Session: FSI-02-02 Flow-induced Vibration II

Paper Number: 62112

Start Time: Wednesday, July 14, 2021, 09:00 AM

62112 - Measurement of Steam-Generator Tube Damping Caused by Anti-Vibration-Bar Supports 

In 2013, Atomic Energy of Canada Limited (now Canadian Nuclear Laboratories) took vibration damping measurements to examine low-frequency damping associated with vibration of a tube parallel to Anti-Vibration Bar (AVB) supports.  These tests were performed to better understand and control in-plane fluidelastic instability of tubes in the U-bend region of  recirculating steam generators.

AVBs are also referred to as flat-bar U-bend restraints or simply flat bars.  These supports are used to restrain tubes in the U-bend regions of many recirculating steam generators.  The bars are aligned in the plane of the U-tubes to allow for thermal expansion of the tubes.  As a result of their alignment, these bars restrict out-of-plane tube movement but do not absolutely prevent in-plane movement.  If static friction between tubes and AVBs is insufficient to prevent in-plane tube vibration, then the outer U‑tubes in large steam generators can vibrate at natural frequencies of less than 5 Hz.  Under these conditions, AVBs only control in-plane vibration by adding support damping.  A correlation commonly used to predict steam-generator tube damping indicates that low-frequency damping varies as the inverse of frequency and, therefore, becomes quite high at low frequencies.  This low-frequency behaviour is attributed to the squeeze-film damping that is associated with drilled-hole and other types of tube supports.  The tests preformed by AECL were largely designed to examine the low-frequency in-plane damping behaviour associated with AVBs.   

In the tests, the damping ratios of a single steam generator tube vibrating parallel to a single pair of AVBs were measured using both a log-decrement and a power-based method.   Non-contacting excitation and position-sensing techniques were employed to improve accuracy.  The tests examined the effects of fluid (water or air), natural frequency, gap width, preload, and vibration normal to the bars.  Additional tests were also done using a drilled hole support and without supports to allow validation against previously published work. 

This paper describes the tests including the test apparatus, test methods, and analysis techniques.  A summary of the results is presented.  These results show that the damping results measured with a drilled-hole support and without any supports are consistent with previously published work and with the damping correlation under considered.   However, the AVBs resulted in no additional viscous or squeeze-film damping for vibration parallel to the flat bars.  Therefore, the in-plane viscous-type support damping caused by AVBs would be significantly over-predicted by the damping correlation.  However, the results also showed that AVBs can introduce significant in-plane Coulomb-type damping as a result of friction and impacting.  

Presenting Author: Paul Feenstra Rhodes Associates Inc.

Authors:

Bruce Smith Rhodes Associates Inc.
Paul Feenstra Canadian Nuclear Laboratories
Michael C. Liu Intertek