Effect of Upstream Unsteady Flow Conditions on Rotor Tip Leakage Flow

Abstract

A study has been conducted, using unsteady three-dimensional Reynolds-averaged Navier-Stokes simulations to determine the impact on rotor performance of the interaction between the stator wakes and rotor tip clearance flow. The key effects of the interaction are: (1) a decrease in loss and blockage associated with tip clearance flow; (2) an increase in passage static pressure rise. Performance benefit is seen in the whole operability range of interest, from near design to high loading. The benefit is modest near design and increases with loading. Significant beneficial changes occur when the phenomenon of tip clearance flow double-leakage is present. Double-leakage occurs when the tip clearance flow passes through the tip gap of the neighboring blade. Double-leakage typically takes place at high loading but can be present at design condition, as well. A benefit due to unsteady interaction is also observed in the operability range of the rotor. A new generic causal mechanism is proposed to explain the observed changes in performance. It identifies the interaction between the tip clearance flow and the pressure pulses, induced on the rotor blade pressure surface by the upstream wakes, as the cause for the observed effects. The direct effect of the interaction is a decrease in the time-average double-leakage flow through the tip clearance gap so that the stream-wise defect of the exiting tip flow is lower with respect to the main flow. A lower defect leads to a decrease in loss and blockage generation and hence an enhanced performance compared to that in the steady situation. The performance benefits increase monotonically with loading and scale linearly with upstream wake velocity defect.