Nonlinear dynamics of thermoacoustic oscillations in syngas combustors*

A combustion system can exhibit large-amplitude, self-excited limit cycle oscillations when the unsteady heat release rate from the flame oscillates in phase with the natural acoustic modes of the combustor. In this experimental study, we show that the dynamical features of these self-excited oscillations can vary as function of the fuel composition. For fuel-lean, methane-air combustion above a critical Reynolds number, the instability is observed at a frequency closer to the fundamental acoustic mode. As the percentage of hydrogen is increased with the concurrent reduction in the methane percentage, we observe that the instability transitions to higher harmonic modes. Further, we find that the presence of multiple frequencies when the fuel is comprised of mixture of methane, carbon monoxide and hydrogen. This study shows that the combustion of fuel having mixture fraction variation can involve multiple time lags in the unsteady heat release rate, causing the transition to multiple harmonic thermoacoustic oscillations.