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 50 Hz, 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.
Experimental study was made to investigate the propagation and extinction characteristics of a stretched cylindrical flame undergoing periodic fluctuation of equivalence ratio near the lean limit. With a lean methane-air and a lean propane-air mixture, burning velocity, flame luminosity and flame stretch rate were measured or evaluated for the fluctuation frequencies of 5Hz and 20Hz. The results were summarized as follows: (1) In some part of a period, burning velocity and flame luminosity of the dynamic flame near the lean limit were possible to become lower than those at the lean flammability limit of the static flame. (2) At the high frequency of 20Hz, the burning velocity took a negative value in a certain time range. In spite of this loss of propagation ability, the flame was not extinguished but sustained, indicating the recovery of the flame intensity due to the dynamic effect of fluctuating flame. (3) Flame recovery phenomenon could occur more easily for the methane flame which was strengthened by the Lewis number effect than the propane flame which was weakened by that effect.
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