Towards large eddy simulations of premixed turbulent flames without a combustion model

TL;DR

This study demonstrates that large eddy simulations of premixed turbulent flames can accurately predict flame surface density and heat release rates without subgrid models if the filtering resolution is sufficiently fine, especially in intense turbulence conditions.

Contribution

The paper provides evidence that filtering at a scale comparable to the laminar flame thickness allows direct evaluation of flame properties without subgrid models in highly turbulent premixed flames.

Findings

Filtering at the flame thickness scale yields accurate flame surface density predictions.

Small-scale turbulence does not significantly alter local flame structure.

Large turbulent eddies primarily strain the flame, affecting its structure.

Abstract

The paper aims at assessing a hypothesis that resolution required to evaluate fuel consumption and heat release rates by directly (i.e., without a subgrid model of unresolved influence of small-scale turbulent eddies on the local flame) processing filtered fields of density, temperature, and species mass fractions should be significantly finer than resolution required to directly compute flame surface density by processing the same filtered fields. For this purpose, box filters of various widths are applied to three-dimensional Direct Numerical Simulation data obtained earlier by Dave et al. (Combust. Flame 196 (2018) 386-399) from a statistically one-dimensional and planar, moderately lean H2/air complex-chemistry flame propagating in a box under conditions of sufficiently intense small-scale turbulence (Karlovitz number is larger than unity and a ratio of laminar flame thickness to Kolmogorov length scale is about 20). Results confirm this hypothesis and show that the mean flame surface density and area can be predicted with acceptable accuracy by processing filtered combustion progress variable fields computed using a sufficiently wide filter. Such an approach does not require a model of the influence of subgrid turbulent eddies on flame surface density provided that filter width and laminar flame thickness are of the same order of magnitude. Good performance of this approach is attributed to inability of small-scale (when compared to the thickness) turbulent eddies to substantially change the local flame structure, which, nevertheless, is significantly perturbed by larger turbulent eddies that strain the local flame.