Numerical investigation of the Soret effect on burning velocity of lean hydrogen flame

2D numerical simulations of a wrinkled lean hydrogen (H2)-air premixed flame are performed with and without considering the Soret effect to investigate the Soret effect on flame propagation. The equivalence ratio and temperature of unburnt premixed gas are 0.5 and 300 K, respectively, and ambient pressure is 1 atm. Results show that neglecting the Soret effect underestimates the burning velocity and flame surface area by approximately 4% and 5.5%, respectively. Neglecting the Soret diffusion of H2 overestimates the Lewis number of H2. This overestimated Lewis number suppresses the growth of the flame front perturbation with a large wavelength and causes the underestimation of the flame surface area. Moreover, neglecting the Soret effect underestimates the total heat release near the maximum of the progress variable. At the flame bulge, the Soret diffusion enhances the transport of H2 toward the convex flame front, increasing the local mixture fraction. This increase in the mixture fraction means that the local equivalence ratio approaches stoichiometry under the current fuel-lean conditions. Therefore, neglecting the Soret effect results in a leaner local mixture and an underestimation of the reaction progress in this region. Additionally, neglecting the Soret diffusion of H results in the underestimation of H supply to burnt gas side. Thus, chain-branching reaction of H + O2 = O + OH and the formation of H2O from intermediate species (H, O, OH) in the burnt gas region at flame bulge are suppressed. Consequently, the heat release in the flame bulge, where the progress variable is near its peak, is also underestimated. The underestimations of flame surface area and heat release result in the underestimation of the burning velocity.