Fluidelastic Instability of a Tube Bundle Preferentially Flexible in the Flow Direction to Simulate U-bend in-plane Vibration

Abstract

Almost all the available data about fluidelastic instability of heat exchanger tube bundles concerns tubes that are axisymetrically flexible. In those cases, the instability is found to be mostly in the direction transverse to the flow. Thus, the direction parallel to the flow has raised less concern in terms of bundle stability. However, the flat bar supports used in steam generators for preventing U-tubes vibration may not be as effective in the in-plane direction as in the out-of-plane direction. The possibility that fluidelastic instability can develop in the flow direction must then be assessed. In the present work, tests were done to study the fluidelastic instability of a cluster of seven tubes much more flexible in the flow direction than in the lift direction. The array configuration is rotated triangular with a pitch to diameter ratio of 1.5. The array was subjected to two-phase (air-water) cross flow. Well-defined fluidelastic instabilities were observed albeit at somewhat higher flow velocities than for axisymetrically flexible tubes. This so far unknown phenomenon may be of concern if some supports become ineffective in the in-plane direction.