Principal component analysis of vibration response of a rod bundle subjected to jet cross-flow

Flow injecting through loss of coolant accident (LOCA) holes in the baffle plates of some pressurized water reactors (PWRs) can potentially induce vibrations of reactor fuel assemblies. For reactor fuel rods in close proximity to LOCA holes, the jet flow transverse to the rod axis may lead to vibration levels significantly higher than those for rods located away from the LOCA holes. The jet cross-flow-induced vibration could, in turn, induce fretting wear of the fuel rod cladding at the fuel assembly supports, also known as spacer grids. Fuel rods have axisymmetric flexibility, testing the real condition of the fuel assembly is important to know the most dominant vibration direction relative to the jet flow. This information could be used to suppress the dominant vibration by increasing the stiffness of spacer grids in the dominant direction. In this study, the effect of jet cross-flow on a 6x6 square rod bundle axisymmetric vibration is experimentally investigated. This research aims to answer the following important question: Does there exist a generalized eigenvector model that could be used to predict rod bundle vibration in jet flows, in addition to characterizing the stability effect of the diameter ratio? The dynamical behavior of the rod bundle is studied for three jet-to-rod diameter ratios; 1.6, 2.3, and 3.0. Flow-induced vibration tests are performed to determine the stability threshold of the bundle as well as to extract the bundle vibration mode shapes for the three nozzles tested. The experimentally measured mode shapes identify the dominant vibration direction which provides valuable insight into the fluid-structure interaction dynamics. The experiments show that increasing the diameter ratio lowers the critical velocity at which fluidelastic instability occurs. The maximum vibration amplitude in the bundle, on the other hand, decreases as the diameter ratio increases. Analysis employing PCA and SVD confirmed the existence of a set of robust modes that exist for the range of system parameters tested. Hence, modes shape obtained with different nozzles are the same for the same acceleration ratio of the jet flow. The existence of these structurally stable (in parameter space) fluid-structure modes has significant implications for the stability analysis of reactor fuel rods subjected to jet cross-flow. The results show that reduced order models of the high-dimensional fluid-structure can be developed and that the models can realistically capture the dynamics of the jet cross flow-induced vibrations of the fuel rod bundle.