Tip Opening and Burning Intensity of Bunsen Flames Diluted with Nitrogen.

Tip opening and burning intensity of Bunsen flames diluted with nitrogen have been experimentally investigated. Methane/air and propane/air flames were used in the experiment. The tip opening limit was mapped as a function of the equivalence ratio φ and added nitrogen ratio X, and the tip temperature was measured. Results show that the burning at the tip was intensified in rich flame for methane and lean flame for propane. The maximum allowable dilution by nitrogen also occurs in rich flame (φ=1.05) for methane and lean flame (φ=0.94) for propane. The tip temperature steeply decreases with increasing X on the lean side for methane and the rich side for propane, while it gradually decreases on the rich side for methane and the lean side for propane. These results at the tip were due to the preferential diffusion and nonunity Lewis number through negative stretch induced by flame curvature. The present flame responses to preferential diffusion and Lewis number are ompletely reversed for those of positively stretched stagnation flames.