Development and Validation of a Thickened Flame Modeling Approach for Large Eddy Simulation of Premixed Combustion

The development of a dynamic Thickened Flame (TF) turbulence chemistry interaction model is presented based on a novel approach to determine the sub-filter flame wrinkling efficiency. The basic premise of the TF model is to artificially decrease the reaction rates and increase the species and thermal diffusivities by the same amount which thickens the flame to a scale that can be resolved on the LES grid while still recovering the laminar flame speed. The TF modeling approach adopted here uses local reaction rates and gradients of product species to thicken the flame to a scale large enough to be resolved by the LES grid. The thickening factor, which is a function of the local grid size and laminar flame thickness, is only applied in the flame region and is commonly referred to as dynamic thickening. Spatial filtering of the velocity field is used to determine the efficiency function by accounting for turbulent kinetic energy between the grid-scale and the thickened flame scale. The TF model was implemented into the commercial CFD code FLUENT. Validation of the approach is conducted by comparing model results to experimental data collected in a lab-scale burner. The burner is based on an enclosed, scaled-down version of the Low Swirl Injector (LSI) developed at Lawrence Berkeley National Laboratory. A perfectly premixed lean methane-air flame was studied as well as the cold-flow characteristics of the combustor. Planar Laser Induced Fluorescence (PLIF) of the hydroxyl molecule was collected for the combusting condition as well as velocity field data using Particle Image Velocimetry (PIV). Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.