Preferential transport effects in premixed bluff-body stabilized CH₄/H₂ flames

Recent studies of bluff-body stabilized premixed CH₄/air flames, using line-imaged Raman/Rayleigh/LIF methods to measure temperature and major species, have revealed significant effects of preferential species transport on scalar structure across the flame and into the recirculation zone. The main observations were that the C/H atom ratio and the equivalence ratio increased going from reactants to products along a radial profile when the reactant flow velocity was sufficiently high to push the flame close to the boundary of the recirculation zone. This phenomenon was attributed to preferential diffusion of H₂ and H₂O toward the reactants followed by convective transport of the hydrogen-species enriched fluid downstream and beyond the recirculation zone, leaving elevated levels of CO₂ and the C/H and C/O atom ratios within the recirculation zone.The present work continues the investigation of this preferential transport phenomenon by considering lean premixed methane flames, with varying levels of H₂ addition, stabilized on the same bluff body burner as in the previous study. Results show similar effects of preferential transport as in the methane flames, with the measured equivalence ratio within the recirculation zone higher than in the reactant stream. However, with increasing levels of H₂ addition in the fuel, H₂ diffuses from reactants toward products rather than the other way around. For the highest level of H₂ addition (40% mole fraction) the C/H atom ratio in the products decreases relative to that in the reactants, while the C/O atom ratio still increases. This indicates that there is still preferential transport of H₂O into the rapidly flowing reactant stream, causing a net loss of oxygen from products in the recirculation zone. However, differential diffusion of H₂ from the reactant stream into the flame more than compensates for this effect on the C/H atom balance.