0
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
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 1

ND45A (50Hz) last stage moving blade

Grahic Jump Location
Figure 2

ND45A LSMB mode description

Grahic Jump Location
Figure 3

Datum LSMB mode shape description

Grahic Jump Location
Figure 4

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

Grahic Jump Location
Figure 5

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

Grahic Jump Location
Figure 6

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

Grahic Jump Location
Figure 7

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

Grahic Jump Location
Figure 8

Assembly of the model turbine

Grahic Jump Location
Figure 10

Magnet assembly installed in upstream blade carrier

Grahic Jump Location
Figure 11

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

Grahic Jump Location
Figure 12

An example of a magnet off decay curve

Grahic Jump Location
Figure 13

The individual δ values from the analysis

Grahic Jump Location
Figure 14

Averaged values for aggregate damping

Grahic Jump Location
Figure 15

Method to determine the aerodynamic damping from the aggregate damping measurements

Grahic Jump Location
Figure 16

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

Grahic Jump Location
Figure 17

Calculated values of aerodynamic damping

Grahic Jump Location
Figure 18

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

Grahic Jump Location
Figure 19

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

Grahic Jump Location
Figure 20

Identification of the predicted location of the onset of blade flutter

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In