Impact of Turbulence Intensity and Equivalence Ratio on the Burning Rate of Premixed Methane-Air Flames

Direct Numerical Simulations (DNS) have been conducted to study the response of intially laminar spherical premixed methane-air flame kernels to successively higher turblence intensities at five different equivalence ratios. The numerical experiments include a 16-species/25-step skeletal mechanism for methane oxidation and a multicomponent molecular transport model. Highly turblent conditions (with integral Reynolds numbers up to 4513) have been accessed. The effect of turbulence on the physical properties of the flame, in particular its consumption speed S-c which is an interesting measure of the turbulent flame speed S-T has been investigated. Local quenching events are increasingly observed for highly turbulent conditions, particularly for lean mixutres. The obtained results qualitatively confirm the expected trend regarding correlations between u'/S-L and the consumption speed: S-c first increases, roughly linearly, with u'/S-L (low turbulence zone), then levels off (bending zone) before decreasing againg (quenching limit) for too intense turbulence. For a fixed value of u'/S-L, S-c/S-L varies with the mixture equivalence ratio, showing that additional parameters should probably enter phenomenological expressions relating these two quantities.