Influence of preferential diffusion on the distribution of species in lean H2-air laminar premixed flames at different equivalence ratios

Abstract

The influence of equivalence ratio on preferential diffusion effects and the resulting changes in the distributions of major species and their reaction rates have been analysed based on 2D simulations of lean ${\mathrm{H_2}}$-air laminar premixed flames, at $ϕ=0.4$ and $0.7$. The enhancements of burning rate, flame surface area, and stretch factor increase with decreasing equivalence ratio with the increase in stretch factor particularly prominent when the burning rate and flame area are evaluated based on normalised mass fraction variation of ${\mathrm{H_2}}$. The preferential diffusion effects have been demonstrated to lead to significant deviations of mass fractions of major species and their reaction rates from the corresponding 1D unstretched laminar premixed flame solution and local variations of equivalence ratio. This tendency is particularly strong for ${\mathrm{H_2}}$ among all the major species. Moreover, mass fractions of ${\mathrm{O_2}}$ and ${\mathrm{H_2O}}$ are found to assume super-adiabatic values at the super-adiabatic temperature zones and this trend is particularly strong for the $ϕ=0.4$ case. It has been demonstrated that the deviations of major species mass fractions and their reaction rates from their corresponding 1D unstretched laminar premixed flame values arise principally due to preferential diffusion effects induced by relative focussing/defocussing of species and heat at the positively and negatively curved regions with the nature of the deviations being opposite to each other depending on the sign of the curvature. The variations of normalised species mass fractions of $\mathrm{O_2}$ and $\mathrm{H_2O}$ are found to be significantly affected by the local equivalence ratio within the 2D laminar flame with $ϕ=0.4$ but these effects weaken with an increase in global equivalence ratio.