Lo-Frequency Augmentor Instability Study.

Abstract : With the advent of the mixed-flow afterburner in turbofan engines, a type of low-frequency instability known as rumble became a serious problem. Rumble occurs mainly at high fuel-air ratios and at flight Mach numbers and altitudes where low duct inlet air temperatures and pressures exist. 'Cut and try' methods of solution during engine development have been partially successful, but very expensive. To aid the development engineer in designing rumble-free afterburners an analytical model has been formed. The model was evolved in conjunction with and checked by two experimental programs. Rumble mechanisms investigated early in this study involved system airflow dynamics, combustion efficiency oscillations, fuel vaporization and recirculation wake energy. The model was then refined and extended to include the mixed flow experienced in a turbofan augmentor. Predictions were made for the effects of altitude, fan stream fuel-air ratio, fan stream temperature, core stream fuel-air ratio, and fan duct pressure loss. The major conclusion from the modeling effort was that the efficiency falloff in the fan stream at high fuel-air ratio causes rumble. This was verified with engine altitude tests at NASA Lewis Research Center with several augmentor configurations. These tests included heat addition to the fan stream, fuel-air distribution changes and spraybar to flameholder length variation. The basic formulation of the model, the mixed flow augmentor model, predictions, and the experimental programs are discussed. (Author)