Abstract

Effects of clearance size and flowrate on the flow around the tip of an axial turbomachine rotor are studied experimentally. Visualizations and PIV measurements in a refractive index-matched facility compare the performance, leakage velocity, and the trajectory, growth, and strength of the tip leakage vortex (TLV) for gaps of 0.49% and 2.3% of the blade chord, and two flowrates. Enlarging the tip clearance delays the TLV breakup in the aft part of the rotor passage at high flowrates but causes earlier breakup under pre-stall conditions. It also reduces the entrainment of endwall boundary layer from the point where the leakage and passage flows meet. Reducing the flowrate or tip gap shifts the location of TLV detachment from the blade suction side (SS) upstream, to points where the leakage velocity is 70-80% of the tip speed. Once detached, the growth rates of total shed circulation are similar for all cases, i.e. varying the gap or flowrate mostly shifts the detachment point. The TLV migration from the SS decreases with increasing gap, but not with flowrate. Two mechanisms dominate this migration: Initially, the leakage jet pushes the TLV away from the blade at 50% of the leakage velocity. Further downstream, the TLV is driven by its image on the other side of the endwall. Differences in migration rate are caused by the smaller distance between the TLV and its image for the narrow gap, and the increase in initial TLV strength with decreasing flowrate and gap.

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