Shell-and-tube heat exchangers are subjected to high-speed flows, which may result in large amplitude structural oscillations and possibly tube failure.Fluidelastic instability (FEI) is recognized as the most important excitation mechanism&nbsp;that must be avoided in nuclear steam generators (SGs). The instability has been the subject&nbsp;of study over the past 50 years. The out-of-plane direction (transverse to the flow, and out of the plane of the tube U-bend) was found to be generally more unstable. Consequently, steam generators were fitted with so-called anti-vibration bars which effectively limit tube vibration in the out-of-plane direction.The failure of the SONGS steam generators in 2012 confirmed for the first time the possibility of inplane fluidelastic instability (IPFEI) in an operating steam generator and the catastrophic consequences.The present paper presents experimental tests on fluidelastic instability in a rotated square geometry tube array. The pitch spacing ratio of approximately P/D=1.6 is somewhat larger than that&nbsp;found in typical array geometries.&nbsp;&nbsp;&nbsp;&nbsp;Experiments were conducted in water flow. The stability of a single flexible tube as well as multiple flexible tubes was investigated. The tubes were free to vibrate in the streamwise direction or the transverse direction relative to the upstream flow.A single flexible tube was found to undergo large amplitude vibration (up to 25% D) in the transverse direction. Initial evaluation suggested possible fluidelastic instability. However, tube vibration response PSDs also indicated the presence of a second frequency component possibility related to vorticity shedding in the array. This potential vorticity-induced-vibrations (VIV) or coupling between VIV and FEI is discussed in the paper.The hypothesis of possible FEI-VIV coupling is supported by comparison with the case of 4 flexible tubes in column. In this case instability occurs before frequency lock-in hence the two mechanisms can be distinguished.Test results for streamwise flow-induced vibrations are also presented. Results in water flow show a possible effect related to VIV at low velocity followed by fluidelastic instability for high flow velocity. Importantly, the apparent streamwise (inplane) fluidelastic instability appears to be limited in velocity range. At significantly high flow velocity, the tubes are found to fully restabilize. Restabilization after FEI has not been reported for the out-of-plane (transverse) direction.The present results suggest that fluidelastic stability of tubes in a rotated square array configuration is significantly more complex than in other geometries, particularly for the streamwise vibration case.

Flow-Induced Vibration Behaviour of a Rotated Square Tube Array Subjected to Cross-Flow

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

ASME 2021 Pressure Vessels & Piping Conference (PVP2021)

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