Soret effects on the diffusion-chemistry interaction of hydrogen-air edge flames propagating in transverse gradient evolving mixing layers

The effects of Soret diffusion (SD) on the hydrogen-air edge flame propagation and the diffusion-chemistry interaction are investigated through simulation facilitated by the numerical code MultiDiffFOAM. The edge flames in this study gradually develop from a flame kernel into a tri-brachial structure in a hydrogen-air mixing layer that temporally evolves due to transverse reactant concentration gradient. We demonstrate that the responses of flame displacement speed S-d to flame curvature K, stretch rate kappa and scalar dissipation rate chi are distinctly influenced by SD. For the linear S-d-K and S-d-kappa correlations, SD would result in a smaller Markstein length. Moreover, SD is shown to lead to shifting of the S-d-chi curve towards the regime with larger chi. Compared with the weak influences of SD on the tangential diffusion component S-d,S-t and normal diffusion component S-d,S-n the chemical reaction component S(d,r )is significantly weakened by SD. The important chemical reactions for edge flame propagation are identified based on sensitivity analysis and their rates are found to be smaller when SD is considered. For the local composition at the flame marker, the mass fraction of H-2 is slightly larger and that of H is obviously smaller when SD is considered. The SD flux of H(2 )j(H2)(SD) and that of H j(H)(SD) are both coupled with the driving force del(lnT) along the mixture fraction coordinate. However, the 4D is mainly concentrated on the unburnt side while the j(H2)(SD) is on the burnt side. The analyses on decomposed fluxes of H-2 and H along the flame normal direction further suggest that SD would enhance the H-2 mass diffusion but weaken the H mass diffusion. Such opposite effects stem from the distribution features that H-2 is mainly on the unburnt side while H on the burnt side.