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

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

[+] Author and Article Information
Bernhard C. Bobusch

e-mail: bernhard.bobusch@tu-berlin.de

Christian Oliver Paschereit

Chair of Fluid Dynamics,
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Street 8,
Berlin 10623, Germany

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 23, 2013; final manuscript received August 29, 2013; published online November 1, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(2), 021506 (Nov 01, 2013) (8 pages) Paper No: GTP-13-1319; doi: 10.1115/1.4025375 History: Received August 23, 2013; Revised August 29, 2013

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatiotemporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The equivalence ratio fluctuations in the reaction zone are measured spatially and temporally resolved using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that, despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

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References

Figures

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Fig. 2

OH* images of the flame: perfectly premixed (a), perfectly premixed with 15 technically premixed (b), and technically premixed (c)

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Fig. 1

Schematic representation of the test rig

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Fig. 3

Flame transfer functions for fuel flow excited flame (circles), acoustically excited perfectly premixed flame (squares), and acoustically excited premixed flame (asterisks)

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Fig. 4

Calibration curve for integrated chemiluminescence ratio

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Fig. 5

Measured and modeled global transfer function for equivalence ratio fluctuations. The solid line with markers denotes measurement points and the dashed line model results.

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Fig. 6

Spatial distribution of the mixing transfer function at 14 Hz: (a) amplitude, (b) phase

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Fig. 7

Spatial distribution of the mixing transfer function at 109 Hz: (a) amplitude, (b) phase

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Fig. 8

Spatial distribution of the mixing transfer function at 189 Hz: (a) amplitude, (b) phase

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