel cycle is further investigated. As the intermediate pressure of the novel cycle decreases in a certain range, the entrainment ratio and pressure lift ratio of the ejector increase and it further leads to the performance improvement of the novel cycle. In a given vapor quality range, the COP of the basic cycle show a monotonic increase tendency with the vapor quality at the condenser outlet, but the novel cycle has an optimal vapor quality to obtain the maximum COP. The simulation results shows that the novel cycle yields an improvement of 40.1-21.4% in the COP over the basic cycle as the vapor quality varies from 0.3 to 0.5. Besides, there exist optimal mixture compositions for the two cycles to obtain the optimum performance. The maximum COP of the two cycles can be obtained under the considered condition when the R134a mass fraction of the zeotropic mixture R23/R134a at the compressor inlet is fixed at about 0.6.

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