Effects of oxidizer preheating and dilution on the extinction and structure of CH₄-O₂-Ar diffusion flames

The effects of oxidizer preheating and dilution levels of fuel and exygen on the extinction and structure of counterflow CH₄-O₂-Ar diffusion flame were investigated experimentally and numerically. The oxidizer temperature of both experiments and calculations were varied up to 1100 K. The oxidizer preheating significantly contributes to expansion of the extinction limits. The oxidizer preheating and the dilution levels are characterized by the magnitude of stoichiometric mixture fraction (F_st). The extinction limit curves can be roughly divided into two regions based on the magnitude of F_st. In the region about F_st<0.5 which the flame location is in the oxidizer flow side, the oxidizer concentration exerts dominant effect on the extinction, and the effects of preheating is clearly observed. In this region, the combustion reaction appears with oxidizer preheating from the upper of oxidizer side. On the other hand, the region about F_st>0.5, which the flame is located to the fuel flow, the expansion of reaction zone with preheating is almost small. Only small effects of the oxidizer preheating to the extinction limits are observed in this region. As the results, the flame location is quite important factor to understand the effects of dilution levels and oxidizer preheating on the extinction phenomena.