The Interaction of Main Stream Flow and Cavity Flows in Turbine Center Frames and Turbine Vane Frames

Abstract

Abstract 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.