A coupled CFD-FEM strategy to predict thermal fatigue in mixing tees of nuclear reactors

Fluctuating stresses imposed on a piping system are potential causes of thermal fatigue failures in energy cooling systems of nuclear power plants [4]. These stresses are generated due to temperature fluctuations in regions where cold and hot flows are intensively mixed together. A typical situation for such mixing appears in turbulent flow through a T-junction. In the present work, turbulent mixing in a T-junction is investigated by a coupled approach of fluid-flow calculations and mechanical calculations. The flow characteristics and the temperatures in the pipe wall are obtained using Large-Eddy Simulations (LES). The corresponding structural stresses, causing the fatigue loading, are determined by means of finite element calculations. In general, LES is very well capable of capturing the mixing phenomena and the accompanied turbulent flow fluctuations in a T-junction. Previous work based on a direct comparison with experimental results [8, 9] showed the accuracy of LES predictions for thermal fatigue assessment in case of adiabatic flow. In this paper, also the effect of heat transfer through the pipe walls is included in the simulations. Temperatures are solved in both fluid and structure. Subsequently, the temperatures in the structure are transferred from the fluid-flow model to the finite element model. Using the finite element method, a mechanical calculation is performed to determine the structural deformations and stresses due to these thermal loads. Based on the resulting stresses, the fatigue life of the T-junction is finally predicted using fatigue curves from a design code. A strategy has been developed to assess thermal fatigue of complex mixing problems in nuclear power plants by coupling LES modelling, finite element modelling and code assessment. A description of this approach along with its application to a specified test case is presented here.