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Modal Stress Efficiency Scaling for Gas Turbine Combustion Systems

[+] Author Affiliations
W. David Day, Scott Auerbach

Power Systems Manufacturing, LLC (PSM), an Alstom Group Company, Jupiter, FL

Paper No. GT2010-22479, pp. 879-887; 9 pages
doi:10.1115/GT2010-22479
From:
  • ASME Turbo Expo 2010: Power for Land, Sea, and Air
  • Volume 6: Structures and Dynamics, Parts A and B
  • Glasgow, UK, June 14–18, 2010
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-4401-4 | eISBN: 978-0-7918-3872-3
  • Copyright © 2010 by Power Systems Mfg., LLC

abstract

Can-annular combustion systems for gas turbines are complex assemblies of relatively thin components made of heat resistant alloys or superalloys. The thermal environment requires minimal structural constraints in order to accommodate deformation due to thermal growth and expansion. As a result, the systems have the potential to be rich in modes within the range of combustion dynamic frequencies. The natural frequencies of the system are extremely sensitive to variations in contact, damping and thickness. Combustion dynamic frequencies vary, to some extent, from can-to-can and engine-to-engine. Frequencies can also change with ambient conditions, engine load and emissions tuning. Tuning the design to avoid resonant frequencies can be extremely difficult or impractical. A method for determining which modes are likely to be responsive to combustion drivers, for obtaining approximate response magnitudes, for identifying limiting locations, and for comparing two design concepts, would be a great aid to combustion system designers. An analytical technique is proposed for scaling modal stresses. This technique, hereafter referred to as modal stress efficiency scaling (or the acronym MOSES), includes the effect of the efficiency with which a traveling sine wave couples with structural modes of differing wavelengths. Excitation is assumed to come from stationary waves impacting exposed surfaces simultaneously as well as waves traveling in all 3 orthogonal directions. Excitation due to base motion in 3 axis is also considered. In order to quickly evaluate multiple sources of excitation, this technique assumes the driver is at resonance. It is assumed that the modes are widely spaced so no modal superposition is considered. As a result of these simplifications, the response is simple rather than complex. No phase relationships are calculated. The result is the mode shape scaled by the calculated efficiency factor. The method was applied to a combustion system component with a history of cracking, and a low emissions redesign expected to eliminate the problem. The technique identified modes most likely to be excited, and predicted substantially lower responses and stresses in the new design. Results were validated through instrumented engine testing conducted with multiple accelerometers and pressure transducers on engines with each design. Acceleration measurements from these tests confirmed MOSES’s identification of the harmful response and demonstrated the robustness of the redesigned component. The MOSES technique is demonstrated to be a useful tool for root cause analysis as well as for design of combustion hardware.

Copyright © 2010 by Power Systems Mfg., LLC

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