Experimental Study And Numerical Simulation Of Flow-induced Vibration Characteristics Of Grids

Flow-induced vibration occurs when fuel assemblies in nuclear reactor are flushed by high-velocity coolant,and the vibration of rod bundles and grids will lead to abrasive wear.The fuel assembly is the core component of the reactor core.Radioactive material leakage caused by the damage of fuel rod cladding may lead to severe consequences.Therefore,the flow-induced vibration characteristics and numerical simulation of the grid strips are of great importance to the design and development of the grid.In general,the researches on flow induced vibration mainly focus on cylinders,square cylinders and plates,and the effects of dimensionless parameters of structure and fluid on amplitude,wake vortex shape,natural frequency and resonance phenomena are obtained.However,very few experimental data have been reported about the flow induced vibration of fuel assemblies,especially the vibration characteristics of the grid strips.In order to confirm the safety,it is necessary to experimentally and numerically investigate on the flow-induced vibration characteristics of the grid strip in the reactor,so as to understand its physical mechanisms and develop appropriate numerical simulation models which will help to design and develop the rod bundle grid.The flow-induced vibration characteristics of flat strips and real grid strips were experimentally studied on the flow induced vibration experimental platform in the Institute of Nuclear Science and Engineering,Shanghai Jiao Tong University.The effects of natural frequency,vibration shape and size of the grid strips on the vibration characteristics of the strips were obtained.The experimental results show that: 1)The frequency of the vortex-induced vibration varies linearly with the velocity of the flow.When the frequency of the vortex-induced vibration approaches the natural frequency of the strips,frequency locking and resonance will occur.2)The strips size will affect the St value,natural frequency and turbulent excitation.3)With the increase of flow velocity,the natural frequencies of each order will be excited in turn,and the vibration patterns of different size strip under the natural frequencies are different.4)The vibration spectrum characteristics of strip for the real grids are consistent with that of the flat strip,which agrees with the mechanism of vortex-induced vibration.The real grid strips structure has more natural frequencies,and the existence of the agitator will affect the frequency locking,eddy-induced vibration and turbulent-induced vibration.In this study,the flow-induced vibration characteristics of strip flushed by fluid are numerically simulated by the wet modal analysis of acoustic-solid coupling method.The underwater modes of flat strip and grid strip,including natural frequencies and modes,are analyzed and studied based on the commercial software ANSYS Workbench.Compared with the experimental data,the errors predicted by the simulation are less than 7%,and the simulated mode shapes are well agreed with the experimental data.