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Turbine Research for Aerodynamical Vane-frame Improvements in Advanced Two-spool Arrangements

Periodic Reporting for period 3 - TRAVIATA (Turbine Research for Aerodynamical Vane-frame Improvements in Advanced Two-spool Arrangements)

Reporting period: 2021-03-01 to 2021-09-30

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.
The first part of TRAVIATA was used to perform all the work which was necessary to prepare for the TVF test campaign. Also the operating conditions were chosen in close cooperation with GE in order to ensure save rig operation with avoiding any risks. To guarantee this it was found that a new suction blower in the exhaust line of the test facility was needed. TU Graz was in charge of the selection, the procurement and the implementation of this device. A radial blower with a power of 560 kW has been selected. To be able to measure the power of the downstream LPT very precisely it was also necessary to equip the test turbine with a new torquemeter flange between LPT rotor disc and LPT shaft. Only in this way the tare losses of bearings and seals are not included in the measured torque. The new torquemeter was specially developed together with a new shaft using a preloaded bearing arrangement and better seals to provide high precision together with enough stiffness to ensure save rig operation in terms of rotordynamics.

TU Graz was also in charge of all the other test preparations including assembly, instrumentation and commissioning. After that a prolonged seven-month testing period of the TVF setup took place. The experimental campaign uses as planned conventional but also additional new advanced measurements techniques such as a Two-Sensor-FRAPP and concentration measurements with seed gas in order to track the path of purge air through the duct and study their interaction with other components. Therefore, it has to be mentioned that for the application of the concentration measurement technique the rig was also adapted in terms of seed gas insertion and the test hardware equipped with arrays of surface taps.

The measured test data has been shared with GE and analysed in detail in regular team meetings. GE leveraged the measured dataset to compare against the design intent of the TVF and LPT test vehicle. Comparisons were made with measured total pressure profiles at the TVF inlet and exit, with the TVF exit contours and also for TVF static pressure profiles, consistently showing good agreement between measured data and CFD results. This rig test successfully validated the design intent, one of the most important objectives of the entire project. The gained measurement results and the application of new measurement techniques and data analysis methods were the basis of several conference and journal publications. The dissemination of the outcomes is not yet finished, further articles are planned for upcoming turbomachinery conferences as well as for scientific journals.
Within the Clean Sky 2 program, significant steps towards the reductions in fuel burn, emissions, and perceived noise are targeted for the aviation sector (75% reduction in CO2 emissions, a 90% reduction in NOx emissions and a 65% reduction of the noise compared to a reference of 2000). The Ultra High Propulsive Efficiency (UHPE) engine concept requires the development of new technologies for geared aero engines planned to be introduced into the market around 2025. TRAVIATA was able to make valuable contributions to this goals.

The project enabled TU Graz to continue the research work on intermediate turbine structures. In combination with already executed projects it has improved the expertise of TU Graz and strengthend the position as valuable strategic partner for the aeronautical industry.
Two PhD positions were provided over the whole scheduled timeline. One PhD thesis has already been finished, the second one is well on the way. Therefore not only the topic manager GE and TU Graz benefit from Clean Sky 2. Also the community will gain general new insights through these doctoral theses and the related publications from the project TRAVIATA. This will support TU Graz known as attractive employer for scientific personnel such as post-docs and PhD-students also in the future.

TRAVIATA was part of the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation program under grant agreement No 785313.
Logo of the Project Traviata
New LP Shaft with Torquemeter
Inspection of Rotor Assembly
Laser-Sintered Kiel-Head Rake
Test 2 Setup Mounted in the Test Facility