空気流速の周期変動に対する伸長円筒状拡散火炎の応答特性（速度変動振幅が火炎応答に及ぼす影響）

抄録

An experimental study investigated the frequency characteristics of a stretched cylindrical diffusion flame in response to sinusoidal air flow velocity oscillation. The flame has a convex curvature with respect to the air stream. The fuel was methane diluted with nitrogen, and oxidizer air. The flame was formed in the air stream side from the stagnation surface. The oscillation frequency f ranged from 10 Hz to 250 Hz. In order to clarify the influence of an oscillation amplitude A of the air flow velocity on the flame responses, three kinds of amplitude were chosen. The flame was photographed by the high speed video camera, and flame radius r_f, flame thickness δ, and flame luminosity L_f were analyzed. The air flow velocity v_a at the nozzle outlet was measured by using particle image velocimetry. Though velocity gradient of the air flow g_a responds quickly to the changing v_a regardless of f and A, the gradient of the fuel flow g_f delays with increasing f and A. Here, g_a correlates to the flame stretch effect, and g_f impacts the fuel transportation. Response of r_f to varying v_a also delays with increasing f and A. δ/r_f, which relates to the flame curvature effect, responds complicated with the change in f and A. Oscillation amplitudes of L_f have maximum values with respect to f regardless of A. These maximum values are greater than those of a steady flame, over the same air velocity fluctuation ranges. This complex change in the L_f with respect to f is closely related to the phase difference in the respective time variations in δ/r_f and g_f, and the magnitude of these values.