Infrared radiation intensity measurements are used to identify and characterize stable and unstable combustion regimes of turbulent premixed flames anchored on a bluff body. An infrared camera was utilized to acquire time-dependent narrowband radiation intensity measurements of the recirculation zone of flames with varying equivalence ratios (0.54 to 1.20) and Reynolds numbers (15,000 and 32,000). High frequency acoustic measurements were acquired along the length of the combustor for comparison with the infrared radiation data. Turbulent radiation statistics are examined to gain insights into premixed flame stabilization and lean blowout phenomena. Time-dependent infrared images reveal oscillatory behavior in the flow for stoichiometric and fuel rich equivalence ratios. The characteristic frequencies and magnitudes of the combustion oscillations are quantified using temporal correlations and power spectral density functions of the infrared radiation intensity fluctuations. Conclusions ascertained from the infrared signal are in quantitative agreement with those determined from the acoustic measurements. For fuel lean equivalence ratios, occasional low infrared signals are evident and indicate localized extinction in the flames. Lean blowout ultimately occurs when localized extinction regions form, grow, and propagate upstream to the bluff body as demonstrated by infrared images of the blowout process. Infrared imaging coupled with quantitative analysis is becoming a combustor diagnostic tool as illustrated in this work.
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