Experimental assessment of two-phase nozzle performance for low-GWP refrigerants

Abstract

The performance of two-phase ejectors critically depends on the efficiency of motive nozzles, which govern the critical mass flow rate and overall system operation. However, accurately predicting nozzle performance under two-phase flow conditions remains challenging due to the complex interplay of thermodynamic and flow dynamics. This study addresses this issue by evaluating the performance coefficient of motive nozzles for three refrigerants: CO₂, R600a, and R1234ze(E). Beyond ejector applications, understanding two-phase flow dynamics is essential for optimizing other key components, such as control valves and safety valves, which operate under similar conditions. Experiments conducted on a versatile test bench revealed significant differences in nozzle performance among the refrigerants. The performance coefficient ranged from 0.85 to 1.35 for CO₂, 0.90 to 1.15 for R600a, and 0.92 to 1.22 for R1234ze(E). The Henry-Fauske model, used to predict critical mass flow, demonstrated an average deviation of 30 % for CO₂, while deviations were much lower for R600a (9 %) and R1234ze(E) (4 %). The results highlight the sensitivity of the performance coefficient to the refrigerant thermodynamic properties, with CO₂ exhibiting the most complex flow behavior due to its lower critical temperature and higher compressibility. This study provides quantitative insights into the performance of motive nozzles under two-phase flow and validates the applicability of simplified models for predicting critical flow rates. The findings contribute to optimizing ejector and valve design, emphasizing the need for further validation with additional refrigerants to enhance model universality.