Modelling two-phase flow-excited fluidelastic instability in heat exchanger tube arrays.

摘要

This thesis reports on the investigation of flow induced vibration in heat exchanger tube arrays. This work is in support of nuclear steam generator design, where attention is focussed on the tubes in the upper U-bend region which are subjected to cross-flow, and are therefore most susceptible to the destructive effects of flow-induced vibration. Results for this study are reported for a dynamically scaled model tube array in a parallel triangular layout, with a pitch over diameter ratio of P/D = 1.44, mounted in a clamped-free arrangement. This tube bundle was subjected to single and two-phase cross-flows of refrigerant R-11.; The main motivation of this work is to present experimental results on fluidelastic instability, the mechanism that usually causes the most damaging vibrations, and to make comparisons with other data on the basis of non-dimensional similitude parameters to determine the effect of using different fluid mixtures to simulate the actual steam-water flow in the steam generator. A new method is proposed for calculating the average fluid density and equivalent flow velocity of the two-phase fluid, using a newly developed void fraction model which accounts for the difference in velocity between the gas and liquid phases. The fluidelastic data of several researchers, who used a variety of fluids, is re-examined using this new void fraction model and the results show a remarkable difference in trend between two common flow regimes, bubbly and intermittent flow. The latest flow regime map, developed by other researchers for predicting the two-phase flow regimes in the shell-side cross-flows in tube bundles, was applied to the fluidelastic results. This analysis showed that the sudden change in stability behavior which appeared in various data sets were directly related to a predicted change in flow regime from bubbly to intermittent flow.; The two-fluid model used in the fluidelastic data analysis was developed by the present author from experimental measurements of void fraction in the horizontal tube bundle using the gamma densitometer.; Measurements of the damping effect of two-phase flows were obtained in the present study and are presented and compared with previous data using an existing analysis technique. Damping measurements showed that the tube damping peaks at about 75% to 80% HEM void fraction, and decreases at lower and especially higher void fraction.; The turbulence buffeting response of the tubes was measured and from this the non-dimensional spectrum of turbulent forces was determined and compared with existing data. In single phase flow, the data of the present study appears to plot higher than the extensive data points of Taylor et al. (1996), but are mostly within the upper bound determined by Taylor and Pettigrew (1999). In two-phase flow, the turbulent forces were analysed according to the new data reduction method of deLangre and Villard (1998). (Abstract shortened by UMI.)