Aamir Shabbir

Design and Test of an Aspirated Counter-Rotating Fan

The design and test of a two-stage, vaneless, aspirated counter-rotating fan is presented in this paper. The fan nominal design objectives were a pressure ratio of 3:1 and adiabatic efficiency of 87%. A pressure ratio of 2.9 at 89% efficiency was measured at the design speed. The configuration consists of a counter-swirl-producing inlet guide vane, followed by a high tip speed (1450 ft/s) nonaspirated rotor and a counter-rotating low speed (1150 ft/s) aspirated rotor. The lower tip speed and lower solidity of the second rotor result in a blade loading above conventional limits, but enable a balance between the shock loss and viscous boundary layer loss; the latter of which can be controlled by aspiration. The aspiration slot on the second rotor suction surface extends from the hub up to 80% span. The bleed flow is discharged inward through the blade hub. This fan was tested in a short duration blowdown facility. Particular attention was given to the design of the instrumentation to measure efficiency to 0.5% accuracy. High response static pressure measurements were taken between the rotors and downstream of the fan to determine the stall behavior. Pressure ratio, mass flow, and efficiency on speed lines from 90% to 102% of the design speed are presented and discussed along with comparison to computational fluid dynamics predictions and design intent. The results presented here complement those presented earlier for two aspirated fan stages with tip shrouds, extending the validated design space for aspirated compressors to include designs with conventional unshrouded rotors and with inward removal of the aspirated flow.

Aeromechanics Analysis of a Boundary Layer Ingesting Fan

Abstract Boundary layer ingesting propulsion systems have the potential to significantly reduce fuel burn but these systems must overcome the challenges related to aeromechanics—fan flutter stability and forced response dynamic stresses. High-fidelity computational analysis of the fan aeromechanics is integral to the ongoing effort to design a boundary layer ingesting inlet and fan for fabrication and wind-tunnel test. A three-dimensional, time-accurate, Reynolds-averaged Navier Stokes computational fluid dynamics code is used to study aerothermodynamic and aeromechanical behavior of the fan in response to both clean and distorted inflows. The computational aeromechanics analyses performed in this study show an intermediate design iteration of the fan to be flutter-free at the design conditions analyzed with both clean and distorted in-flows. Dynamic stresses from forced response have been calculated for the design rotational speed. Additional work is ongoing to expand the analyses to off-design conditions, and for on-resonance conditions.

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

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.Copyright

LES Loss Prediction in an Axial Compressor Cascade at Off-Design Incidences With Free Stream Disturbances

It is well known that an axial compressor cascade will exhibit variation in loss coefficient, described as a loss bucket, when run over a sweep of incidences, and that higher levels of free stream turbulence are likely to suppress separation bubbles and cause earlier transition (see e.g. [23]). However, it remains difficult to achieve accurate quantitative prediction of these changes using numerical simulation, particularly at off-design conditions, without the added computational expense of using eddy-resolving techniques. The aim of the present study is to investigate profile losses in an axial compressor under such conditions using wall-resolved Large Eddy Simulation (LES) and RANS. The work extends on previous work by Leggett et al.[11] with the intention of furthering our understanding of loss prediction tools and improving our quantification of the physical processes involved in loss generation. The results show that while RANS predicts losses with good accuracy the breakdown of these losses are attributed to different processes, meaning that optimisation of a compressor cascade profile, based solely on RANS, may be hard to achieve.