Purge air, injected through seals in the hub of axial turbines, is necessary to prevent hot gas ingestion into endwall cavities, but generates losses by viscous interaction with the mainstream flow. Recent work has shown that for a given purge air mass flow rate, introducing swirl into the purge flow can reduce these losses. This paper investigates the effect of introducing such swirl on the ability of purge flow to prevent ingestion. In particular, it is observed that in the presence of the rotating external pressure non-uniformity due to a downstream blade row, swirled purge flow is much less effective in sealing a turbine disk rim cavity compared to non-swirled purge flow. This is reflected in higher purge air mass flow rates necessary to seal a given cavity, and that in turn diminishes the positive effect of pre-swirling purge flow in the first place. It is shown that this will occur whenever the circumferential pressure disturbance associated with the downstream rotating blades is the dominant driver for externally induced ingestion. It is reasoned that swirled purge flow moves with the rotating pressure non-uniformity and responds to it more readily than non-swirled purge flow, which sees the averaged effect of multiple blade passing events. A flow model based on this physical principle is developed, showing good agreement with computational results. The model yields an ingestion criterion with a parametric dependence on purge flow design parameters. The analysis is extended to an unsteady situation, whereby the effects of both stationary and rotating pressure non-uniformities, from vanes and blades respectively, are taken into account simultaneously. This unsteady flow model points to an optimal design space, in the context of minimizing purge flow losses while maintaining an appropriate margin with regard to hot gas ingestion.
Experimental and Analytical Assessment of Cavity Modes in a Gas Turbine Wheelspace