Ultrashort Nacelles for Low Fan Pressure Ratio Propulsors

Abstract

As the propulsor fan pressure ratio (FPR) is decreased for improved fuel burn, reduced emissions and noise, the fan diameter grows and innovative nacelle concepts with short inlets are required to reduce their weight and drag. This paper addresses the uncharted inlet and nacelle design space for low-FPR propulsors where fan and nacelle are more closely coupled than in current turbofan engines. The paper presents an integrated fan-nacelle design framework, combining a splinebased inlet design tool with a fast and reliable body-force-based approach for the fan rotor and stator blade rows to capture the inlet-fan and fan-exhaust interactions and flow distortion at the fan face. The new capability enables parametric studies of characteristic inlet and nacelle design parameters with a short turn-around time. The interaction of the rotor with a region of high streamwise Mach number at the fan face is identified as the key mechanism limiting the design of short inlets. The local increase in Mach number is due to flow acceleration along the inlet internal surface coupled with a reduction in effective flow area. For a candidate short-inlet design with length over 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 off-design conditions relative to a long-inlet propulsor with L/D = 0.5. As a consequence, the rotor locally operates close to choke resulting in fan efficiency penalties of up to 1.6 % at cruise and 3.9 % at off-design. For inlets with L/D < 0.25, the benefit from reduced nacelle drag is offset by the reduction in fan efficiency, resulting in propulsive efficiency penalties. Based on a parametric inlet study, the recommended inlet L/D is suggested to be between 0.25 and 0.4. The performance of a candidate short inlet with L/D = 0.25 was assessed using full-annulus unsteady RANS simulations at critical design and off-design operating conditions. The candidate design maintains the propulsive efficiency of the baseline case and fuel burn benefits are conjectured due to reductions in nacelle weight and drag compared to an aircraft powered by the baseline propulsor. This document has been publicly released 2 NOMENCLATURECopyright © 2014 by ASME BACKGROUND & INTRODUCTIONNext-generation turbofan engine designs for commercial transport aircraft seek higher bypass ratios (BPR) and lower fan pressure ratios (FPR) for improved fuel burn and reduced emissions and noise [1][2][3][4], increasing fan bypass stream propulsive efficiencies and enabling higher overall pressure ratios and turbine inlet temperatures [4]. In addition, significant cabin and far-field noise benefits can be achieved by potentially avoiding buzz-saw noise, reducing fan broadband and rotorstator interaction noise, and enabling steeper take-off profiles due to excess thrust capability [5][6][7].Reductions in fan pressure ratio can be realized for example through geared low-speed fans. First-generation geared turbofan engines with BPR = 12 and FPR ≈ 1.4 for s...

Fan–Intake Interaction Under High Incidence

“…The size of the separation determines the distortion at the fan face which will influence the performance of the fan and its compatibility. studies can be broadly classified into the following categories, which explored the effect of the: (a) Off-design conditions like high-incidence and crosswinds on the intake-only configuration [1] and the hysteresis phenomena associated with flow separation and reattachment [2]; (b) Inlet distortion on the stability of the fan [3] ; (c) Fan-intake interaction on the incidence tolerance [4], [5] and the subsequent optimization of the intake shape [6].…”

“…It was shown that the fan stage is beneficial in increasing the tolerance to flow incidence and suppressing the flow distortion. Peter et al [6] presented an integrated fan-nacelle design framework based on CFD with the fan incorporated using a bulk body force model (BFM). A parametric study of the influence of intake-fan interaction on overall engine performance was conducted.…”

Prediction of Crosswind Separation Velocity for Fan and Nacelle Systems Using Body Force Models: Part 2: Comparison of Crosswind Separation Velocity with and without Detailed Fan Stage Geometry

“…This improves propulsive efficiency; however, it comes with the negative trade off of larger and heavier nacelles. To combat this negative side effect, manufacturers are using shorter inlets with thinner nacelle lips [1]; however, these changes increase the chance of flow separation occurring in the inlet. The airframers' ability to determine the impact of these changes is important during design.…”

“…The airframers' ability to determine the impact of these changes is important during design. To model such situations, the airframer must be able to also model the engine fan stage as its operation will greatly impact the intake performance [1]; however, two major concerns arise while modelling these flows. The first is that it requires full wheel simulations to capture the non-uniform flow caused by inlet separation; using traditional bladed full wheel simulations to model non-uniform flow is very computationally expensive.…”

Quasi 3D Nacelle Design to Simulate Crosswind Flows: Merits and Challenges

“…Modern engine architectures are trending towards higher bypass ratios and lower fan pressure ratios, which offer further improvements in propulsive efficiency with consequential reduction in emissions and noise. To circumvent the consequent increase in the weight and drag, engine manufacturers are exploring the possibility of employing shorter intakes and slimmer nacelle lips [1,2]. Such designs are however much more susceptible to flow separation due to the reduced diffusion capability of the intake, specifically under the off-design conditions of high crosswinds and high-incidence.…”