Unsteady hydraulic characteristics in pipe with elbow under high Reynolds condition

In the design of Japan Sodium-cooled Fast Reactor (JSFR), coolant velocity is beyond 9 m/s in the primary hot leg pipe of 1.27 m diameter. The Reynolds number in the piping reaches 4.2x10⁷. Moreover, a short-elbow (r/D=1.0, r: curvature radius, D: pipe diameter) is adopted in the hot leg pipe in order to achieve compact plant layout and reduce plant construction cost. Therefore, the flow-induced vibration (FIV) arising from the piping geometry may occur in the short-elbow pipe. The FIV is due to the excitation force which is caused by the pressure fluctuation on the wall. The pressure fluctuation on the pipe wall is closely related with the flow fluctuation. In this study, water experiments using two types of 1/8 scaled elbows with different curvature ratio, r/D=1.0 and 1.5 (short-elbow and long-elbow), were conducted in order to investigate the mechanism of velocity and pressure fluctuation in the elbow and its downstream. The experiments were carried out at Re=5.4x10⁵ conditions. Measurement of velocity fluctuation and pressure fluctuation in two types of elbows with different curvature revealed that behavior of separation region and the circumferential secondary flow affected the pressure fluctuation on the wall of the elbow greatly. (author)