Aerodynamic Analysis of a Fan for Future Ultra-High-Bypass-Ratio Aeroengines

Kurzfassung

A model scaled fan being representative for a propulsor of a future high bypass-ratio aeroengine was designed, tested and aerodynamically investigated. The focus of the experimental and theoretical work was mainly to evaluate the fan performance with respect to the changing requirements of modern fans being typical for engines with a bypass ratio in the order of 12 or higher. The fan design work, also carried out at DLR, comprised aeromechanical aspects of the bladed disk rotor and the fully 3D aerodynamic fan design. Besides three dimensional design features of the rotor and stator blades also technological considerations were taken into account during the fan design. In this context the tip circumferential velocity was limited (compared to a conventional state-of-the art design) in order to avoid the evolvement of strong shocks in the rotor tip region. This is expected to drastically reduce the noise emission of this fan and was one of the main design goals. A typical near tip sweep or a so called S-shape leading edge of the rotor blade was found as the best compromise to reach high efficiency, 10 % higher specific mass flow rate and reasonable stall margin. A 1:3 scaled rig was built and experimentally investigated at the Multistage Two Shaft compressor test facility M2VP at DLR cologne (Figure 1, left). The measurements presented in this paper mainly comprised probe measurements upstream of the rotor and downstream of the OGV in order to measure fan performance in terms of efficiency and total pressure ratio. The experimental data, covering the complete potential flight envelope, fully confirmed the design goal in all aspects such as the maximum achieved pressure ratio and corresponding massflow (Figure 1, right). Apart from the experimental effort detailed numerical studies were carried out during all phases of the project. The results in Figure 1 (right) show the comparison of the pre-test computations with the numerical setup used during the design iterations as well as the results of post-test predictions with a much finer computational grid and boundary conditions derived from the experimental data. A detailed analysis of the aerodynamic flow features, also in comparison with existing experimental data at selected operating conditions (near stall, peak efficiency and choked) will be presented and discussed in the paper. This comparison includes global performance data as shown in Figure 1 as well as results from the probe measurements (mainly total pressure and temperature) upstream and downstream of the fan stage (see an example in Fig. 2). The analysis of the fine grid computations will focus on two different speedlines in the stage performance map, one at 100% mechanical speed and one at 43% speed, which is representative for an approach condition of the corresponding engine. Ongoing investigations, results of which will partly be included into the final version of the paper, are focussing on aeroacoustic experimental analysis as well as stability enhancement using active flow control, such as flow injection in the rotor region.