Experimental investigations of the aerodynamic damping of compressor blades are usually performed by forcing the blades externally to a motion that is similar to a given mode shape and inter-blade phase angle (IBPA) while measuring the motion-induced unsteady pressure distribution. Evaluating this to an aerodynamic work entry from the fluid to the blade, at least a local contribution to the flutter (in-)stability can be determined. Test rigs are usually non-rotating linear or rotating annular cascade wind tunnels. In the latter case, besides measuring in and transmitting data out of the rotating system, the excitation of the blades themselves is still a challenge. In the present case a blisk rotor with realistic fan blade geometries and flow conditions was investigated aeroelastically. For the excitation of the 1st bending (1F) mode shape of the blading a sector of five blades was chosen. In this sector the natural vibration of the blading, represented by constant vibration amplitudes and a given IBPA should be simulated. Therefore the blades were equipped with Macro Fiber Composites (MFC). These foils of piezoelectric material expand and contract due to the applied high voltage. A control system was developed to adjust the amplitude and the IBPA of the blade vibration. For the transmission of the high voltage, a separate data transmission system on the base of liquid metals was chosen. The blade vibration was measured by strain gauges and additionally monitored by a specific rig system. The aeroelastic investigations were carried out in the compressor test facility M2VP of the DLR Institute of Propulsion Technology in Cologne. During the measurement, the MFCs were able to excite the blades to a certain extent. The paper will present the technique to excite the blades of a compressor blisk by means of MFCs as well as achieved vibration amplitudes and limitations under engine-like operating conditions.

This paper presents the design of an aspirated stator of a four-stage high-speed axial compressor. The aspiration of near-wall fluid at the suction side of the first stator is designed numerically by means of DoE (Design of Experiments). The design objective is a reduction or complete suppression of hub corner separation at off-design conditions. As operating point for the CFD-based design process, the last numerically stable operating point near the stall limit of the reference configuration at 80% of the design speed is chosen. As DoE factors the aspiraton velocity, the chordwise position, and the axial by radial dimensions of the aspiration slot are varied. Their effects on the two target values overall isentropic efficiency and total pressure ratio are investigated. All evaluated configurations show significant improvements in stage performance of all compressor stages be-cause of a reduction of hub corner separation. Based on the DoE correlation factors an optimization is performed. The optimum configuration shows an increase in overall isentropic efficiency Δηis of 1.3% to 90.98% whereas the total pressure ratio π is raised by 0.08 to 2.08.

Reducing the fuel consumption is a main objective in the development of modern aircraft engines. Focusing on aircraft for mid-range flight distances, a significant potential to increase the engines overall efficiency at off-design conditions exists in reducing secondary flow losses of the compressor. For this purpose, Active Flow Control (AFC) by aspiration or injection of fluid at near wall regions is a promising approach. To experimentally investigate the aerodynamic benefits of AFC by aspiration, a 4½-stage high-speed axial-compressor at the Leibniz Universitaet Hannover was equipped with one AFC stator row. The numerical design of the AFC-stator showed significant hub corner separations in the first and second stator for the reference configuration at the 80% part-load speed-line near stall. Through the application of aspiration at the first stator, the numerical simulations predict the complete suppression of the corner separation not only in the first, but also in the second stator. This leads to a relative increase in overall isentropic efficiency of 1.47% and in overall total pressure ratio of 4.16% compared to the reference configuration. To put aspiration into practice, the high-speed axial-compressor was then equipped with a secondary air system and the AFC stator row in the first stage. All experiments with AFC were performed for a relative aspiration mass flow of less than 0.5% of the main flow. Besides the part-load speed-lines of 55% and 80%, the flow field downstream of each blade row was measured at the AFC design point. Experimental results are in good agreement with the numerical predictions. The use of AFC leads to an increase in operating range at the 55% part-load speed-line of at least 19%, whereas at the 80% part-load speed-line no extension of operating range occurs. Both speed-lines, however, do show a gain in total pressure ratio and isentropic efficiency for the AFC configuration compared to the reference configuration. Compared to the AFC design point, the isentropic efficiency ηis rises by 1.45%, whereas the total pressure ratio Πtot increases by 1.47%. The analysis of local flow field data shows that the hub corner separation in the first stator is reduced by aspiration, whereas in the second stator the hub corner separation slightly increases. The application of AFC in the first stage further changes the stage loading in all downstream stages. While the first and third stage become unloaded by application of AFC, the loading in terms of the De-Haller number increases in the second and especially in the fourth stage. Furthermore, in the reference as well as in the AFC configuration, the fourth stator performs significantly better than predicted by numerical results.

The decrease of fuel consumption is a main objective in the development of modern aircraft engines and heavy‐duty gas turbines. Especially at off‐design conditions, one promising approach to suppress flow losses and to increase the efficiency of the compressor is Active Flow Control (AFC) by aspiration or injection. Aerodynamically, the compressor flow of a gas turbine responds more sensitively to volatile flow conditions than the turbine flow because of the positive pressure gradient in a compressor achieved by flow deceleration. In a decelerating flow, particularly at off‐design operating conditions, the compressor flow tends to separate from the blade surfaces. This flow separation causes unstable operating conditions inside the flow path resulting in low overall engine efficiency. Thus it is obvious that counter‐measures against increased flow losses at off‐design operation should be concentrated on the compressor. Considering industrial objectives, both the performance increase and the operating range enhancement are subjects of current compressor research, as is a reduction of vanes or even entire stages. While a reduced vane count reduces cost, even greater benefits can be gained if entire stages could be eliminated and thus the number of rotor discs reduced, which further reduces cost along with reducing the length of the rotor which could also improve rotor dynamics. For all of these purposes, different AFC approaches were implemented in a four stage axial compressor (4AV) and were experimentally examined. This paper presents an overview of the past and ongoing AFC research at the Institute of Turbomachinery and Fluid Dynamics (TFD). (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)