Aerodynamic Comparison Between Circumferential and Wing-Embedded Inlet Distortion for an Ultra-High Bypass Ratio Fan Stage

Abstract

Abstract Future aircraft design concepts often show a somewhat wing embedded ultra-high bypass ratio engine. The aircraft concept of the Coordinated Research Centre 880 (CRC880) is a single-aisle configuration with engines partly integrated over the aircraft wing. The aircraft is designed to take off and land on regional airfields with low noise and fuel emissions to address the guidelines set by the ACARE. As a result of the engine installation, the inlet induces a non-axisymmetric boundary layer ingestion into the fan stage. In experimental setups, inlet distortion has often been seen as a 60-degree circumferential inlet stagnation pressure distortion. However, the fan stage inlet flow of the prescribed engine installation of the CRC880 differs to a great extent from a 60-degree sector. In this paper, an aerodynamic comparison between a realistic inflow situation and a similar 60-degree inlet distortion for the same ultra-high bypass ratio fan stage is given. The realistic inflow situation is a result of the flow moving over the aircraft wing suction side and entering the nacelle. As non-axisymmetric inlet geometry remains the same for both cases, therefore, only the total pressure boundary condition at nacelle inlet was changed between both cases. Hence, full annulus simulations are required. Both inlet distortion cases are equivalent by matching average 60-degree distortion coefficient. This study points out that the method, by using averaged 60-degree segment values, excludes specific inflow characteristics. For the same averaged 60-degree distortion coefficient, the local distortion of the embedded case is up to four times larger at rotor tip compared to the segmental approach. For constant mass flow, fan pressure ratio and isentropic efficiency drop by more than five and eight percent respectively. At peak efficiency operating condition, the decrease is even more significant with more than nine percent in stage efficiency. For future embedded aircraft engine configurations, the fan designer has to take into account the non-axisymmetric local flow changes. Hence, the 60-degree segment method is not sufficient when investigating experimental boundary layer ingesting fans and therefore, further method developments are necessary.