Experimental Operating Range Extension of a Twin-Spool Turbofan Engine by Active Stability Control Measures

Abstract

Modern engine operation is guided by the aim to broaden the operating range and to increase the stage loading allowing the stage count to be reduced. This is possible by active stability control measures to extend the available stable operating range. Different strategies of an active control system like air injection and air recirculation have been applied. While in the past mainly results have been published regarding the stability enhancement of compressor rigs or single-spool engines, this experimental study focuses as well on the stability as also on the operating range extension of a twin-spool turbofan engine as an example for a real engine application on an aircraft. The objective of this investigation is the analysis of the engine behavior with active stabilization compared to not supported operation. For this purpose high frequency pressure signals are used and analyzed to investigate the effects of air injection with respect to the instability onset progress and the development of any instabilities like rotating stall and surge in the LP Compression system. These Kulite signals are fed to a control system. Its amplified output signals control fast acting Direct Drive Valves circumferentially distributed ahead of the LPC. For the application of air injection described in the paper, the air is delivered by an external source. The control system responsible for air injection is a real-time system which directly reacts on marked instabilities and their pre-cursors. It allows the LP Compression System to recover from fully developed rotating stall by asymmetric air injection based on the pressure signals. Additionally, a delayed appearance of instabilities can be provoked by the system. Air injection guided by this control system resulted in a reduction of the required amount of air compared to constant air injection. Also, disturbances travelling at rotor speed can be detected, damped and eliminated by this control system with a modulation of the injected air in such a way that the injection maximum travels around the 10 injection positions.