DAMPING PREDICTION METHOD FOR ASYNCHRONOUS VIBRATION MODES AND VERIFICATION BY ELECTROMAGNETIC EXCITATION FOR FULL SCALE TURBINE BLADE

The blades at the end stage of low-pressure turbine (LP-End) become thinner and longer to increase the efficiency and power of steam turbines even under severe conditions. Integral shrouded blades are often used in the low-pressure turbine to increase blade stiffness and get frictional damping to reduce the blade vibration. It is important to predict the frictional damping characteristics of such shrouded blades analytically during the design phase, and many analytical methods have been vigorously proposed. However, the behavior of the friction damping is complex due to nonlinearities such as contact and sliding, it is difficult to make a high-fidelity model to express the friction. For example, it is important to consider micro-slip phenomena, to initiate sliding due to small excitation force in localized areas, and formulating the contact stiffness. In this paper, with the aim of incorporating damping prediction into the design process, reasonable micro- slip model was developed by predicting the contact conditions during operation in advance, and calculating the damping considering the contact areas of shroud and stub, as well as the distribution of contact forces. And then, full-scale testing of turbine blades with the electromagnetic excitation system was conducted to validate the analytical method, and vibratory stress and damping of the blades were measured by strain gauges in the test facility. As the result, it was confirmed that the analytical predictions matched well with the measured damping of the high nodal diameter (ND) of the 1st mode by asynchronous excitation.