Thermal fatigue may occur downstream from mixing tee junctions where hot and cold water are turbulently mixed. Depending on the momentum of the incoming flows, different characteristic eddies are formed which create a local hot spot on the inner surface of the pipe. The hot spot have an unstable discrete boundary that spatially oscillates and this creates an oscillating multidimensional thermal load that induces high cycle thermal fatigue.
To predict the thermal fatigue lifetime of a pipe subject to this kind of thermal load, transient FEM simulations with an ideal hot spot in two and three dimensions were conducted. The crack lengths induced by the frequency of the oscillating boundary was analysed and it was found that the most harmful frequency for a pipe is in the nondimensional frequency of 0.01 to 1.0. Additionally, the simulation method was validated against data publications of FAT3D experiments.
The simulations have been compared to the current proposed Japanese design code for mixing tee junctions. The study shows that this multidimensional effect induces larger crack lengths than the current design code. The reason of this was analysed.
