Studies show that Boundary Layer Ingesting (BLI) propulsion systems can provide significant fuel burn reduction relative to pylon-mounted turbofan engines. However, together with great potential, this type of propulsion can lead to serious difficulties and engineering challenges. A low-speed fan has been designed for a uniform flow at the inlet using an open source design code with optimization. Numerical analyses of the fan with a uniform flow at the inlet, and one exposed to the distorted non-axisymmetric flow were performed. These required the use of full-annulus unsteady time-marching CFD model. The model was validated with experimental data obtained in a test rig which was designed and built specially for that research.
The first crucial negative effect of the distorted inlet velocity profile is a variable incidence angle of the rotor. Both the CFD calculations and the experiment show the axial velocity non-uniformity and induced non-uniform swirl in front of the rotor, due to boundary layer ingestion and flow redistribution. As a result, significant changes in the angle of attack of the blades and corresponding unsteady rotor loads are observed. The simulations and measurements were performed for different operating conditions to obtain performance curves and assess the BLI influence at the design point and beyond. This demonstrated the second problem arising from the operation of the fan with the distorted flow at the inlet, which is a reduction of the stall margin (48% reduction from experimental results). The whole stage was previously optimized for axisymmetric flow in order to simultaneously ensure a required value of stall margin and the highest possible efficiency at the design point. The experimental efficiency of the rig at the design point with distorted inlet was 2.05% lower than the undistorted inlet. The CFD model show a 1.25% reduction. Similar reductions are shown for other operating points.