Influence of inlet velocity condition on unsteady flow characteristics in piping with a short elbow under a high-Reynolds-number condition

In the design of the Advanced Sodium-cooled Fast Reactor in Japan, the mean velocity of the coolant is approximately 9 m/s in the primary hot leg (H/L) piping, which has a diameter of 1.27 m. The Reynolds number in the H/L piping reaches 4.2 × 107. Furthermore, a short elbow, which has Rc/D = 1.0 (Rc: curvature radius, D: pipe diameter), is used in the H/L piping to achieve a compact plant layout and reduces plant construction costs. In the H/L piping, flow-induced vibration (FIV) is a concern due to the excitation force caused by pressure fluctuation in the short elbow. In a previous study, the relation between flow separation and pressure fluctuations in the short elbow was investigated under the specific inlet condition of a flat velocity profile of time-averaged axial velocity and relatively low velocity fluctuation intensity. However, the inlet velocity condition of the H/L in a reactor may be a highly turbulent non-uniform profile owing to the complex geometry in the reactor vessel (R/V). In this report, the influence of inlet velocity condition on the unsteady velocity characteristics in the short elbow was studied. Although the flow around the inlet of the H/L in the R/V could not be simulated completely, the inlet velocity conditions were controlled by installing a perforated plate, which appropriately plugged the flow holes. Then, controlled flow patterns were established at a position 2D upstream of the elbow inlet. Observed flow structures by particle image velocimetry indicated that the inlet velocity profiles affected a circumferential secondary flow, which then affected an area of flow separation at the elbow. It was also found that the velocity fluctuation at low frequency components observed upstream of the elbow could remain in downstream of the elbow though its intensity was attenuated.