2019 Volume 27 Issue 4 Pages 77-85
To improve the performance of the two-phase flow nozzle is very important for the development of the ejector refrigeration system. The velocity of refrigerant in the small nozzle has not yet been measured at near the critical point of the CO2. The purpose of this study is to visualize the CO2 refrigerant flowing in a convergent-divergent nozzle with short divergent length using the shadowgraph method, and to verify whether the droplet velocity can be calculated from the captured video. In this study, three types of convergent-divergent nozzles with the same cross-sectional area at the inlet, throat and outlet and different divergent lengths and angles were tested. The measurements of the flow rate and the droplet velocity of CO2 refrigerant in the nozzle were carried out at the subcooled liquid or supercritical inlet conditions. For the case of the nozzle inlet temperature of 20 ℃, the results of flow rate measurement showed that the measured flow rate increased by about 5% for all inlet pressure compared with the theoretical flow rate, unlike the other inlet temperature conditions. It is mainly because the influence of boiling delay according to thermodynamic non-equilibrium phenomena is larger than the other inlet temperature cases. Also, the results of the droplet velocity measurement elucidated that the droplets in the nozzle were scarcely accelerated in the path from the throat to the outlet. The CO2 refrigerant flow in the nozzle is expected to be at the homogeneous equilibrium condition because the decompression boiling can start easily near at the critical point. However, the experimental results at the inlet temperature of 20 ℃ showed that the refrigerant flow in small nozzle deviated from the homogeneous equilibrium flow because the measured droplet velocity was not accelerated more than the theoretical equilibrium velocity of homogeneous equilibrium flow model.
