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

Abstract

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 (LSMBs) 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 an unshrouded LSMB, can lead to destabilizing aerodynamic forces during vibration. At a sufficiently high mass flow, the destabilizing 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 program is presented, with the purpose of identifying the margin for safe operation with respect to blade flutter. Unsteady computational fluid dynamics predictions are utilized 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 that demonstrate the identification of the aeroelastic stability margin for a new LSMB are presented. The stability margin predicted from the measurements demonstrates a significant margin of safety.

Advanced Flutter Analysis of a Long Shrouded Steam Turbine Blade

“…These long blades are susceptible to flutter because of their low structural frequency and supersonic tip speeds. Although no steam turbine blade failure (loss of blade) due to flutter has been reported in the literature [1], blade flutter is a concern for the manufacturers of steam turbines [1,2] and blade root cracking at the last stage of a steam turbine due to flutter has been reported [3].…”

“…The results of three-dimensional unsteady flow calculations performed for the purpose of flutter analysis of steam turbine blades have been presented previously. These methods solved the inviscid flow equations [1,3,4] and the URANS flow equations [2] in the time domain.…”

Non Contact Measurement System with Electromagnets for Vibration Tests on Bladed Disks

“…Electromagnets allows to release the excitation frequency from the rotation speed by using an AC feeding (Prchlik et al, 2009) in spite of a more complexity of the excitation system: more room is required to place the EMs, heat must be dissipated (which can be a critical issue in vacuum), an amplifier system and a signal generator must be provided. Hybrid solution can be also found in literature as in (Rice et al, 2007) where magnetic excitation is used as an extra source of excitation in engine tests with gas flow in order to explore flutter onset at different engine orders for the same rotation speed i.e., for the same aerodynamic load, air mass flow and centrifugal effects. Moreover in (Szwedowicz et al, 2007) a really cheap exciter is constituted by an air jet simply generated by the pressure drop between room condition and the vacuum chamber.…”

Analysis of an experimental setup for structural damping identification

“…The used test rig, in combination with the analysis method, offers the possibility to identify the decay curve of one mode shape in order to calculate the damping values. From this, decay fitting is used (see Rice et al, 2007 and. Then further experimental results are presented, which investigate in addition to the influence of ambient pressure also the influence of the clamping conditions on the determined specimen specific damping value of stainless steel.…”