CO2, ejector, two-phase flow, subcritical
Carbon dioxide is high pressure fluid. Therefore difference between condensation and evaporation pressures is very high which involves large throttling loss in expansion device. This aspect causes low efficiency of these systems in comparison with conventional compression systems operating with most of synthetic or natural refrigerants. Therefore, improving of carbon dioxide cycle efficiency is one of the most important issue for refrigeration and heat pump engineering. Â Most of investigations presented in literature were focused on the analysis of the whole system including ejector of assumed geometry applied. This makes unfavourable conditions for the estimation of the possibilities of the cycle efficiency improvement since they also depend on the performance of the ejector applied in the system. The last one is undoubtedly complicated and separate issue since there is lack of theoretical model of two-phase ejector that was proved experimentally and could be applied in such analysis. Authors of the paper proposed own approach based on the pure thermodynamic analysis of the compression-ejection cycle. In this method it is possible to analyse the operating parameters of the ejector that are required by the refrigeration system in order to improve system efficiency. The analysis refers to determine the requirements of the refrigeration system for the ejector operation. The analysis presented in the paper is focused on the cases which have not been discussed in previous studies, i.e. application of two-phase ejector for subcritical cycles and application of two-phase ejector as a second step compressor in wide range of operating parameters covering both subcritical and supercritical cycles. Â The results shows that the application of the two-phase ejector in subcritical/supercritical low stage of the cascade refrigeration system can be considered as efficient way for COP improvement. The testing stand is presented in the paper. The exemplary experimental results for two-phase ejector were presented. The simple theoretical model of two-phase ejector along with the experimental correlations describing the velocity coefficients for the suction chamber, motive nozzle, mixing chamber, and diffuser were presented.