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Intermediate Compressor Case Duct Aerodynamics

Periodic Reporting for period 1 - IDA (Intermediate Compressor Case Duct Aerodynamics)

Reporting period: 2018-04-01 to 2019-09-30

Climate change is a global problem. The Advisory Council for Aviation Research and innovation in Europe (ACARE) has set out stringent targets in its Flight Path 2050 document to reduce the environmental impact of air travel. Key technologies must be developed including innovative new geared turbofan engines with very-high bypass ratios and ultra-high-pressure ratios. This will increase the engine efficiency reducing fuel-burn and CO2 emissions. However, these new engines such as the Rolls-Royce UltraFan provide significant design challenges. To maximise efficiency the compression system is generally split into two or three parts. These low, intermediate and high-pressure compressors are mounted on separate shafts (or spools) allowing each to rotate at an optimum speed and hence deliver an optimal work balance. The diameter of each spool must reduce as the air density increases through the compression system and s-shaped ducts are used connect the three spools. The design of these ducts is challenging due to their conflicting aerodynamic and structural requirements. Reducing length by improving integration within the compression system will lead to weight and fuel savings thereby reduced emissions. However, the fundamental aerodynamic function of the ducts must not be compromised, and flow separation must be avoided. The IDA (Intermediate Compressor Case Duct Aerodynamics) project is focussed on improving the understanding of the aerodynamics of these compressor transition ducts with respect to future engine configurations. Through this the IDA project will deliver an experimentally validated duct design for the UltraFan demonstrator engine, advanced, accurate and efficient numerical design tools and finally, future more integrated duct technologies for very high bypass ratio and geared turbofans.
The work carried out in the first 18 months of IDA is separated into two work packages addressing, respectively, experimental and computational activities. For the experimental work, carried out by Loughborough University (UK), an existing low-speed test facility has been modified to add a new single stage axial compressor and compressor transition duct representative of the UltraFan demonstrator engine. The system aerodynamics were extensively studied by taking measurements through the compressor and duct using miniature pneumatic five-hole probes and constant temperature anemometry. Importantly the data show that the aerodynamic design is valid. Subsequently, an engine representative bleed flow was added between the compressor and duct. Experimental data provided a better understanding of system interaction highlighting the acceptable limits of bleed. This information has been fed back to the design team and will be used to inform/design a second test configuration. In the computational work, carried out at Chalmers University of Technology (Sweden), the goal is to develop advanced, accurate and efficient numerical design tools for compressor transition ducts. Hence, time has been spent exploring advanced CFD (Computational Fluid Dynamic) approaches. This has included examining parameters such as the turbulence model, mesh strategy, boundary conditions, and interfaces between stationary and rotating components. An initial computational study had been performed which examines the Loughborough University experiment using commercial CFD solver, ANSYS CFX. Initially both steady and unsteady RANS (Reynolds Averaged Navier Stokes) predictions were made using a k−ω SST two equation turbulence model. This was then extended to a hybrid, scale-resolving approach using a SBES (Stress Blended Eddy Simulation). The k−ω SST was again used for the RANS regions whereas for the in the LES (Large Eddy Simulations) the sub-grid-scale turbulence was modelled using the WALE algebraic model. The predicted data from the three CFD simulations showed good agreement in the time-averaged flow with the uRANS and SBES each giving more information on the time-resolved flow. At the current time these results are being compared to the Loughborough University experiments.
In line with the project goals the IDA project, thus far, has aerodynamically validated the compressor duct design for the UltraFan demonstrator engine. This is a new compressor duct configuration for a new engine architecture and therefore represents progress against current technologies. The inclusion of a system bleed flow in an experiment, for the first time, has also provided new understanding which will be used to refine the engine design as the project progresses. The unique experimental data will be used to validate the more advanced, accurate and efficient numerical design tools being developed as part of IDA. Ultimately, knowledge, tools developed in IDA will used in the development of the next generation of low-emission turbofans in a more-timely and cost-effective manner. IDA will therefore contribute to enabling the EU aerospace industry to meet its commitment to Flightpath 2050 as part of the ongoing Clean Sky 2 initiative. For example, the goal of the UltraFan engine is a minimum 25% fuel burn improvement, a minimum reduction of 25% in CO2 and a significant reduction in NOx and other emissions such as smoke particulates.
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