Mixing in high-pressure flows: the influence of the number of species

Mixing of several species in high-pressure (high-p) turbulent flows is investigated to understand the influence of the number of species on the flow characteristics. Direct Numerical Simulations are conducted in the temporal mixing layer configuration at approximately same value of the momentum ratio for all realizations. The simulations are performed with two, three, five and seven species for various compositions, and two values of the free-stream pressure, p_0, which is supercritical for each species. The extensive database thus obtained allows the study of the influence of the composition, p_0, and the initial density ratio. The species effective Schmidt number is computed and it is found that negative values occur for all secondary species - particularly for H_2 - thus indicating uphill diffusion, while the primary species only experience regular diffusion. The probability density function (p.d.f.) of the species-specific effective Schmidt number shows strong variation with p_0 but weak dependence on the number of species; however, the p.d.f. substantially varies with the identity of the species. In contrast, the p.d.f. of the effective Prandtl number indicates dependence on both p_0 and the number of species. Similar to the species-specific effective Schmidt number, the species-specific effective Lewis number p.d.f. depends on the species and for all species the mean is smaller than unity, thus invalidating one of the most popular assumptions in combustion modeling