Tip Leakage Flows and Stall Suppression in Axial Turbomachines
Abstract: This presentation summarizes a series of experimental studies aimed at characterizing the flow and turbulence in the tip region of a one and a half stage axial liquid turbomachine with a compressor-like geometry. The experiments have been performed using transparent blade rows in a refractive-index-matched facility and included performance tests, stereo PIV (SPIV) measurements, and flow visualization using cavitation. Data analysis follows the evolution the tip leakage flow, its rollup into a tip leakage vortex (TLV), the migration and breakup of this vortex, development of secondary flows, as well as the evolution of turbulence in the rotor passage. Experiments aimed at characterizing the precursors to rotating stall show that it involves intermittent formation of large-scale backflow vortices (BFVs) that extend diagonally upstream, from the suction side of one blade at mid-chord to the pressure side near the leading edge of the next blade. The BFVs originate form radial gradients in circumferential velocity occurring under the TLV center, at the transition between the region affected by the backward tip leakage flow and the main passage flow. When the BFVs penetrate to the next passage, either across the tip gap or by circumventing the leading edge of the next blade, they trigger a similar phenomenon there. In the stall regime, the number and size of these vortices increase. Skewed semicircular axial casing grooves, which partially overlap with the rotor blade’s leading edge and the rest extending upstream, reduce the stall flow rate by as much as 40%, but degrade the performance near the best efficiency point (BEP). These grooves entrain most of the TLV, prevent the formation of BFVs, and cause periodic variations in the incidence angle near the rotor blade leading edge. In contrast, near BEP, secondary flows generated within the grooves are entrained back into the passage by the TLV. A series of grooves with the same inlet but different outlet angles have been tested in an attempt to alleviate the performance degradation. Aligning the outflow circumferentially against the blade rotation using U-shaped grooves is very effective in suppressing the stall, but degrades the BEP performance. Conversely, aligning the outflow with the blade rotation using S-shaped grooves achieves more moderate (25%) stall suppression, but does not degrade the BEP performance. Causes for these trends are elucidated by examining the groove-passage flow interactions. For example, the better stall suppression by the U grooves is attributed to higher periodic variations in flow angle at low flow rates, while the performance degradation and increased turbulence at BEP is caused by flow jetting out from the downstream end if this groove.