Intrinsic thermoacoustic harmonic switching in hydrogen–methane–ammonia flames

In practical combustion systems, co-firing of ammonia ( normal upper N normal upper H Subscript 3 N H 3 NH₃ ) with blends of methane ( normal upper C normal upper H Subscript 4 C H 4 CH₄ ) and hydrogen ( normal upper H 2 H 2 H₂ ) offers a promising pathway towards low-carbon combustion. However, the nonlinear thermoacoustic oscillations arising from such multi-fuel blends remain poorly understood, particularly in systems supporting intrinsic thermoacoustic (ITA) modes. In this study, we experimentally investigate the effects of varying fuel composition on thermoacoustic instability in a partially premixed swirl-stabilised combustor. Operating at a fixed thermal power ( k equals 4100 normal upper W k = 4100 W k=4100\,W ) and equivalence ratio ( phi equals 0.85 ϕ = 0.85 φ =0.85 ), we systematically vary the fuel composition across blends of normal upper C normal upper H Subscript 4 C H 4 CH₄ , normal upper H 2 H 2 H₂ and normal upper N normal upper H Subscript 3