Ultra-short nacelles for low fan pressure ratio propulsors

Abstract

This thesis addresses the uncharted inlet and nacelle design space for low pressure ratio fans for advanced aeroengines. A key feature in low fan pressure ratio (FPR) propulsors with short inlets and nacelles is the increased coupling between fan and inlet. The thesis presents an integrated fan-nacelle design framework, combining a spline-based tool for the denition of inlet and nacelle surfaces with a fast and reliable body-force-based approach for the fan rotor and stator blade rows. The new capability captures the inlet-fan and fan-exhaust interactions and the ow distortion at the fan face and enables the parametric exploration of the short-inlet design territory. The interaction of the rotor with a region of high streamwise Mach number at the fan face is identied as the key aerodynamic mechanism limiting the design of short inlets. The local increase in streamwise Mach number is due to ow acceleration along the inlet internal surface coupled with a reduction in eective ow area. For a candidate short-inlet design with inlet length to fan diameter ratio L=D = 0:19, the streamwise Mach number at the fan face near the shroud increases by up to 0:16 at cruise and by up to 0:36 at o-design conditions relative to a long-inlet baseline propulsor with L=D = 0:5. As a consequence, the rotor locally operates close to choke, resulting in fan eciency penalties of up to 1:6% at cruise and 3:9% at o-design. For inlets with L=D < 0:25, the benet from reduced nacelle drag is offset by the reduction in fan eciency, resulting in propulsive eciency penalties. Based on a parametric inlet study, the recommended inlet L=D for engine propulsive eciency benefits is suggested to be between 0:25 and 0:4. A candidate design with L=D = 0:25 maintains the cruise propulsive efficiency of the baseline case without jeopardizing fan and LPC stability at o-design conditions. On the aircraft system level, fuel burn benefits are conjectured to be feasible due to the reductions in nacelle weight and drag compared to an aircraft powered by the long-inlet baseline propulsor.