Experimental study on the primary flow expansion characteristics in transcritical CO_2 two-phase ejectors with different primary nozzle diverging angles

摘要

This paper presented an experimental study to explore the primary flow expansion characteristics in the transcritical CO2 two-phase ejectors for various primary nozzle diverging angles (NDAs) and secondary flow pressure. The phase change of the primary flow was visualized, and the effect of the primary flow expansion state on the entrainment ratio was evaluated. The results disclosed that the primary flow changed from the under-expanded state to the over-expanded state with increasing the NDA, and the primary flow expansion state was insensitive to the secondary flow pressure. The visualization images showed that the phase change position moved towards the nozzle exit visibly when the NDA was 0.0 degrees, and the effect of secondary flow pressure on the phase change position was insignificant. Besides, the results revealed that the over-expanded state of primary flow had a negative effect on the entrainment ratio, and the entrainment ratio obtained the maximum when the NDA was 2.0 degrees. The present work is indispensable to realize a comprehensive understanding of the primary flow expansion mechanism inside the transcritical CO2 ejector, and the insights gained from this study may be of assistance to optimize the ejector structure and improve the ejector performance. (C) 2019 Elsevier Ltd. All rights reserved.