Response of flame displacement speeds to oscillatory stretch in wall- stagnating flow

The response of the flame displacement speeds of stagnating flat premixed flames to the periodical fluctuation of stretch is investigated experimentally, regarding two mixtures with different Lewis numbers: lean C₃H₈/air and lean CH₄/air in the range of the Strouhal number 2.3 to 4.8 (24 to 51 Hz). The stagnation wall is oscillated sinusoidally along the stagnation streamline. The oscillation of the stagnation wall induces the periodic fluctuations of flow velocity and flame stretch, and hence the fluctuations of flame displacement speed. The amplitude of the flame stretch fluctuation increases with increasing frequency of the wall in the Strouhal number greater than unity owing to the increase in the amplitude of the oscillatory wall velocity, when the stagnation wall is oscillated along the stagnation streamline at constant amplitude. The displacement speed is measured as the propagating velocity of the flame relative to the unburned gas velocity at the cold edge of the flame by using the history of flow velocity fluctuation. The response of the flame displacement speed is discussed from two viewpoints: its amplitude response and phase response. The significant dependence of the flame displacement speed on the flame stretch has been shown by the amplitude response. It is shown that the sinusoidal fluctuations of flame displacement speeds for both mixtures follow the sinusoidal fluctuation of flame stretch with a phase delay from about 20°to 90° in the present frequency range. Hence, the displacement speeds for both mixtures, although these mixtures have the contrary ratio of thermal to mass diffusivity against unity, show the same increasing dependence on unsteady stretch because the flow divergence affects the displacement speed with unsteady stretch as well as steady stretch. (C) 2000 by The Combustion Institute.