Modelling of fatigue due to thermal mixing by using a spectrum method

Turbulent mixing of cold and hot fluids can lead to high-cycle thermal fatigue in T-junctions, but modelling of the thermal load induced to the structure is challenging. The spectrum method has been recently proposed as an efficient modelling method, which uses a theoretical turbulence spectrum to represent the fluctuating thermal load. In this work, further development and application of the method is presented. Estimation of the frequency of the largest-scale temperature fluctuations is considered in particular, as this is an essential input parameter for the model spectrum. The method is applied to a T-junction including a mixer pipe and a high temperature difference that induces significant buoyancy effects. The frequency scale of the fluctuations is determined for the flow case based on temperature spectra obtained from a Computational Fluid Dynamics (CFD) simulation. A formula for the frequency scale is proposed, which can be used in approximating the frequency scale for other flow conditions. Fatigue and crack growth analysis of the T-junction wall are carried out by using the spectrum method, and the results are compared with those obtained with sinusoidal and CFD temperature loads. The results indicate improvement in accuracy over the highly simplified sinusoidal method.