Large-Eddy Simulation of a partially premixed gas turbine model combustor using artificially thickened flames with Flamelet Generated Manifolds

Kurzfassung

Lean combustion within aircraft engines is considered to tackle today`s restrictions regarding a fuel-efficient and less air pollutant combustion process. However, the lean combustion concept remains challenging, inter alia, regarding the flame stability and cooling air distribution leading to a great demand of research. The German Aerospace Center (DLR) has conducted the "Low NOx Oxide Ceramic Combustor for Aero-Engines" (LOCCA) project to undertake experimental investigations on wall-flow-field interactions of a lean partially premixed model combustor. The LOCCA combustor is based on a swirl stabilized flame and runs with a lean, partially premixed mixture of natural gas and air. Within this work a numerical framework has been developed in order to conduct LES simulations of a partially premixed reacting flow field using Flamelet Generated Manifolds together with an Artificially Thickened Flame (ATF) turbulence-combustion closure. The premixed flamelet tables have been generated using the software Cantera and the LES solver is based on the framework OpenFOAM. The numerical framework has been validated on laminar premixed flames and then applied to the LES of the LOCCA combustor. The aim was to reconstruct the experimentally observed flow field by inert and reactive LES simulations and to undertake further investigations on the flow structure within the burner and combustor. The simulated results have shown a good agreement with the experiment. Differences within the flow field occur, in particular, regarding the flame position in the combustor. Therefore, the impact on the flow field by the chosen homogeneous boundary condition, used to model the effusion cooling, has been analysed. A parameter study on the homogeneous boundary condition, wherein the streamwise inflow velocity and streamwise momentum flux have been significantly decreased, revealed that this boundary condition has no significant impact on the flow field and flame position. In order to estimate the flame structure, observations on the intermediate product carbon monoxide have shown that two separated reaction zones occur within the combustor. By applying a lean air-fuel ratio, investigations on the Mixing process revealed that the dominant reaction zone within the combustor does not react in completely lean but rather stoichiometric condition. Thus, the mixing process within the burner is not sufficient to provide an overall lean mixture when entering the combustor. Finally, a flame index analysis has confirmed the suitability of the partially premixed approach used to simulate the LOCCA combustor.