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 UT, 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 DH2 of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with κ/DH2 to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in UT due to differences in molecular transport coefficients even at Karlovitz number Ka 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 TcFcF conditionally averaged at the local value of the combustion progress variable cF and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high Ka, the TcFcF-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.