Thermoacoustic Instability Considerations for High Hydrogen Combustion in Lean Premixed Gas Turbine Combustors: A Review

Hydrogen is receiving increasing attention as a versatile energy vector to help accelerate the transition to a decarbonised energy future. Gas turbines will continue to play a critical role in providing grid stability and resilience in future low-carbon power systems; however, it is recognised that this role is contingent upon achieving increased thermal efficiencies and the ability to operate on carbon-neutral fuels such as hydrogen. An important consideration in the development of gas turbine combustors capable of operating with pure hydrogen or hydrogen-enriched natural gas are the significant changes in thermoacoustic instability characteristics associated with burning these fuels. This article provides a review of the effects of burning hydrogen on combustion dynamics with focus on swirl-stabilised lean-premixed combustors. Experimental and numerical evidence suggests hydrogen can have either a stabilising or destabilising impact on the dynamic state of a combustor through its influence particularly on flame structure and flame position. Other operational considerations such as the effect of elevated pressure and piloting on combustion dynamics as well as recent developments in micromix burner technology for 100% hydrogen combustion have also been discussed. The insights provided in this review will aid the development of instability mitigation strategies for high hydrogen combustion

Experimental Investigation of Forced Intermittency in Swirl-Stabilised Turbulent Methane-Air Flames

Thermoacoustic oscillations continue to be a major problem affecting combustor performance and operation in gas turbines. Acoustic forcing is often employed to simulate dynamical states in a combustor, however reproducing intermittent-like behaviour can be difficult. This paper, for the first time, reports a forced intermittent behaviour in a laboratory-scale, swirl-stabilised combustor burning methane-air mixtures that mimics the configuration of an industrial gas turbine. A frequency sweep using loudspeakers to generate velocity fluctuations identified 90, 220 and 320 Hz as the dominant frequencies of the burner. Amplitude sweeps were performed at these same frequencies and the flame response was obtained. At low forcing amplitude the acoustic flame response was in the linear range and the dominant frequency of the response matched the forcing frequency. Intermittency was observed beyond an amplitude threshold at 320 Hz, consisting of bursts of high-amplitude oscillations corresponding to broadband excitation around the forcing frequency. Further amplification resulted in excitation of a low frequency band. Wavelet analysis showed intermittent bursts to have a frequency signature matching the forcing frequency, and pressure and heat release oscillations were found to be coupled. In low amplitude regions between bursts, signals were found to be decoupled and response at forcing frequency was weak. Phase-space reconstruction showed acoustic modes to be consistent with those of self-excited flames found in literature. The outcomes of this work could provide further understanding on the causes of intermittency and its relationship with limit cycle oscillations

Experimental Investigation of Combustion Instabilities in a Laboratory-Scale, Multi-Can Gas Turbine Combustor

Can and can-annular (can-type) combustors are widely employed in stationary gas turbines. While majority of combustion instability research so far has focused on single can combustors, studying combustion dynamics in multi-can configurations holds more practical relevance. In can-type combustors, the annular gap between the transition ducts and first stage nozzles, known as the ‘cross-talk’ region, promotes strong can-to-can acoustic interactions. In this study, coupled interactions between neighbouring cans are experimentally investigated in a laboratory-scale, atmospheric, two-can combustor rig. Results demonstrate that an out-of-phase longitudinal mode, characterised by pressure anti-nodes in the cans and a pressure node in the cross-talk area, is preferentially excited in the multi-can combustor. Investigation into the effects of combustor flow rate and cross-talk geometry on the can-to-can dynamics revealed an increase in the amplitude of the limit cycle oscillations with mass flow rate, cross-talk volume and cross-talk exit area reduction. However, these factors exerted a weak influence on the frequency of the oscillations and did not impact the mode shape of the resonant can-to-can mode. These findings would help better understand the complex dynamical interactions that occur in multi-can systems towards model validation and tool development, while highlighting the significance of factoring in thermoacoustic aspects in the design of the combustor-turbine interface

Assessment of the Dynamical Behaviour of Hydrogen Combustors With Recurrence Quantification Analysis (RQA)

Hydrogen has immense potential as a future energy vector for power and propulsion applications. However there are several challenges, including flame stability, flashback and emissions, that impede its widespread use in modern energy systems. Hydrogen systems are required to be operated at fuel-lean conditions to avoid high NOₓ emissions and flashback issues, however lean operation makes systems more susceptible to thermoacoustic instabilities and lean blow-off (LBO). Understanding these dynamical behaviours is key to develop robust operational strategies. Recurrence Quantification Analysis (RQA) is an effective tool to study nonlinear dynamics, and has been applied in this study to characterise hydrogen flame behaviour in industry-relevant laboratory-scale combustion systems. RQA was conducted on two different configurations — hydrogen lean direct injection (LDI) combustor and a swirl-stabilised combustor operated on methane-hydrogen blends. RQA was carried out on time-resolved data of (i) pressure measurements in the combustor and (ii) integrated OH* chemiluminescence signals. These commonly used measurements in combustion diagnostics were selected to develop a tool which can be applied easily to other industrial systems. The effect of H₂ blend, bulk velocity, and global equivalence ratio (ϕg) were investigated in this study, and four quantifiable parameters such as Shannon entropy, recurrence rate, determinism, and laminarity were extracted from the RQA, to determine different dynamical behaviours — stable, intermittent, and limit-cycle. High values of Shannon entropy and recurrence rate indicated stable conditions for the swirl-stabilised combustor, while a combination of Shannon entropy and determinism were found to best capture the hydrogen flame dynamics in the LDI combustor. The outcome of this work will allow development of efficient predictive tools for nonlinear data analysis