Date of Award
Doctor of Philosophy (PhD)
Dr. D. S. Weaver
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-emphirical 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 has 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. The model is based on concepts developed by Lever and Weaver, as well as Yetisir and Weaver, but is extended to include some crucial factors. Velocity and pressure fluctuations, caused by tube vibration were obtained by using continuity and momentum equations in curvilinear coordinates. Rather than the nonlinear function previously used, a linear area perturbation decay function was introduced to account for the decay of disturbances away from an oscillating tube. Thus, an analytical solution could be obtained. The resulting explicit instability expression is a more convenient tool to analyze effects of various design parameters. The difference between the linear and nonlinear decay functions was found to have a negligible effect on the stability threshold. Critical velocity in both streamwise and transverse directions was calculated, and the latter is lower in the high mass damping ratio range. Therefore, only the instability in the transverse direction needs to be analyzed in that range. On the other hand, there is no clear trend in the low mass damping ratio range, which agrees with many previous research results. This model is a multiple flexible tube model. Tubes directly two rows upstream and downstream of a central tube, neglected in the Yetisir and Weaver model, but are included in the present model. Significant improvement is obtained for the parallel triangular tube arrays. In contrast, little improvement is achieved on the other tube patterns due to the reduced influence of the upstream tube wake. The velocity distribution and mode shape along the tube span were introduced into the model. This is made possible because of the explicit instability equation. Therefore, the present model, in which fluid flow in two-dimensional, but considering tube mode shape in the third dimension, can be used to calculate the fluidelastic instability for multi-span tube arays. The comparison between the theory and experimental data agrees well.
Li, Ming, "An Experimental and Theoretical Study of Fluidelastic Instability in Cross Flow Multi-Span Exchanger Tube Arrays" (1997). Open Access Dissertations and Theses. Paper 1151.