Experimental Investigation of Tip Clearance Flow in a Transonic Compressor With and Without Plasma Actuators

Blade tip losses represent a major performance penalty in low aspect ratio transonic compressors. This paper reports on the experimental evaluation of the impact of tip clearance with and without plasma actuator flow control on performance of an U.S. Air Force-designed low aspect ratio, high radius ratio single-stage transonic compressor rig. The detailed stage performance measurements without flow control at three clearance levels, classified as small, medium and large, are presented. At design-speed, increasing the clearance from small to medium resulted in a stage peak efficiency drop of almost 6 points with another 4 point drop in efficiency with the large clearance. Comparison of the speed lines at high-speed show significantly lower pressure rise with increasing tip clearance, the compressor losing 8 percent stall margin with medium clearance and an additional 1 percent with the large clearance. Comparison of the stage exit radial profiles of total pressure and adiabatic efficiency at both part-speed and design-speed and with throttling are presented. Tip clearance flow-control was investigated using Dielectric Barrier Discharge (DBD) type plasma actuators. The plasma actuators were placed on the casing wall upstream of the rotor leading edge and the compressor mapped from part-speed to high-speed at three clearances with both axial and skewed configurations at six different frequency levels. The plasma actuators did not impact steady state performance. A maximum stall margin improvement of 4 percent was recorded in this test series. The large clearance configuration benefited the most with the plasma actuators. Increased voltage provided more stall margin improvement. Plasma actuator power requirements were almost halved going from continuous operation to pulsed plasma. Most of the improvement with the plasma actuators is attributed to the reduction in unsteadiness of the tip clearance vortex nearstall resulting in additional reduction in flow prior to stall. NOMENCLATURE

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