Research Papers

System Level Analysis of Acoustically Forced Nonpremixed Swirling Flames

[+] Author and Article Information
Uyi Idahosa

GE Global Research Center,
1 Research Circle,
Niskayuna, NY 12309

Saptarshi Basu

Department of Mechanical Engineering,
Indian Institute of Science,
Bangalore 560 012, India
e-mail: sbasu@mecheng.iisc.ernet.in

Ankur Miglani

Department of Mechanical Engineering,
Indian Institute of Science,
Bangalore 560 012, India

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received January 13, 2013; final manuscript received March 14, 2014; published online April 11, 2014. Assoc. Editor: Alexander L. Brown.

J. Thermal Sci. Eng. Appl 6(3), 031015 (Apr 11, 2014) (15 pages) Paper No: TSEA-13-1018; doi: 10.1115/1.4027297 History: Received January 13, 2013; Revised March 14, 2014

This paper reports an experimental investigation of dynamic response of nonpremixed atmospheric swirling flames subjected to external, longitudinal acoustic excitation. Acoustic perturbations of varying frequencies (fp = 0–315 Hz) and velocity amplitudes (0.03 ≤ u′/Uavg ≤ 0.30) are imposed on the flames with various swirl intensities (S = 0.09 and 0.34). Flame dynamics at these swirl levels are studied for both constant and time-dependent fuel flow rate configurations. Heat release rates are quantified using a photomultiplier (PMT) and simultaneously imaged with a phase-locked CCD camera. The PMT and CCD camera are fitted with 430 nm ±10 nm band pass filters for CH* chemiluminescence intensity measurements. Flame transfer functions and continuous wavelet transforms (CWT) of heat release rate oscillations are used in order to understand the flame response at various burner swirl intensity and fuel flow rate settings. In addition, the natural modes of mixing and reaction processes are examined using the magnitude squared coherence analysis between major flame dynamics parameters. A low-pass filter characteristic is obtained with highly responsive flames below forcing frequencies of 200 Hz while the most significant flame response is observed at 105 Hz forcing mode. High strain rates induced in the flame sheet are observed to cause periodic extinction at localized regions of the flame sheet. Low swirl flames at lean fuel flow rates exhibit significant localized extinction and re-ignition of the flame sheet in the absence of acoustic forcing. However, pulsed flames exhibit increased resistance to straining due to the constrained inner recirculation zones (IRZ) resulting from acoustic perturbations that are transmitted by the co-flowing air. Wavelet spectra also show prominence of low frequency heat release rate oscillations for leaner (C2) flame configurations. For the time-dependent fuel flow rate flames, higher un-mixedness levels at lower swirl intensity is observed to induce periodic re-ignition as the flame approaches extinction. Increased swirl is observed to extend the time-to-extinction for both pulsed and unpulsed flame configurations under time-dependent fuel flow rate conditions.

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Fig. 1

Nonpremixed burner geometry [26]

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Fig. 2

Experimental setup [26]

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Fig. 3

Unpulsed (fp = 0 Hz) isothermal velocity field

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Fig. 4

Pulsed (fp = 105 Hz) isothermal streamlines—constant fuel flow rate (C1 and C2) configurations

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Fig. 5

Binned FFT spectra at burner base microphone (p1′)

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Fig. 6

Magnitude square coherence (γ2)

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Fig. 8

CWT spectra | fp = 0 Hz (unpulsed)

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Fig. 9

CWT spectra | rich (C1) flame configuration

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Fig. 10

CWT spectra | lean (C2) flame configuration

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Fig. 11

Transient CH* chemiluminescence images | fp = 0 Hz (unpulsed)

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Fig. 12

Signals time trace and heat release CWT for captured CH* image | fp = 0 Hz

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Fig. 13

Time averaged flame response metrics [26]

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Fig. 14

Heat release time series for exponentially decaying (C3) fuel flow rates

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Fig. 15

CH* chemiluminescence images|exponential decay fuel flow rate (C3)

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Fig. 16

CWT spectra|exponential fuel flow rate (C3) flame configuration




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