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Clean Sky's UHPE demonstrator progressing at full throttle

To reach the ACARE environmental objectives and align with the European Union’s Green Deal goals, aircraft engines need to operate with more propulsive efficiency, cutting fuel burn and reducing CO2 and NOx emissions. Engines with high by-pass ratios have become a mainstream paradigm to increase efficiency, but with the prospect of the next generation of aero-engines on the horizon there's an opportunity to introduce further efficiencies with a new kind of engine architecture.

LP Turbine Rig @ SAFRAN
LP Turbine Rig @ SAFRAN

To reach that objective Safran Aircraft Engines is developing a number of technologies within Clean Sky 2 for the engine known as an Ultra High Propulsive Efficiency (UHPE), alongside a next generation of low pressure turbines. Ultra High Propulsive Efficiency (UHPE) engine is one of the major demonstrators within Clean Sky's Engines Integrated Technology Demonstrators (ITDs).

The UHPE is a geared engine which means it's possible to vary the rotational speed of the turbine. The final architecture will be selected during 2022. This presents the opportunity to increase the turbine speed and therefore perform the same function with fewer stages, making it possible to build a lighter and shorter turbine. Part of the technical strategy to achieve this is to optimise the engine by combining the functions of the turbine centre frame (TCF) and the first nozzle into a turbine vane frame (TVF). This leads to a highly loaded turbine with increased rotational speeds. 

The bottom line: more miles with less fuel. It's a win-win for lowering aviation's environmental impact, while positioning European aeronautics advantageously in the fiercely contested global aero-engine market.

Transitioning innovation off the drawing board

It's essential to confirm what works by testing on demonstrators in line with the ethos of Clean Sky, to transition innovation off the drawing board in readiness for exploitation to meet Europe's societal needs and to provide an industrial advantage. 

As of 2020 a stack of achievements has been accomplished in this endeavour. These include a series of experimental test campaigns which elicited insights into the specificities of high speed turbines in order to prepare the design for ground testing. 

‘At Safran Engines we are preparing a ground test demo engine within Clean Sky 2, and to achieve these results, we have a technology maturation plan to prepare the key enabling technologies identified for the ground test demo,’ says Christophe Diette, Clean Sky Programme Manager at Safran.

‘The purpose of this technology maturation is to decrease the technical risks on the ground test demo engine and prepare for potential next generation technologies that will be needed for future improvements,’ he adds.

The purpose of this technology maturation is to decrease the technical risks on the ground test demo engine and prepare for potential next generation technologies that will be needed for future improvements

Part of this technology maturation process focuses on the low pressure turbine and the propulsive system, which includes the fan and the transmission. And because the turbine is quite different from today's engine architectures, there are some thermo-mechanical as well as aerodynamic challenges. 

‘The turbine blade and its interaction with the high pressure turbine and low pressure turbine is quite new and we need to study it in detail – in fact, the whole aerodynamics of the turbine is quite different due to the higher pressure changes at each stage and higher Mach numbers,’ says Paul Labuttie, Module Manager for the UHPE high speed low pressure turbine maturation for UHPE Ground Test Demo project at Safran.

A lot of component tests have been underway with partners within the Clean Sky network to improve comprehension of the phenomenon in the high speed turbine.

‘We already finished several cascade campaigns which we performed with the help of the Von Karman Institute for Fluid Dynamics (VKI) in Belgium to understand the high speed turbine aerodynamics at the fundamental level,’ says Labuttie.  

Also, in collaboration with the Von Karman Institute plus COMOTI (Research Institute on Gas Turbines in Romania) and in the context of Clean Sky 2, Safran is preparing several test campaigns to understand the flow interactions at several locations within the engine. 

These component tests are complemented by turbine module tests performed at SafranAE premises, in Villaroche, to help confirm the performance of the turbine.

SafranAE evaluated the performance of a turbine vane frame (TVF) with a 2-stage high pressure turbine (HPT)+TVF test rig at high speed low pressure turbine (HSLPT) engine representative conditions. Fine airstream mapping and turbulence were measured upstream and downstream of the TVF, and good correlation was found with the CFD predictions.

Subsequently, SafranAE has designed, manufactured and put a TVF + 3-stage HSLPT rig on the test bench. The test campaign started mid-2020 and is still ongoing. The rig is heavily instrumented with representative engine conditions, making it possible to perform a comprehensive exploration of the turbine map. 

A second build is scheduled to be tested within Clean Sky 2 to evaluate geometries updates.  

In 2021 there will be a full turbine test campaign during which time the clearances control tests will be completed with the University of Florence. Safran is also preparing for upcoming GAPs (Clean Sky 2 “partner” grants) with the Von Karman Institute, expected in 2022.

Two complementary projects have supported the UHPE demonstrator engine: VOLTAYRE which focuses on the design and manufacture of an innovative high flow direct drive oil valve, and InVIGO, which looks at intake vortex ingestion during aircraft ground manoeuvres.