The thickened flame approach for non-premixed combustion: Principles and implications for turbulent combustion modeling

Abstract Modeling turbulent non-premixed combustion remains a challenge in the context of Large Eddy Simulation (LES) in complex geometries and for realistic conditions, taking into account all physical phenomena impacting the flame such as heat loss, dilution, or liquid fuel atomization and evaporation. In this work, the Thickened Flame concept, which allows to resolve the flame front on the LES grid while preserving the consumption speed, and initially derived for premixed combustion, is adapted to diffusion flames. It is demonstrated that the concept holds for these flames, with however, a different formulation of the model due to but their specific nature and properties. In particular, in the high-Damkohler regime, the thickening factor is applied only to the diffusion coefficients. The behavior of thickened diffusion flames is illustrated on laminar steady strained flames for both simple and complex chemistry, showing how the Thickened Flame concept applies. Based on these results, an expression for the thickening factor related to mesh coarsening is derived. For a complete turbulent combustion model, the thickening factor should also describe the sub-grid scale flame-turbulence interaction, which is left for future work.