Ignition of hydrogen/air mixtures by a heated kernel: Role of Soret diffusion

Effects of Soret diffusion on the ignition of hydrogen/air mixtures by a heated kernel, and the structure and dynamics of the embryonic flame that is subsequently formed, were investigated numerically with detailed chemistry and transport. Results show that Soret diffusion leads to larger (smaller) minimum ignition energy (MIE) for relatively rich (lean) mixtures, that this effect is mainly engendered by the Soret diffusion of H₂ while that of the H radical is almost negligible, and that Soret diffusion also leads to an increase (decrease) of the Markstein length for rich (lean) mixtures. Satisfactory agreement with literature experimental data on the MIE is shown, especially for the critical states near lean and rich flammability limits. Evolvement of the flame structure shows that before the self-sustained flame is formed, the high temperature gradient associated with the ignition kernel has driven the H₂ in the mixture towards the ignition kernel and formed a locally high H₂ concentration region, which consequently renders lean (rich) mixtures easier (harder) to ignite. It is further shown that Soret diffusion of both H and H₂ affect the propagation dynamics of the stretched spherical flame that is subsequently formed, from its embryonic state until that of free propagation, in that Soret diffusion of H₂ is the dominant mode at small flame radius with the large strain rate, while that of H is the dominant mode at large flame radius with the small strain rate similar to that of the unstretched adiabatic planar flame.