Fluidelastic Instability of an Array of Tubes Preferentially Flexible in the Flow Direction Subjected to Two-Phase Cross Flow

Almost all the available data about fluidelastic instability of heat exchanger tube bundles concerns tubes that are axisymmetrically 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 generator for preventing U-tube vibration may not be as effective in the in-plane direction than 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. Fluidelastic instability was observed when the flexible tubes were located at the center of the test section and also when the seven flexible tubes were placed over two adjacent columns. No instability was found for a single flexible tube in a rigid array nor for the case where the seven flexible tubes were placed in a single column. Tests were also done with tubes that are axisymmetrically flexible for comparison purposes. It was found that fluidelastic instability occurs at higher velocities when the tubes are flexible only in the flow direction. These results and additional wind tunnel results are compared to existing data on fluidelastic instability. Two-phase flow damping results are also presented in this paper.