Aerodynamic Analysis of a Boundary-Layer-Ingesting Distortion-Tolerant Fan

Abstract

The paper describes the aerodynamic CFD analysis that was conducted to address the integration of an embedded-engine (EE) inlet with the fan stage. A highly airframe-integrated, distortion-tolerant propulsion preliminary design study was carried out to quantify fuel burn benefits associated with boundary layer ingestion (BLI) for “N+2” blended wing body (BWB) concepts. The study indicated that low-loss inlets and high-performance, distortion-tolerant turbomachines are key technologies required to achieve a 3–5% BLI fuel burn benefit relative to a baseline high-performance, pylon-mounted, propulsion system. A hierarchical, multi-objective, computational fluid dynamics-based aerodynamic design optimization that combined global and local shaping was carried out to design a high-performance embedded-engine inlet and an associated fan stage. The scaled-down design will be manufactured and tested in NASA’s 8′×6′ Transonic Wind Tunnel. Unsteady calculations were performed for the coupled inlet and fan rotor and inlet, fan rotor and exit guide vanes. The calculations show that the BLI distortion propagates through the fan largely un-attenuated. The impact of distortion on the unsteady blade loading, fan rotor and fan stage efficiency and pressure ratio is analyzed. The fan stage pressure ratio is provided as a time-averaged and full-wheel circumferential-averaged value. Computational analyses were performed to validate the system study and design-phase predictions in terms of fan stage performance and operability. For example, fan stage efficiency losses are less than 0.5–1.5% when compared to a fan stage in clean flow. In addition, these calculations will be used to provide pretest predictions and guidance for risk mitigation for the wind tunnel test.

Stall Inception in a Boundary Layer Ingesting Fan

“…Flow field measurements and simulations have shown that distortion leads to a large-scale flow redistribution upstream of a compressor rotor, leading to non-uniform velocity and swirl angles at its leading edge, [20][21][22][23][24]. In many of these studies the rotor is found to operate at high incidence in localised regions of the annulus, with a flow coefficient below the critical clean-flow value, while the machine as a whole remains stable.…”

A Review of Concepts, Benefits, and Challenges for Future Electrical Propulsion-Based Aircraft

“…To reap the benefits of BLI, a stronger distortion tolerant fan design with blades designed to straighten out the swirl flow is required [74]. UTRC and NASA Glenn Research Centre (GRC) made collaborative efforts in designing a fan aiming to limit the efficiency deterioration within 1-3% [85]; Figure 3 displays the wind tunnel test for a distortion tolerant propulsor fan to observe the impact from varying wind speeds on the operability, structure, and performance of the fan.…”

Nonaxisymmetric Stator Design for Boundary Layer Ingesting Fans

“…In regions of reduced mass flux and higher incidence, the Mach number will be reduced and this could help reduce the stator losses behind the distorted region. Such effects are visible in [1,2,4] and this suggests the amount of non-axisymmetric design variation required in a transonic fan may be lower than for a low speed case. circumference at midspan, the maximum wake thickness occurs within -60° < θ < 30°.…”

“…Similar features have been observed in highspeed fans and compressors operating with inlet total pressure distortion. For example, in [2] full unsteady calculations were performed of a coupled inlet and transonic fan operating with BLI. The fan pressure ratio was 1.35 and the inlet distortion had a similar form to that used in the current study.…”