Dynamics of an inline tube array in steam-water flow. Part 3. Fluidelastic instability tests and comparison with theory

Comprehensive flow induced vibration tests have been conducted on an inline array. The responses of three essentially isolated flexible cylinders at different depths within the array were simultaneously measured. Tube excitation was provided by steam-water two-phase flow. The effect of various important parameters in the tube responses was investigated. Tests reported here were conducted simultaneously with the damping tests reported in Part 1 of this study. At the highest pressures (3.0 and 5.8 MPa), strong instabilities, in homogeneous flow akin to single phase flow occurred. The test tube located deep within the array was the most susceptible to instability. This was attributed partly to reduce two-phase damping deep in the array, while differences in local fluid forces at different locations in the array is not discounted. Vastly different tube response occurred for 0.5 MPa pressure. The flow in this case was non-homogeneous intermittent slug type flow. Strong turbulence excitation obscured clear fluidelastic instability. This report is the third of a three-part series. Unsteady fluid dynamic forces were presented in Part 2. Vibration tests reported here provide ideal data for testing the accuracy of an unsteady fluid dynamic model for fluidelastic vibration. Preliminary calculation results for the case of P = 5.8 MPa show very reasonable agreement between the measured instability velocities and the theoretically predicted stability boundary