Equivalence Ratio Effects in Turbulent, Premixed Methane-air Flames

Turbulent methane flames exhibit a change in Markstein number as the equivalence ratio changes. In this paper, we demonstrate how these changes in Markstein number are related to shifts in the chemistry and transport. We focus on the analysis of simulations of two-dimensional premixed turbulent methane flames at equivalence ratios φ=0.55 and φ=1.00 computed using the GRI-Mech 3.0 mechanism. The simulations were performed using a low Mach number adaptive mesh refinement algorithm coupled to an automatic feedback control algorithm that stabilizes the flame on the computational grid. The flames are characterized in terms of curvature, strain and a local fuel-consumptionbased flame speed. Joint probability density functions show correlations of the local flame speed with curvature and a lack of correlation with the tangential strain rate. We then introduce a pathline diagnostic that follows parcels of fluid through the flame, enabling us to quantify the associated reaction and diffusive transport processes. Using this diagnostic, we examine the differences in the chemical and transport properties of the two flames, and isolate those responsible for the shift in the correlation of local flame speed with flame surface curvature.