Study On Fluid Structure Coupling Vibration Characteristics Of U-type Heat Transfer Tube Of Steam Generator

As the key equipment of the first and second loops in the pressurized water reactor nuclear power plant,the safety of steam generator must be guaranteed.The heat transfer tube is the weakest structure in the steam generator,which may cause failure due to vibration,and causing serious consequences.The vibration characteristics of the heat transfer tube are affected by the secondary side flow field,the natural frequency of the tube and the coupling instability of the tube array.Comprehensive study of the variation characteristics of each influencing factor has important guiding significance for the safety operation of the steam generator.In this paper,the FLUENT software was used to analyze the thermal-hydraulic characteristics of the secondary side flow field of steam generator under design conditions,the influence of the hot and cold side water supply ratio and anti-vibration bar offset were studied.The results show that the effect of the secondary side fluid on the tube bundle is concentrated in the inlet and the elbow area.The water supply ratio has a significant influence on the fluid force at the inlet area.The offset of single anti-vibration bar have less effect on the overall distribution of the flow field,but the local flow velocity will increase.The linear equivalent treatment of anti-vibration bar support was carried out,and the support equivalent stiffness value and the two-direction stiffness ratio range were obtained by the vibration excitation experiment when the support gap is in the range of 0.025 mm to 0.3 mm.The modal analysis method was used and the inner direction of the support surface was restrained by spring connection to obtain the modal frequency and mode shape of the heat transfer tube.The results show that when the dimensionless gap is less than 0.4,the dimensionless equivalent stiffness ratio of the support is not more than 0.1.Increase the number of loosely supports or decrease the support stiffness will reduce the lowest natural frequency in the elbow plane,and the minimum value is 5.4 Hz.Fluid elastic instability vibration experiments of unequal stiffness tube arrays were carried out by using the water tunnel experimental system.The effect of the tube bundle on the stiffness ratio in the downstream direction compared to the cross flow direction,the pitch-to-diameter ratio of the tube array,and the arrangement of the elastic tube were analyzed.The results show that the natural frequency of the tube bundle in the downstream direction has a significant effect on the vortex shedding frequency.The coupling between tube bundles is more obvious in small pitch-to-diameter ratio.Under the experimental conditions,the fluid elastic instability of the tube bundle is mainly occurred at the cross flow direction.The relevant results provide reference for life prediction,structural improvement and safety analysis of steam generators,which are of great significance for ensuring the safety of nuclear power systems.