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

Abstract

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 Sd to flame curvature K, stretch rate κ and scalar dissipation rate χ are distinctly influenced by SD. For the linear Sd-K and Sd-κ correlations, SD would result in a smaller Markstein length. Moreover, SD is shown to lead to shifting of the Sd-χ curve towards the regime with larger χ. Compared with the weak influences of SD on the tangential diffusion component Sd,t and normal diffusion component Sd,n, the chemical reaction component Sd,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 H2 is slightly larger and that of H is obviously smaller when SD is considered. The SD flux of H2jH2SD and that of H jHSD are both coupled with the driving force ∇(lnT) along the mixture fraction coordinate. However, the jH2SD is mainly concentrated on the unburnt side while the jHSD is on the burnt side. The analyses on decomposed fluxes of H2 and H along the flame normal direction further suggest that SD would enhance the H2 mass diffusion but weaken the H mass diffusion. Such opposite effects stem from the distribution features that H2 is mainly on the unburnt side while H on the burnt side.