Modal dynamics and spectral signatures of ultra-lean H2–air annular flames

Lean hydrogen-air flames have the practical benefit of emitting less NOx and reaching lower temperatures than stoichiometric flames, but they exhibit intrinsic instabilities that affect flame dynamics and propagation. The goal of this study is to analyze the spatio-frequential structure of ultra-lean, burner stabilized, hydrogen-air flames in two distinct situations. The first corresponds to the presence of intrinsic instabilities only, whereas, in the second, longitudinal pulsations related to the burner acoustic arise. The analysis uses measured time resolved schlieren data and the spectral proper orthogonal decomposition technique (SPOD). The SPOD is a reduction technique for isolating patterns and structures in complex flows, commonly applied in aeroacoustics and jet studies to represent flow structures. The results for the intrinsically unstable flame show a monotonic energy decay with frequency, with an exponential decay. The spatial distribution remains similar across frequencies, depicting intercalated wave packets across the burner, resembling the intrinsic instability’s transversal movement, as observed in schlieren images. For the longitudinal pulsation case, the energy spectrum reveals multiple peaks, with the first mode at the harmonic frequencies exhibiting significantly higher energy. From the second mode on, an exponential decrease is observed. The spatial distribution varies with frequency, where the first mode at harmonics displays longitudinally intercalated wave packets, whereas the second mode appears to capture a superposition from both instabilities. The SPOD analysis highlights the distinct interactions between intrinsic instabilities and longitudinal pulsations due to burner acoustic modes across different frequencies and spatial modes. Novelty and significance statement: The present work introduces a novel analysis of the spatio-temporal organization of intrinsic and acoustically coupled instabilities characteristic of ultra-lean hydrogen–air flames stabilized on a newly designed annular slit burner. While both intrinsic thermo-diffusive instabilities and acoustically coupled flame oscillations are known phenomena, the present study is novel in its combination of a controlled annular slit burner and a SPOD-based analysis of instantaneous schlieren images, which allows for a quantitative and reproducible distinction between transverse intrinsic modes and longitudinal acoustic responses. Its significance is twofold: first, it provides a rigorous and reproducible methodology to distinguish transverse intrinsic modes from longitudinal acoustic responses, enabling a physically grounded interpretation of flame fluctuations; second, it establishes a foundation for reduced-order modeling and predictive analysis of hydrogen combustion systems, thereby supporting the development of low-emission, stable, and scalable hydrogen-based energy technologies.