Flow-induced Vibration And Fatigue Life Analysis For U-tube Elbow Of Steam Generator

For PWR nuclear power plants,the flow-induced vibration of heat transfer tubes is a long-standing problem in the steam generator,which may cause damage to the heat transfer tube.Therefore,the study of the flow-induced vibration characteristics and fatigue life analysis of U-tube is of great significance to the safety analysis of the steam generator in nuclear power plant.Based on ANSYS Workbench platform,the bi-directional fluid-structure interaction calculation method was used to numerically study the flow-induced vibration characteristics and fatigue life of the elbow for full-size steam generator heat transfer U-tube.In this paper,different turbulence models are used to verify the fluid-induced vibration characteristics of straight pipe,including Transition SST,Realizable k-e,Low-Reynolds k-e and LES.According to the structural dimensions of the steam generator heat transfer tubes of the Daya Bay Nuclear Power Plant,the calculation domain of the flow field and solid structural were established.And the fluid-induced vibration characteristics of U-bend in one-phase flow with different velocity and two-phase flow with different mass fraction of steam are studied respectively,and its modal analysis,stress analysis and fatigue analysis of the bend structure are also carried out.The numerical results show:The velocity vortices occur alternately in the downstream of the U-tube,corresponding to the alternate shedding of vortices on the outer surface of the tube and the moving back and forth of boundary layer separation point,so that the pressure on both sides of the tube change periodically,which is the basic reason for the flow-induced vibration of the U-tube;In the monophasic flow with a flow rate of 3～7m/s,the natural frequencies of each order can be regarded as fixed values,and the first-order natural frequency of the U-tube is 139.9 Hz;And in the two-phase flow field with a vapor mass content of 20～50%,the natural frequencies of each order can also be approximately regarded as fixed values,and the first-order natural frequency is 140.32 Hz;The maximum stress occurs at the position where the U-tube contacts the tube sheet;In one-phase flow,the average stress increases with the increase of the flow velocity,and the stress ratio decreases.The damage of the U-tube elbow during the design life will increase and the fatigue life will decrease accordingly;The higher the mass vapor content in the two-phase flow,the less accumulated damage of the U-tube elbow during the design life,and the higher its fatigue life;Four different mean stress theories were used in the fatigue analysis,and it was found that the results obtained by SN-Gerber correction method were the closest to those obtained by SN-None,while the results obtained by SN-Soderberg correction method were the most conservative.