Hydrogen-induced changes in flame topology, coherent structures, and instability development in a premixed swirl-stabilized combustor

Hydrogen is increasingly being used in the aerospace sector as a clean energy source. The premixed combustion of hydrogen–air mixtures in porous media has received considerable attention, with the aim of enhancing hydrogen energy conversion efficiency and mitigating environmental impacts. This work experimentally investigates the influence of hydrogen enrichment on the morphology, spectral response, and temporal coherence of premixed methane–hydrogen combustion flames in a swirl-stabilized combustor at atmospheric pressure. Three fuel compositions (0%, 30%, and 40% H₂ by volume) were tested at constant thermal power (4.6 kW) and equivalence ratio (ϕ = 0.8). Hydrogen addition produced more compact, intense, and symmetric CH* and OH* chemiluminescence fields, indicating increased reactivity and stronger localization of the reaction zone. However, this morphological compactness was accompanied by a marked increase in dynamic instability. The main acoustic peaks rose from about 225 and 195 Pa at 240 and 300 Hz in the methane flame to 526 and 496 Pa at 30% H₂, while at 40% H₂ the dominant peak shifted to 310 Hz and reached 581 Pa, together with a broader spectral distribution. SPOD revealed a progressive shift from low-frequency coherent structures to fragmented higher-frequency modes, with CH* and OH* dominant frequencies extending up to 207.5 Hz at 40% H₂. Temporal autocorrelation further showed a transition from stable periodic behavior at 0% H₂ to strong mid-frequency coherence at 30% H₂ and rapid decorrelation at 40% H₂. The results demonstrate that hydrogen enrichment simultaneously enhances flame compactness and amplifies unsteady flame–acoustic interactions, defining a clear trade-off between combustion reactivity and dynamic stability.