Flow Induced Vibration Of Slender Cylinder And Cylinders Bundle Subjected To Axial Flow

With the vigorous development of nuclear energy in the national energy system,reactor safety has always been put in the first place.As the first safety barrier of reactor,fuel assembly is very important to ensure its integrity and reliability during its service life.At present,flow induced vibration(FIV)is the main factor of fuel assembly failure.Therefore,aiming at the flow induced vibration of fuel rods and rod bundle,the flow induced vibration model of slender cylinder and cylinders bundle subjected to axial flow is established in this paper.The main research contents are as follows:At first,the stability and nonlinear dynamic characteristics of a single rod are studied.Based on the Pa?doussis linear model,the nonlinear motion equation of a slender cylinder under pulsating flow is established under reasonable assumptions and simplification.The effects of system parameters such as dimensionles velocity,damping ratio,axial tension and other parameters on the stability of the region where the equilibrium point exists and the zero equilibrium point are analyzed when the system is subjected to steady flow,as well as the nonlinear dynamic behavior caused by pulsating flow parameters after instability.Secondly,based on the potential flow theory and slender body,the added mass coefficient model of cylinder bundle is established,and the calculation program of added mass coefficient is verified as well.Through the calculation of the added mass coefficient of the rod bundle,it is found that the added mass coefficient and natural frequency tend to be constant when the gap between cylinders increases,and the critical velocity calculated by the dynamic equation of cylindrical beam coupling system in axial steady flow is in good agreement with the experimental results.Furthermore,the dynamic equation of cylinders bundle coupling system subjected to pulsating flow is established by micro element force analysis method and Hamilton principle.The law that the amplitude of each cylinder changes linearly with the pulsating amplitude parameters is obtained by numerical calculation.In addition,taking the fuel element parameters of China Experimental Fast Reactor(CEFR)as the calculation parameters,the fretting wear and vibration fatigue of contact between the fuel rod and the wire,which is caused by flow induced vibration,are analyzed.In the first step,the theoretical calculation model is verified,and then the flow field in the fluid domain of wire wrapped rod bundle is calculated by using the large eddy simulation method to obtain the wall pressure spectrum function.Then,according to the flow induced vibration model of rod bundle,the relationship between the wall pressure spectrum and the displacement power spectrum is obtained,so as to establish the fretting wear and vibration fatigue analysis model of wire wrapped rod bundle.The above investigations can provide engineering reference for both the fuel element design and the life prediction of China initial accelerator driven system(Ci ADS).Finally,the finite element program user-defined function(UDF)is embedded into the computational fluid dynamics software,and the fluid structure coupling calculation is carried out in combination with the dynamic grid technology.The motion trajectory of the cylinder at the subcritical velocity and the buckling instability characteristics,flutter instability and two-dimensional trajectory characteristics of the circular cylinder section at high velocity are analyzed,and the critical velocity of buckling instability and flutter instability of the cylinder is given,the variation of the flow field around the cylinder at different times in the flutter state is analyzed.In summary,the nonlinear dynamic characteristics of slender cylinder subjected to pulsating axial flow,the added mass coefficient of cylinder beam in fluid and the influence of pulsating flow parameters on cylinder amplitude are studied in this paper.Combined with practical engineering,the structural failure analysis model of wirewrapped cylinder caused by flow induced vibration,namely fretting wear and vibration fatigue,is provided.In addition,the influence of more accurate flow field on the flow induced vibration of slender cylinder is studied by numerical simulation,which verifies the feasibility of the dynamic grid fluid structure coupling numerical method in calculating the flutter motion of cylinder at high velocity,and gives the motion trajectory and nonlinear characteristics of cylinder at different inlet velocities.