Stabilization mechanisms of CH4 premixed swirled flame enriched with a non-premixed hydrogen injection

The impact of non-premixed hydrogen addition on the flame shape and stabilization mechanisms of a swirled methane/air flame was studied by high-fidelity large eddy simulations (LES). An analytically reduced chemistry mechanism was used to achieve a detailed description of CH4/air-H2 chemistry. First, a validation of this kinetic scheme against the detailed GRI-Mech 3.0 mechanism was presented considering both simplified and complex transport properties. When hydrogen was added to the mixture, large variations of the mixture Prandtl and of the N2 Schmidt numbers were observed depending on the local species concentrations, features that are missed by simplified models. LES was then employed to investigate the structure and stabilization mechanisms of a lean (Φ = 0.8) premixed CH4/air swirled flame enriched with hydrogen by using different transport modeling strategies. The fully premixed CH4/air case was considered and the results were found to validate the LES approach. In agreement with experiments, H2 injection was found to have a marginal impact on the general structure of the swirled V-shaped flame, leading only to a more intense reaction at the flame root and along the outer side of the flame facing the central recirculation zone. Hydrogen enrichment was achieved injecting 2% of the CH4 thermal power with a central fuel injection lance. Both premixed and diffusion flame branches were present in this case, impacting flame stabilization and flame angle. The flame root of the main premixed flame was stabilized by a diffusion flame kernel created by the injected hydrogen reacting with the oxygen in excess of the premixed stream. Moreover, the H2 consumed with the remaining oxygen in burnt gases led to the formation of a second flame branch inside the CRZ which was responsible for an increase of the flame angle. Given the high concentration of hydrogen, an impact of the molecular transport models was observed on the flame lift-off height highlighting the importance of using complex transport properties in any LES involving hydrogen combustion.