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 [Formula: see text], root mean square velocity [Formula: see text], and turbulence length scale [Formula: see text]. For each set of [Formula: see text], to explore the influence of molecular transport coefficients on the turbulent burning velocity [Formula: see text], four cases were designed: (i) mixture-averaged diffusivities; (ii) diffusivities equal to the heat diffusivity [Formula: see text] 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 [Formula: see text] of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with [Formula: see text] to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in [Formula: see text] due to differences in molecular transport coefficients even at Karlovitz number [Formula: see text] 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 [Formula: see text] conditionally averaged at the local value of the combustion progress variable [Formula: see text] and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high [Formula: see text], the [Formula: see text]-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.