The Physical Mechanism of Tip Leakage Loss Reduction by Means of Passive Injection in Low Mach Number Flows

Tip clearance losses represent a major source of efficiency losses in turbomachinery. A novel method based on passive injection for reducing tip leakage losses in axial turbine cascades has been proposed in 2007 by Willinger and co-workers, and first experimental demonstrations of the potential of this approach were reported recently. However, these experimental tests were limited to linear cascade experiments w6ith stationary endwalls, and they were not able to explain the underlying physical mechanism of the observed loss reduction. In the present contribution, results of detailed experimental and numerical analyses of the tip gap flow, the interaction of the passive tip injection jet with the gap and the main flow, and the effect of the moving endwall are presented. Both numerical and experimental results indicated that the beneficial effect of the passive injection mainly results from a weaker tip gap vortex and not so much from a simple blockage of the tip gap flow itself.