Theoretical Model Development For Fluidelastlc Instability of Heat Exchanger Tube Arrays

Abstract

<p>A study is carried out to investigate the cross-flow induced instabilIties In heat exchanger tube arrays. For this purpose. the shortcomings of the Lever and Weaver unsteady theoretical model for a single flexible tube are dealt with and the modified model is extended to a multiple flexible tube analysis. Among the significant modifications is the introduction of a decay function to take into account the decay of the perturbations. This model predicts both static and dynamic instabiIities in the transverse and longitudinal directions. It was found that a single flexible tube become tends towards divergence at high values of the mass-damping ratio. This phenomenon is associated with smaller vibration frequencies than the natural frequency of the heat exchanger tube and approaches zero (divergence) at very high mass-damping ratios. The single flexible tube model is extended to a multiple flexible tube model to investigate the effect of the motion of neighboring tubes. It was found that this effect is very important at high values of the mass-damping ratio where the instability is dominated by stiffness terms. The decay function is investigated experimentally. Velocity fluctuations are measured up to 4 tube rows upstream the Vibrating tube. The experimentally determined decay function is used to predict the critical velocities for the dynamic instability. Equations of the theoretical model are solved numerically and the agreement between the experimental data and the theoretical predictions is reasonable for all array configurations.</p>