Numerical Study On Flow-induced Vibration Characteristics Of Fuel Assembly With Complex Structure

Flow induced vibration is widely used in engineering applications,especially in nuclear reactors,which directly affects the safe operation.The high speed flow of coolant may cause the vibration of the fuel element.The fluid-solid coupling vibration often causes the deformation and wear of rod and even the radioactive material leakage.The study of the flow induced vibration can further reveal the flow induced vibration characteristics of fuel elements in axial flow,and it lays a foundation for more accurate prediction of fretting wear.The paper would study on the fluid solid coupling vibration behavior of a fuel rod with a locating element(spacer grids or wires),focusing on fuel elements in nuclear reactor.The flow field is calculated by Fluent,and the modal analysis is used to solve the dynamic characteristics of the structure.The wet modal analysis is also carried out,whose results are compared with the published experimental data to prove the reliability of numerical calculation method.The following results are concluded: the vibration frequency of the fuel rod with wire is slightly increased in the low velocity lead bismuth alloy than that in the vacuum,while that is slightly lower among the light rod.When the fluid flows through the wire,the turbulent intensity around is obviously enhanced,and the turbulent boundary layer pressure fluctuates violently.as the force is uneven,there is a large equivalent deformation.The flow field is calculated by Fluent,and the transient dynamic solution of the solid domain and the dynamic mesh technique are used to calculate the two-way fluid-solid coupling.The results are compared with the published experimental data,and the accuracy of the numerical method is verified.The fluid-solid coupling dynamic response of the fuel rod with spacer grids and wires is studied.And the influence of different parameters on the vibration of fuel rods is analyzed.The inlet velocity,the fluctuation of the force and the turbulence kinetic energy is proportional to the vibration displacement,respectively,while the wall shear force and stress is inversely proportional to the vibration displacement.By means of the analysis of spectrum and statistical,it is found that the vibration frequency bands of the two different positioning methods are the same,and the random vibration coincides with the normal distribution.At the same time,the calculation and analysis of 5×5 fuel rod bundles with spacer grids are carried out.A comparative analysis of the fuel rod bundles with and without spacer grids is carried out by the two-way fluid-solid coupling.Results showed that the existence of the spacer grids increases pressure difference between the radial sides and obviously strengthens the intensity of its vibration.The vibration flow field with different axial sections is compared with that which is without the vibration flow field,it shows that the cross section distribution characteristic of the vibration convection field is very small,which can be ignored in the thermal hydraulic study.