Chain mechanisms in the overall reaction orders in laminar flame propagation

The laminar flame speeds of methane-oxygen-nitrogen mixtures as functions of the flame temperature T_ad and system pressure p have been experimentally determined by using the counterflow, twin-flame technique. These data are then compared with numerically-calculated values obtained by using an independently validated kinetic scheme. Results show that the overall reaction order n and activation energy E_a are far from being constants, that n decreases with increasing p and decreasing T_ad whereas E_a increases with increasing p, and that n can actually assume negative values; the last result implies that the mass burning rate of some weakly burning flames may decrease with increasing pressure. By further identifying the crucial reaction steps through sensitivity analysis, the present results are interpreted on the basis of the influence of chain branching-termination mechanisms on the overall reaction rate.