Flame-vortex interaction driven combustion dynamics in a backward-facing step combustor

Journal Article · Fri May 15 00:00:00 EDT 2009 · Combustion and Flame

DOI:https://doi.org/10.1016/J.COMBUSTFLAME.2009.02.003· OSTI ID:21169002

Altay, H Murat; Speth, Raymond L; Hudgins, Duane E; Ghoniem, Ahmed F ^[1]

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1. Massachusetts Institute of Technology, Department of Mechanical Engineering, Cambridge, MA 02139 (United States)

The combustion dynamics of propane-hydrogen mixtures are investigated in an atmospheric pressure, lean, premixed backward-facing step combustor. We systematically vary the equivalence ratio, inlet temperature and fuel composition to determine the stability map of the combustor. Simultaneous pressure, velocity, heat release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When fuel is injected far upstream from the step, the equivalence ratio entering the flame is temporally and spatially uniform, and the combustion dynamics are governed only by flame-vortex interactions. Four distinct dynamic regimes are observed depending on the operating parameters. At high but lean equivalence ratios, the flame is unstable and oscillates strongly as it is wrapped around the large unsteady wake vortex. At intermediate equivalence ratios, weakly oscillating quasi-stable flames are observed. Near the lean blowout limit, long stable flames extending from the corner of the step are formed. At atmospheric inlet temperature, the unstable mode resonates at the 1/4 wavemode of the combustor. As the inlet temperature is increased, the 5/4 wavemode of the combustor is excited at high but lean equivalence ratios, forming the high-frequency unstable flames. Higher hydrogen concentration in the fuel and higher inlet temperatures reduce the equivalence ratios at which the transitions between regimes are observed. We plot combustion dynamics maps or the response curves, that is the overall sound pressure level as a function of the equivalence ratio, for different operating conditions. We demonstrate that numerical results of strained premixed flames can be used to collapse the response curves describing the transitions among the dynamic modes onto a function of the heat release rate parameter alone, rather than a function dependent on the equivalence ratio, inlet temperature and fuel composition separately. We formulate a theory for predicting the critical values of the heat release parameter at which quasi-stable to unstable and unstable to high-frequency unstable modes take place. (author)

• OSTI ID: 21169002
• Journal Information: Combustion and Flame, Journal Name: Combustion and Flame Journal Issue: 5 Vol. 156; ISSN CBFMAO; ISSN 0010-2180
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ATMOSPHERIC PRESSURE
CHEMICAL COMPOSITION
COMBUSTION KINETICS
COMBUSTORS
DIAGRAMS
FLAMES
FUELS
FUNCTIONS
Flame-vortex interactions
HEAT
HYDROGEN
INSTABILITY
MIXTURES
PROPANE
SOUND WAVES
STABILITY
TEMPERATURE DEPENDENCE
Thermoacoustic instability
VELOCITY
VORTICES