Effects of fuel staging on combustion oscillations in bluff body non-premixed flames

Bluff body non-premixed flames are widely used in afterburners of aviation engines, where combustion oscillations pose significant challenges to operational stability. A self-sustained combustion oscillation system and a flame dynamic response measurement system were employed to analyze the effects of fuel staging on oscillation amplitude, frequency, and flame dynamic response. The results demonstrated that increasing the primary fuel ratio from 0.6 to 0.88 effectively suppressed combustion oscillations, reducing pressure oscillation amplitude by 65.6%, while the dominant oscillation frequency remained nearly unchanged in the range of 176–180 Hz. Mechanistically, adjustments in fuel staging significantly modified the flame length, fuel injection depth, and recirculation zone size, resulting in pronounced spatial variations in heat release rate fluctuations. These alterations introduced damping regions within the flame, reducing the coherence of heat release oscillations. Additionally, flame response analysis via the flame transfer function and flame describing function showed that increasing the primary fuel ratio shifted the phase delay of heat release rate fluctuation relative to velocity fluctuation, fundamentally altering the thermoacoustic coupling. The results indicate that the primary fuel ratio is the key control parameter for oscillation suppression in the present study, whereas the oscillation frequency is determined mainly by the combined effect of combustor acoustic characteristics and flame-response phase behavior. The study provides essential insights into optimizing fuel staging strategies and offers guidance to suppress combustion oscillation in afterburners.