Aerodynamics of Boundary Layer Ingesting Fans

Abstract

Boundary Layer Ingesting (BLI) turbofan engines could offer reduced fuel burn compared with podded engines, but the fan stage must be designed to run continuously with severe inlet distortion. This paper aims to explain the fluid dynamics and loss sources in BLI fans running at a cruise condition. High-resolution experimental measurements and full-annulus unsteady CFD have been performed on a low-speed fan rig running with a representative BLI inlet velocity profile. A three-dimensional flow redistribution is observed, leading to an attenuation of the axial velocity non-uniformity upstream of the rotor and to non-uniform swirl and radial angle distributions at rotor inlet. The distorted flow field is shown to create circumferential and radial variations in diffusion factor with a corresponding loss variation around the annulus. Additional loss is generated by an unsteady separation of the casing boundary layer, caused by a localised peak in loading at the rotor tip. Non-uniform swirl and radial angles at rotor exit lead to increased loss in the stator due to the variations in profile loss and corner separation size. An additional CFD calculation of a transonic fan running with the same inlet profile is used to show that BLI leads to wide variations in rotor shock structure, strength and position and hence to loss generation through shock-boundary layer interaction, but otherwise contained the same flow features as the low-speed case. For both fan geometries, BLI was found to reduce the stage efficiency by around 1–2% relative to operation with uniform inlet flow.

Influence of Upstream Total Pressure Profiles on S-Duct Intake Flow Distortion

“…Results expressed in the rotor frame of reference revealed that the flow distortion increased the relative incidence angle by up to approximately 10 in the outer annuli of the duct at the fan face plane. A similar test case [31] showed that absolute swirl angles could increase up to 10 in the main coswirling and counter-swirling regions. Prior studies [3][4] have demonstrated that the peak swirl angles at the S-duct outlet are proportional to the duct offset ratio (h/Din).…”

Optimality considerations for propulsive fuselage power savings

“…While vehicular efficiency quantification (equation (32)) necessitates an instantaneous # at the considered operating point, the evaluation of fuel burn based on the solution of the Breguet-Coffin equation presented in equation (30) requires aircraft gross weight to be determined at the end of ÁR. Hence, equation (32) can be used to solve equation (30) for both aircraft and to analytically express the consumed fuel mass of the PFC aircraft Ám f,PFC for a given range segment ÁR as a function of the determined PSC, the vehicular efficiency of the reference aircraft ov Á L=D ð Þ Ref as well as the aircraft gross weight ratios at the representative operating point and at the end of range segment, # and # end…”

Simulation of Indexing and Clocking with Harmonic Balance

“…Both rotor and stator are comprised of 15 blades/vanes. The inflow distortion (depicted in Figure 5b) models an ingested boundary layer of an airplane fuselage and is similar to the ones presented by [18,19]. It has the form of an ideal turbulent boundary layer that covers 50% of the inlet duct height in the lower part of the annulus.…”

“…Figure 6b,c show a well-known influence mechanism of ingested boundary layers on fan stages: due to reduced inflow velocity in the distorted area, the overall mass flow rate is reduced while the global total pressure ratio rises due to an increased inflow and hence incidence angle [20]. For the same reason, the distorted inflow reduces the isentropic efficiency of the fan stage [18]. The results show that the decrease in mass flow and the increase in total pressure ratio are predicted correctly by the HB simulations as well as by the mixing plane results with inlet distortion.…”