Azimuthal Instabilities of Non-swirling bluff body stabilised Methane/Hydrogen Flames in a Pressurized Annular Combustor

The present study experimentally investigates the modal dynamics of the azimuthal mode with twelve methane/hydrogen blended non-swirl bluff body stabilised flames in a lab-scale pressurized annular combustor. The hydrogen power fraction was set to 40% which corresponds to 67% per volume. Equivalence ratio was varied between 0.5 to 1.1. Air mass flow was maintained at 85.0 g/s. The chamber pressure was ranged between 2.0 to 2.4 bar, which leads to fully reflecting boundary conditions. Longitudinal modes were observed for the lower equivalence ratio conditions. As the equivalence ratio is increased, the system couples to the azimuthal mode with a pressure oscillation amplitude of around 4.2 kPa. The fundamental mode at around 1680 Hz features a standing mode with pressure amplitude of 1.6 kPa. Interestingly, harmonic components are characterized by a time varying nature of the modes, with both counterclockwise and clockwise spinning mixed modes observed. Flame imaging analysis reveals that a periodic strong transverse flapping motion clearly dominates the flame structure and dynamics. This implies the presence of a significant effect of the flame structure on the nature of the azimuthal mode.