Aerothermal Effects of High-Pressure Turbine Rim Seals and Blade Tip Geometries in the Presence of Cooling Flows

The primary scope of this thesis is to contribute to improving the aerothermal performance of turbine sections in modern gas turbines for electricity production and aircraft propulsion in aero engines. The high-pressure turbine components in such machines are typically subjected to significant thermal loads induced by the continuously increasing turbine entry temperatures. Sensitive components, such as rotor disks and blades, are consequently supplied with cooling flows that, in turn, tend to have a detrimental impact on the aerodynamic performance of the turbine. In that context, geometrical features, like the rim seals and the rotor blade tips, bear great potential for optimization. Furthermore, the designers aim to reduce the number of turbine blades to cut the cooling requirements. Within that design space, this work targets providing guidelines for optimizing the rim seal space and rotor blade tips to efficiently use and reduce the expensive cooling flows while maintaining high aerodynamic performance levels. Potential unfavorable concomitant effects on the turbine characteristics (such as turbine noise) are elucidated in the context of the optimization process. The present work was carried out within the scope of a joint academic and industrial research effort to improve the aerothermal performance of high-pressure turbines by integrating novel geometrical features such as advanced rim seal designs and rotor blade tips in combination with non-axisymmetric end wall contouring. In total, three different 1.5-stage turbine configurations with multiple sub-assemblies, each consisting of specific rim seal and tip designs, were probed for different cooling flow rates in the axial turbine facility “LISA” at ETH Zurich. A unique experimental dataset is presented consisting of inter-row pneumatic and fast-response probes as well as cavity wall-mounted pressure and temperature measurements. The unsteady flow perturbations inside the delicate rim seal space were resolved using miniature-sized, piezo-resistive pressure transducers installed both on the stator- and rotor-sided hub cavity walls. A purpose-made hub cavity heat transfer setup consisting of thin-film heater and double-sided heat flux gauges allowed for changes in the thermal boundary conditions during turbine operation and thereby offered the determination of heat transfer and ingestion quantities. A modular multipurpose rotor onboard telemetry system was used, enabling the acquisition and subsequent transfer of pressure and temperature data from the rotor relative frame of reference. The test rig capabilities were extended by a low-temperature bypass flow system, which facilitated increasing the temperature difference between cavity fluid and main annulus flow. A modular, bladed-disk design was introduced to allow for a rainbow rotor setup featuring different blade tip designs for the same test run. The experimental work was complemented by extensive unsteady computational fluid dynamics simulations using the in-house developed explicit solver MULTI3. The complex flow field in the rim seal space was traced by fast-response instrumentation and unsteady calculations. The significance of specific non-synchronous low-frequency flow perturbations (cavity modes) for the aerothermal turbine design was elaborated, indicating that the main annulus ingestion behavior, as well as the turbine noise emissions, are directly affected by these flow structures. Transient full-annular numerical modeling presented a reasonable prediction of the low-frequency modes. The extensive probing of different turbine operating conditions and rim seal designs provides measures to attenuate the hub cavity asynchronous flow excitation. A novel rim seal design concept, termed as “purge control features,” is proposed, which was found to improve the aerodynamic performance for an already optimized turbine stage in the presence of rim seal purge flow. In comparison to a baseline case, the absolute stage efficiency increase were experimentally determined to be 0.4% points. Complementary beneficial effects on the ingestion behavior and the attenuation of low-frequency cavity modes were experimentally and computationally quantified. A unique experimental dataset for a combined study of turbine ingestion and hub cavity convective heat transfer is provided for a systematic variation of rim seal purge flow rates. The sensitivity analysis of the ingestion and heat transfer with respect to purge air highlights the importance of cooling flow rates to the overall improvement of the aerothermal performance of a turbine stage. An increase in the local convective heat transfer coefficient by a factor of 2–3 on the rotor-sided hub cavity wall was detected for a 1% increase in purge flow injection rate. The resultant heat transfer coefficients were compared to existing empirical correlations, indicating the limitations of such approaches in providing accurate predictions. Extensive probing of a substantially reduced blade count rotor featuring an optimized blade and end wall design revealed the potential to improve the turbine stage performance by 0.4% points. However, the achievements were compromised by a significant increase in turbine tonal noise emissions by up to 13dB. An additional absolute performance gain, up to 0.1% points, was seen by integrating a set of advanced blade tip geometries that were experimentally cross-compared using a rainbow rotor setup. The combined absolute stage performance gain was found to be 0.5% points for a 22% reduction in rotor blade count, resulting in an overall higher aerothermal performance.