Hydro-thermal-mechanical analysis of thermal fatigue in a mixing tee

摘要

This paper covers work carried out by the CEA to study the mechanisms leading to cracking of piping as a result of thermal loading in flow mixing zones. The main goal of the work is to analyse, by calculation, the thermal loading caused by turbulent mixing in tees and to understand the mechanism of initiation and propagation of cracks in such components. This work is supported by IRSN. This thermal fatigue phenomenon is still not fully understood. One of the main obstacles to its understanding resides in the multi-domain nature of the loading and associated damage, involving three complementary scientific disciplines: thermal-hydraulic field, thermo-mecham'cal field and materials science. This paper describes the approach adopted by the CEA to establish natural mechanisms (turbulence, pulsing and instability) which might be the cause of any substantial thermo-mechanical loading in the piping. Although turbulence may be the cause of the thermal stripping (presence of high-frequency thermal fluctuations on the inner surface of the component), it cannot alone explain the propagation of deep cracks. The main reason is the "high-pass filter" effect of convection. The wall cannot be subjected to convection-related thermal fluctuations and frequencies less than the inverse of the turbulence transit time. A straightforward frequency-based analysis of the loading, carried out as a first stage, made it possible to establish the limits of the loading created by these high-frequency events. However, turbulence can give rise to flow instability (such as pulsing) of lower frequency. But this cannot explain everything. The geometry upstream of the tee, particularly the sequence of straight sections and bends can, in certain cases, damp the pulses or greatly amplify them. The use of suitable thermal-hydraulic modelling is discussed in the second part of this article. The final result of the thermo-hydro-mechanical link-up on application to the complex 3D geometry of the Civaux unit 1 case (which includes a mixing tee, bends and straight sections) enabled the observations made in this plant case to be highlighted and correlated. One of the originalities of this study is to carry out the overall analysis (thermal-hydraulic and thermo-mechanical) with a single computer code, the CAST3M code developed by the CEA.