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

Abstract

Abstract Stereo-PIV 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. The flow is examined at two operating conditions, namely pre-stall flow rate and near the best efficiency point (BEP) of the untreated casing. Previous studies in this machine have shown that the ACGs improve the stall margin significantly but reduce the peak efficiency. At pre-stall and with untreated casing, intermittent reverse axial flow near the casing induced by backflow vortices (BFVs), tip leakage vortex (TLV), and the leakage flow extend upstream of the rotor leading edge. Although more probable in front of the blade, this backflow occurs along the entire circumference. Inside the rotor, the tip region blockage is characterized by low axial and high circumferential momentum. This region expands radially inward as the flow evolves axially, extending to half of the span by the time it reaches the rotor trailing edge. In contrast, inward of the region affected by blockage, the axial velocity exceeds the spatially averaged value by well above 60%. This extreme nonuniformity diminishes rapidly as the flow enters the stator, resulting, except for the blade wakes, in a nearly uniform axial velocity distribution at the exit from the stator. Previous measurements focusing on the effect of ACGs on tip flows have established that they entrain portions of the TLV and the backward leakage flow, and the outflow from them causes periodic variations in flow angle near the blade tip leading edge. The current data reveals that the outflow with negative axial and circumferential velocity jetting out from the grooves upstream of the rotor generates axially aligned vortices on both sides of each jet. These vortices act as homogenizers by entraining the faster mid-span flow into the tip region, substantially reducing the axial velocity deficit in the spaces between grooves. Consequently, the blockage in the tip region and flow non-uniformity in the passage are reduced substantially. Indeed, passage-averaged velocity profiles obtained by averaging the data circumferentially highlight that the ACGs modify the flow structure over the entire span, increasing the axial velocity at the tip, and decreasing it near the hub. Near BEP, the blockage is confined to the tip region and differences in flow structure with or without ACGs become more subtle. The flow jetting out of the grooves, and axial vortices are weaker but do not vanish. Furthermore, interactions of the TLV with secondary flows entrained from the grooves into the passage cause circumferential expansion of its signature, which presumably affects the compressor performance adversely.