Tube Row and Damping Parameter Effects on Tube-array Stability

Abstract

<p>Modern heat-exchangers are susceptible to damage due to the excessive tube vibrations caused by the shell side fluid flow. The present investigations seeks to further the understanding of flow induced vibrations due to fluid-elastic instability in tube arrays.</p> <p>A low-speed wind-tunnel having 305 x 305 mm, working-section, was used to conduct the experiments. The tube-array was a parallel-triangle having a pitch/diameter = 1.375. The array was 18 rows deep with 5 tubes in each row. Nineteen elastically mounted movable tubes in the front of the array were especially designed so that natural frequency and damping could be controlled precisely over a wide range of values. Positions for as many as nine rows of fixed tubes in the front of the tube-array were available in order to facilitate studying the effect of tube-bundle size on tube response.</p> <p>The experiments have indicated that the first tubes to become unstable in a particular tube-bundle are in the first few rows, and that the third and fourth rows are the critical ones. From the fluid dynamic damping-flow velocity results obtained for the array tested, it suggests the use of the static damping measured in still fluid for the estimate of the velocity for the onset of instability. For the first time a fluidelastic stability boundary as a function of the mass ratio has been determined experimentally for the array. This result along with the result obtained by Grover for the fluidelastic stability boundary as a function of the damping only, show that the mass ratio and the damping behave independently of each other, i.e., they should not be lumped together in the damping factor.</p>