Correlation Of Flame Speed With Stretch In Turbulent Premixed Methane Air Flames

In the flamelet approach of turbulent premixed combustion, the flames are modeled as a wrinkled surface whose propagation speed, termed the[open-quotes]displacement speed, [close-quotes] is prescribed in terms of the local flow field and flame geometry. Theoretical studies suggest a linear relation between the flame speed and stretch for small values of stretch, S[sub L]/S[sub L][sup 0]= 1 - MaKa, where S[sub L][sup 0] is the laminar flame speed, Ka=[kappa][delta][sub F]/S[sub L][sup 0] is the nondimensional stretch or the Karlovitz number, and Ma= L/[delta][sub F] is the Markstein number. The nominal flame thickness, [delta][sub F], is determined as the ratio of the mass diffusivity of the unburnt mixture to the laminar flame speed. Thus, the turbulent flame model relies on an accurate estimate of the Markstein number in specific flame configurations. Experimental measurement of flame speed and stretch in turbulent flames, however, is extremely difficult. As a result, measurement of flame speeds under strained flow fields has been made in simpler geometries, in which the effect of flame curvature is often omitted. In this study we present results of direct numerical simulations of unsteady turbulent flames with detailed methane/air chemistry, thereby providing an alternative method of obtaining flame structure and propagation statistics. The objective is to determine the correlation between the displacement speed and stretch over a broad range of Karlovitz numbers. The observed response of the displacement speed is then interpreted in terms of local tangential strain rate and curvature effects. 13 refs., 3 figs.