Nonlinear dynamics and thermoacoustic intermittency of a hydrogen-powered sequential combustor

Abstract

This study experimentally investigates the coupling between thermoacoustic instabilities and autoignition kernel formation in Constant Pressure Sequential Combustors (CPSCs). Two fuel types are examined: a less reactive methane–hydrogen blend ($F_{{\mathrm{CH}}_4}$) and pure hydrogen ($F_{H_2}$). By increasing the thermal power of the first stage, thermoacoustic instabilities arise in both configurations, albeit with distinct behaviors. $F_{{\mathrm{CH}}_4}$ exhibits a gradual onset of instability, whereas $F_{H_2}$ undergoes a subcritical Hopf bifurcation, characterized by abrupt, intermittent transitions between a linearly stable state and limit cycles at intermediate first-stage power. Distinct acoustic pressure spectra are observed during instability: $F_{{\mathrm{CH}}_4}$ features a single dominant peak around 290 Hz, while $F_{H_2}$ displays multiple high-amplitude peaks corresponding to harmonics of the fundamental frequency near 400 Hz. Analysis of acoustic pressure and OH* chemiluminescence during instability reveals a strong coupling between acoustic fluctuations and autoignition kernel formation. With $F_{{\mathrm{CH}}_4}$, the temporal evolution of the OH* chemiluminescence associated with these kernels follows a quasi-sinusoidal profile at the instability frequency, whereas with $F_{H_2}$, it consists of sharp pulses synchronized with the fundamental acoustic mode. Although existing Low-Order Models (LOMs) successfully capture the experimental behavior in $F_{{\mathrm{CH}}_4}$, they fail to replicate the complex dynamics of $F_{H_2}$. To address this, a novel LOM incorporating a strongly nonlinear Heat Release Rate (HRR) feedback term is developed, specifically tailored for configurations with significant coupling between autoignition and thermoacoustics. This model successfully replicates the key spectral features of $F_{H_2}$, underscoring the need for advanced models to accurately reproduce the complex thermoacoustic behavior of sequential combustors. The findings of this study provide a deeper understanding of the challenges associated with sequential combustors operating under autoignition conditions, particularly in the context of decarbonization through 100% hydrogen operation.

Novelty and Significance Statement

This study presents, for the first time, experimental results from a lab-scale constant-pressure sequential combustor fired with pure hydrogen in both stages. The behavior under pure hydrogen fueling is compared with that of a methane–hydrogen blend. In both cases, a strong coupling is observed between autoignition kernel formation and thermoacoustic instabilities. However, the acoustic pressure spectra under pure hydrogen fueling exhibit distinct and atypical features compared to those obtained with the methane–hydrogen blend. The critical role of autoignition kernels in triggering and sustaining these instabilities is highlighted. Additionally, a novel Low-Order Model is proposed, accurately replicating the key spectral features observed in the pure hydrogen case. These findings provide valuable insights for the community, supporting the transition to pure hydrogen fueling in sequential combustors under autoigniting conditions.