In many modern aircraft concepts, civil as well as military ones, the engine is fully integrated into the fuselage. This integration often requires a highly bent intake duct. Due to the high degree of curvature and also the diffusive character of the intake duct, the inflow at the engine’s fan is non-uniform and may feature severe flow distortions. The size, strength, and pattern of these flow distortions may affect the engine’s compressor system and its safety margins. In this paper the flow through a short highly bent intake duct geometry is analysed by means of CFD. The numerical simulations are validated against experimental data, which are obtained in extensive investigations at the institute’s engine test facility. The setup for the numerical investigations is based on previous studies of the aerodynamics of intake ducts at the Institute of Jet Propulsion, where it is shown that the shape of the entrance cross-section of the intake duct has a strong influence on the flow field throughout the entire intake duct. In this paper the flow throughout the duct is analysed in order to gain information on the flow features which cause the flow distortion at the aerodynamic interface plane (AIP) and how these flow features interact. Two main flow distortion patterns exist at the AIP, one of them is a system of two twin vortices, one on each side in the lower part of the AIP. These are caused by the particular shapes of the cross-sections in the front part of the duct. The dominating flow distortion in the AIP is caused by a large flow separation in the rear part of the duct, which resides in the upper half of the AIP and results in a large total pressure loss and axial velocity deficit, combined with a twin swirl. Although no direct interaction between these two flow patterns is present, it was found that the small vortices in the lower part are influenced by the flow separation at the upper wall in the rear section of the intake duct.

In propulsion industry there is an ongoing need to significantly reduce SFC of jet engines resulting in cost reduction and lower emissions. Since the design of most of the engine components is at the limit of today’s technology level further gain of improvement on short term is to be achieved by implementation of new system concepts. Especially the stall safety margin in compression system design holds high potential for the optimization of the overall engine system. Once a reliable and effective stall control system becomes available an extension of present operating range is likely to be achieved by moving the steady operating line towards the stability limit and to intervene only in critical situations. At the Institute of Jet Propulsion at the University of the Federal Armed Forces in Munich, Germany a Larzac 04 twin-spool turbofan engine has already been equipped and tested with an adequate active stabilization system of the low pressure compressor for research purposes. Those investigations revealed a strong dependency of the achievable stabilization effect and the amount and momentum of the injected air mass flow. For flying applications this mass flow has to be delivered by carried on means. Therefore it always penalizes the propulsion efficiency. In the given configuration, redirected air from the last stage of the high pressure compressor is used for injection. Usage of this bleed air directly influences the propulsion efficiency of the engine. In order to optimize the mass flow needed for stabilization, the existing injection system was redesigned to utilize ejector pumps. With this configuration a comparable stabilizing effect could be realized with less redirected air mass flow. In fact the open ejector pump configuration showed an even higher performance at maximum injection rate than the closed injection before. Therefore further investigations with this system focused on the effect of additional flow ports to the engine intake as they are necessary for an ejector pump and their basic influence on the operation stability of the low pressure compressor (LPC). In combination with the already existing stall detection algorithm of the institute a very promising system for increasing the available operating range in turbo compressors could be achieved.

In many modern aircraft concepts, civil as well as military ones, the engine is fully integrated into the fuselage. This integration often requires a highly bent intake duct. Due to the high degree of curvature and also the diffusive character of the intake duct, the inflow at the engine’s fan is non-uniform and may feature severe flow distortions. The size, strength, and pattern of these flow distortions may affect the engine’s compressor system and its safety margins. In this paper five highly bent intake duct geometries are analyzed by means of CFD. They evolve from the same baseline geometry but are defined by different crosssectional shapes. With this variation of the cross-sections, the influence of the cross-sectional shape on the aerodynamics of the intake duct is investigated qualitatively. Based on these analyses a sixth intake duct geometry was created as test vehicle for experimental investigation of intake-compressor interaction within the engine test facility. The defining cross-sectional shapes were selected in order to achieve a flow distortion at the duct outlet plane, that is small enough to ensure a safe engine operation, but is still strong enough to provoke interaction of the distorted flow and the compressor flow. The setup for these fully numerical investigations is based on previous studies of the aerodynamics of intake ducts at the Institute of Jet Propulsion. It is shown that the entrance cross-section has a strong influence on the flow throughout the whole intake duct. Additionally, it could be determined that the flow distortion caused by the strong curvature of the intake duct can be reduced in size and strength by a proper combination of cross-sectional shapes.

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.

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