Session: 03-01 Ammonia as Fuel and Hydrogen Carrier – Combustion, Storage, and Safety

Submission Number: 176982

Influence of Ammonia Cracking Ratio on Thermoacoustic Instabilities in a Swirl-Stabilised Combustor 

Decarbonising energy-intensive sectors requires carbon-free fuels and combustors tolerant to wide variations in fuel properties and reactivity. Depending on the application, promising chemical energy vectors include hydrogen, ammonia, and synthetic liquid fuels. Hydrogen offers high reactivity and near-zero carbon emissions at the point of use, yet increases flashback propensity, lean thermoacoustic susceptibility, and poses storage/transport challenges. Ammonia is carbon-free, liquid-storable and readily transported, but exhibits slow chemistry, low extinction limits, and elevated NOₓ due to fuel-bound nitrogen. Partial cracking of NH₃ to H₂/N₂ upstream of the combustor combines advantages: added H₂ improves ignition, flame stabilisation and lift-off, while the cracking ratio provides a tuneable lever to balance reactivity, stability and safety alongside injector design and swirl/staging. This work delineates how the cracking ratio reshapes the stability envelope, covering flashback/blowout limits and susceptibility to limit cycles and mode switching, by linking heat-release dynamics to acoustic coupling in an H₂/NH₃-fuelled model combustor.

The gas-turbine model combustor follows the PRECCINSTA geometry and operates at atmospheric pressure in fully and partially premixed modes. This configuration enables direct comparison with extensive hydrogen- and natural-gas-based datasets for the same geometry. The database is extended to partially cracked-ammonia conditions, emulated using synthetic blends by independently metering compressed air, H₂, NH₃ and N₂ via calibrated Brooks SLA mass-flow controllers and mixing them in a static mixer. Acoustic pressure is measured with three amplitude- and phase-calibrated probes (Brüel & Kjær Type 2670): one in the air plenum and two in the combustion chamber; signals are sampled at 100 kHz. Line-of-sight chemiluminescence of OH*, NH* and NH₂* is recorded using a Photron HSS6 high-speed camera equipped with an intensifier relay optic unit, a 64 mm Halle UV lens (IRO gain 80%, 10 µs gate) and narrow band-pass filters. The image-acquisition rate is 10 kHz. Camera and acoustic acquisition are synchronised to enable phase-resolved analysis, and OH* intensity serves as the primary global heat-release marker.

For the fully premixed case at a fixed thermal output of 10 kW, the neat-ammonia flame shows lean blowout (LBO) at an equivalence ratio (Φ) of 0.65 and rich blowout (RBO) at Φ = 1.10. For cracking ratios crNH₃ = 10–30%, the flammability limits broaden, allowing stable operation up to Φ = 1.20 in fuel-rich conditions and widening the lean operating margin; the flame remains generally stable within this regime. As crNH₃ increases to 40% and above, strong combustion instabilities appear for 0.6 < Φ < 0.8. In the fully premixed configuration, conditions with crNH₃ > 60% are susceptible to combustion-induced vortex breakdown (CIVB) flashback over a wide range of equivalence ratios. Consequently, higher cracking ratios are tested in a partially premixed configuration to mitigate flashback. At crNH₃ = 40%, distinct dynamic behaviours are identified. At Φ = 0.80, the system exhibits period-2 limit-cycle oscillations (LCOs), characterised by a fundamental frequency of 270 Hz and a second harmonic at 540 Hz. Reducing Φ to 0.65 transitions the system to intermittent chaos with multiple interacting frequencies. This transition is consistent with a cracking- and equivalence-ratio-dependent change in the phase lag between acoustic-pressure perturbations (p′) and the OH*-derived relative heat-release fluctuations, which in turn modifies the Rayleigh-index estimate. The evolution of the precessing vortex core (PVC) and its interaction with the flame front modulate this phase relation and, in turn, the thermoacoustic feedback loop, providing a mechanistic explanation for the observed switch from periodic LCOs to intermittent chaos. The resulting benchmarks, together with the existing database, sharpen attribution of fuel-property effects and inform practical deployment in aviation, marine and stationary gas-turbine combustors.

Presenting Author: Fabian Hampp University of Stuttgart, Institute of Combustion Technology for Aerospace Engineering (IVLR)

Presenting Author Biography: Dr. Fabian Hampp leads the DFG Emmy Noether Junior Research Group at the Institute of Combustion Technology for Aerospace Engineering, University of Stuttgart. His research advances low-emission, fuel-flexible combustion by pioneering additively manufactured injection and combustion systems, pushing multi-scale laser diagnostics, and developing open-source machine-learning tools. He previously held postdoctoral appointments at DLR Stuttgart and Imperial College London, where he also earned his PhD. Dr. Hampp is the 2025 recipient of the Wilhelm-Jost Prize and a past Sugden Prize laureate for significant contributions to combustion research.

Authors:

Jihwan Ahn University of Stuttgart, Institute of Combustion Technology for Aerospace Engineering (IVLR)
Andreas Fiolitakis German Aerospace Center (DLR), Institute of Combustion Technology
Fabian Hampp University of Stuttgart, Institute of Combustion Technology for Aerospace Engineering (IVLR)