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DNS of auto-ignition in turbulent diffusion H2/air flames

Direct numerical simulation (DNS) is used to study auto–ignition of turbulent diffusion flames. A novel, all–Mach number algorithm developed by Doom et al is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H2 and Air from Mueller at el. 2 Simulations of three dimensional turbulent diffusion flames are performed. Isotropic turbulence is superimposed on an unstrained diffusion flame where diluted H2 at ambient temperature interacts with hot air. Both, unity and non–unity Lewis number are studied. The results are contrasted to the homogeneous mixture problem and laminar diffusion flames. Results show that auto–ignition occurs in fuel lean, low vorticity, high temperature regions with low scalar dissipation around a most reactive mixture fraction, ζMR (Mastorakos et al.). However, unlike the laminar flame where auto-ignition occurs at ζMR, the turbulent flame auto–ignites over a very broad range of ζ around ζMR, which cannot completely predict the onset of ignition. The simulations also study the effects of three-dimensionality. Past two–dimensional simulations (Mastorakos et al.) show that when flame fronts collide, extinction occurs. However, our three dimensional results show that when flame fronts collide; they can either increase in intensity, combine without any appreciable change in intensity or extinguish. This behavior is due to the three–dimensionality of the flow.