Exploring the effect of ammonia enrichment on longitudinal thermoacoustic instability in an annular combustor

This study investigates the characteristics of thermoacoustic instability in an annular combustor fueled with ammonia-methane mixtures, with a particular focus on the impact of ammonia enrichment on longitudinal instability. The experiments revealed distinct transitions in combustion states as the equivalence ratio increased, following the sequence: lean blowout – stable combustion – intermittent oscillation – limit cycle oscillation. Ammonia enrichment was found to reduce oscillation amplitudes while increasing the lean blowout limit. Using integrated experimental diagnostics of acoustic pressure, OH* emissions, flame structure, and phase-averaged flame dynamics, the mechanisms underlying the effect of ammonia addition on thermoacoustic coupling were analysed. The instability maps identified at various equivalence ratio and ammonia content suggested that ammonia addition remarkably weakened the thermoacoustic coupling, which was attributed to the significant increase in convection delay time and ignition delay time by ammonia addition. This work highlights the role of ammonia in mitigating thermoacoustic instability and provides valuable insights for optimizing ammonia-methane combustion systems.