STV-3 Flow-Induced Vibration in Tube Bundles with a Pitch Ratio of 1.42
A series of flow-induced vibration experiments have been completed in full bundles with a tube pitch-to-diameter ratio of 1.42 and six equal crosspasses produced by segmental baffles. The results of these water-flow tests are compared with tests of similar bundles with a tube pitch-to-diameter ratio of 1.25. It had been anticipated that the vibration characteristics would be similar with only differences in the flows required for vibration to start. This was not the case.
For bundles with the narrower pitch ratio, there was a sharp rise in vibration amplitude with a modest increase in flow, characteristic of "classic" fluidelastic instability vibration. The vibration always started in tubes with the lowest natural frequency and in the same region of these bundles with the baffles of the same cut orientation. In contrast, for bundles with the wider pitch ratio, the start of vibration for water-flow tests was more difficult to identify. "Classic" fluidelastic instability was not always the type of vibration first observed. Sometimes, vibration started in tubes with more support and higher natural frequencies. In addition, the orientation of the baffle cut relative to the axis of the nozzles was found to produce significant differences in the flows when vibration started, in which tubes vibrated, and at which mode.