Effect of a small amount of NO x on extinction limit of lean premixed counterflow flame

The effect of a small amount of NO x (NO and NO2) on the extinction of CH4 and C2H4 flames was experimentally and numerically investigated. The extinction stretch rates of counterflow premixed flames when NO2 was added to the mixture were measured at pressures ranging from 0.1 to 0.3MPa. Calculations by using one-dimensional flame code with full chemistry were also conducted to clarify the effect of NO x on the extinction limit and burning velocity. Extinction stretch rates of mixtures near the flammability limit increased by adding NO x at high pressure in the experiments and the calculations. The catalytic effect of NO x for CH4 was via the low temperature chemistry involving CH3, CH3O, CH3O2, and HO2, and that for C2H4 was via reactions that included HO2 species. The change in extinction stretch rates with NO x addition reflected the behavior of the burning velocity with NO x addition. The burning velocities of mixtures near the flammability limit increased with the addition of NO x at high pressure, the same as the extinction stretch rate.