  Implementing multicomponent diffusion models in numerical combustion studies
is computationally expensive; to reduce cost, numerical simulations commonly
use mixture-averaged diffusion treatments or simpler models. However, the
accuracy and appropriateness of mixture-averaged diffusion has not been
verified for three-dimensional, turbulent, premixed flames. In this study we
evaluated the role of multicomponent mass diffusion in premixed,
three-dimensional high Karlovitz-number hydrogen, n-heptane, and toluene
flames, representing a range of fuel Lewis numbers. We also studied a premixed,
unstable two-dimensional hydrogen flame due to the importance of diffusion
effects in such cases. Our comparison of diffusion flux vectors revealed
differences of 10-20% on average between the mixture-averaged and
multicomponent diffusion models, and greater than 40% in regions of high flame
curvature. Overall, however, the mixture-averaged model produces small
differences in diffusion flux compared with global turbulent flame statistics.
To evaluate the impact of these differences between the two models, we compared
normalized turbulent flame speeds and conditional means of species mass
fraction and source term. We found differences of 5-20% in the mean normalized
turbulent flame speeds, which seem to correspond to differences of 5-10% in the
peak fuel source terms. Our results motivate further study into whether the
mixture-averaged diffusion model is always appropriate for DNS of premixed
turbulent flames.
