Effects of Velocity Oscillation of Air Flow on Counterflow Diffusion Flames

Effects of sinusoidal velocity oscillation of air flow on counterflow diffusion flames of nitrogendiluted methane against air has experimentally been investigated. The burner exit velocity was 0.7∼1.7m/s. The unsteady flow characteristics and the extinction limit were measured by varying the frequency (5∼500 Hz) and the amplitude (burner exit velocity×0.3, 0.5) of the flow oscillation. Furthermore, the behavior of diffusion layer was observed by using Laser Tomography method. The result shows that the extinction limit of unsteady flame is determined as a function of the maximum strain rate regardless of amplitude of velocity oscillation and it almost coincides with that of the steady state flame. The extinction limit of unsteady-state flame depends on the frequency of velocity oscillation as well. The strain rate at the extinction becomes minimum at the frequency where the amplitude of fluctuation of diffusion layer width becomes maximum. This suggests that the variation of the structure of diffusion flame zone plays a key role on the flame extinction.