Experiments on Tube/Support Interaction With Feedback-Controlled Instability

Abstract

Due to tube-support gaps in heat-exchangers, low-frequency modes may develop and become unstable at comparatively low flow velocities. This kind of linear fluidelastic instability results in a negative modal damping value, which is a function of the flow velocity. The response amplitude of the unstable tubes increases steadily until tube-support impact becomes unavoidable. These extremely nonlinear vibratory motions have a high-risk potential, as tube velocities and impact forces can be of very considerable magnitude. This paper reports results on a series of laboratory experiments, intended to validate nonlinear calculations on vibro-impact dynamics of heat exchanger tube bundles under fluidelastic instability. The test model was designed for unidirectional motion and the results obtained should be fairly representative of the actual behavior of the U-bend portion of the heat exchanger tube bundles. The system instability is generated by a velocity feedback loop. This method presents significant advantages due to simplicity of the setup and the controllability of the system parameters, in particular concerning the negative damping ratio of the unstable model. A comparison of experimental and computed system dynamics is presented for several values of the instability growth rate and for various initial conditions of the motion. Influence of other parameters, such as tube-support gap magnitude and gap symmetry, is asserted for realistically ranged values. Results show that several steady motion regimes may arises, depending on the system parameters and initial conditions of the motion, which is a fact of engineering significance. Furthermore, a satisfactory qualitative and quantitative agreement was obtained between theoretical predictions and test data.