Abstract
Thermoacoustically-induced flashback (FB) of pure hydrogen flames is examined in a laboratory-scale trapped vortex combustor. The FB limits of the main flame are explored both with and without the presence of a pilot flame. At low to medium main-stage thermal power, the flame propagates upstream of its design stabilization position and anchors temporarily in the annular channel supplying the main-stage reactants. This enables the estimation of the turbulent burning velocity for these pure hydrogen flames and comparison with semi-empirical scaling laws. Such predictive guidelines for the turbulent burning rate in boundary-layer FB are also shown to capture with satisfactory accuracy the experimentally measured FB limits, despite the configuration complexity. Moreover, it is observed that high-amplitude thermoacoustic instabilities are excited at high main-stage thermal powers, resulting in a pulsating FB pattern. The flame oscillation velocity is quantified through high-speed imaging, and correlated with the velocity difference between the estimated and measured FB velocities. This analysis reveals a relationship between thermoacoustic instability magnitude and the pulsating FB behavior, providing predictive capabilities for the occurrence of flashback. Novelty and significance statement This work’s novelty lies in the investigation of thermoacoustically-induced flashback (FB) in premixed hydrogen-air flames, which is a key outstanding challenge in hydrogen-firing of gas turbines. Limited experimental studies exist on undiluted hydrogen-air flames FB observed in a combustor geometry of industrial relevance featuring full optical access, and even fewer consider trapped vortex configurations and the influence of thermoacoustic instabilities on flashback limits. The present study quantifies the link between thermoacoustically-induced flame front displacement and modifications in measured FB limits. The study is performed in a newly developed fully optically-accessible trapped vortex combustor, modeled on a fuel-staged combustor configuration of industrial relevance. Furthermore, this study assesses the accuracy of semi-empirical correlations for the turbulent propagation velocity of hydrogen premixed flames, demonstrating their usefulness. In summary, the present results offer guidance for the design of hydrogen-fired combustion systems, particularly for staged configurations, supporting the development of FB-resilient, clean and efficient gas turbines combustors. © 2026 The Authors.