Numerical simulation of long-period fluid temperature fluctuation at a mixing tee for the thermal fatigue problem

Thermal fatigue cracks may be initiated at mixing tees where high and low temperature fluids flow in and mix. According to a previous study, damage by thermal fatigue depends on the frequency of the fluid temperature fluctuation near the wall surface. Structures have the time constant of structural response that depends on physical properties of the structure and the gain of the frequency response tends to become maximum at the frequency lower than the typical frequency of fluid temperature fluctuation. Hence the effect of the lower frequency, that is, long-period temperature fluctuation is important for the thermal fatigue assessment. The typical frequency of fluid temperature fluctuation is about St = 0.2 (nearly 6 Hz), where St is Strouhal number and means non-dimensional frequency. In the experimental study by Miyoshi et al. (2014), a longer-period fluctuation than St = 0.2 was also observed. Results of a fluid-structure coupled analysis by Kamaya et al. (2011) showed this long-period temperature fluctuation causes severer damage to piping. In the present study, a large eddy simulation was carried out to investigate the predictive performance of the long-period fluid temperature fluctuation more quantitatively. Numerical simulation was conducted for the WATLON experiment which was the water experiment of a mixing tee performed at the Japan Atomic Energy Agency. Four computational grids were used to confirm grid convergence. In the short time (9 s) simulations, tendencies of time-averaged and fluctuated velocities could be followed. Time-averaged temperature distributions were also reproduced, although overestimation appeared near the wall. The fluid temperature fluctuation intensity near the wall surface could be predicted qualitatively, while the peak value was overestimated. From the engineering viewpoint, it was concluded that the numerical simulation provided results that were conservative and on the side of safety. From the grid convergence study, the coarsest grid for which grid convergence was almost attained was selected and the simulation was continued until 100 s. Frequency analysis of the fluid temperature fluctuation showed that the long-period fluctuation appeared as well as the well-known typical frequency (St = 0.2). This indicated that numerical simulation could reproduce the long-period temperature fluctuation at the mixing tee.