Numerical Modelling of Two-Phase Critical Flow in a Steam Generator Guillotine Tube Rupture Scenario

Abstract

Critical flow is a phenomenon that occurs when the mass flow from an opening reaches a choked state, and the downstream pressure does not affect the flow rate. This phenomenon is well understood in fluids that do not undergo a phase change during rapid depressurization. It is crucial to understand critical flow for the safe operation of nuclear power plants (NPP) and validated safety analysis modelling. Precise measurement and modelling of two-phase critical flow (TPCF) at the break location are essential for designing reliable emergency core cooling systems (ECCS) and containment safety systems. It is also necessary for advanced reactor designs that rely on passive ECCS. In this work, simulations of TPCF are carried out using the Apros system code and the ANSYS FLUENT CFD code. This paper compares numerical results gathered pre-commission of the CRAFTY facility used for TPCF experiments at Lappeenranta-Lahti University of Technology LUT University. This facility studies critical flow discharges during a primary-to-secondary leak scenario caused by a guillotine break in a steam generator tube. The Apros simulations are based on a 6-equation model. The CFD simulations utilize the Eulerian-Eulerian two-fluid approach. Two different discharge tube inner diameters, D=13 mm and 17 mm, are simulated. Several L/D ratios are studied. The results reveal the critical pressure ratio at which two-phase critical flow conditions occur in tubes with varying L/D ratios. The study offers valuable insights into the impact of L/D ratios on critical mass flow, void fractions, pressures, and flow velocities.