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

Convective Scaling of Intrinsic Thermo-Acoustic Eigenfrequencies of a Premixed Swirl Combustor

[+] Author and Article Information
Alp Albayrak

Fakultät für Maschinenwesen,
Technische Universität München,
Boltzmannstraße 15,
Garching D-85747, Germany
e-mail: albayrak@tfd.mw.tum.de

Thomas Steinbacher

Fakultät für Maschinenwesen,
Technische Universität München,
Boltzmannstraße 15,
Garching D-85747, Germany
e-mail: steinbacher@tfd.mw.tum.de

Thomas Komarek

Fakultät für Maschinenwesen,
Technische Universität München,
Boltzmannstraße 15,
Garching D-85747, Germany

Wolfgang Polifke

Fakultät für Maschinenwesen,
Technische Universität München,
Boltzmannstraße 15,
Garching D-85747, Germany
e-mail: polifke@tfd.mw.tum.de

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 July 28, 2017; final manuscript received August 1, 2017; published online November 7, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(4), 041510 (Nov 07, 2017) (9 pages) Paper No: GTP-17-1407; doi: 10.1115/1.4038083 History: Received July 28, 2017; Revised August 01, 2017

Spectral distributions of the sound pressure level (SPL) observed in a premixed, swirl stabilized combustion test rig are scrutinized. Spectral peaks in the SPL for stable as well as unstable cases are interpreted with the help of a novel criterion for the resonance frequencies of the intrinsic thermo-acoustic (ITA) feedback loop. This criterion takes into the account the flow inertia of the burner and indicates that in the limit of very large flow inertia, ITA resonance should appear at frequencies where the phase of the flame transfer function (FTF) approaches π/2. Conversely, in the limiting case of vanishing flow inertia, the new criterion agrees with previous results, which state that ITA modes may arise when the phase of the FTF is close to π. Relying on the novel criterion, peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. Various combustor configurations are investigated over a range of operating conditions. It is found that in this particular combustor, ITA modes are prevalent and dominate the unstable cases. Remarkably, the ITA frequencies change significantly with the bulk flow velocity and the position of the swirler but are almost insensitive to changes in the length of the combustion chamber (CC). These observations imply that the resonance frequencies of the ITA feedback loop are governed by convective time scales. A scaling rule for ITA frequencies that relies on a model for the overall convective flame time lag shows good consistency for all operating conditions considered in this study.

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Figures

Grahic Jump Location
Fig. 1

Intrinsic thermo-acoustic feedback loop of a velocity sensitive flame: heat release q′ responds to fluctuations of upstream velocity u′c, thereby generating a characteristic wave g, which propagates upstream, where it perturbs velocity u′c. The resulting intrinsic feedback loop does not involve reflection of acoustic waves at combustor inlet or outlet. R and T represent coefficients of reflection and transmission by the discontinuity of acoustic impedance across the flame, respectively.

Grahic Jump Location
Fig. 2

Sketch of the Beschaufelter-Ring-Spalt (BRS) test rig

Grahic Jump Location
Fig. 3

Stable operating condition with LCC = 300 mm. Vertical lines indicate ITA frequencies according to −π criterion (- . -) and refined criterion (- - -), respectively. Top three rows: measurements (○) versus model (––––) of FTF. Row 1 and 2: gain and phase of FTF. Row 3: absolute value of dispersion relation, solution of FM−χ=0 (•), local minima of |F−χ| (×). Row 4: measured sound pressure levels.

Grahic Jump Location
Fig. 4

SPL for unstable 30 kW case. Left/right: front/rear swirler position. Vertical lines: refined criterion - - -.

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Fig. 5

Flame intrinsic configuration

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Fig. 6

Flame and burner placed in an anechoic environment

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Fig. 7

Block diagram for a thermo-acoustic system

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Fig. 8

Phasor diagrams for flame-only (left) and burner-flame (right) ITA feedback loops. Phase lags of acoustics and flame response are represented by circular arrows indicated by Zh/Zc and 1+θ/F, respectively.

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Fig. 9

Axial distribution of normalized OH* intensity emission –––I. 30 kW - - -, 50 kW, and 70 kW - . -.

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Fig. 10

Normalized variation of the dominant ITA frequency over the bulk flow velocity inside the burner. ── model from Eq. (20), - - - lowest ITA frequency of swirler rear setup, - . - lowest ITA frequency of swirler front setup.

Grahic Jump Location
Fig. 11

Relevant length scales of the burner. LF is the flame length and LD is the duct length from the swirler to the combustion chamber.

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