Schlüsselwort: Ständer (Technik)

In order to increase the efficiency of turbomachines, as they are used in propulsion systems for aircrafts or gas turbines in power plants, the possibilities of passive means to optimize the flow conditions appear to be exhausted by now. However, active flow control has proven its significance and enormous potential to influence and optimize flow processes in various investigations in recent years. In this study the possibilities of the technology are investigated to reach a further increase in performance of a commercial jet engine. The application of active flow control methods may help to suppress or at least mitigate the characteristic separation phenomena and thereby improve the aerodynamic behavior. The compressor is an elementary component of a jet engine and contributes to a major part of the total weight and the overall length. By reducing the number of necessary compressor stages while providing the same total pressure increase, the jet engines of the future can be built more compact and efficient. By that, the construction as well as the maintenance costs could be reduced. Furthermore, less fuel would be needed and the environment would benefit, too. To achieve this aim it is vital to increase the compression ratio per stage, which consists of a rotor and a stator element. Thus, the efficiency can be enhanced without intervening into the thermodynamic cycle of the engine. The present study demonstrates different methods of closed-loop control that enable an effective reduction of flow separation processes within a highly loaded compressor cascade as well as an axial fan. At the axial fan it is possible to delay rotating stall and increase the effective flow through section area by injecting pulsed air into the gap between the tips of the rotor blades and the casing. By means of an H∞-controller it is possible to increase the attainable pressure rise and enhance the usable aerodynamic range significantly. For this reason the engine can be run at operating points that are usually unstable. Adaptive control methods enable the automatic stabilization of the system and are additionally capable of compensating disturbances whereby an unstable operation can be avoided. Due to the overcritical deflection of the blades of the compressor cascade a pressure induced sepa-ration as well as corner vortices develop in the passage flow field, which lead to significant aerody-namic losses. By means of robust closed-loop control the secondary flow structures can be mitigated and disturbances in the wake flow can be compensated. By this, the blockade of the passage flow field is decreased and the attainable pressure rise increases. In order to control both separation phenomena simultaneously, two control variables reflecting the actual flow conditions can be derived. As a result, a multiple-input multiple-output control task has to be solved. Both robust control approaches and classical decoupling controllers are employed. In either case it can be demonstrated that the proposed control strategy is capable of increasing the pressure rise at the leading edge of the stator blade significantly and that heavy disturbances can be compensated quickly. Moreover, extremum-seeking controllers are applied to detect energy efficient actuation parameters.