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

Abstract

Effects of species diffusion models on the laminar burning velocity SL 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 (Tu) conditions. Results show that the MA diffusion well reproduces the results of MS diffusion including SL while saving computational cost to three-fourths. The unity Lewis number diffusion overestimated SL by 30 % because of the underestimation of the mass flux of H2. By considering the species thermal diffusion, SL decreases by 6.4 % and 3.0 % under the reference (P = 0.1 MPa and Tu = 300 K) and HPT (P = 2 MPa and Tu = 673 K) conditions, respectively, and increases by 1.3 % under the HP condition (P = 2 MPa and Tu = 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 SL. 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 SL. Under the HP condition, the recombination reaction of H + O2 + 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 SL.

Keywords

• hydrogen flame
• premixed flame
• species diffusion
• thermal diffusion
• soret effect
• dufour effect
• laminar burning velocity
• numerical simulation