The Near-Field Structure of a Planar Methane Jet Flame

Abstract

Direct numerical simulations (DNS) are conducted of a preheated planar methane jet diffusion flame for various flow conditions. The fuel stream is a mixture of methane and nitrogen, and the oxidizer stream is air. The chemistry is modeled via the 1-step global mechanism of Bhui-Pham (1992). The flame behavior is assessed for various oxidizer stream temperatures, fuel stream velocities and nitrogen dilutions of the fuel stream. Consistent with experimental results, the root mean square (rms) values of temperature show two local maxima and a local minima on either side of the jet centerline and the probability density function (PDF) of temperature displays bimodality within the intermittent flow regions. Analyses of the post-ignition region of the flame in mixture fraction space indicate that as the conditional average values of the temperature increase downstream, those of the reaction rate decrease. The near-field characteristics of the flame are strongly influenced by the dilution of the fuel stream. An increase in the fuel dilution results in the increase in flame-vortex interactions, flame thickness and finite-rate chemistry effects. Peak values of the tangential strain rate and the curvature, calculated on the flame surface, are also increased. The correlations between the scalar dissipation rate and the strain rate improve significantly when the interactions between the flame and the vorticity field increase. The analyses of the fiowfield show that the laminar flamelet model compares favorably with the DNS in the regions where the flame curvature is small.