Intermediate turbine ducts (ITD) represent the flow path between the high pressure turbine (HPT) and the low pressure turbine (LPT) of a high-bypass ratio aero engine. Caused by the different rotational speeds of high and low pressure spool, these components have to diffuse and guide the flow safely to a larger diameter without disturbances or boundary layer separations. The large radial offset between in- and outlet of ITDs leads to a pronounced S-shape. The trend for further increased bypass ratios will require more attention to this component since its shape influences the overall weight of engine and nacelle considerably. The complicated aerodynamics of these ducts has to be understood to realize shorter designs. The design of ITDs with integrated flow turning aerofoils (so called TMTF or TVF configurations) has recently received a considerable amount of attention with the introduction of the geared turbo fan (GTF) engine architecture. It has been demonstrated that the first LP vane row downstream of the TVF can be completely avoided thus decreasing weight, costs and fuel burn. The GTF is one approach to realize future engines, where the ITD is characterized by smaller radial offsets, counter-rotating HP and LP shafts and faster high-speed LPTs.
Equipped with a unique test turbine facility, Graz University of Technology has been at the forefront of European research and development of turbine intermediate duct aerodynamics going back 10-15 years and has also contributed in the testing of different TMTF developments. In the aerodynamics of TVF modules the interaction with the HPT and LPT rotor is one of the major key factors for loss generation and has to be accounted for already in the design process. The TVF inlet flow is driven by the HPT flow effects including wakes, secondary flow effects and tip leakage as well as purge flows. In order to provide relevant test data to guide the TVF aerodynamic design, it is critical that engine-relevant TVF inlet and exit flow conditions are provided. Therefore the main objective of the project TRAVIATA was to execute rig tests of TVF aerodynamic designs, coupled with an upstream HPT stage and downstream LPT blade, in a flow environment representative of future GTF aero-engine applications. Since the performance of any HPT-LPT transition duct is impacted by the level of the incoming flow effects, a variation of HPT tip gap and purge flow levels was planned.
The two-spool transonic test turbine facility at TU Graz equipped with a secondary air system (SAS) was used for performing these investigations. Besides conventional measurement with rakes and pneumatic probes advanced instrumentation such as fast response pressure probes, hot-wire and optical measurement techniques as well as concentration measurements were used. In this way not only the pure component performance, such as pressure loss, was evaluated but also the unsteady three-dimensional interaction between the neighbouring components. With the help of the SAS the influence of different independent purge flows and their variations onto the TVF aerodynamics were investigated for the first time. The improved understanding of the flow effects made it possible to rise the technology readiness level from TRL4 to TRL5 and to provide the input for a Ground Test Demo (TRL6) within the Clean Sky 2 programme.
