Flame structures, blowout limits and emissions of swirl-stabilized premixed methane-air flames were studied experimentally in a small atmospheric combustor rig. Combustion sections with rectangular cross section (30mm by 40mm) and circular cross section (inner diameter = 39mm) were used to investigate effects of combustor geometry on the flame's performance. Flame structures and instabilities were obtained from CH~* chemiluminescence captured by a high speed intensified CMOS camera. Maps of flame blowout limits (Φ_(BO)) versus total mass flow rates (m = 70 ～130 standard liter per minute, SLPM) were obtained with the combustor inlet flow temperature (T_(in)) kept at T_(in) = 397 ± 5K and a flow swirl number of S = 0.6. Emission data of mole fraction of CO in the exhaust gas versus equivalence ratio was obtained under the conditions of T_(in)= 293 ± 5K and S = 0.66. It is found that the flame became longer and more unstable with decreasing equivalence ratio or increasing total mass flow rates. A strong high-amplitude and low-frequency oscillation was found to be the reason for the flame blowout. A possible reason for flame instability and blowout is presented in the paper. Within the parameters investigated in this study, the equivalence ratio had the strongest impact on flame stabilities and CO emission. Both in the rectangular and circular combustors, when the flame length increased to a critical value (L_(IBO), which was approximately the same for these two combustors), flame could not be stabilized anymore and blowout occurred. Compared with the rectangular combustor, the circular one had lower blowout limits and was better in stabilizing the flame. Combustor geometry did not significantly affect CO emission in the current study.
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