Estimation of the Fluidelastic Instability Boundary for Steam-Generator Tubes Subjected to Two-Phase Flows

The estimation of tube wear due to vibration-induced tube-support impacting remains an important problem for operating steam generators. For fluidelastic excitation this requires a realistic model for the excitation fluid forces. Recently the authors (Mureithi et al., 2008) have presented detailed force measurements in a rotated triangular tube bundle subjected to air-water two-phase flow. The data provides new potential to designers to estimate expected fluid dynamic loads for two-phase flow conditions. In particular, the data may be used for quantitative estimation of critical velocities for fluidelastic instability — the most important excitation mechanism for steam generator tube bundles. In the work reported here the fluid force field is employed along with the quasi-steady vibration stability model, adapted to two-phase flows, to model the two-phase flow problem and predict the critical instability velocity for a steam-generator U-tube subjected to non-uniform two-phase flow. The implication of the present model, with respect to improved estimation of tube wear is discussed in the paper.