Design Optimization of a rear Fuselage Fan for different Boundary Layer Profiles

Kurzfassung

With regard to the aircraft propulsion system, the current industrial approach for reducing flight emissions is to continuously improve the efficiency of aircraft engine components. However, to achieve further progress in the future, alternative propulsive concepts should be considered. One possible concept is the additional implementation of a boundary layer ingesting and turbo-electric powered engine at the rear fuselage of the aircraft, a so-called aft fan. Due to a reduced inlet momentum at the engine inlet face and thus, less necessary power input for thrust production, the fuel consumption and hence the CO2 emissions can be reduced. This paper shows the effects of imposing a radial boundary layer stemming from the aircraft fuselage, on a conventional clean inflow design of the aft fan. Therefore, the rear fuselage geometry of a conventional single aisle medium range aircraft configuration was adapted in preliminary studies to design and integrate a turbo-electric powered fan stage. In a first approach the fan annulus geometry was designed using throughflow and CFD calculations. The annulus is designed to introduce contraction in the areas of rotor and OGV blades and to achieve the necessary amount of flow diffusion up- and downstream of the blades. This was necessary to reduce the hub radius to its targeted value. Next, the fan stage was optimized with 3D-RANS calculations for clean inflow conditions, but with averaged inlet boundary conditions as in case of boundary layer ingestion (BLI). This design builds the reference case in terms of the maximal possible fan performance in BLI case. For investigating the differences between averaged and radial boundary inlet conditions, aerodynamic data from outer aerodynamics CFD calculations are used. Therefore, the whole aircraft was simulated with 3D-RANS calculations using a Body Force model in the fan stage. Within the paper there are two comparisons shown: On the one hand the reference case is compared to the results from RANS calculations with 1D circumferential averaged engine front face inlet boundary conditions. On the other hand, the reference case is compared to results from several 1D boundary conditions at different circumferential engine front face positions. Both comparisons show compromised fan stage performance by applying a radial boundary layer to the clean inflow design. Especially the annulus design leads to flow separations at the hub areas. Upcoming investigations include results from an automated optimization of the annulus and blade design for the circumferential averaged 1D inlet boundary conditions. Further optimization of the aircraft fuselage inlet area and the nozzle outlet area take place in cooperation with Airbus and the DLR Institute of Aerodynamics & Flow Technologies to handle the distorted boundary layer conditions from an external and internal aerodynamics perspective.