Axial Flow-induced Vibration Of Asymmetrically Arranged Cylinder Cluster At Different Turbulent Intensities

The fuel rods in nuclear reactor are placed in the axial flow coolant.Although the vibration amplitude caused by the axial flow is small.Even the small vibration will cause a certain degree of wear for the fuel rods which are tightly arranged and rigidly constrained.The existence of the positioning grid in the fuel rod will lead to the disorder of the incoming flow and the increase of the turbulence.Then the lateral random vibration of the rod increase,further aggravate the fatigue damage of the fuel rod.If not found in time,it may also lead to catastrophic accidents.In addition,the study of axial flow induced vibration can also be applied to heat exchanger,steam generator,marine mining pipe and drill string.Therefore,it is of practical significance to study the vibration characteristics of axial flow and the effect of turbulence intensity on it.In this paper,the effects of flow velocity,gap ratio and turbulence intensity on the dynamic characteristics of asymmetrically arranged cylinder cluster are studied.Three cylinders are arranged in rows(arrangement A)and in 90 °(arrangement B).The experiment was carried out in a vertical water tunnel with a maximum Reynolds number of 30566.The range of dimensionless velocity is 0.63-6.98.The gap ratio(the ratio of spacing between two cylinders center to cylinder diameter)is 1.21-1.71.Experiments were carried out at high and low turbulence intensity levels,among which low turbulence intensity level is 0.71-0.80% and high turbulence intensity level is 2.30-2.91%.The vibration information is obtained by Laser Doppler Vibrometer(LDV)and Fiber Bragg Grating sensors(FBG).The flow field information is obtained by Particle Image Velocimetry(PIV).In arrangement A,the experimental results show that fix one side gap ratio at 1.57,the vibration in the direction has cylinders first decreases as the other side gap ratio decrease,and then increases at gap ratio﹤1.57.That is,consistent with the symmetrical layout,there are R_Ⅰ region(P~* ≥ 1.57)and R_Ⅱ region(P~* < 1.57).At fixed gap ratio 1.28,the vibration decreases with increasing the other side gap ratio in both directions.When the other side gap ratio is 1.57,the displacement is found to be the minimum.The fixed gap ratio at one end is 1.28,the dominent vibration frequency increases with the decrease of gap ratio in the direction with rigid cylinder,which is due to the added mass effect.The vibration at high turbulent intensity exceeds greatly than that at low turbulent intensity given the same volecity.The vibration is almost the same in two arrangements at the same gap ratio.This is because the number of rigid cylinder is the same.The arrangement only affects the vibration direction.In addition,the maximum turbulence intensity of active grid is 3.45%,which is 27.8% higher than that of passive grid.Counterflow injection increases the effective solidity of the grid then increase the turbulent intensity.The turbulent intensity can be increased by further increasing the injection angle.