Effects of Tip Gap Size on the Flow Structure in the Tip Region of an Axial Turbomachine

Abstract

This experimental study examines the effect of tip gap size on the flow structure and turbulence in the tip region of an axial turbomachine. The experiments have been performed in the Johns Hopkins University (JHU) optically index-matched facility using an axial compressor settings designed based on the geometry of the inlet guide vanes (IGV) and the first stage of the Low Speed Axial Compressor (LSAC) facility at NASA Glenn. Two sets of rotor blades with similar cross sections, but with tip gap sizes of 0.49% and 2.3% of the blade chord (or 1.1% and 5.4% of the blade span) have been installed and tested. The measurements include performance tests, visualization of the tip leakage vortex (TLV) using cavitation, and stereo PIV (SPIV) measurements in several meridional planes. Increasing the tip gap size causes a substantial reduction in pressure rise across the machine for the same flow rate. The cavitation images, whose trends agree with the velocity and vorticity distributions obtained by the SPIV measurements, show that TLV rollup in the less loaded blade occurs at later chordwise location, and that the vortex remains located closer to the suction side (SS) corner of the originating blade. The delayed detachment from the blade with increasing gap is attributed to the increase of distance of the ‘image vortex’ (wall interaction) from the TLV. The wider gap also reduces the entrainment by the TLV of the endwall boundary layer after it separates at the point where the backward leakage flow meets the main passage flow. The previously observed TLV breakup, which is evident for the narrow gap in the aft part of the rotor passage, is delayed significantly for the wider gap. Consistent changes also appear in the distributions of turbulent kinetic energy, which peaks in the vicinity of the TLV core, the endwall boundary layer separation, and in the shear layer connecting the TLV center to the SS corner of the blade tip.