High-pressure-drop two-phase critical flow with a non-homogeneous equilibrium model

Two-phase critical flow, where mass flow rate depends solely on upstream conditions, is a common phenomenon in pipeline systems of nuclear power plant, such as nozzles and throttle valves. In the automatic depressurization system, pressure variations in two-phase critical flow can reach 5–8 MPa, significantly affecting the enthalpy–temperature relationship. This renders the conventional energy equation, which simplifies the relationship between enthalpy and temperature, inadequate for addressing the thermodynamic nonequilibrium. To simulate high-pressure-drop two-phase critical flow with instantaneous phase change, maintaining fluid temperature near the saturation is essential to prevent numerical issues from temperature deviations. This paper develops a simulation method based on the non-homogeneous equilibrium model. Given that the energy exchange between phases is rapid enough to satisfy local thermal equilibrium, the fluid temperature depends on the pressure and gas volume fraction. In this method, a temperature-sensitive Lee model is utilized to adjust the phase distribution deviating from temperature limit, thereby ensuring reasonable accuracy of the model. Using this method, a three-dimensional numerical investigation of two-phase critical flow through the AP600 orifice plate is conducted. The results indicate that the proposed method effectively captures the critical flow characteristics, including rapid depressurization caused by orifice throttling and pressure recovery caused by shock wave. Under design conditions, the pressure drop can reach up to 8 MPa, while the pressure recovery can reach up to 2.5 MPa. This study provides an effective approach for the numerical simulation and hydraulic analysis of high-pressure-drop two-phase critical flow.