Effects of H₂ Enrichment and Inlet Velocity on Stability Limits and Shape of CH₄/H₂-O₂/CO₂ Flames in a Premixed Swirl Combustor

The stability characteristics of swirl-stabilized hydrogen-enriched CH₄-O₂-CO₂ premixed flames were experimentally investigated to determine the effects of hydrogen addition and inlet velocity on flame stability under stoichiometric conditions (φ = 1.0). The stability limits in terms of blowout and flashback were identified over wide ranges of hydrogen fraction (HF: %H₂ in H₂-CH₄ mixture) and oxygen fraction (OF: %O₂ in O₂-CO₂ mixture) at fixed inlet velocity of 6 m/s. The lines of stability limits were plotted against the contours of adiabatic flame temperature (AFT), power density (PD), and Reynolds number (Re) to understand the physics behind the flame extinction mechanism. The effects of inlet velocity on flame stability limits under hydrogen enrichment were investigated by comparing the stability maps of the combustor for different inlet velocities, namely, 4.4, 5.2, and 6.0 m/s. The results show a stable combustion zone in the OF-HF space in the ranges of OF from 16 up to 44% and HF from 0 (pure CH₄) up to 90%. However, increasing HF restricts the range of both OF and mixture Re for stable flame operation. Flame shapes at different inlet velocities were captured using a high-definition camera and compared to investigate the effects of OF and HF on flame stability. Flame visualizations near flashback and blowout limits were recorded to explore the physics behind flame extinctions mechanisms. The effect of reaction kinetic rates on the flame stability was investigated by recording flame shapes at fixed adiabatic flame temperature. The results show that the flames become gradually shorter and more compact with the increase in HF because of the enhanced reaction rates within the combustion zone. Insignificant changes in the flame shape were observed at fixed AFT or fixed Re operation.