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

Interaction Between Swirl Number Fluctuations and Vortex Shedding in a Single-Nozzle Turbulent Swirling Fully-Premixed Combustor

[+] Author and Article Information
Nicholas A. Bunce

e-mail: nab5001@psu.edu

Bryan D. Quay

e-mail: bdq100@psu.edu

Domenic A. Santavicca

e-mail: das8@psu.edu
Center for Advanced Power Generation,
Department of Mechanical and
Nuclear Engineering,
The Pennsylvania State University,
University Park, PA 16802

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 8, 2013; final manuscript received July 30, 2013; published online October 28, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(2), 021503 (Oct 28, 2013) (11 pages) Paper No: GTP-13-1242; doi: 10.1115/1.4025361 History: Received July 08, 2013; Revised July 30, 2013

Flame response to imposed velocity fluctuations is experimentally measured in a single-nozzle turbulent swirling fully-premixed combustor. The flame transfer function is used to quantify the flame's response to imposed velocity fluctuations. Both the gain and phase of the flame transfer function are qualitatively similar for all operating conditions tested. The flame transfer function gain exhibits alternating regions of decreasing gain with increasing forcing frequency followed by regions of increasing gain with increasing forcing frequency. This alternating behavior gives rise to gain extrema. The flame transfer function phase magnitude increases quasi-linearly with increasing forcing frequency. Deviations from the linear behavior occur in the form of inflection points. Within the field, the current understanding is that the flame transfer function gain extrema are caused by the constructive/destructive interference of swirl number fluctuations and vortex shedding. Phase-synchronized images of forced flames are acquired to investigate the presence/importance of swirl number fluctuations, which manifest as fluctuations in the mean flame position and vortex shedding in this combustor. An analysis of phase-synchronized flame images reveals that mean flame position fluctuations are present at forcing frequencies corresponding to flame transfer function gain minima but not at forcing frequencies corresponding to flame transfer function gain maxima. This observation contradicts the understanding that flame transfer function gain maxima are caused by the constructive interference of mean flame position fluctuations and vortex shedding, since mean flame position fluctuations are shown not to exist at flame transfer function gain maxima. Further analysis of phase-synchronized flame images shows that the variation of the mean flame position fluctuation magnitude with forcing frequency follows an inverse trend to the variation of flame transfer function gain with forcing frequency, i.e., when the mean flame position fluctuation magnitude increases, the flame transfer function gain decreases and vice versa. Based on these observations it is concluded that mean flame position fluctuations are a subtractive effect. The physical mechanism through which mean flame position fluctuations decrease flame response is through the interaction of the flame with the Kelvin–Helmholtz instability of the mixing layer in the combustor. When mean flame position fluctuations are large the flame moves closer to the mixing layer and damps the Kelvin–Helmholtz instability due to the increased kinematic viscosity, fluid dilatation, and baroclinic production of vorticity with the opposite sign associated with the high temperature reaction zone.

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References

Figures

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Fig. 1

Schematic of the combustor; flow is from left to right

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Fig. 2

Cross section of the nozzle and combustion chamber

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Fig. 3

Schematic drawing illustrating the image processing procedure

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Fig. 4

Example of the flame transfer function gain and phase versus the forcing frequency: Pc = 0.1 MPa, Tin = 373 K, u¯ = 30 m/s, φ = 0.60

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Fig. 5

Flame transfer function (a) gain, and (b) phase versus f* = f/fGmin1: all operating conditions

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Fig. 6

Phase between the swirl number and axial velocity fluctuations at flame transfer function gain extrema frequencies: all operating conditions

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Fig. 7

(a) Ratio of the heat release fluctuation magnitude in the upper and lower windows, and (b) the phase between the upper and lower window heat release fluctuations versus the percentage of total heat release in the lower window at first gain minimum: Pc = 0.1 MPa, Tin = 373 K, u¯ = 25 m/s, φ = 0.75

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Fig. 8

(a) Ratio of the heat release fluctuation magnitude in the upper and lower windows, and (b) the phase between the upper and lower window heat release fluctuations versus the percentage of total heat release in the lower window at gain maximum: Pc = 0.1 MPa,Tin = 373 K, u¯ = 25 m/s, φ = 0.75

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Fig. 9

Example of the revolved image with the mean flame position indicated by the pink dashed line

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Fig. 10

Envelope of the mean flame position over one period of forcing at (a) first gain minimum, (b) gain maximum, and (c) second gain minimum: Pc = 0.1 MPa, Tin = 373 K, u¯ = 25 m/s, φ = 0.75. The flame base is defined as the area to the left of the green line.

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Fig. 11

Envelope of the mean flame position near the flame base over one period of forcing at all forcing frequencies: Pc = 0.1 MPa, Tin = 373 K, u¯ = 25 m/s, φ = 0.75

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Fig. 12

Mean normalized fluctuations in the flame base, flame downstream, and total heat release versus the forcing frequency: Pc = 0.1 MPa, Tin = 373 K, u¯ = 25 m/s, φ = 0.75

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