Preliminary Aerodynamic Design and Assessment of Boundary Layer Ingesting Fans

Abstract

Boundary Layer Ingestion (BLI) is a promising technology for reducing fuel burn and environmental impact in civil aviation. With BLI, the fan is exposed to circumferentially and radially varying inflow conditions. The consideration of BLI during preliminary aerodynamic design is challenging as the calculation of BLI flow is either very time consuming or the methods provide simplified performance data instead of detailed information about the flow. Therefore, both types of approaches are not suitable for the application within the preliminary aerodynamic design. In this thesis a preliminary design and assessment methodology for boundary layer ingesting fans is developed. Within the methodology, a Reynolds-averaged Navier-Stokes (RANS) approach and a streamline curvature approach are coupled. The former is dedicated to the calculation of flow redistribution. The latter is extended by a stream tube contraction model and accounts for the local fan operation around the circumference. During the development of the methodology, the flow redistribution and local performance data are comprehensively verified by unsteady RANS (uRANS) calculations, the unsteady effects related to work input and loss generation are discussed and the limitations of the methodology are presented. Overall, the methodology is able to reduce the numerical effort for a BLI assessment from 10000 CPUh for state-of-the-art uRANS calculations to approximately 3 CPUh. The methodology is then applied to two different integration scenarios. Fuselage embedded turbofans are dealing with a once per revolution distortion. In this integration scenario, three spanwise fan pressure ratio distributions are investigated with the preliminary design methodology and the data is again verified by uRANS calculations. In a subsequent step, 32 individual fans differing in meridional Mach number and blade tip speed are evaluated. Based on the findings of this study, distortion tolerant fuselage embedded fans have a higher fan pressure ratio than conventional fans, the spanwise fan pressure distribution requires a reasonably high fan pressure ratio within the distortion to re-energise the low momentum fluid, and the tip section needs to be slightly unloaded to avoid high tip gap related losses. The second integration scenario is an aft-propulsor that ingests boundary layer fluid over the entire circumference of a fuselage. The propulsor has a descending spanwise fan pressure ratio to mitigate distortion. The new methodology is again used to assess the fan stage at a preliminary design level and the performance is verified by uRANS calculations. The results demonstrate the applicability of the methodology to this integration scenario. In a subsequent step, the effect of superimposed swirl distortion is investigated. The distortion tolerant fan shows only slightly lower fan efficiencies due to swirl distortion. The (corrected) mass flow and the fan pressure ratio, as well as the fans ability to attenuate the distortion, are mainly unaffected by the swirl distortion. Therefore, it is sufficient to consider only the stagnation pressure distortion in the preliminary design. Finally, a fan assessment along an iso speed line is performed to demonstrate the efficient applicability of the methodology to assess various off-design conditions.