Identification of the Stability Margin Between Safe Operation and the Onset of Blade Flutter

The introduction of longer last stage blading in steam turbine power plant offers significant economic and environmental benefits. The modern trend, adopted by most leading steam turbine manufacturers, is to develop long last stage moving blades (LSMB) that feature a tip shroud. This brings benefits of improved performance due to better leakage control and increased mechanical stiffness. However, the benefits associated with the introduction of a tip shroud are accompanied by an increased risk of blade flutter at high mass flows. The shroud is interlocked during vibration, causing the first axial bending mode to carry an increased, out of phase, torsional component. It is shown that this change in mode shape, compared to a unshrouded LSMB, can lead to destabilising aerodynamic forces during vibration. At a sufficiently high mass flow, the destabilising unsteady aerodynamic work will exceed the damping provided by the mechanical bladed-disk system, and blade flutter will occur. Addressing the potential for flutter during design and development is difficult. Simple tests prove inadequate as they fail to reveal the proximity of flutter unless the catastrophic condition is encountered. A comprehensive product validation programme is presented, with the purpose of identifying the margin for safe operation in respect to blade flutter. Unsteady CFD predictions are utilised to identify the mechanisms responsible for the unstable aerodynamic condition, and the particular modes of vibration that are most at risk. Using this information, a directed experimental technique is applied to measure the combined aerodynamic and mechanical damping under operating conditions. Results are presented which demonstrate the identification of the aeroelastic stability margin for a new LSMB. The stability margin predicted from the measurements demonstrates a significant margin of safety.