Active Stabilization of the Compression System in a Twin-spool Turbofan Engine at Inlet Distortions

摘要

This paper presents the active control of a twin-spool turbofan engine to stabilize the compression system, also at inlet distortions. For this purpose combined inlet distortions consisting of total pressure distortions and swirl distortions are generated by special devices in the inlet duct. The described active control engagements consist either of air injection or of air recirculation. In the case of air injection air is delivered by an external source while for air recirculation the injected air is taken from the bleed ports behind the final stage of the compression system. For both applications fast acting direct drive valves are used to inject the air at the tip region just ahead of the 1st compressor rotor. The valves allow either constant or modulated air injection or air injection guided by a controller. The air is routed inside injection channels with pivoted nozzles at their end to vary the injection angle. Low and high frequency measurement technique is applied to gather all informations with respect to benefits in surge margin, additional operating range and reasons for instabilities and their pre-cursors. The profits are displayed in compressor maps and zoomed extracts of them, the increasing instabilities and the corresponding damping effects are shown in so-called waterfall diagrams. For this purpose FFT-analysis is done and compressor map investigations are conducted. A significant extension of the possible operating range of a twin-spool turbofan engine is demonstrated along with the evidence that based on the illustrated active control system an enhanced surge avoidance system respectively surge retardation system is generated. This allows a further reduction of the design surge margin in the lay-out process of a future turbofan engine when the described active stabilization method is applied in combination with a digital control system. Also engine deterioration or intake distortions can be covered with this application.