Numerical simulation for thermal fluctuation and thermal stress fluctuation of a tee junction under turbulent mixing of hot and cold fluids based on a coupled CFD-FEM method

摘要

Thermal fatigue of the pipelines is a threat to the safe operation of nuclear power plants (NPPs).Tee junctions are common pipeline structures in NPPs, and turbulent mixing of hot and cold flows in tee junctions is recognized as one of the possible cause of thermal fatigue.Thermal fatigue is a significant long-term degradation mechanism, and the root cause of thermal fatigue is thermal stress fluctuation which caused by thermal fluctuation of the fluid in the pipeline.In practical applications, it is not an easy work to evaluate thermal stress and thermal fatigue of a tee junction under turbulent flow mixing because of the thermal loads acting at fluid-structure interface of the pipe are so complex and changeful.In this paper, a coupled Computational Fluid Dynamic-Finite Element Method (CFD-FEM method) based on the ANSYS Workbench 15.0 software has been developed to simulate thermal fluctuation and thermal stress fluctuations of a tee junction under turbulent mixing of hot and cold fluids.The Large-eddy simulation (LES) model on the FLUENT platform are employed to capture thermal fluctuations, and the obtained temperature fields in the Fluid-Solid Interface (FSI) are delivered to the inner wall of the tee junction for the calculation of thermal stress fluctuations in the ANSYS Workbench platform.The normalized mean values and the root-mean square (RMS) values of temperature are employed to represent the amplitude of temperature fluctuation.The Power Spectral Density (PSD) values of temperature are used to represent the frequency of temperature fluctuation.With this method, more insight is obtained in the physical phenomenon causing thermal fatigue due to turbulent fluid mixing, and the obtained thermal stress can then be used to evaluate thermal fatigue with the rainflow counting method.The coupled CFD-FEM method in this paper will supply valuable information for establishing a methodology on thermal fatigue, and provide a more iutuitive and direct ways of generating thermal stress and evaluating thermal fatigue than the previous methods which evaluate thermal fatigue only by studying the temperature fluctuations.