BEHAVIOR OF EXTINCTION LIMITS OF LEAN PREMIXED COUNTERFLOW FLAMES AT ELEVATED TEMPERATURES

The effects of unburned gas temperature (Tu), mole fraction of oxygen (C(O-2)) in a mixture and pressure on the extinction limit of lean CH4/O-2/N-2 premixed counterflow flames were experimentally and numerically investigated. Increments of extinction stretch rates with an increase in T-u were measured at a range of 300 K < T-u < 700 K, and those with an increase in C(O-2) were measured at a range of 17% < C(O-2) < 22%. The extinction stretch rate of mixtures with low C(O-2) decreased with pressure. This decreasing trend changed into an increasing trend with increasing Tu or C(O-2). When the density-weighted stretch rate was plotted in place of the normal stretch rate, the extinction limit monotonically expanded with pressure even for weakly burning flames. Moreover, the study investigated the applicability of the local Karlovitz number (Ka(L)) and the density-weighted stretch rate to the global description of behavior of stretched flames in various conditions. The Ka(L) remained at almost unity in wide ranges of T-u and C(O-2).