In the present paper an experimental investigation of the effects of hub purge flow on the performance of a high pressure axial turbine stage is presented. The test configuration consists of a one-and-a-half stage, unshrouded, highly loaded axial turbine, with a blading representative of high pressure gas turbines. The test configuration has the capability of integrating purge flow from the cavity under the rotor/stator platform.

Efficiency measurements for various purge flow injection levels have been carried out with pneumatic probes at the exit of the rotor and show a reduction of isentropic total-to-total efficiency of 0.8 % per percent of injected mass flow.

For three purge flow conditions the unsteady aerodynamic flow field at rotor inlet and rotor exit has been measured with the in-house developed Fast Response Aerodynamic Probe (FRAP): one condition is with sucking at a mass flow fraction of −0.1 % and two conditions are with injection at a mass flow fraction of 0.8 % and 1.2 %. The time-resolved data shows the unsteady interaction of the purge flow with the secondary flows of the main flow and the impact on the radial displacement of the rotor hub passage vortex.

Steady measurements at off-design conditions show the impact of the rotor incidence and of the stage flow factor on the resulting stage efficiency and the radial displacement of the rotor HPV.

A comparison of the effect of purge flow and of the off-design conditions on the rotor incidence and stage flow factor shows that the detrimental effect of the purge flow on the stage efficiency caused by the radial displacement of the rotor hub passage vortex is dominated by the increase of stage flow factor in the hub region rather than by the increase of negative rotor incidence.

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