As bypass-ratio in modern aero engines is continuously increasing over the last decades, the radial offset between low pressure compressor (LPC) and high pressure compressor (HPC), which needs to be overcome by the connecting s-shaped intermediate compressor duct (ICD), is getting higher. Due to performance and weight saving aspects the design of shorter and therefore more aggressive ducts has become an important research topic. In this paper an already aggressive design (with respect to current aero engines) of an ICD with integrated outlet guide vane (OGV) is used as a baseline for an aerodynamic optimization. The aim is to shorten the duct even further while maintaining it separation free. The optimization is broken down into two steps. In the first optimization-step the baseline design is shortened to a feasible extent while keeping weak aerodynamic restrictions. The resulting highly aggressive duct (intermediate design), which is shortened by 19 % in axial length with respect to the baseline, shows separation tendencies of low momentum fluid in the strut/hub region. For the second step, the length of the optimized duct design is frozen. By implementing new design features in the process of the optimizer, this optimization-step aims to eliminate separation and to reduce separation tendencies caused by the aggressive shortening. In particular, these features are: a nonaxisymmetric endwall contouring and parametrization of the strut and the OGV to allow for changes in lift and turning in both blade designs. By comparison of the three designs: Baseline, intermediate (separating flow) and final design, it can be shown, that it is possible to decrease length of the already aggressive baseline design even further, when adding a nonaxisymmetric endwall contouring and changes in blade shape of the strut and OGV. Flow separation can be eliminated while losses are kept low. With a more aggressive and therefore shorter duct the engine length and weight can be reduced. This in turn leads to lighter aircrafts, less fuel consumption and lower CO2 and NOx emissions.

As part of Clean Sky 2 (Engines ITD), a working group, formed by the DLR Institute of Propulsion Technology, MTU Aero Engines and GKN Aerospace, is setting up a complete compression system module (LPC/ICD/HPC). The aim of the project is to shorten the intermediate compressor duct (ICD), which connects the low pressure compressor (LPC) and high pressure compressor (HPC). This leads to a reduction of the axial length and weight of the overall engine and thus a reduction of the specific fuel consumption. In order to identify measurement-technologies and mitigate risks for this experimental set-up, a non-rotating testrig was set up in advance to investigate two different test vehicles. In a first step an ICD demonstrator was set up, which had a reduced axial length of 25%, compared to currently operating designs. This design predicted no separation or performance losses inside of the aggressive s-shaped contour. In a second step another demonstrator was installed with an even shorter axial length of 50% axial length reduction, compared to todays state of the art configurations. In this demonstrator the design was chosen to exceed the functional limit according to today’s CFD prediction and thus explore the potential for further length reduction in the future and to create a data base for numerical tool validation. The two configurations were tested at DLR Cologne. The test channel offers the possibility to use a variety of different measurement techniques to determine the flow behavior inside the ICD. This paper presents the oil streak pattern taken on the hub area between the struts and the ones taken on the pressure and suction side of the outlet guide vanes (OGV- LPC) for both demonstrators.