Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 1137–1142 THE EFFECTS OF FLAME STRUCTURE ON EXTINCTION OF CH4–O2–N2 DIFFUSION FLAMES

The effects of flame structure on the extinction limits of CH4–O2–N2 counterflow diffusion flames were investigated experimentally and numerically by varying the stoichiometric mixture fraction Zst. Zst was varied by varying free-stream concentrations, while the adiabatic flame temperature Tad was held fixed by maintaining a fixed amount of nitrogen at the flame. Zst was varied between 0.055 (methane-air flame) and 0.78 (diluted-methane-oxygen flame). The experimental results yielded an extinction strain rate Kext of 375 s!1 for the methane-air flame, increasing monotonically to 1042 s!1 for the diluted-methane-oxygenflame. Numerical results with a 58-step C1 mechanism yielded 494 s!1 and 1488 s!1, respectively. The increase in Kext with Zst for a fixed Tad is explained by the shift in the O2 profile toward the region of maximum temperature and the subsequent increase in rates for chain-branching reactions. The flame temperature at extinction reached a minimum at Zst " 0.65, where it was 200!C lower than that of the methane-air flame. This significant increase in resistance to extinction is seen to correspond to the condition in which the OH and O production zones are centered on the location of maximum temperature