Nonaxisymmetric Stator Design for Boundary Layer Ingesting Fans

Abstract

In a boundary layer ingesting (BLI) fan system, the inlet flow field is highly nonuniform. In this environment, an axisymmetric stator design suffers from a nonuniform distribution of hub separations, increased wake thicknesses, and casing losses. These additional loss sources can be reduced using a nonaxisymmetric design that is tuned to the radial and circumferential flow variations at exit from the rotor. In this paper, a nonaxisymmetric design approach is described for the stator of a low-speed BLI fan. First, sectional design changes are applied at each radial and circumferential location. Next, this approach is combined with the application of nonaxisymmetric lean. The designs were tested computationally using full-annulus unsteady computational fluid dynamics (CFD) of the complete fan stage with a representative inlet distortion. The final design has also been manufactured and tested experimentally. The results show that a 2D sectional approach can be applied nonaxisymmetrically to reduce incidence and diffusion factor at each location. This leads to reduced loss, particularly at the casing and midspan, but it does not eliminate the hub separations that are present within highly distorted regions of the annulus. These are relieved by nonaxisymmetric lean where the pressure surface is inclined toward the hub. For the final design, the loss in the stator blades operating with BLI was measured to be 10% lower than that for the original stator design operating with undistorted inflow. Overall, the results demonstrate that the nonaxisymmetric design has the potential to eliminate any additional loss in a BLI fan stator caused by the nonuniform ingested flow field.