Numerical investigation of refrigerant flashing flow in electronic expansion valves

Understanding more about complex two-phase flow dynamics is key to optimizing the design of the electronic expansion valve (EEV) and reducing refrigerant flow-induced noise. An extended two-fluid model (E-TFM) is first developed to simulate liquid–vapor bubbly and droplet co-existing flows during refrigerant passes through an EEV. The Lee model is employed to evaluate the vapor generation rate during the flashing process. The R32 flashing process inside an EEV has been successfully simulated using the developed five-equation extended two-fluid model. Furthermore, the refrigerant mass flow rate is reasonably predicted using this model. It is found that violent vaporization of liquid R32 begins immediately after the valve throat owing to abrupt depressurization, this evaporation continues up to the valve exit. The flashing process absorbs liquid refrigerant sensible heat and reduces the mixture temperature. The evaporation coefficient determines the intensity of the flashing process, whereas the flashing capacity of an EEV is bounded. The refrigerant mass flux increases with increases in the inlet–outlet pressure difference or with the EEV pin opening. Both ‘along-the-wall flow’ and ‘liquid-core jet flow’ immediately after the EEV throat are numerically predicted. The along-the-wall flow occurs only when the concentric cylindrical annulus is long enough for the pin to push the liquid R32 to the vertical wall of the bellmouth. It is concluded from simulations that the developed flashing CFD model is capable of simulating flashing two-phase flow in an EEV and/or other throttling organs and nozzles.