| An experimental study was conducted to investigate fluidelastic instability in multi-span heat exchanger tube arrays. This work is in support of nuclear steam generator design, especially with regard to the U-bend and inlet regions, where tubes are subjected to non-uniform cross flow. The design guidelines defined in the current ASME codes and other recommended semi-empirical formulas for fluidelastic instability have been based on the extension of experimental results from single span tube bundles. In this study, a specially designed multi-span tube array test rig was used to investigate the effects of partial flow admission. Using this test rig, the water flow can pass across any location along the tube span. Various end supports were used in the different experimental set-ups. Therefore, not only the first mode but also the higher vibration modes can be excited, depending on the location of the flow and tube-support configurations. It was been found that vibration modes higher than the third mode do not have significant vibration displacement. The experiments show that the fluid energy is additive along the span, regardless of the tube mode shape. Response peaks were observed prior to the ultimate fluidelastic instability. By analyzing the corresponding Strouhal numbers, it was found that both vortex shedding and secondary instability mechanisms exist. These two different phenomena may interact and enhance each other. Therefore, high amplitude displacement can be reached even before the ultimate fluidelastic instability. The previous and present experimental data suggest that the energy fraction is a representative parameter in the analysis of the flow induced vibration caused by nonuniform flow velocity distribution. However, existing design guidelines do not always give conservative predictions for the critical velocity. This research reveals that a single correlation of reduced velocity versus mass damping ratio does not follow the same trend in air and liquid flows. An improved design guideline is suggested, which gives consistent conservative flow velocity predictions in multi-span tube arrays.; In parallel, an analytical model was developed for the prediction of fluidelastic instability in cross flow multi-span heat exchanger tube arrays. (Abstract shortened by UMI.)... |
