A study on precise estimation of laminar burning velocity of lean hydrogen-air premixed flame (Effect of species diffusion models)

Effects of species diffusion models on the laminar burning velocity S-L of lean hydrogen-air premixed flame are investigated by performing one-dimensional numerical simulations of lean hydrogen-air premixed flames at an equivalence ratio of 0.5. Maxwell-Stefan (MS) diffusion, mixture-averaged (MA) diffusion, and unity Lewis number diffusion are compared as the concentration diffusion models at a pressure of 0.1 MPa. Moreover, the contribution of the species thermal diffusion is also investigated under three different pressure (P) and unburnt gas temperature (T-u) conditions. Results show that the MA diffusion well reproduces the results of MS diffusion including S-L while saving computational cost to three-fourths. The unity Lewis number diffusion overestimated S-L by 30 % because of the underestimation of the mass flux of H-2. By considering the species thermal diffusion, S-L decreases by 6.4 % and 3.0 % under the reference (P = 0.1 MPa and T-u = 300 K) and HPT (P = 2 MPa and T-u = 673 K) conditions, respectively, and increases by 1.3 % under the HP condition (P = 2 MPa and T-u = 300 K). Under the reference and HPT conditions, the species thermal diffusion removes the H radical from the reactive region. This mitigates the chain-branching reaction of H + O2 -> OH + O and decreases S-L. On the other hand, under the HP condition, the species thermal diffusion supplies the H radical toward the reactive region. This enhances the aforementioned chain-branching reaction and increases S-L. Under the HP condition, the recombination reaction of H + O-2 + M -> HO2 + M is enhanced by a high molar concentration of third body M because of high gas density. The large contribution of this reaction to the consumption of H radicals in the high-temperature region makes peak positions of mass fraction of H and mass flux of H by the species thermal diffusion lower temperature side, which leads to the supply of H radicals to the reactive region and the increase in S-L.