Modal contribution of bundle oscillation to induced flow

Abstract

Bundle motion of a pressurized water reactor core under seismic excitation induces a pulsatile oscillatory flow which can strongly affect the behaviour of the whole reactor. Understanding these phenomena is thus a primary concern for the nuclear industry. An index matched experimental facility was recently built allowing the study of a surrogate fuel bundle under axial flow undergoing a seismic excitation. The use of a fluid which is refractive index matched with the transparent surrogate bundle allows to perform non-intrusive particle image velocimetry measurements over the whole fluid domain. This paper focuses on experiments performed with a shaking table frequency close to the first resonance mode of the bundle under otherwise stagnant water. The local flow induced by the oscillation of the fuel bundle is then observed. Three significant dataset are analysed to understand how very small changes in the structural behaviour of the fuel bundle can lead to important changes in the flow field. A proper orthogonal decomposition of the bundle excitation shows that the higher modes (even if they have small amplitudes) can contribute significantly to the flow induced in the channel. A numerical simulation based on the potential flow theory simulates the experimental conditions and provides a physical interpretation of the observed phenomena.