58777 - Transient Thermoacoustic Responses of Ch4-H2 Flames in a Pressurized Annular Combustor 

Flexible, hydrogen fired gas turbines play a vital role in accelerating the energy transition by providing stability to the grid through dynamic operation enabling increased penetration of intermittent renewable power sources. Nonetheless, the effect of dynamic operation on azimuthal combustion instabilities has not been investigated under well-defined acoustic boundary conditions before. The present article experimentally investigates the triggering and transient growth of azimuthal instabilities during equivalence ratio ramps in a pressurized laboratory-scale annular combustor featuring twelve CH4-H2 flames. The hydrogen content of the fuel blend and the ramping rate of equivalence ratio are varied to examine their effect on transient thermoacoustic responses and driving mechanisms. The phenomena are investigated with phase space reconstruction and high-speed flame imaging analysis. The results reveal that although the flame deformations from stable to unstable state are observed to be similar for all cases, routes from mixed to spinning azimuthal mode limit cycle oscillations are markedly different, depending on operating and ramping conditions.
With increasing hydrogen content, the duration of the mode transition from mixed mode to spinning mode increases. This behaviour is shown to be insensitive to the ramp time. The mode transition occurs at lower equivalence ratio for increasing ramping rates and lower hydrogen contents. The mechanisms dominating the transition behaviour are highly associated with the growth of the flame sheet wrinkling attributed to shear layer vorticity oscillations. This behaviour is responsible for the growth of the azimuthal oscillations which are related to the spinning nature of the mode.