Impact of Operating Conditions and Axial Casing Grooves on the Evolution of Flow Structure Across Blade Rows in an Axial Compressor

Abstract

Stereo particle image velocimetry (SPIV) measurements in a series of axial planes investigate the impact of operating conditions and semicircular axial casing grooves (ACGs) on the evolution of flow structure across multiple blade rows in an axial compressor. The field of view extends radially from the hub to the tip and circumferentially over entire blade passages. Previous studies in this machine have shown that the ACGs improve the stall margin significantly but reduce the peak efficiency. At pre-stall flowrate and without ACGs, intermittent reverse axial flow near the casing is induced by backflow vortices, tip leakage vortex (TLV), and the leakage flow extend upstream of the rotor leading edge. Inside the rotor, the tip region blockage, characterized by low axial and high circumferential momentum, expands radially inward as the flow evolves axially. This extreme non-uniformity diminishes rapidly within the stator. In addition to previously shown ACGs effects, the current data reveal that the flow jetting out from the groove upstream of the rotor generates axially aligned vortices on both sides of each jet. These vortices substantially reduce the flow non-uniformity over the entire passage by entraining the faster mid-span flow into the tip region. Near the best efficiency point, the jets become weaker, the blockage is confined to the tip region, and differences between the global flow structure with and without ACGs become subtle. However, interactions of the TLV with secondary flows entrained from the grooves into the passage expand the TLV signature, which has adverse effects on the compressor performance.