Matched refractive-index PIV visualization of complex flow structure in a three-dimentionally connected dual elbow

Flow structure in a three-dimensionally connected dual elbow is visualized using a 1/15-scale experimental apparatus simulating the 1st and 2nd elbows of JSFR cold-leg piping. A matched refractive-index PIV measurement clarifies that a low-velocity region formed on the inner wall side of the 1st elbow develops toward the 2nd elbow. This low-velocity region consists of the following two ones: a flow separation region accompanied mainly with the generation and disappearance of transverse vortices, and a velocity recovery region that has longitudinal vortices with strong unsteadiness. These longitudinal vortices exist as twin vortices in the time-averaged flow field, and their dynamic characteristics highly depend on high-velocity creeping flows generated in the 1st elbow that flow into the velocity recovery region through the side walls. Since the velocity recovery region reaches the 2nd elbow, the geometry of the 2nd elbow has a significant impact on the characteristics of the vortex shedding in the velocity recovery region. On the other hand, obvious flow separation is not observed in the 2nd elbow, whereas high-velocity flow with intense velocity fluctuation is confirmed on the inner wall side. Furthermore, the unsteady vortices shed from the velocity recovery region are transferred to the central area of the 2nd elbow while growing significantly. The visualization of the secondary elbow shortly after the 2nd elbow clarifies that a strong swirling flow is formed in the 2nd elbow. These flow structures are due to the distorted flow formed in the 1st elbow and the shape effect of the 2nd elbow.