Effects of CO2 addition and pressure on an extinction of a counterflow premixed methane/air flame were experimentally investigated. The extinction stretch rate decreased with the mole fraction of CO2 in the pressure range from 0.1 MPa to 0.5 MPa. Therefore, an enhancement of flame strength by the effect of radiation reabsorption did not appear in such conditions. The increase in atmospheric pressure resulted in an instability of the flame surface, though the increase in the stretch rate had a role which stabilizes the flame surface. The effect of pressure on the extinction stretch rate depended on the flame strength. In the case of flame which extinction stretch rate was relatively low, the increase in the pressure resulted in decrease of extinction stretch rate for mixtures with any Lewis number. However, in the case of flame which extinction stretch rate was relatively high, the extinction stretch rate became constant for different pressure or it increased with the pressure. The reason for this result was considered to be suppression of incomplete combustion in the thin flame under high pressure.
Direct numerical simulations of methane-air turbulent premixed flames propagating in two-dimensional homogeneous isotropic turbulence are conducted to investigate the effects of turbulence intensities and fuel species on the structure of turbulent premixed flames. Detailed kinetic mechanism including 49 reactive species and 279 elementary reactions is used to simulate CH4-O2-N2 reaction in turbulence. DNS are conducted for the case of turbulence intensities of about 10, 20 and 30 times of the laminar burning velocity, and the results of DNS are compared with those of the hydrogen-air turbulent premixed flames with same turbulence intensity. At the same turbulence intensity, the enhancement ratio of the burning velocity of methane-air premixed flame is lower than that of hydrogen-air premixed flame. This low enhancement is caused by local flame structure in turbulence. In the case of hydrogen-air turbulent premixed flame, local heat release rate becomes higher than the maximum heat release rate of a laminar flame on the average, because local flame thickness decreases in turbulence and molecular diffusion of O2 is enhanced. On the other hand, in the case of methane-air turbulent premixed flame, the local heat release rate decreases with the increase of the turbulence intensity and become lower than that of laminar flame on the whole. This decrease is mainly caused by H atom reactions in the preheat zone. In turbulence, the local flame thickness decreases due to the large strain rate at the flame front and H atom diffusion into the preheat zone is enhanced. The defection of H atom suppresses the reaction which has a large contribution on the total heat release rate.