Aerodynamic Assessment of Turbine Center Frames and Turbine Vane Frames Under the Influence of Purge Flows

Abstract

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.

Aerodynamics and Sealing Performance of the Downstream Hub Rim Seal in a High-Pressure Turbine Stage

“…While the investigated radial positions show comparable amplitudes at the 𝐵𝑃𝐹, the outermost sensor (𝑥 = 1, outside of the stator seal lip) always measures the highest magnitude at the 𝐵𝑃𝐹 multiples. A minor peak is spotted at 𝑓 ≈ 0.63 ⋅ 𝐵𝑃𝐹, which corresponds to a linear combination of the HPT and LPT blade passing frequencies: Merli et al [34] also found…”

“…While the investigated radial positions show comparable amplitudes at the BPF, the outermost sensor (x = 1, outside of the stator seal lip) always measures the highest magnitude at the BPF multiples. A minor peak is spotted at f ≈ 0.63 • BPF, which corresponds to a linear combination of the HPT and LPT blade passing frequencies: Merli et al [34] also found pressure fluctuations with the mentioned periodicity in the annulus, upstream of the TVF struts. No non-synchronous components, which could be associated with rotating flow modes [18], are detected with the current measurement setup, at least not in the same order of magnitude as the BPF-related components.…”

Performance Evaluation in a Fully Purged High-Pressure Turbine Stage Using Seed Gas Concentration Measurements

“…There are very few works available involving more or different cavities than the upstream hub one; for instance, Zlatinov et al [8] numerically studied the effect of the upstream shroud purge injection, concluding that it was partially recovering the associated losses by inhibiting the tip leakage flow. Also, Merli et al [9] experimentally investigated the total pressure loss across intermediate turbine ducts, varying the injection rates in a fully purged HPT stage. A thorough review of the open literature yielded no publications regarding the performance of a fully purged HPT stage.…”