Self-induced unstable behaviors of CH4 and H2/CH4 flames in a model combustor with a low-swirl injector

Laboratory experiments and modeling analysis have been conducted to gain insight on the self-excited unstable flame behaviors of a model gas turbine combustor that utilizes a low-swirl injector (LSI). The combustor consists of a 5.7 cm I.D. LSI firing into 18 cm diameter by 32 cm or 20.3 cm long cylindrical enclosures at atmospheric condition. The experiments involved lean premixed turbulent CH4 (0.6 < ϕ < 0.7) and 0.9 H2/0.1 CH4 (0.3 < ϕ < 0.4) flames at bulk flow velocities of 12 and 18 m/s. Acoustic spectral information was obtained from pressure transducers. A laser-based flame motion detection method was used to determine the locations and frequencies of flame oscillations. Phase-resolved 2D velocity statistics of the reacting flow fields was measured by particle image velocimetry (PIV). Self-excited unstable flame behaviors were found for the richer flames burning in the 32 cm long chamber at 18 m/s. The incited acoustic frequencies correspond to the first longitudinal mode of the combustor computed by the General Instability Model (GIM) program. Flame oscillations are found primarily in the region along the outer shear layer (OSL) of the LSI rim and the oscillation frequencies are consistent with the acoustic frequencies. Phase-resolved PIV show the OSL convects ring vortices shed from the rim. The trajectories of the vortex centers enable a linear instability analysis by GIM. For the CH4 flame, analysis shows that the roll up vortices from the dump plane are responsible for generating self-excited acoustically coupled flame instability. For the 0.9 H2/0.1 CH4 flame, the GIM analysis suggests a dominance of shedding vortices but is less conclusive due to flame attachment to the LSI rim resulting from the highly reactive H2 fuel and possible unsteady heat release contributions from bulk flow oscillations.