PIV Measurements of the Flow in the Tip Region of a Compressor Rotor

Abstract

An axial turbomachine, adapted from the NASA Glenn Low-Speed Axial Compressor (LSAC), has been assembled for detailed flow and turbulence measurements in the JHU optical refractive index matched facility. The test section consists of a row of twenty inlet guide vanes (IGV), followed by fifteen rotor blades, then twenty stator blades. The blades have the same geometry, but lower aspect ratio as the inlet guide vanes and the first stage of the LSAC facility at NASA Glenn. Although smaller in scale, the Reynolds number based on the tip speed and rotor blade chordlength are comparable to those of the LSAC. The casing, rotor blades, as well as half the IGVs and stator blades are made of transparent acrylic, matched with the refractive index of the working fluid, a concentrated solution of sodium iodide and water. The facility is designed to allow optical flow measurements in all three blade rows and from all directions. Results presented in this paper are based on 2D PIV measurements focusing on the flow structure in the tip region of the rotor blade for two flow rates, one of them just above the stall level. Included are phase-averaged distributions of velocity, circumferential vorticity, and turbulent kinetic energy in several meridional planes dissecting the tip at different chordwise locations. They follow the evolution of the Tip Leakage Vortex (TLV) as it rolls up near the blade suction side, migrates across the rotor passage, and subsequently bursts. Upon bursting, the distinct high vorticity core is replaced by a broad region of elevated vorticity, which occupy a substantial fraction of the passage. Sample instantaneous realizations and higher resolution stereo-PIV data are also provided. The turbulent kinetic energy is high near the vortex core, in the shear layer connecting the vortex to the blade tip, and around the point of flow separation on the endwall casing, where the leakage flow meets the main passage flow.