Assessment of a BLI-Tolerant Aft-Fan at Design and Near Stall Conditions - From Preliminary to Detailed Aerodynamic Design

Abstract Boundary layer ingestion (BLI) is a promising concept to decrease the climate impact of civil aviation. One propulsion unit architecture considers a wake filling propulsor in the rear of the fuselage aiming at a higher fuel-efficiency. The flow entering the propulsion unit features stagnation pressure and swirl distortions. The fuselage boundary layer causes a stagnation pressure variation orthogonal to the fuselage. Moreover, the fuselage upsweep leads to a circumferential non-homogeneous stagnation pressure distribution and causes a swirl distortion. This study focuses mainly on three parts. Firstly, a new and numerically efficient design and assessment methodology for distortion tolerant fans is presented. The new methodology requires approximately 3 CPUh to assess fans in BLI conditions which is four orders of magnitude lower than the assessment by time-resolved calculations. Secondly, a distortion tolerant fan stage is assessed at the aerodynamic design point. The study shows that the new design methodology is able to capture the flow redistribution due to the stagnation pressure distortion and the local operating point variation around the circumference. Compared to the design conditions which feature purely radial distortion, the two-dimensional distortion leads to a fan efficiency decrease of 1.16%. The additional decrease of fan efficiency associated with the swirl distortion is 0.35%. Thirdly, a similar analysis is conducted at near stall conditions. The two-dimensional stagnation pressure distortion leads to a 2.5% lower fan efficiency compared to the purely radial distortion while the swirl distortion leads to a further decrease of 0.44%. Therefore, the fan performance is mainly driven by the stagnation pressure distortion rather than by the swirl distortion. At both operating conditions and both distortions, the fan attenuates the distortion which manifests in a decrease in distortion intensity of minimum 30% up to 40%.