Efficiency improvements for gas turbines are strongly coupled with increasing turbine inlet temperatures. This imposes new challenges for designers for efficient and adequate cooling of turbine components. Modern gas turbines inject bleed air from the compressor into the stator/ rotor rim seal cavity to prevent hot gas ingestion from the main flow, while cooling the rotor disk. The purge flow interacts with the main flow field and static pressure field imposed by the turbine blades. This complex interaction causes non-uniform and jet-like penetration of the purge flow into the main flow field, hence affecting the endwall heat transfer on the rotor.To improve the understanding of purge flow effects on rotor hub endwall heat transfer, an unshrouded, high-pressure representative turbine design with 3D blading and extended endwall contouring of the rotor into the cavity seal was tested. The measurements were conducted in the 1.5-stage axial turbine facility LISA at ETH Zurich, where a state-of-the-art measurement setup with a high-speed infrared camera and thermally managed rotor insert was used to perform high-resolution heat transfer measurements on the rotor. Three different purge flow rates were investigated with regard to hub endwall heat transfer. Additionally, steady-state computational fluid dynamics simulations were performed to complement the experiments.It was found that the local heat transfer rate changes up to ±20% depending on the purge flow rate. The main part of the purged air is ejected at the endwall trough location and swept towards the rotor suction side, which is caused by the interaction of main flow and the cavity extended endwall design. The presence of low momentum purge flow locally reduces the heat transfer rate. Changes in adiabatic wall temperature and heat transfer (depending on purge rate) are observed from the platform start up to the cross passage migration of the secondary flow structures.

Efficiency improvements for gas turbines are strongly coupled with increasing turbine inlet temperatures. This imposes new challenges for designers for efficient and adequate cooling of turbine components. Modern gas turbines use bleed air from the compressor to inject into the stator/rotor rim seal cavity to prevent hot gas ingestion from the main flow, while cooling the rotor disk. This purge flow interacts with the main flow field and static pressure field imposed by the turbine blades. The complex interaction causes nonuniform and jet-like penetration of the purge flow into the main flow field and therefore affects the endwall heat transfer on the rotor. In order to improve the understanding of purge flow effects on rotor hub endwall heat transfer, measurements in the 1.5-stage axial turbine facility LISA at ETH Zurich have been performed. An unshrouded, high-pressure representative turbine design with 3D blading and extended endwall contouring of the rotor into the cavity seal has been tested. A state-of-the-art measurement setup with a high-speed infrared camera and thermally managed rotor insert has been used to perform high-resolution heat transfer measurements on the rotor. Three different purge flow rates were investigated with regard to hub endwall heat transfer. Additionally, steady-state CFD simulations were performed to complement the experiments. It was found that the local heat transfer rate changes up to ±20% depending on the purge flow rate. The main part of the purged air is ejected at the endwall trough location and swept towards the rotor suction side caused by the interaction of main flow and the cavity extended endwall design. The presence of low momentum purge flow is locally reducing the heat transfer rate. Changes in adiabatic wall temperature and heat transfer depending on purge rate are observed from the platform start up to the cross passage migration of the secondary flow structures.

A reduction in rotor blade count in combination with a gain in aerodynamic performance is a desirable design goal for gas turbines to reduce the overall operational costs. Reducing the number of blades provokes inherently an increase in blade loading which drives the secondary flow strength. In the presented experimental work, the results of inter-stage probe measurements in a highly loaded 1.5-stage axial turbine rig show the potential to improve the stage efficiency for a reduced blade count rotor with respect to a baseline configuration with more blades. Time-resolved probe measurements reveal the detrimental effects on the turbine tonal noise level. It is found that the periodic vorticity fluctuations induced by the interaction of the rotor passage secondary flow structures with the potential field of the downstream stator, leads at specific span positions to a strong increase in the noise level at rotor exit with respect to the baseline. Both, the downstream effect of the convected rotor flow structures as well as the periodic interaction of the second stator originated flow structures are found to drive the acoustic field of the turbine. Overall, the stage efficiency benefit achievement of 0.4% for a 22% reduction in rotor blade count is derogated by an increase in tonal noise by up to 13 dB at the second stator exit.

A reduction in rotor blade count in combination with a gain in aerodynamic performance is a desirable design goal for gas turbines to reduce the overall operational costs. Reducing the number of blades provokes inherently an increase in blade loading which drives the secondary flow strength. In the presented experimental work, the results of inter-stage probe measurements in a highly loaded 1.5stage axial turbine rig show the potential to improve the stage efficiency for a reduced blade count rotor with respect to a baseline configuration with more blades. Time-resolved probe measurements reveal the detrimental effects on the turbine tonal noise level. It is found that the periodic vorticity fluctuations induced by the interaction of the rotor passage secondary flow structures with the potential field of the downstream stator, leads at specific span positions to a strong increase in the noise level at rotor exit with respect to the baseline. Both, the downstream effect of the convected rotor flow structures as well as the periodic interaction of the second stator originated flow structures are found to drive the acoustic field of the turbine. Overall, the stage efficiency benefit achievement of 0.4% for a 22% reduction in rotor blade count is derogated by an increase in tonal noise by up to 13 dB at the second stator exit.

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