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

Ruochen Guo, Yunde Su, Yuewen Jiang

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.

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

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.
Paper Structure (16 sections, 16 equations, 20 figures)

This paper contains 16 sections, 16 equations, 20 figures.

Table of Contents

  1. Introduction
  2. Model formulations
  3. Governing Equations
  4. Front propagation formulation model
  5. Evaluation of sub-filter flame speed
  6. Heat loss modeling
  7. Experimental configuration and Numerical Setup
  8. Experimental configuration
  9. Numerical setup
  10. Results and Discussions
  11. Cold flow simulation
  12. Reactive flow simulation
  13. Effects of resolved flame thickness
  14. Evolution of resolved flame thickness and wrinklings
  15. Secondary temperature peaks and flame pockets
  16. ...and 1 more sections

Figures (20)

  • Figure 1: 3-dimensional view (a) and radial section (b) of the computational domain.
  • Figure 2: Radial profiles of mean and fluctuating axial velocities at various axial positions for the isothermal case compared to experimental results.
  • Figure 3: Radial profiles of mean and fluctuating radial velocities at various axial positions for the isothermal case compared to experimental results.
  • Figure 4: Radial profiles of mean and fluctuating azimuthal velocities at various axial positions for the isothermal case compared to experimental results.
  • Figure 5: Radial profiles of mean and fluctuating axial velocities at various axial positions for the reacting case compared to experimental results.
  • ...and 15 more figures