Large eddy simulation of turbulent swirl-stabilized flames using the front propagation formulation: impact of the resolved flame thickness

TL;DR

This study extends the front propagation formulation for LES of turbulent swirl flames, demonstrating the critical impact of resolved flame thickness on accurately predicting flame dynamics and temperature peaks.

Contribution

The paper introduces an extended FPF model that accounts for non-adiabatic effects and improves sub-filter flame speed estimation in LES of swirl-stabilized flames.

Findings

Resolved flame thickness significantly influences flame dynamics predictions.

Proper modeling of chemical steepening prevents over-prediction of flame brush thickness.

Failure to model flame thickness effects leads to missing secondary temperature peaks.

Abstract

This work extends the front propagation formulation (FPF) combustion model to large eddy simulation (LES) of swirl-stabilized turbulent premixed flames and investigates the effects of resolved flame thickness on the predicted flame dynamics. The FPF method is designed to mitigate the spurious propagation of under-resolved flames while preserving the reaction characteristics of filtered flame fronts. In this study, the model is extended to account for non-adiabatic effects and is coupled with an improved sub-filter flame speed estimation that resolves the inconsistency arising from heat-release effects on local sub-filter turbulence. The performance of the extended FPF method is validated by LES of the TECFLAM swirl-stabilized burner, where the results agree well with experimental measurements. The simulations reveal that the stretching of vortical structures in the outer shear layer leads to the formation of trapped flame pockets, which are identified as the physical mechanism responsible for the secondary temperature peaks observed in the experiment. The prediction of this phenomenon is shown to be strongly dependent on the resolved flame thickness, when the filter size is used for modeling sub-filter flame wrinklings. Without proper modeling of the chemical steepening effects, the thickness of the resolved flame brush is over-predicted, causing the flame consumption rate to be under-estimated. Consequently, the flame brush detaches from the outer shear layer, resulting in a failure to capture the flame pockets and the associated secondary temperature peaks.