Thermoacoustic oscillations of laminar premixed hydrogen-rich flames*

Thermoacoustic oscillations are known to occur when the unsteady heat release rate from the flame is in phase with acoustic perturbations inside the combustor. A number of nonlinear dynamical states that arise due to the thermoacoustic feedback of the premixed flames have been reported. In this experimental study, we investigate the effect of fuel composition, particularly hydrogen-enrichment on thermoacoustic oscillations. We systematically increase the percentage of hydrogen (by energy and by volume) in the fuel and measure the system response in terms of acoustic pressure and high-speed chemiluminescence images of the flame. We find that the premixed flame undergo a sequence of transitions in nonlinear dynamical states when the hydrogen percentage is increased. At low hydrogen percentage, the system dynamics show limit cycle oscillations at a frequency closer to the fundamental acoustic mode. As the percentage of hydrogen is increased, we observe the emergence of quasiperiodic state characterized by oscillations at two incommensurate frequencies. We analyze the mode and the flame structure by using dynamic mode decomposition of high-speed chemiluminescence images. Further, the phase-averaged images corresponding to different nonlinear states are investigated. This study shows that moderate percentage of hydrogen modify the spatial distribution of heat release rate and cause the emergence of new acoustic modes.