A loss reduction design strategy for a ducted fan under boundary layer ingestion condition based on a lip bump

Abstract

A distributed fan propulsion system can potentially achieve a 10%-20% fuel burn reduction compared with traditional podded engines. However, the ducted fan is continuously working under inflow distortion conditions caused by boundary layer ingestion, thus resulting in an increase of aerodynamic loss and decreasing the aircraft overall aerodynamic benefit achievement. To control the loss in the boundary layer ingestion ducted fan, a loss reduction design strategy based on the lip-installed bump is developed. The computational results show that the lip-installed bump can effectively suppress the intensity of the distorted inflow with a 9.7% reduction at the rotor inlet. The reason is that a high-pressure area will be presented on the mount of bump and a low-pressure on both sides of the bump compression surface which pushes the low-momentum flow in the boundary layer away from lip. In this way, the bump energizes the low-momentum flow on the downer wall of the lip and reduces the local boundary layer momentum loss thickness. In the meanwhile, as the distortion intensity is decreased near the downer wall at rotor inlet, the local blade load is also reduced, especially near the blade tip. Further, the decrease of the blade load near the tip leads to a reduction of the normal pressure difference between the pressure and suction surfaces which can eliminate the interaction of the main flow and the tip leakage flow, resulting in a 6% decrease of flow loss in the rotor blade tip region. In comparison to the baseline ducted fan, the fan with a lip-installed bump has achieved a 1.3% enhancement of adiabatic efficiency at the re-design point under inflow distortion condition, without a sacrifice of total pressure ratio.