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 (XO2/XCO2 = 0.62) mixture was tested in a micro flow reactor with a controlled temperature profile.
• Three kinds of flame responses, namely normal flame, FREI and weak flame were observed.
• The ignition temperature of the CH4/O2/CO2 (XO2/XCO2 = 0.62) weak flame is close to that of the CH4/air weak flame.
• The effect of CO2 on the CH4/O2/CO2 (XO2/XCO2 = 0.62) weak flame was clarified.

A stoichiometric CH4/O2/CO2 (XO2/XCO2 = 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 (XO2/XCO2 = 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 (XO2/XCO2 = 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 (XO2/XCO2 = 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 (XO2/XCO2 = 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 (XO2/XCO2 = 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).