Flow-induced vibration of nuclear steam generator U-tubes in two-phase flow

Flow-induced vibration characteristics of a U-tube bundle were experimentally investigated in air–water two-phase flow. The test section was equipped with 39 U-tubes, simulating the innermost region of an actual steam generator. The U-tubes were made of Inconel 690 with a diameter of 19.05 mm. The horizontal region of the U-tubes had a rotated square array with a pitch of 31.11 mm and a p/d of 1.633. The U-tubes and supporting structures have almost the same prototypical geometries. Vibration responses of six U-tubes were measured with ten 3-axis accelerometers. Two sets of experiments were performed to investigate an onset of fluid-elastic instability, damping ratio, and hydrodynamic mass of the U-tubes. The experiments were performed for a void fraction of 70–95%. The instability constant (K) of the Connors’ equation for the present U-tube bundle was evaluated to be in the range of 6.5–10.5.