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 ar e 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 result s show that the minimum velocity method fails when the chemical heat releas e is weak. A velocity gradient method is presented and examined. Furthermor e, at low equivalence ratio, the results show that dame thickness increases exponentially with the decrease of stretch rate. This flame thickening res ults in a radiation extinction. An extrapolation of burning velocity to zer o stretch is shown to be inaccurate. At a high equivalence ratio, it is fou nd that two kinds of stable flames exist, normal flame and weak flame, with distinct burning velocities. A G-shaped curve showing the flammable region s is obtained by plotting the burning velocities at extinction limits as a function of equivalence ratio. The results indicate that a simple linear ex trapolation of the extinction limits of stretched Barnes to zero stretch ra te does not give the standard flammability limit.