ON THE INTERACTIONS OF A ROTOR BLADE TIP FLOW WITH AXIAL CASING GROOVES IN AN AXIAL COMPRESSOR NEAR THE BEST EFFICIENCY POINT

摘要

Previous studies have shown that axial casing grooves (ACGs) are effective in delaying the onset of stall, but degrade the performance of axial turbomachines around the best efficiency point (BEP). Our recent experimental study [1] in the JHU refractive index-matched liquid facility have examined the effects of ACGs on delaying stall of a one and half stage compressor. The semicircular ACGs based on Mueller et al. [2] reduce the stall flow rate by 40% with a slight decrease in pressure rise at higher flow rates. Stereo-PIV (SPIV) measurements at a flow rate corresponding to the pre-stall condition of the untreated machine have identified three flow features that contribute to the delay in stall. Efficiency measurements conducted as part of the present study show that the ACGs cause a 2.4% peak efficiency loss. They are followed by detailed characterizations of the impact of the ACGs on the flow structure and turbulence in the tip region at high flow rates away from stall. Comparisons with the flow structure without casing grooves and at low flow rate are aimed at exploring relevant flow features that might be associated with the reduced efficiency. The SPIV measurements in several meridional and radial planes show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the blade suction side (SS). The inflow velocity magnitude is substantially lower than that occurring at a flow rate corresponding to the pre-stall conditions of the untreated machine. Yet, entrainment of the PS boundary layer and its vorticity during the inflow phase generates counter-rotating radial vortices at the entrance to the groove, and a "discontinuity" in the appearance of the tip leakage vortex (TLV). While being exposed to the blade SS, the backward tip leakage flow causes flow separation and formation of a counter-rotating vortex at the downstream corner of the groove, which migrates towards the passage with increasing flow rate. Interactions of this corner vortex with the TLV cause fragmentation of the latter, creating a broad area with secondary flows and elevated turbulence level. Consequently, the vorticity shed from the blade tip remains scattered from the groove corner to the blade tip long after the blade clears this groove. The turbulence peaks around the corner vortex, the TLV, and the shear layer connecting it to the SS corner. During periods of inflow, there is a weak outflow from the upstream end of the groove. At other phases, most of the high secondary flows are confined to the downstream corner, leaving only weak internal circulation in the rest of the groove, but with a growing shear layer with elevated (but weak) turbulence originating from the upstream corner. Compared to a smooth endwall, the groove also increases the flow angle near the blade tip leading edge (LE) and varies it periodically. Accordingly, the magnitude of circulation shed from the blade tip and leakage flow increase near the leading edge. The insight from these observations might guide the development of ACGs that take advantage of the effective stall suppression by the ACGs but alleviate the adverse effects at high flowrates.