Flame Stabilization and Lean Blowout Prediction by LES Combustion Model with Strain and Heat Loss Effects

Modern ultra-low NOx combustors in land based gas turbines operate mostly in lean premixed mode, close to their lean limits making the flame susceptible to instabilities and blow out. There is significant interest in examining and predicting the lean blowout conditions using CFD and other computational tools, which have now become an integral part of combustor design and development process. However, very limited literature is currently available on the CFD prediction of lean blowout (LBO) of turbulent premixed flames. Experimental studies of LBO phenomenon reveal that flame stretch induced local extinction is the primary precursor to blowout. Turbulent Flame Speed Closure model with Strain and Heat Loss Effects (TFC-SHL), and without these effects i.e. the Standard TFC model (STFC), have been employed in LES framework for this computational study. These two models have been evaluated for their capabilities to correctly predict stabilization and blowout of turbulent lean premixed flames. Recognizing that heat loss enhances sensitivity of flame to strain, combined influence of strain and heat loss was thus anticipated to capture the local extinction events and eventually the flame blowout. A simple lab-scale, conical bluff-body combustor configuration from University of Cambridge was selected for computing lean premixed methane-air flames. LBO was approached by reducing the mixture equivalence ratio. LES simulations were performed in ANSYS Fluent 17.0, wherein, a user defined function (UDF) implementing the TFC-SHL model was used. Simulation results were validated against available OH-PLIF and OH* Chemiluminescence experimental data. The Extended TFC model, predicts experimentally observed changes in flame structure, as it approaches blowout with increasingly lean mixtures. and many important aspects of blowout dynamics. On the contrary, the STFC model, gives flawed results with incorrect flame structure and the flame burning strongly at blowout condition. While Standard TFC (STFC) is unable to predict flame extinction, Extended TFC (TFC-SHL) can capture extinction at the experimentally determined blowout point. Improvements in predictive capability of Extended model can be attributed to the inclusion of strain and heat loss effects. This study in general also emphasize that enhancing physics in combustion models can significantly improve CFD predictions of important phenomenon in combustion.