Experiments on Axial-Flow-Induced Vibration of a Cantilever Rod in Two-Phase Flow

Flow-induced vibration (FIV) of a cantilever rod subjected to axial two-phase flow is experimentally investigated. The experimental setup features a stainless steel rod confined inside a vertical tube and clamped at one end- a simple configuration yet informative of water-cooled nuclear fuel assemblies. The rod is filled with lead pellets to produce a linear mass density representative of nuclear fuel rods. Three end-pieces for the rod tip with blunt, hemispherical, and cone shapes are investigated. Experiments are conducted with flow passing the rod axially either in a “clamped-free” or a “free-clamped” configuration. The flow involves two phases resulting from the mixture of air and water, with a homogeneous void fraction ranging between 0-0.5, covering a range of flow regimes from single-phase to void-dominated flows. Water flow rate is varied within a range of superficial annular Reynolds numbers between 24k-73k. A novel electro-magnetic, non-intrusive methodology, based on the Hall effect is developed to measure the rod vibration dynamics in two-phase flows, allowing the first comprehensive investigation of axial-FIVs in such flow conditions. A consistent trend in rod vibration dynamics emerges across all investigated tip shapes, Reynolds numbers, and configurations: increasing the void fraction leads to increased chaotic motion amplitudes, but decreased periodic motion amplitudes. This is due to an increased flow turbulence resulting from air-water phase interactions, as well as increased random fluctuations in forces due to void impingement on the rod surface. At low Reynolds numbers, the increased random excitations lead to increased overall rod vibration amplitudes. However, at high Reynolds numbers, the reduced periodic force excitations can result in reduced overall motion amplitudes.