Experimental Investigation of Stator Blade Vibration Due to Air Impingement in a Axial/Centrifugal Compressor

The purpose which has guided the integration and the development of the quick-start system is the increase of safety during intended single-engine operation for rotorcraft application, leading to an effective reduction of the start-up time of the inactive turboshaft engine. The acceleration torque provided by the system is obtained through a pressurized air injection at the compressor section using an impingement system made up by three De Laval nozzles, integrated into the impeller casing. During maintenance operations on the modified engine at the TUM laboratory, one blade of the last stator stage in the axial compressor has found damaged and the investigation of the possible causes that have lead to this failure was the aim of this work of the thesis. Especially, the attention was focused on the impingement force and the related vibration field originated by the quick-start system, affecting the sixth stator blades and the whole compressor: due to an adverse gradient raised inside the compressor, a backflow has sourced, leading to the excitation of the system. To analyze the effects of this excitement, a deep investigation has developed, involving an initial numerical analysis, followed by several experimental tests. The first mentioned part has involved the creation of a finite-element model of the damaged blade, to find out its natural frequencies and obtain a visual investigation of the vibration modes. Proceeding with the experimental investigations, two blades have been instrumented with strain gauges connected in a half-bridge configuration. Also, two vibration sensors have been placed on the compressor casing and two on the rear bearing. Since the amount of the originated backflow is strongly affected by the pressure reached in the impeller section, some pressure sensors have been installed in different points of the compressor section. The interpretation of the reported results has shown the relevant influence of the quick-start system on the vibration field acting on both blades and compressor casing, emphasizing the sourced of the resonance phenomena. Although the integration of the quick-start system leads to an effective reduction in the starting time, the results have shown that its impulsive action impacts the life of the entire compressor. Additional workload involves the structure of blades, the compressor casing and the bearing, mostly related to the appearance of fatigue phenomena, whose persistence over time strongly compromise the safety operations of the compressor.