Research Papers

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

[+] Author and Article Information
Tim Rice, David Bell, Gurnam Singh

 ALSTOM, Newbold Road, Rugby CV21 2NH, England

J. Turbomach 131(1), 011009 (Oct 17, 2008) (10 pages) doi:10.1115/1.2812339 History: Received June 08, 2007; Revised July 19, 2007; Published October 17, 2008

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.

Copyright © 2009 by ALSTOM
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Figure 1

ND45A (50Hz) last stage moving blade

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Figure 2

ND45A LSMB mode description

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Figure 3

Datum LSMB mode shape description

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Figure 4

Predicted aerodynamic stability (log-dec) for the ND45A and datum LSMB (design mass flow)

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Figure 5

Predicted local aerodynamic damping for the ND45A LSMB (log-dec), defined asδA−local=(aerodynamicworkpercycleperunitspan)2×(overallstrainenergy)

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Figure 6

Predicted aerodynamic stability of the ND45A with the datum LSMB frequency and mode shape applied

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Figure 7

Quasisteady illustration of cross-coupling of unsteady pressure due to axial bending and torsional vibration components

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Figure 8

Assembly of the model turbine

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Figure 10

Magnet assembly installed in upstream blade carrier

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Figure 11

The installed magnet can just be seen through the tip gap over the blade (see arrow)

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Figure 12

An example of a magnet off decay curve

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Figure 13

The individual δ values from the analysis

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Figure 14

Averaged values for aggregate damping

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Figure 15

Method to determine the aerodynamic damping from the aggregate damping measurements

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Figure 16

Extrapolating the measurements back to zero flow gives δM, the projected mechanical damping

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Figure 17

Calculated values of aerodynamic damping

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Figure 18

Comparison of the aerodynamic damping values from the measurements with the CFD predictions

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Figure 19

3D visualization of the measurements obtained and the extrapolated information for mechanical damping and the location of the onset of flutter

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Figure 20

Identification of the predicted location of the onset of blade flutter



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