Large Eddy Simulation of Turbulent Premixed Combustion in Gas Turbines

Abstract

An artificially Thickened Flame (TF) approach based on LES framework is used to model the turbulent premixed combustion in gas turbine combustors. A number of variants of the Thickened Flame approach including a modified version of TF model have been studied in details. In the TF model, the flame front is artificially thickened to resolve on LES computational grid. With this approach, reaction rate modeling does not require any ad-hoc closure assumptions. However, suitable modifications have to be made to compensate for flame thickening. To verify the predictive capability of the models, a stoichiometric methane-air flame on Bunsen burner type geometry has been simulated and the TF model predictions compared with experimental data as well as with other model predictions. As a part of this research, turbulent flow over a backward facing step, isothermal swirling flow in a confined geometry have also been studied and predictions are compared with experiments. Good agreement with data is obtained that validates the LES model used. The validated LES based TF model is used to investigate flashback behavior in hydrogen enriched premixed flame in a swirled combustor. Firstly, non-reacting and reacting flows with natural gas are studied, followed by hydrogen enriched combustion. In general, the LES predictions for both reacting and non-reacting cases are found to be in good agreement with measurements. In non-reacting flow conditions, the recirculation zones (WRZ, CRZ and CTRZ) are clearly observed, especially at high Reynolds number. Moreover, the observations reveal that higher combustibility of hydrogen causes Combustion Induced Vortex Breakdown driven flashback due to complex interaction of chemical reaction in swirled burner, resulting in faster flame propagation into the upstream mixing tube. In particular, combined effect of baroclinic production and vortex stretching accelerates the upstream flame propagation in hydrogen-enriched mixture, while only methane shows stable behavior. Further, the effect of swirl strength, premixedness and geometry has also been studied on flashback behavior of hydrogen enriched mixture. Flame flashback is always observed at higher swirl strength irrespective of level of premixedness and burner geometry, whereas the premixed systems exhibit stable behavior while operating under lower swirl strength.