Numerical Insights of the Two-phase Cross-flow Inside an In-line 5×5 Tube Bundle Configuration
In collaboration with EDF R&D UK
Constantinos Katsamis, Bruce Kakimpa, Ehimen Iyamabo
Steam Generators of Pressure Water Reactor (PWRs) are subjected to complex multi-phase flow phenomena which can cause flow-induced vibrations (FIV) leading to fretting wear and high-cycle fatigue, affecting the structural integrity of the components. Improving current understanding in numerical modelling of FIV flows is important when predicting component failure and developing maintenance strategies in nuclear power plants. Therefore, the present investigation focuses on the Computational Fluid Dynamics (CFD) modelling of an air–water flow mixture with high void fraction inside a square 5x5 tube bundle configuration that includes a single moving cylinder. The work forms part of the contribution to the EU-Horizon project, Go-Viking (Gathering expertise On Vibration ImpaKt In Nuclear power Generation) which targeted enhancing current knowledge and improving predictive methodologies of Flow-Induced Vibration phenomena. The transient CFD 3D simulations focus on representing the flow physics captured in TREFLE (Two-phase flow REgimes and FLuid–structure interaction Experimental facility) in Cadarache (France). The characteristics of the two-phase flow are those of T5 tests with void fraction, α ≈ 0.8, reported from the wire mesh sensor and fast speed camera of the experimental campaigns. The Volume-of-Fluid (VoF) method has been adopted to represent the interface between water and air, modelling the intermittent two-phase flow regime and the two-equation k-ω SST has been used for closing the Reynolds-Averaged Navier-Stokes equations. The investigations include tests on different approaches for representing the inlet experimental conditions of the air bubbles generated and three different methods for the fluid induced vibrations modelling. These include rigid body motion dynamics (spring-mass-damper approach), a sequential-coupling in which the force time series is the input to the Finite Element solver and a two-way coupling for modelling the central vibrating tube. Validation is performed against experimental data which includes void fraction profiles, bubble sizes, power spectra of forces acting on the cylinder and tube acceleration. The CFD predictions of the two-phase flow have shown that bubbles generated at inlet form large structures which are convected across the domain, coalesce and break-up when interacting with the surface of the cylinders inside the tube bundle section. All three different FSI approaches showed acceptable agreement with experimental power spectral data, however the two-way FSI coupling returned the most accurate prediction of the tube dynamic response.