Experimental study on the CO₂ flow characteristics through electronic expansion valves in heat pump

In this study, the influence of key parameters on the EEV mass flow rate was investigated experimentally, including the inlet temperature, inlet pressure and EEV opening. The parametric tests show that a relatively linear variation of CO₂ flow could be maintained with the decrease of inlet temperature at higher pressure and the increase of inlet pressure at lower temperature. However, due to the large density changes above the critical point, significant changes in the CO₂ flow characteristics were observed with the variations of inlet temperature at lower inlet pressure and the variations of inlet pressure at higher inlet temperature. The important role of CO₂ density can be further confirmed by an empirical correlation which incorporates the effects of density, pressure difference and EEV opening, and the maximum deviation of mass flow predictions is below 15.3%.