Behavior of flow distortion within a boundary layer ingestion inlet

One of the critical problems restricting the development of boundary layer ingestion (BLI) propulsion systems is the strong distortion and swirling flow in the inlet, which will threaten the stable operation of the fan. In this study, the effects of the Exit Mach number (M-AIP), the ingested boundary layer thickness (delta) on the internal flow and flow distortion at the aerodynamic interface plane (AIP), are investigated under a freestream Mach number of 0.75. The experimental results indicate that a low total pressure region persistently located at the lower part of the AIP accompanied by a pair of vortices with a maximum swirl angle of 23 degrees With the decrease of exit Mach number, these vortices move toward the center of the AIP, which decreases the total pressure distortion while increases the swirl intensity. However, as the ratio of the ingested boundary layer thickness to the inlet height (delta/h) increases from 18.6 % to 83.8 %, the length-scale of these vortices at AIP increases, leading to a descending trend of the total-pressure recovery coefficient from 0.942 to 0.871. The total pressure distortion gets intensified from 0.34 to 0.42, with the mean swirl intensity (SImean) dropping from 8 degrees to 6.7 degrees The influence of the delta/h on SImean could be attributed to the enhancement of SI in the outer ring of AIP. Meanwhile, the validated numerical results of delta/h = 83.8 % and delta/h = 43.3 % show that the vortices at AIP are originated from the coalescence of two vortices. M-AIP has a substantial influence on the origin of the vortex, separation positions, and pressure distributions of the inlet. The flow separation position is associated with total pressure distortion at AIP. Further optimization design can be considered by controlling the flow separation position.