A thickened flame model for large eddy simulations of turbulent premixed combustion

Abstract

A subgrid scale model for large eddy simulations of turbulent premixed combustion is developed and validated. The approach is based on the concept of artificially thickened flames, keeping constant the laminar flame speed sl0. This thickening is simply achieved by decreasing the pre-exponential factor of the chemical Arrhenius law whereas the molecular diffusion is enhanced. When the flame is thickened, the combustion–turbulence interaction is affected and must be modeled. This point is investigated here using direct numerical simulations of flame–vortex interactions and an efficiency function E is introduced to incorporate thickening effects in the subgrid scale model. The input parameters in E are related to the subgrid scale turbulence (velocity and length scales). An efficient approach, based on similarity assumptions, is developed to extract these quantities from the resolved velocity field. A specific operator is developed to exclude the dilatational part of the velocity field from the estimation of turbulent fluctuations. The combustion model is then implemented in a compressible parallel finite volume–element solver able to handle hybrid grids to simulate a lateral injections combustor (LIC). Results are in agreement with the available experimental data.

Modeling chemical flame structure and combustion dynamics in LES

“…It has been shown in Ref. [20] that when the flame front is artificially broadened, then the flame response decreases. Therefore as δ T > δ T under similar mesh conditions, thickened flames will be always less affected by turbulence than filtered and original flames.…”

“…A solution to propagate a flame front on a coarse grid is to artificially thicken the flame front (Thickened Flame model for LES) and to model subgrid scale wrinkling [20]. In premixed combustion, as this technique can be applied to the progress variable balance equation, coupling TFLES and tabulated chemistry is straightforward [21].…”

A Filtered Tabulated Chemistry Model for Large Eddy Simulation of Reactive Flows

“…29 and 30. It can either be estimated from analytical models [9][10][11] or from the solution of a flame surface density balance equation. 12, 13…”

“…This turbulent flame speed is related to the laminar flame speed by the wrinkling factor Ξ. 9 The model is derived to ensure that the filtered flame front propagates at the turbulent flame speed. The filtered flame thickness is assumed to be only related to the filter size ∆ and is not altered by small-scale eddies.…”

“…To overcome this difficulty, dedicated models have been developed under simplified chemistry assumptions. The main commonly used strategies are to artificially thicken the flame, 5,6 or track the inner layer using level set techniques 7 or thicken by filtering the reaction rate at scales larger than the grid size.…”