| Differential diffusion effects in a turbulent H2-O2 nonpremixed flame are investigated using DNS. A generalized Burke-Schumann formulation 1 that allows for differing mass and thermal diffusivities as well as finite-rate chemistry is used. The formulation is based on a three-step reduced mechanism with H as the only intermediate species. Preferential diffusion of H2 and the intermediate species H are considered. Heat release is assumed small such that there is no feed back on the flow field. The flow field is incompressible decaying isotropic turbulence. The objectives are to study the effect of differential diffusion of both fuel and intermediate species, and the effects of reaction rate, dilution and turbulence on differential diffusion. The effects of differential diffusion are characterized by the difference in element mixture fractions, Δ = ξH − ξO. Comparisons are made with the limiting cases of no reaction and a single step, infinite rate reaction. An evolution equation for Δ is derived for each of the three cases.; With LeH2 ≠ 1 and LeH = 1, the peak conditional average temperature increases over the unity Lewis number case. But with LeH 2 ≠ 1 and LeH ≠ 1, the peak conditional temperature decreases below the unity Lewis number case, but increases on the oxidizer side. The effects of H2 and H differentially diffusing are found to be counteracting.; For the nonreacting case, conditional Δ has a reverse “ S” shape that is characteristic of differential diffusion. With reaction, the flame modifies the shape so a peak appears at the stoichiometric position. Varying the reaction rate is found to have little effect on Δ though reacting scalars that are used in the calculation of Δ are affected.; With increasing dilution, the peak of the conditional average of Δ also increases. The shape of the conditional average of Δ for the lowest dilution was found to approach the reverse “ S” shape of the nonreacting case. With more dilution, Δ exhibits more of a peak at the stoichiometric position.; Increasing turbulence was found to reduce the conditional average of Δ while increasing the fluctuations of Δ.; 1Sanchez, Liñan, and Williams, Combust. Sci. and Tech., 1997... |
