Vibration Response of a U-Tube Bundle with Anti-Vibration Bar Supports due to Turbulence and Fluidelastic Excitations

Abstract A flow test program is described which characterizes turbulence and fluidelastically induced tube responses of a 5 by 12 U-tube bundle with Anti-Vibration Bar (AVB) supports. The tube bundle was tested with three different tube/AVB support-clearance conditions. Individual, isolated tubes were also investigated with various, precisely set boundary conditions. These latter tests provide accurate tube response and tube/tube support interaction data under conditions of flow turbulence and fluidelastical tube excitation. These results are used for verification of nonlinear finite element computer code simulations of tube vibration and wear. Conventional accelerometer and proximity probe instrumentation identified modal tube responses, and in-plane and out-of-plane tube motion behavior in the U-bend region of the tube bundle. Replacing short tube sections with force/motion measurement devices integral with the tube enabled nonintrusive monitoring of contract/impact forces, as well as of tube in-plane sliding motions when the tube was in contact with an AVB. This arrangement allowed for the experimental determination of "work rates" and impact force histograms. During a series of three tests, using increasing support clearance conditions, complex tube vibration patterns were produced that exhibited effects of AVB movement. Centered tubes, subjected to flow, first responded to turbulence excitation, and then, as flow increased, exhibited predictable fluidelastic instabilities at fundamental vibration modes. Further increase in flow resulted in amplitude controlled limit cycles. These cycles showed gradually increasing response frequencies with evidence of mode switching. The threshold velocity for fluidelastic response increases with the number of AVB contact points and the level of preload. At low cross-flow velocities, work rate results vary significantly for different conditions. At higher cross-flow velocities, similar work-rate data were obtained, even though different wear mechanisms (sliding versus impact) existed.