Comparative study on flame instability and combustion characteristics of CH4/O2/CO2 and CH4/O2/N2 mixtures

To investigate flame instabilities and combustion characteristics of oxygen-enriched methane in a closed chamber, a series of combustion experiments on the CH4/O-2/CO2 and CH4/O-2/N-2 mixtures were conducted at the initial temperature and pressure of 301 +/- 2 K and 1.0 atm. The results show that the distorted tulip flames and corrugated flames, which can be differentiated by a 2D plane of firstly wall-touching pressure (P P-wall/(0)) versus flame position (Z(tip)(wall)/L), are for the first time observed in the oxygen-enriched methane mixtures. Particularly, the corrugated flames can emerge by increasing the reactivity of CH4/O-2/N-2 mixtures. There are distinct flame instabilities between the CO2/N-2 diluents. Specifically, the CH4/O-2/CO2 mixtures always show stronger Thermal-Diffusive instability whereas the CH4/O-2/N-2 mixtures always exert stronger Darrieus-Landau instability. There is a linearly positive correlation between the Darrieus-Landau instability and oxygen enrichment ratio for both mixtures. In contrast, the oxygen enrichment ratio plays positive (N-2 dilution) and negative (CO2 dilution) role for the Thermal-Diffusive instability. A linear prediction between the maximum combustion pressures and S-l.E/ l(fg) is proposed by considering the Darrieus-Landau instability. However, the Rayleigh-Taylor instability, as an additional mechanism for flame distortion, makes the linearity less accurate. As the oxygen enrichment ratio is further increased, the Richtmyer-Meshkov instability instead of the Rayleigh-Taylor insta-bility plays a role in converting distorted tulip flames to corrugated flames, and could revalidate this linear prediction.