Infrared Radiation and Acoustic Characteristics of Combustion Instabilities in Turbulent Premixed Flames

C OMBUSTION stability in turbulent premixed flames is desired for many applications including aircraft engines, heat recovery steam generators, and industrial boilers. Anchoring the flames to a bluff body is one technique for stabilizing these flows [1]. The interaction between pressure and heat release fluctuations can destabilize turbulent premixed flames and result in two distinct phenomena: combustion oscillations or localized extinctions near lean blowout limits. Combustion instabilities have been the focus of much research, as reviewed by Lee and Santavicca [2] and Shanbhogue et al. [3] Infrared radiation characteristics of turbulent premixedflames [4] and their relationship to combustion instabilities have received limited attention and are the focus of the present investigation. Combustion oscillations are typically driven by both flame-vortex interactions and equivalence ratio fluctuations from feed-system coupling [2,5]. During flame-vortex interactions the flame is stretched as vortices periodically shed from theflame holder and pass through the flame front [2,6]. The flame stretching increases the flame area (increases heat release rate) or leads to localized extinction (decreases the heat release rate) depending on the flame stretch rate and local equivalence ratio [2]. Feed-system coupling occurs when equivalence ratio fluctuations, resulting from fluctuations in either the fuel or oxidizer flow rate, are advected downstream to the combustion region [2]. If the equivalence ratio and pressure fluctuations arrive at the flame front in phase, the resulting heat release fluctuation amplifies combustion oscillations [2]. Outof-phase equivalence ratio and pressure fluctuations result in the heat release fluctuations damping the combustion oscillations [2]. Localized extinction near lean blowout is another phenomenon that contributes to instabilities in premixed flames. As the equivalence ratio is decreased, localized holes in the flame sheet occurwhere extinction stretch rates are exceeded [3]. Flame straining is the primary mechanism by which localized extinction occurs with secondary contributions by heat losses to the bluff body [3]. Blowout occurs when localized extinctions result in the flame being sufficiently cooled by the reactants entrained into the flow such that reactions are no longer self-sustaining. Acoustic and optical emission fluctuations increase sharply near blowout [1,3,7,8]. Power spectral densities of acoustic measurements showed strong peaks for = LBO 1:02 but not for = LBO 1:19 [1]. Nair and Lieuwen [1] experimentally observed that the fraction of visible images demonstrating localized extinction events was 0% at = LBO 1:12, 8% at = LBO 1:08, and 40% at = LBO 1:02. Localized reductions in OH chemiluminescence have been observed near blowout conditions ( = LBO 1:08), indicating extinction events [7]. The frequency of the extinction events monotonically increased as the lean blowout limit was approached. Motivated by the importance of radiation heat transfer and combustion stability in turbulent premixed combustors, the specific objectives of this work are 1) to measure the narrowband infrared radiation intensity emitted from the recirculation zone of bluff-body stabilized turbulent premixed flames for a range of equivalence ratios; 2) to characterize the combustion stability regimes using highfrequency acousticmeasurements and the infrared radiation intensity measurements; and 3) to observe and report plausible relations between the measured narrowband infrared radiation intensity statistics and measured acoustics. The results show that the midinfrared images yield time-dependent multipoint radiation intensities in emission bands of major combustion products (carbon dioxide, carbon monoxide, and water vapor). Turbulent statistics of the radiation intensity fluctuations can be used in designing infrared sensors for use in active control schemes to suppress combustion instabilities.