Experimental Studies on the Thermoacoustics of Afterburner Screech Combustion Instabilities in a Model Afterburner Test Rig

Considerable efforts have been made by aero-engine manufacturers to understand, detect, attenuate, if not eliminate, high-frequency transverse screech combustion instabilities due to their destructive nature. Combustion dynamic stability problems arise in turbofan afterburners when fluctuations in the combustion energy release rates are coupled with the afterburner duct acoustics. Anti-screech liners have been used to attenuate the consequential pressure oscillations and mitigate the harmful effects of the transverse screech combustion instabilities. A versatile experimental test facility, using a single V-gutter flame holder as a driver to generate predetermined screech frequencies of interest of 1250 and 2000 Hz was used to conduct comprehensive experimental studies. Anti-screech liners of variable effectiveness with porosities of 3.5 and 10.0% were used for this investigation. The attenuation effectiveness of these liners was investigated under simulated test conditions specifically for the 2000-Hz screech frequency. For these comprehensive experimental investigations, a novel methodology to generate the predetermined screech frequency in a model afterburner test rig had been evolved. These anti-screech liners with variable effectiveness were not found to be effective in completely suppressing screech over the entire spectrum of operating conditions, probably because the complex modes present in the screech combustion instabilities, may have not been strictly transverse and the pressure amplitude may have been too large. Mitigation efforts need to be attempted at the source of generation of the screech combustion instabilities.