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Research Papers

Combustion Instability and Emission Control by Pulsating Fuel Injection

[+] Author and Article Information
Christian Oliver Paschereit

Hermann-Föttinger-Institute,  Technical University Berlin, 10623 Berlin, Germanyoliver.paschereit@tu-berlin.de

Ephraim Gutmark

Aerospace Engineering and Engineering Mechanics Department,  University of Cincinnati, Cincinnati, OH 45221-0070ephraim.gutmark@uc.edu

J. Turbomach 130(1), 011012 (Jan 25, 2008) (8 pages) doi:10.1115/1.2749292 History: Received July 21, 2005; Revised January 28, 2007; Published January 25, 2008

Open-loop control methodologies were used to suppress symmetric and helical thermoacoustic instabilities in an experimental low-emission swirl-stabilized gas-turbine combustor. The controllers were based on fuel (or equivalence ratio) modulations in the main premixed combustion (premixed fuel injection (PMI)) or, alternatively, in the secondary pilot fuel. PMI included symmetric and asymmetric fuel injection. The symmetric instability mode responded to symmetric excitation only when the two frequencies matched. The helical fuel injection affected the symmetric mode only at frequencies that were much higher than that of the instability mode. The asymmetric excitation required more power to obtain the same amount of reduction as that required by symmetric excitation. Unlike the symmetric excitation, which destabilized the combustion when the modulation amplitude was excessive, the asymmetric excitation yielded additional suppression as the modulation level increased. The NOx emissions were reduced to a greater extent by the asymmetric modulation. The second part of the investigation dealt with the control of low frequency symmetric instability and high frequency helical instability by the secondary fuel injection in a pilot flame. Adding a continuous flow of fuel into the pilot flame controlled both instabilities. However, modulating the fuel injection significantly decreased the amount of necessary fuel. The reduced secondary fuel resulted in a reduced heat generation by the pilot diffusion flame and therefore yielded lower NOx emissions. The secondary fuel pulsation frequency was chosen to match the time scales typical to the central flow recirculation zone, which stabilizes the flame in the burner. Suppression of the symmetric mode pressure oscillations by up to 20dB was recorded. High frequency instabilities were suppressed by 38dB, and CO emissions reduced by using low frequency modulations with 10% duty cycle.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Experimental facility. The figure in the inset is taken from Zajadatz (27).

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Figure 2

Visualization of phase averaged OH images at two phase angles of 0deg and 180deg. (a), (b) Axisymmetric structure (premixed, St=0.58). (c), (d) Helical structure (premixed, St=1.16). (e), (f) Helical structure (premixed flame, St=7.77).

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Figure 3

Frequency response of pressure and OH fluctuations to an open-loop controller with a symmetric pulsed fuel injection (F∕Fmax=15%)

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Figure 4

Amplitude response of pressure and OH fluctuations to an open loop-controller with a symmetric pulsed fuel injection (St=0.61)

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Figure 5

NOx emissions as a function of frequency using an open-loop controller with a symmetric pulsed fuel injection (F∕Fmax=15%) and with an antisymmetric pulsed fuel injection (F∕Fmax=50%)

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Figure 6

Frequency response of pressure and OH fluctuations to an open-loop controller with an antisymmetric pulsed fuel injection (F∕Fmax=50%)

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Figure 7

Suppression of pressure and OH fluctuations by SPI and its effect on NOx and CO emissions

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Figure 8

Temporal combustion-pressure variation during the control of the periodical onset of high frequency instability by pulsed secondary fuel injection. 20% duty cycle.

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Figure 9

Response of pressure and OH fluctuations to modulations of pilot flame, and the effect on NOx and CO emissions. Pilot fuel at 20% duty cycle.

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Figure 10

Variation of pressure and OH fluctuations, and NOx and CO emissions as a function of the duty cycle. Secondary fuel at 4.4% and forcing frequency: St=0.066.

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Figure 11

Variation of pressure fluctuations with an equivalence ratio for continuous and pulsed secondary fuel injection. Secondary fuel flow rate of 4.4%, 10% duty cycle, and St=0.066.

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Figure 12

Variation of NOx and OH fluctuations with an equivalence ratio for continuous and pulsed secondary fuel injection. Secondary fuel flow rate of 4.4%, 10% duty cycle, and St=0.066.

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Figure 13

Variation of CO with an equivalence ratio for continuous and pulsed secondary fuel injection. Secondary fuel flow rate of 4.4%, 10% duty cycle, and St=0.066.

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Figure 14

Variation of pressure oscillations with a combustion power at a nominal equivalence ratio. Secondary fuel flow rate of 4.4%, 10% duty cycle, and St=0.066.

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Figure 15

Suppression of high frequency pressure and OH fluctuations by continuous pilot fuel injection

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Figure 16

Response of high frequency pressure and OH fluctuations to modulations of pilot flame. Pilot fuel at 20% duty cycle.

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