Keywords: cylindrical rod, bundle structure, fluid-structure interaction, dynamic behaviour, amplitude, multi-span structure.

A cylindrical rod bundle structure is composed of several slender rods in a triangular or square type sub-channel pattern. These bundle structures are supported by several grid structures, which are composed of an egg-crate plate. Therefore, they have a multi-span supported structure. In this bundle, the fluid flows axially through the inside or inside/outside of the cylindrical rod bundle structure for a heat exchange. These flows are caused by the flow-induced vibration (FIV) of the slender cylindrical rods. In this work, the dynamic behaviour of them is analyzed by a computational method using a fluid-structure interaction (FSI) code. It is not a separate code for a fluid and structure, but an integrated FSI code. The fluid was assumed to be as incompressible water at room temperature and atmospheric conditions. Therefore, the fluid affects the structure due to the fluid flow, and vice versa. The dynamic behaviour of the cylindrical rods is obtained from the analysis model developed. On the other hand, the experimental test section was composed of the actual structural geometries for the FIV test. The results from the two methods were compared, and then the analysis results were compared with those of the test. The predicted results were very similar to the experimental results test. So, the developed FSI analysis model and the adopted procedure will be the basic tool for evaluating the FIV behaviour of a structure under an axial and cross fluid flow.

A fluid-structure interaction analysis for a fuel rod with a single span in an axial flow was executed with the commercial code, ADINA. This finite element model and the adopted procedure were applied by using a fluid-structure coupled algorithm. The coolant was modeled with an incompressible fluid, and the fuel rod surrounded grid straps were supported with grid spring or dimple. The analysis results from the FSI model were compared with the other results. After verifying the procedure, this FSI analysis model will be expanded to a multi-span and rod cluster structure. It may be necessary to develop a means for predicting the vibration amplitudes. The wear rate strongly depends on a vibration, and wear is a serious concern. Therefore, the developed FSI analysis model will provide key information on the fretting wear behaviour without an actual scaled FIV test.

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