Properties of lean turbulent methane-air flames with significant hydrogen addition

Abstract

We examine the combustion of mixed H2-CH4-air fuels using two-dimensional simulations that incorporate detailed kinetics and a mixture-averaged model for differential species transport. The mixtures range from lean H2-air at ϕ=0.37 to lean CH4-air at ϕ=0.7. For each mixture, we compute the quasi-steady propagation of a flame into flow with superimposed low-level turbulent fluctuations, so that the resulting flames are in the laminar flamelet regime. We examine the resulting global flame characteristics, and quantify how the chemistry depends on local flame curvature. We then examine in more detail how the methane chemistry is modulated by the presence of hydrogen. In particular, we find that the local methane burning speed shows a strong positive correlation with local flame curvature when sufficient hydrogen is added to the mixture. Moreover, for higher hydrogen concentrations, the mixtures exhibit cellular burning patterns that are traditionally associated with thermodiffusively unstable fuel mixtures. Various pathways for the oxidation of methane are identified and are shown to be amplified considerably in the presence of H2 combustion, which varies considerably along the flame surface.