Experiments that demonstrate the use of endwall recirculation to control the stall of transonic compressor stages are described. Endwall recirculation of a compressor stage is implemented by bleeding air from the casing downstream of a stator blade row and injecting the air as a wall jet upstream of a preceding rotor blade row. The bleed ports, injection ports, and recirculation channels are circumferentially discrete, and occupy only 20–30% of the circumference. The development of compact wall-jet injectors is described first. Next, the results of proof-of-concept steady recirculation tests on a single-stage transonic compressor are presented. Finally, the potential for using endwall recirculation to increase the stability of transonic highly-loaded multistage compressors is demonstrated through results from a rig test of simulated recirculation driving both a steady injected flow and an unsteady injected flow commanded by closed-loop active control during compressor operation at 78–100% of design speed. In this test air from an external source was injected upstream of several rotor blade rows while compressor bleed was increased by an amount equivalent to the injected massflow. During closed loop control, wall static pressure fluctuations were monitored and the injected flow rate was controlled to reduce the stalling mass flow. The use of wall jet injection to study the dynamics of transonic compressor stages is also discussed.

In recent years, a demand for compact, lightweight, solid-state actuation systems has emerged, driven in part by the needs of the aeronautics industry. However, most actuation systems used in turbomachinery require not only elevated temperature but high-force capability. As a result, shape memory alloy (SMA) based systems have worked their way to the forefront of a short list of viable options to meet such a technological challenge. Most of the effort centered on shape memory systems to date has involved binary NiTi alloys but the working temperatures required in many aeronautics applications dictate significantly higher transformation temperatures than the binary systems can provide. Hence, a high temperature shape memory alloy (HTSMA) based on NiTiPdPt, having a transformation temperature near 300 o C, was developed. Various thermo-mechanical processing schemes were utilized to further improve the dimensional stability of the alloy and it was later extruded/drawn into wire form to be more compatible with envisioned applications. Mechanical testing on the finished wire form showed reasonable work output capability with excellent dimensional stability. Subsequently, the wire form of the alloy was incorporated into a benchtop system, which was shown to provide the necessary stroke requirements of ~0.125 inches for the targeted surge-control application. Cycle times for the actuator were limited to ~4 seconds due to control and cooling constraints but this cycle time was determined to be adequate for the surge control application targeted as the primary requirement was initial actuation of a surge control rod, which could be completed in approximately one second.

Active flow control has been applied to the suction surface of stator vanes in a low speed axial compressor. Injection from the suction surface has been shown to reduce separation on vanes that were induced to separate by increasing the vane stagger angle by 3° deg. Various configurations were investigated including injector geometry (slots versus holes) and type of injection (steady versus unsteady). Unsteady injection was realized using two different approaches; external actuation through a high frequency valve and embedded actuation using a fluidic device internal to the vane. Using total pressure loss through the vane passage as a metric, reductions in area-averaged loss of 25% were achieved using injected mass flow rates on the order of 1% of compressor throughflow. The development of a tracking control algorithm was also explored for the purpose of closed-loop control. A reliable method of detecting surface separation was implemented using unsteady pressure measurements on the compressor casing near the vane suction surface.

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