Axial Flow-induced Vibration Of Two Elastic Cylinders With Different Diameters And Twenty-five Cylinder Cluster

With the continuous development in the past several decades,various mature technologies for nuclear energy utilization have become readily available.However,after several nuclear problem,people are attaching more and more importance to the safety of nuclear energy.The fuel rods will be vibrated when the coolant flows at high axial speeds.And the fuel rods are placed side by side,increasing the intensity of the vibrations.Moreover,the fuel rod fixtures will lead to an increase in the turbulence intensity of the coolant,which in turn will generate a greater unstable lateral force on the fuel rod,causing the vibration of the fuel rod to be bigger again.Therefore,by studying the axial flow induced vibration phenomenon of two elastic cylinders with different diameters and 25-cylinders cluster,this paper finds out the main factors affecting their vibration and analyzes the mechanism,which is of great significance to optimize the fuel rod arrangement.This experiment was started for a long time in a vertical circulation water tunnel,whose flow velocity could be controlled between 0-2.13m/s,and whose turbulence intensity will close to 0.71-0.80%.The turbulence intensity could be increased to 2.3-2.9%through the grid.The elastic cylinder’s vibration is measured by laser Doppler velocimeter(LDV),and its vibration intensities,frequencies and phases difference are analyzed.Through the study of two elastic cylinders with different diameters,it can be found that the vibration of the first thin elastic cylinder is larger than that of the second thick elastic cylinder.Moreover,the gap ratio P~*(=P/D,P is the distance between two centers of two cylinder’s cross sections,and D is the equivalent diameter of the whole after the two elastic cylinders touch each other,which ranges from 1.2 to 1.77)has a very important influence on the vibration.The smaller the gap is,the greater the impact of the gap ratio on vibration.When the gap ratio of the two cylinders decreases from P~*=1.31 to P~*=1.2,the RMS value of vibration increases greatly.In addition,with the increase of the water tunnel’s flow velocity,the dominant vibration frequency of the first fine elastic cylinder decreases from 1.46 Hz to 0.97 Hz.The dominant vibration frequency of the second elastic cylinder decreases gradually from 2.19 Hz to 1.71 Hz.At the same time,the second elastic cylinder has another frequency equal to that of the first elastic cylinder when the gap ratio is small.However,the first elastic cylinder will have another frequency equal to that of the second elastic cylinder at high turbulence intensity,small gap ratio and high flow velocity.Through the analysis of the instantaneous vibration waveform,it can be concluded that the reason why the second frequency appears when the elastic cylinder vibrates is that the dominant vibration frequency changes in a short time after being affected by another elastic cylinder.Secondly,after studying the 25 cylinder cluster,we can find that the vibration strength of the elastic cylinder decreases first and then increases with the increase of gap ratio P~*(=P/D,P is the distance between the center of the cross section of adjacent cylinders,and D is the diameter of any cylinder,which ranges from 1.28 to1.93),and the vibration strength is the least when P~*=1.64.The turbulence intensity has more obvious influence on the vibration of elastic cylinder with large gap ratio and high flow velocity.When l~*(=l/L,l is the distance from the top end of the elastic cylinder to the measuring point;L is the total length of the rigid cylinder which is same to elastic cylinder.l~*has three values,which are 1/4,1/2 and 3/4 respectively.)takes different values,it is concluded that the vibration of the elastic cylinder is the greatest when l~*=1/2,followed by l~*=3/4,and the cylinder’s vibration will become to be the least when l~*=1/4.But under different l~*,the first order dominant vibration frequencies of the elastic cylinder are the same,and the phase difference is 0.At l~*=1/4 and l~*=3/4,the second order vibration frequencies are the same,and the phase difference is-π(orπ),while the second order vibration phase difference between l~*=1/2 and l~*=3/4 isπ/2.