Negative flame speed in an unsteady 2-D premixed flame: A computational study

This study analyzes a stoichiometric premixed methane flame perturbed by two-dimensional turbulence using direct numerical simulation. The chemistry is described with a detailed reaction scheme. Differential diffusion is accounted for by prescribing the Lewis number for each species. The turbulent Reynolds number based on the integral scale is 136 and the ratio of rms velocity to laminar flame speed u'/S-L = 12. When regions of steep scalar gradients are curved, a large second derivative is created in the direction tangential to the isoscalar lines. This opposes the distortion of the isolines by the imposed velocity held. Negative burning arises when the magnitude of the diffusive flux exceeds that of the adverse convective flux. In the present flow, this occurs in regions of high positive curvature. In these regions, both reaction and diffusion in the direction normal to the isolines generally contribute to decrease the negative flame speed. However, the contribution of diffusion in the direction tangential to the flame front by far exceeds the other contributions to the flame speed. The direct effect of chemical reaction on the flame speed is negligible compared to the effect of diffusion in the highly curved regions. The significance of using a detailed description of the chemistry and diffusion in the present context is that it allows the gradients in the flame to be computed with sufficient accuracy.