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Flame development characteristics inside a reverse-flow model combustor from early stages of combustion to steady operation Mollahoseini, Zahra


Characteristics of flame development inside a small-scale power generator model combustor featuring a reverse-flow configuration and the influence of such configuration on these characteristics from early stages of combustion to the steady operation of the combustor are investigated experimentally. Simultaneous and high-speed OH* chemiluminescence and pressure measurements are utilized. Two sets of experiments are performed. In the first set, relatively lean premixed methane/air flames with a fuel-air equivalence ratio of 0.7 have been investigated. For these, four experiments pertaining to air flow rates of 100 and 160 standard liters per minute along with two igniter rod positions are performed. For the second set of experiments, the igniter position was fixed, and two fuel-air equivalence ratios of 0.9 and 1 along with two air flow rates of 120 and 130 standard liters per minute are tested. The results pertaining to the first set of experiments suggest that, prior to the appearance of steady thermoacoustic oscillations, three phases, namely, ignition, stabilization, and transition may exist. During the ignition phase, the normalized flame edge velocity can achieve relatively large maximum values owing to the reverse-flow configuration compared to that for closed chambers. The ignition phase is followed by the stabilization phase during which the flame is formed on the flame-holder. During the transition phase, the flame chemiluminescence features one long-period sinusoidal oscillation. Despite significant influence of the reverse-flow configuration on flame dynamics during the ignition and stabilization phases, the spectral characteristics of pressure and flame chemiluminescence oscillations are similar to those of unconfined flames. The results pertaining to the relatively rich flames suggest, depending on the phase of the flame development, the reverse-flow combustor can serve as either a closed or an open system. It was shown that, during the ignition and stabilization phases, the thermoacoustic fluctuations are dominated by small frequencies associated with the structural and acoustic modes. However, during the steady operation, the intrinsic thermoacoustic modes also become apparent and contribute to the thermoacoustic oscillations. The results of this study have implications for improving the ignition quality and mitigating the thermoacoustic oscillations in reverse-flow combustors used for small-scale power generation purposes.

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