Determination of burning velocity and flammability limit of methane/air mixture using counterflow flames

The burning velocity and the flammability limit of a methane/air mixture are investigated numerically using counterflow premixed flames. Two different methods, the minimum velocity method and the upstream temperature boundary method, for the determination of burning velocity are compared. The results show that the minimum velocity method fails when the chemical heat release is weak. A velocity gradient method is presented and examined. Furthermore, at low equivalence ratio, the results show that flame thickness increases exponentially with the decrease of stretch rate. This flame thickening results in a radiation extinction. An extrapolation of burning velocity to zero stretch is shown to be inaccurate. At a high equivalence ratio, it is found that two kinds of stable flames exist, normal flame and weak flame, with distinct burning velocities. A G-shaped curve showing the flammable regions is obtained by plotting the burning velocities at extinction limits as a function of equivalence ratio. The results indicate that a simple linear extrapolation of the extinction limits of stretched flames to zero stretch rate does not give the standard flammability limit.