Effect of Axial Casing Groove Geometry on Rotor-Groove Interactions in the Tip Region of a Compressor

The present experimental study expands an ongoing effort to characterize the interactions of axial casing grooves (ACGs) with the flow in the tip region of an axial turbomachine. In recent work, we have tested a series of grooves with the same inlet geometry that overlaps with the rotor blade leading edge, but with different exit directions. Two geometries have stood out: The U grooves, which have an outflow in the negative circumferential direction (opposing the blade motion) are the most effective in suppressing stall, achieving as much as 60% reduction in stall flowrate, but cause performance degradation around the best efficiency point (BEP). In contrast, the S grooves, which have an outflow in the positive circumferential direction, achieve a milder improvement in stall suppression (36%) but do not degrade the performance near BEP. The ongoing effort focuses on explaining these trends by measuring the flow in the tip region and within the U and S grooves. The stereo-PIV measurements are performed in the JHU refractive index matched facility, which allows unobstructed observations in the entire machine. The measurements have been performed in two meridional planes that intersect with the grooves at different locations, and two radial planes (z, q), the first coinciding with the blade tip, and the second, with the tip gap. For each plane, data has been acquired at fourteen rotor orientations relative to the grooves to examine the rotor-grooves interactions. In addition, data has been recorded in an axial plane located downstream of the grooves, close to the blade trailing edge (86% of the blade chord). At low flow rates, the inflow into both grooves peaks periodically when the blade pressure side (PS) faces the entrance (downstream side) to the grooves. This inflow rolls up into a large vortex that remains within the groove after the blade rotates away. The outflow depends on the shape of the groove. For the S groove, the outflow exits at the upstream end of the groove in the positive circumferential direction, as designed. In contrast, for the U grooves, the fast radially and circumferentially negative outflow peaks at the base of the U. The resulting jet causes substantial periodic variations in the flow angle near the leading edge of the rotor blade. Close to the BEP, it appears that the chordwise location of primary blade loading moves downstream, as expected, but not to the same extent. Consequently, the tip leakage vortices are weaker and remain within the passage. For the S grooves, the rotor-groove interactions seem to be minimal, with little (but not zero) inflow or outflow at both ends, and minimal changes to the flow angle in the passage. In contrast, the U grooves still continue to entrain and ejects fluid into the passage around its downstream end, especially when the blade is not near, disrupting and generating secondary flows in the tip region. These interactions might play a role in the 2% reduction of efficiency at BEP for the U grooves, while the S grooves maintain the same efficiency as the untreated endwall.