Finite rate chemistry effects in non-premixed CH ₄/H ₂ flames

The characteristics of CH ₄/H ₂ flames in a reacting vortex ring are studied via direct numerical simulation (DNS), and a conserved scalar approach (CSA). The combustion of CH ₄/H ₂ is modeled using a detailed kinetic mechanism which consists of 36 species and 217 elementary reactions and involves C ₁, C ₂, and some C ₃ kinetics. Simulations using an infinitely fast reaction (IFR) were also performed. The finite rate flames are categorized based on the methane Damköhler (Da) number. Front, top, and wake flame regions are identified. For high Da flames the front and top flames become quasi-steady shortly after ignition, while the wake flame remains unsteady. Scatter plots for species mole fractions show that the front and top flames are similar to one-dimensional counterflow diffusion flames. For the wake flame thickening effects are large on the fuel rich side and are small on the fuel lean side. The CSA results for the flame structure are in agreement with the DNS for the C ₁ species. For low Da flames, unsteady effects are important at all times for all flame regions. Comparisons of DNS and CSA results show larger H ₂O and CO ₂ mole fraction values for the CSA. Curvature and thickening effects are important for the top and wake flames, and result in significant scatter for the mole fraction values of all species. The conserved scalar approach does not capture the wake flame characteristics for the low Da flames.