Investigation of thermal hydraulic mixing mechanism in T-junction pipe with a 90-degree bend in upstream side for mitigation and controlling of thermal-striping phenomena (Joint research)

Yuki, Kazuhisa*; Hashizume, Hidetoshi*; Tanaka, Masaaki  ; Muramatsu, Toshiharu

In T-junction pipe, temperature fluctuation is induced due to unstable fluid mixing. Development of the relaxation and control techniques for the thermal fatigue is one of the most important issues in the future plant design. If a 90-degree bend exists in the upstream of the mixing tee in the piping system of power plants, a secondary flow formed in the bend makes the fluid mixing phenomena even more complex. This study aims at clarifying effects of curvature ratio of the bend on the non-isothermal fluid mixing in the Tee-junction area and the temperature fluctuation induced by the unstable mixing, by visualizing the flow fields with PIV and measuring fluid-temperature fluctuation in the vicinity of wall. From the visualization, it is clarified that a high-temperature jet flowing out from a branch pipe swings and sways near the wall, which leads to higher temperature fluctuation than that in a case without the 90-degree bend. In addition, in the case that there exists a separation in the bend, the fluid mixing and the temperature fluctuation characteristics including its damping in the downstream direction are completely different from those without the separation. Furthermore, in the case using the bend that doesn't produce the separation, there are cautionary conditions in which the temperature fluctuation is maximized in a transition regime of a stratified flow and a turn-jet flow. It seems that the principal cause for this is repetition generation and disappearance of a circulating flow formed behind the jet due to an interaction between unsteady behavior of a secondary flow in a decay process after the bend and the wakes formed behind the jet, which leads to the vigorous oscillation of jet near the wall. For the prediction of the temperature fluctuation, it's possible to predict it in each fluid mixing pattern.