抄録
In nuclear and various power plants, there are many mixing tees at which different temperature fluids are mixed. At this kind of mixing tee, temperature fluctuation in fluids due to the unstable mixing is transported to the surrounding wall, which leads to high-cycle thermal fatigue of structural materials. Depending on the amplitude and frequency of the temperature fluctuation, it could induce cracks on the pipe surface and in the worst case lead to coolant leakage. For pipe maintainability and its reliability improvement in the future power plans, therefore, it becomes important to control the high-cycle thermal fatigue. Particularly in the case of the mixing tee with a 90-degree bend upstream, an unsteady secondary flow arises in the 90-degree bend, and then strongly affects the fluid mixing. Furthermore, the unsteady secondary flow has the tendency to change its velocity distribution after the bend outlet. Therefore, the location of the mixing tee is one of the important parameters to characterize the mixing structure and the temperature fluctuation near the wall. In this study, the temperature fluctuation and its intensity, including frequency analysis, and the fluid mixing structure above the mixing tee are investigated experimentally in different cases of location of the mixing tee from the 90-dgree bend outlet. In case of closer location of the mixing tee from the 90-degree bend, the fluid mixing area tends to shift to the inward side of the bended pipe. As a result, the temperature fluctuation intensity becomes lower and decays faster in the streamwise direction. This result means that the location of the mixing tee strongly affects the fluid mixing and the temperature fluctuation near the wall. Experimental results suggest that changing the location of the mixing tee closer to the 90-degree bend could be an effective control technique for the high-cycle thermal fatigue.
