Abstract
Water ingress accident is a special type of accident for High Temperature Gas-cooled Reactor (HTGR), and is of serious threat to the safety of the reactor system. As the trigger signal for the operation of the core protection system in a water ingress accident, the amount of water in the primary coolant must be monitored online continuously and accurately. Considering the application of 90° elbows in flow measurement of helium in HTGR, a simultaneous measurement method for measuring the average velocity and density of fluid by utilizing the 90° elbow pipe is proposed. In order to achieve this goal, the characteristics of elbow pipe flow must be deeply studied to reveal the mechanism of the pressure fluctuation in the elbow structure. PIV method was performed to capture the transient process of boundary layer separation occurring near the intrados and the switch of secondary flow downstream of the outlet of the elbow. The strong dynamic coupling effects between the secondary flow and the boundary layer separation were studied, and the formation mechanism of pressure fluctuation near the intrados of the elbow was clarified. The experimental results indicate that the boundary layer separation occurs near the outlet of the elbow intrados and a series of shedding vortexes with different location and scale are induced downstream of the separation point by the separation effects. The two components of the secondary flow changes alternatively as time goes on and one of them dominants the flow pattern. The coupling effects between the boundary layer separation and the secondary flow induces the random process of the pressure fluctuation and finally results in the special transient pressure field in elbow pipe flow.
