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Ignition Delay Time Modulation as a Contribution to Thermo-Acoustic Instability in Sequential Combustion

[+] Author Affiliations
Alexander Ni

ABB Alstom Technology Ltd., Baden-Dättwil, Switzerland

Wolfgang Polifke

Technical University of Munich, Garching, Germany

Franz Joos

Fachhochschule Köln, Köln, Germany

Paper No. 2000-GT-0103, pp. V002T02A023; 9 pages
doi:10.1115/2000-GT-0103
From:
  • ASME Turbo Expo 2000: Power for Land, Sea, and Air
  • Volume 2: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations
  • Munich, Germany, May 8–11, 2000
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-7855-2
  • Copyright © 2000 by ASME

abstract

Pressure pulsations due to combustion instabilities have been encountered in a premixed sequential gas turbine combustor. Measured noise spectra display one or several distinct peaks at Strouhal numbers significantly larger than unity. Height and location of the peaks depend in a sensitve manner on fuel type and/or operating conditions.

The paper identifies a possible mechanism of the observed combustion instability and presents a mathematical model of acoustic self-excitation. The mechanism of self-excitation comprises interactions between the acoustic field in the fuel injector / burner with the ignition delay time of the fuel-air mixture and the heat release intensity:

• pressure drop in the fuel injector nozzle changes with variations of the acoustic pressure in the burner,

• variations of pressure drop and air flow velocity modulate the fuel concentration,

• acoustic perturbations in the pre-flame region influence the delay time for self-ignition and consequently lead to fluctuations of flame velocity and -position.

• fluctuations of flame velocity influence the refracation of acoustic waves at the flame front.

• fuel inhomogeneities modulate the heat release rate and consequently the rate of volume production by the flame.

Based on this structure of a self-excitation mechanism, an analytical model has been developed and used to compute eigenfrequencies and growth rates of instabilities. Some characteristics of the suggested self-excitated instabilities as they are predicted by the model match well with empirical information.

Copyright © 2000 by ASME

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