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Journal of Flow Visualization and Image Processing
SJR: 0.161 SNIP: 0.312 CiteScore™: 0.1

ISSN Imprimer: 1065-3090
ISSN En ligne: 1940-4336

Journal of Flow Visualization and Image Processing

DOI: 10.1615/JFlowVisImageProc.2018025857
pages 79-97

INVESTIGATION OF ROTATIONAL SYMMETRY IN VORTEX-DRIVEN ACOUSTIC OSCILLATIONS IN A LABORATORY-SCALE SWIRL COMBUSTOR

Vikram Ramanan
Department of Aerospace Engineering, National Center for Combustion Research and Development, Indian Institute of Technology, Madras, Chennai-600036, India
Satyanarayanan R. Chakravarthy
Department of Aerospace Engineering, National Centre for Combustion Research and Development, Indian Institute of Technology Madras, Chennai 600036, India

RÉSUMÉ

Combustors in gas turbines exhibit thermoacoustic instability marked by high-amplitude pressure oscillations, during which the characteristics of the flame undergo significant changes. In the present work, a laboratory-scale swirl combustor is characterized by its stable and unstable flame behavior. The chemiluminescence of the CH* radicals in the flame is recorded at high framing rates simultaneously with the measurement of the acoustic pressure excited in the combustor. The combustor is unstable at low fue-air equivalence ratio and high air flow rate (represented in terms of Reynolds number, Re). Under unstable conditions, the high-speed CH chemiluminescence images reveal the evolution of a mushroom-shaped flame structure at the pressure maximum in every acoustic cycle, which eventually flares into an axisymmetric flame convecting downstream. The flame curling associated with the mushroom-shaped pattern is investigated for the signature of vortex-driven combustion instability by means of rotational symmetry in the flame images. A scale-invariant feature tranform algorithm is employed to detect rotational symmetry for this purpose. The results reveal the appearance of rotational symmetry at the zero pressure crossings and pressure maxima. The number of frames bearing the rotational symmetry is correlated with the duration over which peak pressure prevails across several acoustic cycles, to reinforce the role of vortex combustion in exciting pressure oscillations in the combustor.


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