Two-phase expansion waves at nozzle outlet using CO2

Abstract

CO2 has been extensively researched as a refrigerant for refrigeration and air conditioning systems because of its environment-friendly, nontoxic and nonflammable characteristics. In order to increase the COP in using CO2, a new refrigeration system that uses the two-phase flow ejector to recover the expansion energy at the expansion valve is needed. Understanding the characteristics of both waves in two-phase flow is very important in designing the ejector. The purpose of the present study is to elucidate the experimentally determined characteristics of expansion waves of CO2 at the outlet of the two-phase flow nozzle. High-pressure blowdown experiment of CO2 had been carried out. The measured decompression curves of the expansion waves were varied by changing the inlet temperature and pressure and the back pressure of the nozzle. The experimental decompression profiles after the nozzle were more rapidly decreasing than those predicted using Isentropic Homogenous Equilibrium. As the inlet temperature became low, the pressure profiles inside the nozzle and of expansion waves after the nozzle decreased far from the predicted profiles. Almost flat pressure profiles, which were increasing in length with decreasing inlet temperature, also appeared right after the nozzle. Expansion waves could reach the opposite wall after those flat profiles because the flow was supersonic. Considerably, the effective length of expansion region after the nozzle was also becoming longer with decreasing inlet temperature. The lengthening of the expansion region of CO2 for low inlet temperature was the same as that observed in the experimental pressure profiles of steam expansion waves.