Boundary Layer Ingestion (BLI) could reduce aircraft fuel burn by up to 15% but requires robust fan systems that can operate efficiently with high levels of inlet distortion.
This research aims to understand the detailed aerodynamics of boundary layer ingesting fans and to develop designs of distortion tolerant fans.
The approach involves high resolution steady and unsteady measurements within a low-speed fan rig combined with unsteady full-annulus CFD of complete fan system designs operating within inlet distortion representative of BLI.
Upstream of the fan rotor a three-dimensional flow redistribution is observed, leading to an attenuation of the axial velocity distortion and to non-uniform swirl and radial angle distributions at rotor inlet. This inlet flow field creates circumferential and radial variations in rotor diffusion factor with a corresponding loss variation around the annulus. Additional loss is generated by an unsteady separation of the casing boundary layer, caused by a localised peak in loading at the rotor tip. Non-uniform swirl and radial angles at rotor exit lead to increased loss in the stator due to variations in profile loss and corner separation size. These variations around the annulus are shown in figure 3.
upper end of BLI potential fuel savings
Overall, BLI has been found to reduce the fan stage efficiency by only 1-2% relative to operation with uniform inlet flow. In addition, the impact of BLI on fan stability has been found to be small.