Abstract
dc:description.abstractVariable pitch fans could enable reduced fuel burn, low fan pressure ratio turbofan engines by extending fan stage operating range and reducing weight and drag by eliminating thrust reversers. The work in this thesis looks to further understanding of the aerodynamic performance of variable pitch fans in forward and reverse thrust operation. In forward operation, the aims are to evaluate the aerodynamic performance variation with stagger setting at design and take-off flight conditions, and how the engine design point defines the utility of a variable pitch fan. In reverse thrust operation, the objectives are to describe the previously unresolved fan-stage flow field, and to investigate the influence of fan-stage design for a fixed forward design point. Computational studies are carried out on the Advanced Ducted Propulsor (ADP) variable pitch fan with a pressure ratio of 1.29. For changes in flow coefficient in forward operation, adjusting stagger setting to match rotor incidence can maintain peak rotor efficiency. Between the high flight Mach number at design, and the low value at take-off, a larger drop in flow coefficient occurs for higher design flow coefficients. At a fixed condition on the fan working line, closing the rotor blade introduces off-design incidence and blockage effects, which reduce work input. Low loss operation is achieved at a wider range of stagger settings and rotor incidence at low flight-speed conditions, which grants freedom to vary fan performance to accommodate flight condition-specific concerns, such as noise production or bypass stator incidence. The change in rotor stagger setting and stator inlet conditions from design to take-off depend primarily on the fan pressure ratio. Design loading coefficient is found to influence off-design matching of core compressors. For all reverse thrust operation operating conditions achieved by restaggering the rotor through feather, the ADP fan radial blade angle variation causes a high-blockage hub recirculation region, large pressure surface separations under negative incidence near midspan, and a high flow and work region near the tip. More closed settings observe lower flow, higher pressure surface separation losses, and a greater proportion of work input through radius change. Loss grows in the high work region at high fan rotational speeds through the combination of high inlet Mach numbers and forward section curvature, limiting maximum thrust output. Varied hub blocked flow area with stagger setting defines distinct reverse working lines, which require an engine-external flow field dependent back pressure and fan performance characteristics to estimate. Greater than 50% take-off gross thrust is obtained by the ADP fan in reverse operation, but the engine braking force falls towards a lower bound of momentum transfer under a high forward Mach number flow field. Design changes are made that maintain forward performance, but alter reverse thrust operation. Moving loading from the tip to mid-span is found to cost around half of reverse gross thrust, compared to 4% by reducing rotor pitch-to-chord ratio. The radial loading distribution change enhances blockage through pressure surface separations, and reduces tip operability by raising incidence. Limiting peak relative Mach numbers extends high rotational speed pressure rise and mass flow rate with lower pitch-to-chord or moving section camber forward. The results motivate fans with high loading at the tip for application in reverse thrust operation.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2019
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Williams, Timothy Stephen
- Advisor dc:contributor.advisor
-
- Hall, Cesare
Subjects
dc:subject × 4Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.43280
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/296236