Lewis number effects on turbulent burning velocity

Experimental values of turbulent burning velocities for propane, hydrogen and iso-octane mixtures with air are reported under conditions of high turbulence and high turbulent Reynolds number. The measurements were made by the double kernel method during explosions in a fan-stirred bomb, with four fans, capable of speeds of up to 10,000 rpm. The ratio of turbulent to laminar burning velocity, u t /u l , is correlated primarily with the ratio of r.m.s. turbulent velocity to laminar burning velocity, u′/u l and a Karlovitz stretch factor given by the ratio of a strain rate u′/λ to a flame gradient given by u l /δ l , where λ is the Taylor microscale and δ l the laminar flame thickness. Asymptotic analyses of strained laminar flames, together with the two-eddy theory of turbulent burning, show the additional importance of Lewis number effects. These result in lean hydrocarbon mixtures being quenched more readily than rich ones, with an opposite effect for H 2 mixtures. This was observed in the experiments. However, full quantitative agreement between theory and experiment was not achieved, due to the inherent limitations of the two theories, which are discussed.

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