Instability control and emission characteristics of methane/hydrogen non-premixed flames under DC electric fields

This study experimentally investigated the Kelvin–Helmholtz instability in methane/hydrogen non-premixed flames induced by velocity differences between the fuel jet and co-flow. Using a co-flow burner, various fuel and co-flow velocity conditions were tested to identify the regimes under which oscillatory flames develop. The hydrogen mole fraction was varied from 0.0 to 0.5 to examine the effects of fuel composition. A DC voltage of up to ± 10.0 kV was applied to the fuel nozzle to generate an electric field aligned with the direction of flame propagation. High-speed imaging and PIV analysis demonstrated that the formation and advection of outer vortices are closely linked to the onset of flame oscillation. Under positive voltage, the ionic wind aligned with the flame flow direction, weakening the outer vortex influence and promoting stabilization. In contrast, under negative voltage, the ionic wind derived in the opposite direction, disturbing the outer flow field and intensifying flame instability. In hydrogen-blended flames, the overall flame response to electric field polarity was similar to that of pure methane flames. Emission measurements showed reductions in both CO and NO X concentrations under electric field. These reductions are attributed to suppressed carbon oxidation and shortened residence time in high-temperature regions due to the electric field. Although hydrogen blending altered ion generation pathways and slightly reduced electric field sensitivity, stabilization and emission control effects were maintained, suggesting the applicability of electric-field-based combustion control in hydrogen-blended systems.