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

The response of the flame displacement speeds of stagnating Bat premixed fl ames to the periodical fluctuation of stretch is investigated experimentall y, regarding two mixtures with different Lewis numbers: lean C3Hg/air and l ean CH4/air in the range of the Strouhal number 2.3 to 4.8 (24 to 51 Pit). The stagnation wall is oscillated sinusoidally along the stagnation streaml ine. The oscillation of the stagnation wall induces the periodic fluctuatio ns of flow velocity and flame stretch, and hence the fluctuations of flame displacement speed. The amplitude of the flame stretch fluctuation increase s with increasing frequency of the wall in the Strouhal number greater than unity owing to the increase in the amplitude of the oscillatory wall veloc ity, when the stagnation wall is oscillated along the stagnation streamline at constant amplitude. The displacement speed is measured as the propagati ng 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 re sponse of the flame displacement speed is discussed from two viewpoints: it s amplitude response and phase response. The significant dependence of the flame displacement speed on the flame stretch has been shown by the amplitu de response. It is shown that the sinusoidal fluctuations of flame displace ment speeds for both mixtures follow the sinusoidal fluctuation of flame st retch with a phase delay from about 20 degrees to 90 degrees in the present frequency range. Hence, the displacement speeds for both mixtures, althoug h these mixtures have the contrary ratio of thermal to mass diffusivity aga inst 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.