Closed loop control of stall flutter of turbo machinery blades
Final Report Abstract
One of the main current goals of engine design is to increase efficiency and thereby reduce greenhouse gas emissions. One way to achieve this is to reduce weight by decrease the number of stages in the compressor. In order to maintain the required pressure build-up, the individual compressor stages are aerodynamically loaded more strongly. Natural consequences are increasing blade vibration. An approach to reduce these vibration is active flow control, where blade vibrations are measured and countered with a suitable actuation. A novel actuation technology are plasma actuators, which actuation strength was always a limiting factor for the application but made significant improvements in recent years. Plasma actuators consist of two electrodes separated by a dielectric barrier. With a high applied current the surrounding air can be locally ionized, which allows the induction of an force into the flow field. The project wants to answer what effect on blade vibrations can be achieved with plamsa actuation. The transonic flow around the test rotor was calculated with CFD. An existing plasma actuation model is adapted in a novel way to calculate the plasma actuation forces. Two additional equations are solved with finite volume method to resolve these plasma actuation forces, which are then implemented into CFD. To assess the possible change of the exiting aerodynamic forces on the rotor blades, at first the actuation effect is assessed for a continuous actuation. Therefore 2 plasma actuators are placed on the suction surface of the blade/blades. The effect for different actuation positions as well as on neighbouring (not plasma actuator equipped) blades is shown. The resulting changes of aerodynamic blade forces remain small in relation to steady state blade forces. The actuation forces lead to opposing effects on the actuated blade and the neighboring blade on the suction side. This effect is disadvantageous for the manipulation of the blade vibration mode, where all blades oscillate in the same manner and phase but beneficial to the manipulation of the blade vibration mode, where neighbouring blades oscillate in the same manner with an 180° phase shift to each other. This can be found also in the assessment of the change of unsteady aerodynamic forces. One benefit of plasma actuation is the nearly instant response from the point of changing the applied current until the maximum force is induced into the flow. Nevertheless, the flow reacts with a short delay, this delay time is in the range of half the blade oscillation period for the bending dominated blade oscillation pattern, decreasing the actuation effect for higher frequency oscillation pattern. Reference aerodynamic forces for blade oscillations with a maximum amplitude of 0.5mm are taken as a reference. The overall results show that the highest observed influence on unsteady exiting blade forces is around 4% for the case of the bending dominated blade oscillation pattern with an phase shift of 180° for neighbouring blades.
Publications
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Aeroelastic load manipulation of turbomachinery blades via plasma actuation, GPPS-TC-2022-20, ISSN 2504-4400, 2022
P. Neumann & D. Peitsch
