Abstract
In this study, a new set of direct numerical simulations is generated and used to examine the influence of mixture composition heterogeneities on the propagation of a premixed iso-octane/air spherical turbulent flame, with a representative chemical description. The dynamic effects of both turbulence and combustion heterogeneities are considered, and their competition is assessed. The results of the turbulent homogeneous case are compared with those of heterogeneous cases which are characterized by multiple stratification length scales and segregation rates in the regime of a wrinkled flame. The comparison reveals that stratification does not alter turbulent flame behaviors such as the preferential alignment of the convex flame front with the direction of the compression. However, we find that the overall flame front propagation is slower in the presence of heterogeneities because of the differential on speed propagation. Furthermore, analysis of different displacement speed components is performed by taking multi-species formalism into account. This analysis shows that the global flame propagation front slows down due to the heterogeneities caused by the reaction mechanism and the differential diffusion accompanied by flame surface density variations. Quantification of the effects of each of these mechanisms shows that their intensity increases with the increase in stratification’s length scale and segregation rate.
Highlights
In this study, a new set of direct numerical simulations is generated and used to examine the influence of mixture composition heterogeneities on the propagation of a premixed iso-octane/air spherical turbulent flame, with a representative chemical description
We demonstrated that the probability density function (PDF) of ξ evolves to a Gaussian-like distribution skewed toward leaner values
A new two-dimensional direct numerical simulation (DNS) database considering homogeneous and heterogeneous flames in turbulent iso-octane/air flow was generated taking into account a representative chemical description with 29 species and 48 elementary reactions
Summary
The low Mach number (LMN) approximation of the fully compressible Navier–Stokes equations (NSE) is considered. Thanks to the LMN approximation, the compressible NSE can be simplified such that the thermochemical state of the flow is decoupled from the momentum equation [16]. In this framework, the mass conservation equation is expressed as. Γ is the ratio of the specific heats, λ is the thermal conductivity and ωhs is the resulting heat release rate This rate is expressed in terms of the species production rates, ωk , and the standard enthalpies of formation ∆h0f ,k as ωhs = −. The variable-coefficient Poisson equation for pressure differences are solved using the multigrid method provided by the HYPRE library [27]
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