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

Effects of Acoustic Excitation on a Swirling Diffusion Flame

[+] Author and Article Information
Michael E. Loretero

Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, R.O.C.

Rong F. Huang1

Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, R.O.C.rfhuang@mail.ntust.edu.tw

1

Corresponding author.

J. Eng. Gas Turbines Power 132(12), 121501 (Aug 20, 2010) (9 pages) doi:10.1115/1.4001768 History: Received August 10, 2009; Revised April 20, 2010; Published August 20, 2010; Online August 20, 2010

A swirling double concentric jet is commonly used for nonpremixed gas burner application for safety reasons and to improve the combustion performance. Fuel is generally spurted at the central jet while the annular coflowing air is swirled. They are normally separated by a blockage disk where the bluff-body effects further enhance the recirculation of hot gas at the reaction zone. This paper aims to experimentally investigate the behavior of flame and flow in a double concentric jet combustor when the fuel supply is acoustically driven. Laser-light sheet assisted Mie scattering method has been used to visualize the flow, while the flame lengths were measured by a conventional photography technique. The fluctuating velocity at the jet exit was measured by a two-component laser Doppler velocimeter. Flammability and stability at first fuel tube resonant frequency are reported and discussed. The evolution of flame profile with excitation level is presented and discussed, together with the reduction in flame length. The flame in the unforced reacting axisymmetric wake is classified into three characteristic modes, which are weak swirling flame, lifted flame, and transitional reattached flame. These terms reflect their primary features of flame appearances, and when the acoustic excitation is applied, the flame behaviors change with the excitation frequency and amplitude. Four additional characteristic modes are identified; e.g., at low excitation amplitudes, wrinkling flame with a blue annular film is observed because the excitation induces vortices in the central fuel jet and hence gives rise to the wrinkling of flame. The central jet vortices become larger with the increase in excitation amplitude and thus lead to a wider and shorter flame. If the excitation amplitude is increased above a certain value, the central jet vortices change the rotation direction and pacing with the annular jet vortices. These changes in the flow field induce large turbulent intensity and mixing and therefore make the flame looks blue and short. Further increase in the excitation amplitude would lift the flame because the flow field would be dramatically modified.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental setup

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Figure 2

Characteristic modes of the unexcited flame at swirling wake. (a) Shutter speed, 1/1000 s, Rec=2386. (b) Dimensionless flame length in the domain of annular swirl strength.

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Figure 3

Flame behaviors during acoustic excitations: Rec=2386, S=0.316(Rea=364), and f=180 Hz. (a) Far field images; shutter speed varies from 1/350–1/725 s. Near field images varies from 1/500–1500 s. (b) Dimensionless reduction of flame length in the domain of jet exit Strouhal number corresponding to the excitation level.

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Figure 4

Rec=2386. (a) Regimes of each flame modes in the domain of excitation level and frequency. S=0.316, Rea=364. (b) Root mean square of velocity fluctuations at the central jet exit excited at the first fuel tube resonant frequency (f=180 Hz).

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Figure 5

Flammability and stability chart at first fuel tube resonant frequency (f=180 Hz)

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Figure 6

Flame length at peak excitation level excited at first fuel tube resonant frequency (f=180 Hz)

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Figure 7

Mixing mechanism of flame mode A at Rec=2386, Rea=364(S=0.316), f=180 Hz, and 0.79>Std>0.62. (a) Near field image of flame and flow; shutter speed is 1/1000 s. (b) Chronological images of flame and flow. (c) Elucidations of flow feature. (d) Central jet velocity fluctuations and phase angle.

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Figure 8

Mixing mechanism of flame modes B, C, and D with jet exit Strouhal numbers respectively equal to (a) 0.44 (Eexc=2 V), (b) 0.29 (Eexc=4 V), and (c) 0.19 (Eexc=8 V). Rec=2386, S=0.316(Rea=364), and f=180 Hz. Colored picture shutter speed, 1/1000 s.

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