This paper presents a recently proposed analysis of the uncertainties involved in the thermal fatigue assessment of pipes under turbulent fluid mixing. The assumed overriding contributor to the uncertainty of the predicted fatigue life is the variability inherent in the temperature histories of the turbulently mixing fluids, which intrinsically depends on the length of temperature histories considered in the assessment.
The fluid temperature histories are generated with an improved spectral loading approach within the temperature variation space, which includes all physically possible temperature fluctuations and all flow regimes. The results of the presented analysis are compared with the typical sinusoidal method, emphasizing the well-known over-conservatism of this method.
This paper extends the analyses to the initial thermo-mechanical transient of the pipe wall around the mixing fluids and its influence on fatigue life predictions. The high stresses arising at the beginning of the transient yield, generally, conservative life time results. In a separate sensitivity analysis, the impact of the heat transfer coefficient on fatigue predictions is also studied. This is the first attempt to correlate the uncertainties in the physical fluid mixing conditions and heat transfer to the estimated fatigue life.