Unsteady Flow and Combustion Characteristics Downstream of a Bluff-Body Flameholder in an Afterburner: An Experimental Study

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Abstract
Unsteady flow and heat-release fluctuations downstream of afterburner flameholders can trigger combustion instability. Experiments were conducted to investigate pressure fluctuations, velocity fluctuations, and flame evolution behind a bluff body. Under non-reacting conditions, the dominant fluctuation frequency increased with inlet Mach number and temperature, while the Strouhal number remained within 0.23–0.26. Velocity fluctuations were mainly concentrated in the shear layers, near the end of the recirculation zone, and in the wake. The dominant mode exhibited alternating vortex shedding with opposite phases on the two sides, whereas a second-harmonic response appeared near the centerline. Under reacting conditions, the time-averaged flame exhibited M-shaped and V-shaped configurations depending on inlet Mach number and fuel–air ratio, reflecting the coupled effects of atomization, evaporation, mixing, and wake entrainment. Flame pulsations were synchronized with wake vortex shedding, but their dominant frequency was slightly lower than that of the corresponding non-reacting flow, indicating that heat-release-induced thermal expansion and density gradients weaken large-scale vortex development. These results identify bluff-body wake vortex shedding as an important hydrodynamic source of unsteady heat release and combustion instability in afterburners.

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