High-temperature combustion in autoigniting non-homogeneous hydrogen/air mixtures

Abstract

The burning modes (premixed vs. diffusion burning) in autoigniting non-homogeneous mixtures of hydrogen in heated air are studied using direct numerical simulations (DNS). The simulations show that high-temperature combustion follows an initial autoignition stage in fuel-lean, low-dissipation kernels. These kernels propagate initially as lean premixed fronts. As they expand into richer mixtures, diffusion flames develop in the wake of rich premixed flames along stoichiometric isocontours. These flames are initially stabilized by diffusion of radicals (H) and excess fuel from the rich premixed flames' side against excess radicals (O and OH) and oxidizer from the earlier passage of lean premixed fronts. In time, diffusion flames detach from the rich premixed flames, and their burning intensity is reduced accordingly. Triple flames also form at the interfaces of the rich and lean premixed flames with the stoichiometric mixture isocontours. However, their contribution to the stabilization and burning intensity of the diffusion branches is insignificant. Analysis of the contribution of lean and rich premixed flames and that of the diffusion flames to the volumetric heat release show that the dominant contribution is attributed mainly to the premixed flames: while the dominant contribution to NO formation is attributed to diffusion flames. The results also show that the relative contribution of the different burning modes is strongly dependent on the mixture distribution and the scalar dissipation rate field. We believe that these parameters affect the diffusion flames' structures and their rates of detachment from the rich premixed flames.