A mechanistic study of Soret diffusion in hydrogen-air flames

The separate and combined effects of Soret diffusion of the hydrogen molecule (H₂) and radical (H) on the laminar flame speed in freely propagating planar premixed flames, and the strain-induced extinction response of premixed and nonpremixed counterflow flames, were computationally studied for hydrogen-air mixtures using detailed reaction mechanism and transport properties. Results show that, except for the conservative freely propagating flame, Soret diffusion of H₂ increases the fuel concentration entering the flame structure and thereby the mixture stoichiometry and flame temperature, which could lead to substantial changes in the flame response. On the other hand, Soret diffusion of H actively modifies its concentration and distribution in the reaction zone, which in turn affects the individual reaction rates. In particular, the reaction rates are increased for diffusion flames because the nonmonotonic temperature distribution localizes the H concentration to the flame region which has the maximum temperature. However, the reaction rates of premixed flames can be increased for lean flames but decreased for rich flames, whose active reaction regions at near-extinction states are respectively located at, and away from, the stagnation surface.