Response of a stretched cylindrical diffusion flame to a sinusoidal velocity oscillation of air stream

Abstract

Experimental study was performed to reveal the response characteristics of a stretched cylindrical diffusion flame to sinusoidal oscillation of air flow velocity. The cylindrical flame used in this study has a convex curvature with respect to air stream and is formed in air stream. The fuel is methane, diluted with nitrogen, and the oxidizer is air. Oscillation frequency f is varied from 5 Hz to 250 Hz. Velocity at the outlet of air supplying nozzle was changed sinusoidally with four speakers. Velocity at the fuel nozzle outlet was kept constant. The air velocity at the nozzle outlet is measured using particle image velocimetry. Flame radius r_f, flame thickness δ, and flame luminosity L_f are obtained by using high speed video camera. Results are summarized as follows: Though fluctuation amplitude of the velocity gradient of air stream Δg_a is constant with increasing f, that of the fuel stream Δg_f increases. The fluctuation amplitude of r_f varies to quasi-steady at low frequency, while the fluctuation amplitude is reduced with increasing f. L_f does not respond to quasi-steady at low frequency. The fluctuation amplitude of L_f has maximum value at 50 Hz and is larger than that for the steady flame corresponding to the velocity fluctuation. It is considered that this complex change of the flame luminosity with respect to f is related to δ/r_f which is associated with the flame curvature effect, g_a which affects the flame stretch effect, and g_f which impacts fuel transport.