Study on flame responses and ignition characteristics of CH₄/O₂/CO₂ mixture in a micro flow reactor with a controlled temperature profile

A stoichiometric CH₄/O₂/CO₂ (X_O2/X_CO2 = 0.62) mixture, which has the same laminar burning velocity (36.9 cm/s) as a stoichiometric CH₄/air mixture, was tested in a micro flow reactor with a controlled temperature profile. Two kinds of steady flame responses, namely normal flame and weak flame, were observed experimentally in high and low inlet flow velocity regimes, respectively. Dynamic flames termed as FREI (flame with repetitive extinction and ignition) were observed in an intermediate inlet flow velocity regime. The flame responses of the CH₄/O₂/CO₂ (X_O2/X_CO2 = 0.62) mixture are similar to those of the CH₄/air mixture, however, the transition velocities from normal flame to FREI and FREI to weak flame of these two mixtures are different. Moreover, the local wall temperature at the weak flame position of the CH₄/O₂/CO₂ (X_O2/X_CO2 = 0.62) mixture was found to be 1235 K experimentally, which is 10 K higher than that of the CH₄/air weak flame. The local wall temperatures at the weak flame positions of CH₄/O₂/CO₂ (X_O2/X_CO2 = 0.62) and CH₄/air mixtures, obtained by one dimensional computations with GRI-Mech 3.0, are close to those measured by experiments. The flame structure, reaction path and important reactions of the weak flames were analyzed to compare the difference between the CH₄/O₂/CO₂ (X_O2/X_CO2 = 0.62) and CH₄/air weak flames. Although significant chemical effects of CO₂ on combustion characteristics of methane premixed flames were reported elsewhere [32-34], it was found that the CH₄/O₂/CO₂ (X_O2/X_CO2 = 0.62) and CH₄/air weak flames do not show significant differences since the chemical effect of CO₂ is not strong at relatively low temperatures (below 1300 K).