Numerical study on the Re effects on the tip flow structures of transonic fan under inlet distortion

Abstract

Compared to conventional Pylon-mounted engines, a 5-15% reduction in aircraft fuel consumption is possible with the use of Boundary Layer Ingestion (BLI) propulsion system. However, this type of propulsion system uses a fan that has to operate under continuous inlet distortion and over a wide range of altitude within the flight envelope, which forces researchers to consider the Re effects on the BLI distorted fan stall. For typical spike-type stall, tip leakage flow (TLF) contributes crucially to its generation. Thus, this paper aims to investigate the Re effects on the tip unsteady flow structures under the circumferential distorted inflow near the stall point. In this study, a full-annulus 3D steady/unsteady CFD calculation has been employed to calculate the performance of the NASA 67 transonic fan stage with a 60-deg circumferential distortion at different Re. The results show that the Re effects change the location of peak amplitude of blade tip unsteady flow, without altering the frequency properties. For the further study of the Re effects on the tip unsteady flow, time series analyses of detailed flow structure features and tip leakage vortex (TLV) evolution are performed. At low Re, the angle between the TLF and the axis increases from 77 to 81 deg by comparing the shock structure at 0.99 span, and the secondary flow is enhanced with a marked tendency of blade leading edge spillage. The results indicate that TLV has greater size and strength for low Re condition, and the TLV is more pronounced in the distortion sector than it in the clean sector. Further analysis reveals that the change of shock structure affects the periodic variation in rotor differential pressure on the both surfaces, enhancing the circumferential momentum of TLF near the leading edge, which is the major contributor for changing the vortex structure and causing the stall advance in low Re condition.