A numerical investigation of the effects of hydrogen addition on combustion instability inside a partially-premixed swirl combustor

Large eddy simulations (LES) of a hydrogen enriched partially premixed gas turbine combustor are performed for various operating conditions for examining the fuel condition effects. The LES turbulence model and multi-step chemical mechanism of hydrogen-methane combustion are applied for simulating the three-dimensional swirling flame in the combustor. To confirm the adequacy of the numerical simulation, a comparison with the existing experimental data is conducted which indicates that hydrogen-enriched methane fuel conditions trigger unstable pressure fluctuations in the combustor. The analysis on pressure spectra and harmonic mode on the unsteady flame is performed to investigate the driving source of combustion instability. In the unstable case, rigorous vortex burning appears as unburnt mixture is periodically supplied into the recirculation zone. By considering the fuel composition and flow rate modifications, it is shown that hydrogen content and fuel flow rate play a dominant role in determining the flame structure and its combustion instability. If hydrogen content is increased or fuel flow rate is decreased, the flame structure changes and the flow field is stabilized. Then, vortex burning does not appear as flame is formed in a narrow region near the dump plane. Thus, the present investigation has identified the change of flame structure and its interaction with vortex field as the key features for understanding the combustion instability.