Abstract A quasi–cubic meso-scale air/methane whirl flow combustion chamber, with no moving parts, is analyzed by means of high-fidelity Large-Eddy Simulations (LES). In order to consider partially premixed flames and differential diffusion process into the combustor, 3D LES computations include semi-detailed chemical kinetics mechanism and complex transport. They show that the reacting flow structure, flame topology and global efficiency are in line with previous experimental and less detailed numerical modeling. Combustion is stabilized by a Central Recirculation Zone (CRZ) and mainly takes place in premixed lean regime. Wall heat losses are also estimated and incomplete combustion zones are identified. Performances improvement was tested with hydrogen addition to fuel mixture. A small amount of hydrogen shows a global efficiency rise while keeping the whirl flow topology. The addition of a large amount of hydrogen implies a change in flame topology leading to a less complete combustion and pollutant formation. These results are in agreement with experimental data.