Kumar, AD; Ezenwajiaku, C; Balachandran, R; Ducci, A; Talibi, M; Massey, JC; Swaminathan, N; (2024) Dynamical Systems Characterisation and Reduced Order Modelling of Thermoacoustics in a Lean Direct Injection (LDI) Hydrogen Combustor. In: Proceedings of the ASME Turbo Expo 2024: Turbomachinery Technical Conference and Exposition. Volume 3A: Combustion, Fuels, and Emissions. ASME: London, UK.

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Abstract

Hydrogen is a promising zero-carbon fuel for decarbonised energy and transportation sectors. While carbon emission is not a concern for hydrogen combustion, its higher adiabatic flame temperature poses challenges of mitigating thermal NOx emissions. The wide flammability limits of hydrogen allow a fuel-lean operation, which can reduce NOx emissions. However, lean operation makes the combustion chamber susceptible to thermoacoustic oscillations. In this study, the thermoacoustic instabilities of partially premixed hydrogen flames in a lean direct injection (LDI) multi-cluster combustor are characterised using dynamical systems theory. The combustor was operated at a range of bulk velocities (30 - 90 m/s) and equivalence ratios(0.2-0.6), and time-resolved pressure oscillations and integrated OH* chemiluminescence measurements were taken. The thermoacoustic system reveals a variety of dynamical states in pressure such as period-1 Limit Cycle Oscillation (LCO) with a single characteristic frequency, period-2 LCO with two characteristic frequencies, intermittent, quasi-periodic and chaotic states as either bulk velocity or equivalence ratio is varied. At a bulk velocity of 30 m/s, as the equivalence ratio is gradually decreased from 0.6 to 0.2, the dynamical behaviour follows a sequence from an intermittent state to a period-1 LCO, then to a quasi-periodic state, and eventually reaches a chaotic state. As the equivalence ratio is decreased for a bulk velocity of 60 m/s, the pressure oscillations evolve from a period-2 LCO to quasi-periodic state before flame blows off. The emergence of period-2 and quasi-periodic states indicates the presence of strong non-linear interactions among the cavity acoustic modes. These modes and their spatial behaviour are investigated using a reduced order model which solves the 3D inhomogeneous Helmholtz equation with an n-tau flame model. The analyses show that the period-2 and quasi-periodic states can arise due to the interaction between the plenum and combustion chamber modes indicating that hydrogen flames may excite a wide range of cavity acoustic modes.

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