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Implementing multicomponent diffusion models in numerical combustion studiesis computationally expensive; to reduce cost, numerical simulations commonlyuse mixture-averaged diffusion treatments or simpler models. However, theaccuracy and appropriateness of mixture-averaged diffusion has not beenverified for three-dimensional, turbulent, premixed flames. In this study weevaluated the role of multicomponent mass diffusion in premixed,three-dimensional high Karlovitz-number hydrogen, n-heptane, and tolueneflames, representing a range of fuel Lewis numbers. We also studied a premixed,unstable two-dimensional hydrogen flame due to the importance of diffusioneffects in such cases. Our comparison of diffusion flux vectors revealeddifferences of 10-20% on average between the mixture-averaged andmulticomponent diffusion models, and greater than 40% in regions of high flamecurvature. Overall, however, the mixture-averaged model produces smalldifferences in diffusion flux compared with global turbulent flame statistics.To evaluate the impact of these differences between the two models, we comparednormalized turbulent flame speeds and conditional means of species massfraction and source term. We found differences of 5-20% in the mean normalizedturbulent flame speeds, which seem to correspond to differences of 5-10% in thepeak fuel source terms. Our results motivate further study into whether themixture-averaged diffusion model is always appropriate for DNS of premixedturbulent flames.

Assessing the impact of multicomponent diffusion in direct numerical simulations of premixed, high-Karlovitz, turbulent flames