Lewis number and preferential diffusion effects in lean hydrogen-air highly turbulent flames

Abstract

Unsteady three-dimensional direct numerical simulations of highly turbulent, complex-chemistry, lean hydrogen-air flames were performed by changing the equivalence ratio φ, root mean square velocity u ′, and turbulence length scale L. For each set of φ, u ′, L, to explore the influence of molecular transport coefficients on the turbulent burning velocity U T, four cases were designed: (i) mixture-averaged diffusivities; (ii) diffusivities equal to the heat diffusivity κ of the mixture for all species; (iii) mixture-averaged diffusivities for all species with the exception of O2, whose diffusivity was equal to the diffusivity D H 2 of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with κ / D H 2 to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in U T due to differences in molecular transport coefficients even at Karlovitz number K a as large as 565. The increase is documented in cases (i) and (iii) but is not observed in case (iv) - indicating that this phenomenon is controlled by Lewis number effects, whereas preferential diffusion effects play a minor role. The phenomenon is more pronounced in leaner flames, with all other things being equal. While the temperature profiles T c F c F conditionally averaged at the local value of the combustion progress variable c F and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high K a, the T c F c F-profiles sampled from the leading edges of the same flame brushes show significant increase in the local temperature in cases (i) and (iii) characterized by a low Lewis number.