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 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> </mml:msub> </mml:math> ${\rm NH}_3$ ) with blends of methane ( normal upper C normal upper H Subscript 4 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:math> ${\rm CH}_4$ ) and hydrogen ( normal upper H 2 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math> ${\rm H}_2$ ) 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 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:mi>k</mml:mi> <mml:mo>=</mml:mo> <mml:mn>4100</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mrow> <mml:mi mathvariant="normal">W</mml:mi> </mml:mrow> </mml:math> $k=4100\,\mathrm{W}$ ) and equivalence ratio ( phi equals 0.85 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:mi>ϕ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.85</mml:mn> </mml:math> $\phi =0.85$ ), we systematically vary the fuel composition across blends of normal upper C normal upper H Subscript 4 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:math> ${\rm CH}_4$ , normal upper H 2 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math> ${\rm H}_2$ and normal upper N normal upper H Subscript 3