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 phi and added nitrogen ratio chi, and the tip temperature was measured. Results show that the burning at the tip was more intense in rich flame for methane and lean flame for propane. The maximum allowable dilution by nitrogen also occurs in rich flame (phi=1.05) for methane and lean flame (phi=0.94) for propane. The tip temperature decreases steeply with increasing chi if the methane flame is lean or the propane flame is rich, while it decreases gradually if the methane flame is rich or the propane flame is lean. These results at the tip were due to the preferential diffusion and nonunity Lewis number resulting from negative stretch induced by flame curvature. The flame responses presented here to preferential diffusion and Lewis number are the reverse those of positively stretched stagnation flames.