Mass flowrate characteristics of supercritical CO2 flowing through an electronic expansion valve

• Both chocked flow and non-chocked flow are possible when CO2 flowing through EEV.
• The conventional method is effective in judging the choked flow for supercritical CO2.
• An empirical correlation is presented and has been validated with experimental data.

Electronic expansion valves (EEV) are employed increasingly in transcritical CO2 systems. In order to investigate the mass flowrate characteristics of supercritical CO2 flowing through an EEV, the effects of EEV outlet pressure on the mass flowrate of CO2 is studied under constant EEV inlet pressure and inlet temperature. The two-phase flow patterns of supercritical CO2 flowing through an EEV are revealed, and the conventional choked flow judgment formulas is proved effective for supercritical CO2. Based on Bernoulli Equation with expansion factor, the mass flowrate correlation of supercritical CO2 flowing through an EEV is presented. The influence of the expansion coefficient on the mass flowrate is considered in the correlation in which the correction coefficient is only affected by the EEV opening, the changing rate of the density and the pressure difference in the entrance region. The mass flowrate predicted by the correlation in this paper shows a good agreement with experimental data. The maximum relative errors are within 10%. The average and standard deviations are 0.76% and 5.9%, respectively.