Instability and Flame-Pressure Coupling in Downward Propagating Premixed H₂/CO/CO₂/Air Flames

This study investigates the instability and flame-pressure coupling characteristics of downward propagating premixed H₂/CO/CO₂/air flames under varying hydrogen volume fractions (HVF). Particular attention was paid to flame morphology evolution, overpressure development, thermoacoustic characteristics, and reaction sensitivity. The results show that increasing HVF markedly modifies flame dynamics. Smooth and laminar-dominated flame fronts are observed at low HVF (5%-10%), whereas pronounced wrinkling, finger-like structures, and cellular instabilities emerge when HVF ≥ 15%, indicating a transition to instability-controlled propagation. Correspondingly, overpressure responses intensify with hydrogen enrichment, evolving from weak low-frequency oscillations to strong nonlinear pressure fluctuations with distinct acoustic mode locking. The dominant frequency shifts from a low-frequency regime to a tube-governed longitudinal acoustic mode when HVF ≥ 10%, demonstrating enhanced flame-acoustic coupling. Sensitivity analysis reveals that CO oxidation (CO + OH ≤ > CO₂ + H) remains the primary heat-release pathway across all conditions, while chain-branching reactions become increasingly dominant at higher HVF. The reaction network gradually shifts toward an OH-centered, strongly coupled kinetic structure, promoting flame acceleration and pressure amplification. A critical transition region is identified around HVF ≈ 10%-15%, beyond which the system evolves into a highly reactive and instability-dominated combustion regime. These findings provide mechanistic insight into hydrogen-enriched syngas combustion and its associated explosion risks in confined configurations.