DELAYING THE ONSET OF ROTATING STALL IN AN AXIAL COMPRESSOR

The optimal performance of axial compressors is crucial in a number of appli- cations, including power plants and gas turbine engines. Their efficiency is limited by the onset of stall, when the pressure rise across the compressor drops suddenly and destructive flow patterns emerge. Because of this, all compressors operate at conditions away from the stall point, but with an accompanying penalty in efficiency. The objective of this study is to examine the detailed fluid dynamic processes lead- ing to stall and devise a scheme of active flow control to delay stall onset, allowing compressors to operate safely much closer to the stall point and at greater efficiency. This is done on the low speed axial compressor rig in the Fluid Dynamics Research Center at the Illinois Institute of Technology. Results indicated that the stall cell appears at a flow coefficient, φ = 0.292 and that it is small, located close to the casing, rotating with the rotor blade row, but at 55% of the rotor rotation rate. Detailed stall inception investigation revealed that small amplitude modal waves instigated flow breakdown upstream of the rotor blade row, causing a spike stall cell to develop. This spike cell rotates very quickly (70%) and grows rapidly into a finite stall cell, slowing its rotation rate as it becomes fully developed. Stagnation pressure in the wake of the rotor blades was phase locked to both the rotor and stall cell rotation, allowing phase-averaging techniques to reveal the averaged stall cell structure. A new actuator was designed for rotating stall control. Based on the stall inception results, a disturbance rejection scheme was devised to interact with the incipient stall cell, preventing its growth into a finite stall cell. The single blade was studied in a number of open loop control experiments, which indicated that pulsing at 2psi over a range of frequencies can inhibit stall inception, extending the operating range by 0.4% mass flow, thus stabilizing a previously unstable operating point.M.S. in Mechanical and Aerospace Engineering, May 201