Abstract

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 a 2% decrease in efficiency 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. This paper focuses on explaining these trends by measuring the flow in the tip region and within the U and S grooves. The stereo-PIV (SPIV) measurements are performed in the JHU refractive index matched facility, which allows unobstructed observations in the entire machine. Data has been acquired in two meridional planes that intersect with the grooves at different locations, and two radial planes (z, θ), 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. 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 and lingers within the groove long after the blade clears the groove. 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, the chordwise location of primary blade loading moves downstream, as expected. The inflow into the grooves occurs for a small fraction of the blade passing period, and most of the tip leakage vortex remains in the main flow 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, for the U groove, the inflow into and outflow from the groove reverse direction (compared to the low flowrate trends), entering at the base of the U, and exiting mostly at its downstream end, especially when the blade is not near. The resulting entrainment of secondary flows from the groove into the passage are likely contributors to the reduced efficiency at BEP for the U grooves.

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