Thermoacoustic instability in two acoustically coupled hydrogen-enriched combustors

Abstract

This study experimentally investigates the potential of tuning acoustic coupling to passively suppress thermoacoustic oscillations in lean-premixed hydrogen-enriched can-annular combustors. Our findings demonstrate that thermoacoustic oscillations in the coupled system can be suppressed by up to 90% compared to the decoupled self-excited baseline, achieved by deliberately mismatching the flame response and chamber acoustics. This mismatch is achieved through hydrogen enrichment and modifications to the acoustic coupling configurations and combustor geometry. As the hydrogen volume fraction increases, the flame preferentially responds to higher frequencies, while the overall “$\Pi$-shaped” acoustic chamber formed by coupling the two identical combustors via cross-talk (XT) sections favors lower acoustic eigenfrequencies, particularly for longer combustors or when XT sections are located further downstream. The amplitude and frequency of the dominant half-wave anti-phase longitudinal mode (i.e., a push-pull mode) are strongly influenced by this mismatch, and a regime of oscillation suppression emerges when the mismatch is maximized, specifically at the highest hydrogen volume fraction and the longest combustor length. The axial location of the most upstream XT defines the total effective length of the “$\Pi$-shaped” acoustic domain, whereas multiple XTs increase the effective acoustic interaction area between the combustors, thereby reducing acoustic resistance and enhancing coupling. This intensified coupling strengthens or triggers the push-pull mode, resulting in pronounced thermoacoustic oscillations and highlighting the importance of accounting for such effects when assessing the stability of individual combustors for integration into can-annular configurations. In summary, this study underscores the critical role of both flame response and acoustic coupling in governing thermoacoustic behavior and demonstrates that careful tailoring of these factors offers a simple yet effective passive strategy to suppress instabilities in hydrogen-enriched can-annular combustion systems, thereby supporting the development of cleaner and more stable heavy-duty gas turbines.