The mechanism of acoustic natural resonant characteristics affecting self-sustained combustion oscillations in bluff body diffusion flames

Bluff body diffusion flames are widely employed in the afterburners of aircraft engines. However, the combustion oscillations induced by these flames poses significant risks to the operational stability and structural integrity. This study investigates the influence mechanism of the combustion oscillations in bluff body diffusion flames from the perspective of combustion chamber acoustic characteristics. Experimental methods combined with acoustic simulations were used to analyze the effects of changes in the acoustic characteristics on combustion oscillations. The results show that the dominant frequency of combustion oscillation is affected by the system's acoustic characteristics but deviates from the natural acoustic frequency of the system by >60 Hz. This deviation is jointly determined by the phase delay of heat release rate oscillations and the acoustic pressure feedback. The length of the combustion chamber and the position of the flame significantly impact the characteristics of combustion oscillations. Shortening the chamber length reduces low-frequency acoustic pressure feedback, suppresses oscillation amplitude, and shifts the dominant frequency. Notably, relocating the flame position reduces oscillatory pressure amplitude by >95 %, effectively eliminating the limit cycle oscillation state. The phase delay in the dynamic response of the flame is identified as a critical factor in determining thermoacoustics. This study reveals the coupling mechanism between acoustic characteristics and combustion oscillations in bluff body diffusion flames, providing technical support for suppressing combustion oscillations in afterburners.