Numerical Modeling and Design Challenges of Boundary Layer Ingesting Fans

Abstract

Studies show that boundary layer ingesting (BLI) propulsion can provide significant fuel burn reduction relative to pylon-mounted turbofan engines. However, this type of propulsion can lead to serious difficulties and engineering challenges. Numerical analyses of the fan with a uniform flow at the inlet and that exposed to the distorted flow were performed. These required the use of a full-annulus unsteady time-marching computational fluid dynamics (CFD) model, which was validated with experimental data obtained at a test rig. The first negative effect of the distorted inlet velocity profile is a variable incidence angle of the rotor. Both the calculations and the experiment show the nonuniform axial and tangential velocity profiles in front of the rotor. As a result, significant changes in the angle of attack of the blades and corresponding unsteady rotor loads are observed. The research was performed for different operating conditions to obtain performance curves and assess the BLI influence. 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 in the experiment). The whole stage was optimized for the axisymmetric flow to 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 the distorted inlet was 2.05% lower than the undistorted one. The CFD model shows a 1.25% reduction. Similar reductions were obtained for other operating points.