Fluidelastic Instability Modeling of Loosely Supported Multispan U Tubes in Nuclear Steam Generators

Abstract

Steam generators in nuclear power plants have experienced tube failures caused by flowinduced vibrations. Two excitation mechanisms are responsible for such failures; random turbulence excitation and fluidelastic instability. The random turbulence excitation mechanism results in longterm failures due to frettingwear damage at the tube supports, while fluidelastic instability results in shortterm failures due to excessive vibration of the tubes. Such failures may require shutdowns, which result in production losses, and pose potential threats to human safety and the environment. Therefore, it is imperative to predict the nonlinear tube response and the associated frettingwear damage to tubes due to fluid excitation. In this paper, a numerical model is developed to predict the nonlinear dynamic response of a steam generator with multispan Utubes and antivibration bar supports, and the associated fretting wear due to fluid excitation. Both the crossflow turbulence and fluidelastic instability forces are considered in this model. The finite element method is utilized to model the vibrations and impact dynamics. The tube bundle geometry is similar to the geometry used in CANDU steam generators. Eight sets of flatbar supports are considered. Moreover, the effect of clearances between the tubes and their supports, and axial offset between the supports are investigated. The results are presented and comparisons are made for the parameters influencing the frettingwear damage, such as contact ratio, impact forces, and normal work rate. It is clear that tubes in loose flatbar supports have complex dynamics due to a combination of geometry, tubetosupport clearance, offset, and misalignment. However, the results of the numerical simulation along with the developed model provide new insight into the flowinduced vibration mechanism and fretting wear of multispan Utubes that can be incorporated into future design guidelines for steam generators and large heat exchangers.