A Comparison of RANS and LES of an Industrial Lean Premixed Burner

LES and RANS simulations of a Siemens scaled combustor are compared against comprehensive experimental data. The steady RANS simulation modeled one quarter of the geometry with 8M polyhedral cells using the SST-k-ω model. Unsteady LES simulations were performed on the quarter geometry (90°, 8M cells) as well as the full geometry (360°, 32M cells) using the WALE sub-grid model and dynamic evaluation of model coefficients. Aside from the turbulence model, all other models are identical for the RANS and LES. Combustion was modeled with the Flamelet Generated Manifold (FGM) model, which represents the thermo-chemistry by mixture fraction and reaction progress. RANS simulations are performed using Zimont and Peters turbulent flame speed (TFS) expressions with default model constants, as well as the kinetic rate from the FGM. The flame speed stalls near the wall with the TFS models, predicting a flame brush that extends to the combustor outlet, which is inconsistent with measurements. The FGM kinetic source model shows improved flame position predictions. The LES predictions of mean and rms axial velocity, mixture fraction and temperature do not show improvement over the RANS. All three simulations over-predict the turbulent mixing in the inner recirculation zone, causing flatter profiles than measurements. This over-mixing is exacerbated in the 900 case. The experiments show evidence of heat loss and the adiabatic simulations presented here might be improved by including wall heat-loss and radiation effects.