Coupled Computational Fluid Dynamics (CFD) and Finite Element (FE) Thermal Stress Analysis of Thermal Fatigue Phenomena within a T-junction

Abstract

The assessment of thermal fatigue phenomena in T-junctions, within the primary and secondary thermal hydraulic circuits of nuclear power plants (NPPs), remains a significant computational challenge. To accurately model the complex physics of fluid thermal mixing, conjugate heat transfer (CHT) and thermal fatigue, it is necessary to use high-fidelity, one-way coupled, finite volume (FV) computational fluid dynamics (CFD) and finite element (FE) thermal stress analysis, modelling and simulation (M&S) methods. In this paper, we compare different M&S approaches to performing one-way coupled CFD, CHT and stress analyses of the thermal stress within the pipe wall of a T-junction for a temperature transient test case with fixed mass flows. These high-fidelity multiphysics models are a necessary first-step in developing mechanistic approaches for analysing thermal fatigue due to cyclical thermal stresses induced by the transient thermo-fluid behaviour within T-junctions. In the first approach, a one-way coupled, CFD, CHT, and thermal stress analysis is performed using only the commercial off-the-shelf (COTS) CFD code STARCCM+. We compare the results with a second approach that uses a one-way coupled sequential STARCCM+ CFD simulation and Abaqus FE thermal stress simulations via the SIMULIA Co-Simulation Engine. The comparison between these two approaches shows small differences for the predictions of von Mises stresses, normal stresses, and displacements. Values predicted by STAR-CCM+ around the pipes’ junction, where strong thermal mixing occurs within the fluid, are in very good agreement with the values predicted by Abaqus and within few percentage points of one another. These differences increase at locations corresponding to the system constraints such as the flange locations.