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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Effect of Fuel System Impedance Mismatch on Combustion Dynamics

[+] Author and Article Information
Geo A. Richards

 National Energy Technology Laboratory, U.S. Department of Energy, PO Box 880, Morgantown, WV 26505gricha@netl.doe.gov

Edward H. Robey

 National Energy Technology Laboratory, Parsons, PO Box 880, Morgantown, WV 26505eroby@netl.doe.gov

Recall that the resonator lengths were normalized by the quarter wavelength at 300Hz. Thus, the resonator mouth has very high impedance at length 2 only at 300Hz. This corresponds to exciting a quarter-wave tube at twice its natural frequency.

J. Eng. Gas Turbines Power 130(1), 011510 (Jan 16, 2008) (7 pages) doi:10.1115/1.2771249 History: Received June 30, 2006; Revised July 21, 2006; Published January 16, 2008

Combustion dynamics are a challenging problem in the design and operation of premixed gas turbine combustors. In premixed combustors, pressure oscillations created by the flame dynamic response can lead to damage. These dynamics are typically controlled by designing the combustor to achieve a stable operation for planned conditions, but dynamics may still occur with minor changes in ambient operating conditions or fuel composition. In these situations, pilot flames or adjustment to fuel flow splits can be used to stabilize the combustor, but often with a compromise in emission performance. As an alternative to purely passive design changes, prior studies have demonstrated that adjustment to the fuel system impedance can be used to stabilize combustion. Prior studies have considered just the response of an individual fuel injector and combustor. However, in practical combustion systems, multiple fuel injectors are used. In this situation, individual injector impedance can be modified to produce a different dynamic response from individual flames. The resulting impedance mismatch prevents all injectors from strongly coupling to the same acoustic mode. In principle, this mismatch should reduce the amplitude of dynamics and may expand the operating margin for stable combustion conditions. In this paper, a 30kW laboratory combustor with two premixed fuel injectors is used to study the effect of impedance mismatch on combustion stability. The two fuel injectors are equipped with variable geometry resonators that allow a survey of dynamic stability while changing the impedance of the individual fuel systems. Results demonstrate that a wide variation in dynamic response can be achieved by combining different impedance fuel injectors. A base line 7% rms pressure oscillation was reduced to less than 3% by mismatching the fuel impedance.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Geometry of combustor model, showing the nomenclature for a single fuel injector. Multiple fuel injectors are analyzed in the same manner.

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Figure 2

Gain and phase of transfer function for two fuel injectors showing the effects of resonator tuning. Diagonal and horizontal lines show equal tuning and one unequal situation.

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Figure 3

Plot of calculated transfer function gain for various resonator length combinations. Base line compared to observed minimum and maximum gain.

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Figure 4

Layout of combustor using two premix fuel injectors

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Figure 5

Details of fuel premixer, with variable length resonator

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Figure 6

Observed combustor rms pressure and oscillating frequency versus equivalence ratio at various bulk velocities. Resonator position (0,0).

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Figure 7

Contour plots of the rms pressure (a) and oscillating frequency (b) as a function of the resonator lengths. The resonator length is normalized by the 1∕4 wavelength at 300Hz(0.371m).

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Figure 8

Plot of the cotangent term in Eq. 5 showing the repeating impedance value for normalized lengths greater than 2

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