Response of a Cylindrical Premixed Flame to Periodic Concentration Fluctuation

The response of a cylindrical premixed flame to periodic concentration fluctuation was investigated. The flame was formed in a porous cylinder by percolating a lean methane-air mixture uniformly through the cylinder wall. The burner used here was devised so as to cause fluctuations of the mixture concentration (equivalence ratio) only in the radial direction of the flow (vertically to the cylindrical flame surface) without varying the flow field. Using this burner, the time variations of burning velocity, burnt gas temperature and flame luminosity were measured for the mixture in a range of fluctuation frequencies from 3 to 50Hz, and the results were examined from the viewpoint of flame curvature effects. The results show that the variation width of the burning velocity of the dynamic flame is larger than that of the static flame in the concentration range corresponding to the fluctuation. Burnt gas temperature and flame luminosity also exhibit similar tendencies. The magnification ratio of the variation width depends on the flame curvature, and a large flame curvature results in a sustainable flame, even for a mixture leaner than the flammability limit of the static flame.