This paper reports on experimental and numerical investigations on circumferential grooves in an axial single-stage transonic compressor. Total pressure ratio and efficiency speedlines were taken at design speed and three off-design conditions. The experiments comprise four different configurations with deep and shallow grooves and variable coverage of the projected rotor axial chord. All casing treatments proved to have a beneficial effect on stall range while maintaining high levels of efficiency, even at off-design operation. Deep grooves extending almost to the trailing edge showed the biggest potential: the mass flow at stall inception for design speed could be strongly reduced, and the operating range could be enlarged by 56.1%. When three shallow grooves were applied to the compressor, the stage efficiency at design speed was shifted to slightly higher values. A possible explanation could be a favorable change in stator aerodynamics due to the reduction of corner separation. For a closer look into the physical effects of grooves on the tip leakage flow, a rotor-only CFD analysis has been carried out using a steady state calculation. A multi-block grid with approximately 1.2 million nodes was used. The numerical simulations reveal strong effects of circumferential grooves on the rotor flow field at tip. Mach-number contours, axial velocity distributions and particle traces for the smooth casing and six deep grooves are presented at stall mass flow. Compared to the smooth wall case, the treated casing significantly reduces blockage in the tip area and weakens the roll-up of the core vortex. These mechanisms prevent an early spillage of low momentum fluid into the adjacent blade passage and delay the onset of rotating stall.

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