Gas-Liquid Flow of R290 in the Integrated Electronic Expansion Valve and Vapor Injection Loop for Heat Pump

Vapor injection (VPI) can significantly enhance the heating performance of electric vehicle (EV) heat pump systems under low ambient temperatures, making the integrated design and control of the VPI loop essential. This study uses R290 as the working fluid and investigates the gas–liquid flow characteristics of the vapor-injection electronic expansion valve (VPI-EXV) in the VPI loop. The evaporation coefficient in the Lee model is calibrated using four typical operating conditions, keeping the relative errors of both total mass flow rate and injection ratio predictions within 10%. Results show that valve opening is the dominant factor: as the opening increases from 10% to 100%, the injection ratio rises from 0.24 to 0.83, while increasing outlet pressure from 0.58 MPa to 0.78 MPa and inlet subcooling from 0 °C to 10 °C reduces it by about 18% and 9%, respectively. The 90° turning structure inside the VPI-EXV induces recirculation and high turbulent kinetic energy downstream of the throttling region, modifying the outlet gas-liquid distribution, based on which an injection ratio control strategy with valve opening as the primary variable is proposed.