Vibration Damping in Multispan Heat Exchanger Tubes

Abstract

Heat exchanger tubes can be damaged or fail if subjected to excessive flow-induced vibration, either from fatigue or fretting-wear. Good heat exchanger design requires that the designer understands and accounts for the vibration mechanisms that might occur, such as vortex shedding, turbulent excitation or fluidelastic instability. To incorporate these phenomena into a flow-induced vibration analysis of a heat exchanger requires information about damping. Damping in multispan heat exchanger tubes largely consists of three components: viscous damping along the tube, and friction and squeeze-film damping at the supports. Unlike viscous damping, squeeze-film damping and friction damping are poorly understood and difficult to measure. In addition, the effect of temperature-dependent fluid viscosity on tube damping has not been verified. To investigate these problems, a single vertical heat exchanger tube with multiple spans was excited by random vibration. Tests were conducted in air and in water at three different temperatures (25, 60, and 90°C). At room temperature, tests were carried out at five different preloads. Frequency response spectra and resonant peak-fitted damping ratios were calculated for all tests. Energy dissipation rates at the supports and the rate of excitation energy input were also measured. Results indicate that damping does not change over the range of temperatures tested and friction damping is very dependent on preload.