Design and Performance of a Boundary Layer Ingesting Fan

Abstract

A Boundary Layer Ingesting fan is designed to function in a tail cone thruster configuration on an existing aircraft. This means that the fan ingests part of the boundary layer developing over the fuselage all around the circumference. While the fuselage drag induced on the ingested flow makes it possible to obtain a higher propulsive efficiency, it also means that the fan will operate in a severely distorted flow. In the configuration studied here the incoming flow will generally have a lower impulse near the hub, but also substantial non-axisymmetric components. The incoming flow profile is evaluated from a CFD model of a complete Fokker 100 aircraft modified with a tail cone thruster installed. Having the aircraft modeled in detail allows the extraction of the flow entering the fan inlet, which makes up the inlet boundary condition to design for. In order to make a rational design of the fan, the incoming flow is circumferentially averaged at each radial location to form the radial profile used in the design. A fan map is created to evaluate critical points in the operating envelope in order to demonstrate that the given design is stable in operation. Operation of the fan in static ground conditions is within the operating envelope of the fan without variable nozzle area.

Design of a sub-scale fan for a boundary layer ingestion test with by-pass flow

“…The fan design will therefore need to be mechanically robust as well as aerodynamically tolerant to variations in operating conditions. Design conditions for the sub-scale fan are derived from the BLI fan integrated with a Fokker 100 aircraft as described in [14] and [12].…”

“…For the “Flat Profile” the inlet total pressure is simply constant as an idealisation of the conditions that would be relevant for the airplane at static conditions. These inlet profiles are derived from the study of the full scale fan reported in [13]. The “scaled baseline” was taken from the same study but re-normalised for comparison with the sub-scale fan results.…”

“…Integrating the fan with the aircraft in order to gain the improvements of BLI, increases the complexity of the fan design, as each possible installation will generate a unique distortion pattern that the fan needs to cope with. The starting point for this study is a design of a BLI fan specifically for the Fokker 100 aircraft [13]. For future developments an understanding of how different distortion patterns affect the design is needed.…”

Fan design assessment for BLI propulsion systems

“…The design with lowest tip speed shows highest changes in isentropic efficiency of − 0.47% and an increase of DF 16 Left: blade-to-blade view of a rotor passage including inflow and outflow velocity triangles as well as chord length s, pitch t and airfoil thickness d max . Right: rotor outflow velocity triangles at the hub for different rotational speeds of 0.103 at 80% span (see Fig.…”

“…The resulting fan stage has a wider operating range at improved efficiency levels compared to the baseline design. Mårtensson and Laban [16] presented a design and performance assessment study for an aft-propulsor. This propulsor ingests the fuselage boundary layer of a Fokker 100 a/c.…”