An investigation of thermal stress characteristics by wall temperature measurements at a mixing tee

Thermal fatigue cracking may initiate at a tee pipe where high and low temperature fluids flow in. The mixing flow causes temperature fluctuation near the pipe wall and may result in fatigue crack initiation. In this paper, characteristics of the wall temperature and thermal stress fluctuation at a mixing tee were investigated by tests, in which wall temperature was measured with 148 thermocouples. Then, the thermal stress fluctuation was estimated by finite element analysis (FEA) based on the measured wall temperatures. The range of the stress and temperature fluctuations became large locally on the main pipe inner surface. The spatial distribution of the stress was similar to that of the wall temperature. The large stress fluctuation was caused by movement of a hot spot, which was heated by the hot flow from the branch pipe, and the frequency of the movement was about 0.1 Hz. The position for the maximum fatigue damage according to the rain-flow counting method was close to that for the maximum stress fluctuation range. The fatigue damage was accumulated mainly by the large stress fluctuation which appeared with the low-cycle frequency fluctuation of about 0.1 Hz.