Role of soret diffusion in the ignition of hydrogen/air mixtures by a heated kernel

Effects of Soret diffusion on the minimum ignition energy of hydrogen/air mixtures by a heated kernel, and the associated flame structure, were investigated numerically with detailed chemistry and transport. Results show that Soret diffusion leads to larger (smaller) minimum ignition energy for relatively rich (lean) mixtures, and that this effect is mainly engendered by the Soret diffusion of H₂, while that of the H radical is almost negligible. Mechanistically, Soret diffusion of H₂ increases the fuel concentration entering the ignition kernel and consequently renders rich (lean) mixtures harder (easier) to ignite. The evolvement of the flame structure shows that before the H radical is substantially accumulated to exert an influence on the reaction zone, the high temperature gradient of the ignition initiation has driven H₂in the mixture moving towards the ignition kernel. For the stretched spherical flame of the ignition, it is shown that Soret diffusion of both H and H₂ should be included such that the stretched flame speed can be accurately predicted. At small flame radius (large strain rate), the Soret diffusion of H₂ is the dominant mode of Soret diffusion; at large flame radius (small strain rate), the influence of Soret diffusion is similar to that on the unstretched adiabatic planar flame where the Soret diffusion of H radical plays the major role.