Date of Award
Doctor of Philosophy (PhD)
Of the various excitation mechanisms causing excessive tube vibrations in tube and shell heat exchangers, fluidelastic instability is the most dangerous, and therefore has received the most attention. The objective of this research is to advance the current understanding of the fluidelastic instability in tube arrays through a fundamental investigation of the phenomenon experimentally, numerically and analytically. The concept of using a single flexible tube in a rigid array to investigate fluidelastic instability has been critically reviewed. It was found that the fluidelastic instability threshold in tube arrays is significantly affected by array geometry, pitch ratio, mass ratio and tube row location in the array. The results showed that, in general, fluidelastic instability in tube arrays is caused by a combination of the damping and the stiffness mechanisms. It was concluded that while the use of a single flexible tube in a rigid array provided a useful model for fundamental research and physical insights, it must be cautioned that it is not generally adequate for determining the experimental stability limits of tube arrays. The outcomes of this critical review helped in the design of a new experiment which facilitated precise control of the system parameters, and provided more comprehensive measurements.
Experimental investigation of the interaction between tube vibrations and fluid forces was conducted using surface pressure measurements at the tube surface. The results showed that, there is a finite time delay between tube vibration and the associated fluid forces acting on the tube. The resulting phase lag was found to increase as the mean gap velocity increased, and ultimately the fluid forces became in phase with the tube velocity during the onset of instability. Velocity measurements of the interstitial flow perturbations associated with tube vibrations were carried out along the flow path in the array. It was found that the flow perturbation amplitude is most pronounced at the flow separation point from the vibrating tube, and that the flow perturbation amplitude decays continuously to a negligible amplitude at about one and a half rows upstream. This suggests that the flow perturbations are caused by the flow separation from the tube and the associated vorticity shedding and convection. The phase lag measurements between tube vibrations and flow perturbations support this conclusion, and show that the flow perturbations propagate upstream and downstream at different rates. Computational Fluid Dynamics modeling of the tube array was developed to assist in understanding the experimental results. The CFD models were validated using experimental data from both the literature and from the present research. It was found that there are two propagation mechanisms for the flow perturbations associated with tube vibrations. The first mechanism is caused by the pressure pulsation due to tube vibrations. This mechanism is dominant at lower reduced velocities, and propagates at the speed of sound. The second mechanism is caused by the flow separation and the associated vorticity shedding, and this mechanism is dominant at higher reduced velocities. The transition between the two mechanisms occurs at a reduced velocity of about (Ur≈2). Mathematical models of the flow perturbation phase lag and amplitude decay were developed. The new models were coupled with the semi-analytical model after modifying its geometrical parameters according to the flow visualizations in the literature. The resulting stability maps show a significant improvement to the current prediction of the fluidelastic instability data in the literature. The outcomes of the present work can contribute to improve the future design guidelines for tube and shell heat exchangers to achieve extended service time with higher efficiency.
Khalifa, Ahmed, "Fluidelastic Instability in Heat Exchanger Tube Arrays" (2012). Open Access Dissertations and Theses. Paper 6630.
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