Downward propagation of syngas/CO _<b>2</b> flames in a vertical cylindrical tube

The downward propagation of CO-rich syngas flames with fixed 5% CO2 dilution was systematically investigated in a vertical cylindrical tube under varying hydrogen volume fractions (5%–30%). Flame front morphology, propagation stability, and overpressure responses were analyzed through high-speed imaging and pressure measurements. Key quantitative results include: the laminar burning velocity increases from 33.43 to 74.40 cm/s, flame thickness decreases from 0.86 to 0.55, and characteristic flame development time reduces from 1324 to 247 ms as the hydrogen fraction rises from 5% to 30%. Flame morphology transitions from smooth laminar fronts to strongly perturbed finger-like structures, while the dominant overpressure frequency shifts from ≈ 50 Hz to 277–285 Hz, indicating enhanced thermoacoustic coupling. Sensitivity analysis (FFCM-1) reveals a transition from a HO2-dominated weak chain-reaction network to an OH-centered strongly coupled network. The interplay of reduced flame thickness, increased Lewis number, and enhanced radical concentrations explains the intensification of Darrieus–Landau and thermal-diffusive instabilities. These findings clarify the role of hydrogen in regulating flame dynamics and pressure interactions in syngas combustion.