A numerical contrast on the adjustable and fixed transcritical CO2 ejector using exergy flux distribution analysis

Abstract

An adjustable ejector with a needle is deemed as an effective device to improve the performance of the transcritical CO2 refrigeration cycle with a wide operating conditions. In this paper, a homogeneous CFD model is built up to compare the performance of the adjustable and fixed ejectors with the same key structural sizes. By validating the numerical results with the experimental data, the k-ε RNG and k-ε Realizable are considered to predict the performance of the CO2 ejector well. Furthermore, an exergy analysis model is proposed to obtain the exergy fluxes of the primary and secondary flows are obtained, respectively. The results discover that the main exergy transfer (interaction between the primary and secondary flows) locates in the suction chamber. Meanwhile, the higher exergy loss is observed in the suction chamber of the adjustable ejector due to flow separation along the needle and higher viscous dissipation of oblique shock wave, which results in 5%–11% lower entrainment ratio than that of the fixed ejector. However, the higher velocity in the fixed ejector results in higher friction of the wall and more intense flow separation in the diffuser which causes higher exergy loss in the mixer and diffuser. Accordingly, the exergy efficiency of the adjustable ejector is only 0.5% lower than that of the fixed ejector. In spite of the special case in this paper, the adjustable ejector is considered to be able to achieve a similar exergy efficiency with the fixed ejector.