The purpose of the paper is to characterize the aerodynamic behavior of a rotor-downstream hub cavity rim seal in a high-pressure turbine (HPT) stage. The experimental data are acquired in the Transonic Test Turbine Facility at the Graz University of Technology: the test setup includes two engine-representative turbine stages (the last HPT stage and first LPT stage), with the intermediate turbine duct in between. All stator-rotor cavities are supplied with purge flows by a secondary air system, which simulates the bleeding air from the compressor stages of the real engine. The HPT downstream hub cavity is provided with wall taps and pitot tubes at different radial and circumferential locations, which allows the performance of steady pressure and seed gas concentration measurements for different purge mass flows and HPT vanes clocking positions. Moreover, miniaturized pressure transducers are adopted to evaluate the unsteady pressure distribution, and an oil flow visualization is performed to retrieve additional information on the wheel space structures. The annulus pressure asymmetry depends on the HPT vane clocking, but this is shown to have negligible impact on the minimum purge mass flow required to seal the cavity. However, the hub pressure profile drives the distribution of the cavity egress in the turbine channel. The unsteady pressure field is dominated by blade-synchronous oscillations. No non-synchronous components with comparable intensity are detected.
The efficiency assessment of a high-pressure turbine (HPT) stage is complicated by the presence of upstream and downstream purge flows. In fact, the efficiency calculation is often based on mass flow-averaged values of total temperature at the stage inlet and outlet planes. Moreover, the purge flow distribution in the annulus is usually unknown and therefore assumed to be uniform. This paper presents and applies an alternative method to calculate the efficiency of a fully purged HPT stage. Such a definition relies on seed gas concentration measurements at the HPT stage outlet plane to determine the outlet purge flow distribution. After comparing the alternative method to the standard definition (based on the assumption of uniform purge) for the nominal purge case, the efficiency variation between the case with nominal purge and the case without purge is investigated.
This paper focuses on the interaction between the last high-pressure turbine (HPT) stage purge flows and the intermediate turbine duct (ITD) in modern turbofan engines. Two state-of-the-art ITD concepts are analyzed in this work: the Turbine Center Frame (TCF), which is supported by symmetric strut fairings and generally adopted in conventional dual-spool engines; the Turbine Vane Frame (TVF), which features turning struts and splitters and is typical of geared turbofan engines. The measurement campaigns for both setups are carried out in the Transonic Test Turbine Facility (TTTF) at Graz University of Technology. The test vehicles consist of an HPT stage, the ITD (TCF or TVF) and the first LPT vane or blade row. All the HPT stator-rotor cavities are supplied with purge flows by a secondary air system, with independent mass flow and temperature control for each purge line. Five-hole probe data are acquired at the inlet and outlet sections of the ITDs, to characterize the aerodynamic flow field entering and leaving the duct. Seed gas concentration measurements are performed in the same planes, to track down the cavity air in the main stream and investigate its post-egress behavior. Finally, detailed post-test CFD results are presented to get additional insight into the flow phenomena developing through the strut passage.
This paper focuses on the interaction between the last high-pressure turbine (HPT) stage purge flows and the intermediate turbine duct (ITD) in modern turbofan engines. Two state-of-the-art ITD concepts are analyzed in this work: the Turbine Center Frame (TCF), which is supported by symmetric aerodynamic strut fairings and generally adopted in conventional dual-spool engines; the Turbine Vane Frame (TVF), which features turning struts and splitters and is typical of geared turbofan engines. The measurement campaigns for both setups are carried out in the Transonic Test Turbine Facility (TTTF) at Graz University of Technology. The test vehicles consist of an HPT stage, the ITD (TCF or TVF) and the first LPT vane or blade row. The same HPT stage is used for both ducts, to enable consistent, engine-representative inlet conditions between the two solutions. All the HPT stator-rotor cavities are supplied with purge flows by a secondary air system, with independent mass flow and temperature control for each purge line. Five-hole probe data are acquired at the inlet and outlet sections of the ITDs, to characterize the aerodynamic flow field entering and leaving the duct. In addition to the pneumatic probe tests, seed gas concentration measurements are performed in the same planes, to track down the cavity air in the main stream and investigate its post-egress behavior. Finally, detailed post-test CFD results are presented to get additional insight into the flow phenomena developing through the strut passage. The concentration effectiveness field at the inlet of the ducts shows the same characteristics in both configurations: the upstream purge flows are entrained in the HPT rotor secondary flows, leading to high-concentration spots that influence large portions of the channel. On the other side, the downstream purge air is confined into a thin concentration boundary layer in close proximity to the endwalls. The thickness of this boundary layer is affected by the circumferential pressure distribution from the HPT vanes and struts. At the TCF and TVF outlet, the upstream purge forms a circumferentially uninterrupted band, shaped by the secondary vortices evolving through the duct. The downstream purge interaction with such vortices leads to the formation of well-bounded lobes, whose size, count, and position are inherently related to the secondary structures and thus differ significantly between the two cases.
This paper investigates and compares the aerodynamics of two state-of-the-art configurations for the intermediate turbine duct (ITD) in a turbofan engine: the turbine center frame (TCF), which is typical of conventional dual-spool engines and features symmetric aerodynamic strut fairings, and the turbine vane frame (TVF), which integrates a set of turning struts and splitters directly in the duct, thus enabling length and weight benefits at engine system level. The measurement data utilized for the analysis are a product of almost ten years of research at Graz University of Technology, involving multiple test campaigns with either TCF or TVF setups at consistent inlet conditions. The experimental tests are carried out in the Transonic Test Turbine Facility at the Institute of Thermal Turbomachinery and Machine Dynamics (Graz University of Technology). All test vehicles include not only the ITD (TCF or TVF), but also the last High-Pressure Turbine (HPT) stage and the first Low-Pressure Turbine (LPT) vane or blade row, in order to ensure engine-representative conditions at the duct inlet and outlet sections. For the same purpose, the test facility supplies all the stator-rotor cavities with purge air, with independent control of temperature and mass flows for each cavity. The measurements are performed with pneumatic probes (five-hole probes, Kiel-head rakes) at the inlet and outlet of the ITDs, for three different HPT purge flow rates. The aerodynamic comparison between TCF and TVF setups is based on three key topics: duct inlet and outlet flow fields, duct total pressure losses and duct aerodynamic excitation on the LPT rotor blades. For each one of them, the sensitivity to HPT purge variation in both configurations is evaluated. Due to the presence of turning struts and splitters inside the ITD, the TVF shows a more complex outlet flow field than the TCF, characterized by the interaction of HPT and TVF secondary phenomena. The TVF total pressure loss is less sensitive to purge variation compared to an advanced TCF design with high casing slope. While the weaker TVF loss derivative to HPT purge may provide off-design operating point benefits relative to a TCF-based engine, the increased level of flow nonuniformity at the TVF exit, distributed over a wider range of engine orders, represents a design challenge for the first-stage LPT rotor.
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