Flame structure of lean premixed and diffusion combined flames near extinction limit

Extinction and flame structure of lean premixed and diffusion combined flames formed in a counter flow are investigated experimentally and numerically. The extinction limits can be divided into two regions. One is a diffusion flame dominant extinction region (DF-DE) in which equivalence ratio of LPF side (φL) at extinction limit is decreasing linearly with increasing fuel concentration of DF side (χU). The other is a lean premixed flame dominant extinction region (LPF-DE) in which effect of χU on extinction φL is increasing with increasing χU. In these two regions, the flame structures and its φL dependence are different greatly. In the DF-DE, the temperature has symmetrical profile regardless of φL and the temperature peak is located near DF. Since DF has higher temperature than LPF, LPF is thermally supported than DF. Temperature gradients between two reaction zones are decreasing with increasing φL, while laminar burning velocity (SL) and burnt gas width (WB) are constant regardless of φL. On the other hand, in the LPF-DE, the temperature has asymmetrical profile and the temperature peak leans to the LPF side. That is, the temperature gradient of LPF side is very steep compared with the DF side. Since LPF has higher temperature than DF, DF is thermally supported than LPF. Temperature gradients between two reaction zones are constant regardless of φL, while SL and WB are increasing with increasing φL.