Rotating Stall Inception Inside the Low Pressure Compressor of a Twin-Spool Turbofan Engine

Abstract

In order to preserve fossil resources aviation industry faces major challenges to reduce engine fuel consumption. Therefore efforts are concentrated to increase efficiency of any engine component. Investigations at the Institute of Jet Propulsion at the University of Federal Armed Forces in Munich focus on the compressor module. Especially the compression system of a gas turbine is designed to operate at very high aerodynamic loads. This makes it one of the most critical components during transient engine operation or inlet flow distortion. Rotating stall and surge have to be avoided in any situation during engine operation. For this reason a detailed knowledge of the flow phenomena of the compressor in normal conditions as well as near the stability limit is essential. Often those research activities are carried out at compressor rigs but not in by utilizing real turbo engines. As a research test vehicle at the Institute of Jet Propulsion the Larzac 04 C5 twin-spool turbofan engine is operated at the engine test facility. The gas turbine is equipped with additional instrumentation and control systems exceeding those of conventional engine monitoring systems by far. Especially a set of high frequency pressure transducers has been installed above the tip of the first stage of the low pressure compressor in order to investigate tip flow phenomena. Besides the information on the flow phenomena in the tip region of the compressor blades these signals can also be used to detect the upcoming of rotating stall precursors. A special algorithm which was developed at the institute is able to estimate the stall and to trigger an active countermeasure. This was demonstrated successfully for a wide range of operating points. Stall inception in different speed ranges is crucial to be detected reliably. More than all high spool speeds challenge an active stabilization system. With the stall typically rising in time periods of less than three rotor revolutions, the requirements regarding high speed data processing are enormous. Since computer technology now provides systems, which are capable to handle such a task and still are compact and robust enough to be used in the rough environment of engine test beds, the challenge remains to set up fitting actuator systems. The test vehicle at the Institute of Jet Propulsion is therefore equipped with fast acting valves, which feed an injector casing mounted closely upstream of the low-pressure compressor. Test series have been performed, which proof the stabilizing capabilities of the entire system. Even at high spool speeds the stall was sufficiently suppressed and a stable operation of the engine was guaranteed.