Abstract
dc:description.abstractFuture low pressure ratio fan systems require an extended operating range and variable pitch fans can achieve this by re-pitching the rotor blades. Moreover, a variable pitch fan has the potential of reducing fuel burn if sufficient reverse thrust can be generated, such that the engine weight and drag associated with heavy cascade-type thrust reversers can be eliminated. Previous studies have shown the installation can have a significant impact on the variable pitch fan performance in reverse thrust. This thesis aims to study the effects of inlet distortion on variable pitch fan performance in reverse thrust operation, to find the most influential distortion components and to determine the way to operate a variable pitch fan with inlet distortion. Steady RANS and rig measurements are used in this study to achieve the research aims. Both the simulations and the experiments show that typical distortion from the engine installation leads to a very different flow field compared to the case with uniform inflow at uninstalled conditions. The distortion causes significant changes in flow structure, mass flow, thrust and power distribution in the engine. In particular, the flow at the fan rotor breaks down and forms a strong radial flow within the rotor passage. This leads to a recirculating jet penetrating into the bypass duct, forming large recirculation regions between the rotor and the bypass inlet. The recirculation regions significantly restrict the net mass flow through the engine and the fan is regarded as stalled in this particular state. It is found that almost all of the power from the fan is used to drive the recirculating flow, which has high loss and total temperature. The VPF operating with recirculating flow is a significantly less efficient thrust reverser than the VPF at uninstalled conditions as the expected net reverse thrust generation for the former is only around 20% of the take-off thrust, which is notably lower than the value of 35% for the latter. Radial pitch angle distortion is found to be the most influential component among radial and circumferential distortion components in yaw angle, pitch angle, total pressure and total temperature. Because of its blockage effect at the bypass inlet, the radial pitch angle distortion alone leads to the recirculating flow in the bypass duct. This is unmatched by other distortion components, as each of them alone is insufficient for causing recirculating flow at the rotor. The radial total pressure distortion is also important as it governs the amount of pressure force acting on the flow through the engine. The lower average inlet total pressure in the distortion increases the size of recirculation and reduces the jet momentum. With all the radial distortion components from the engine installation, the reverse thrust can be recovered through closing the rotor blades and increasing the fan speed if the total pressure deficiency at the bypass inlet can be reduced from 3.5% of the freestream total pressure to an acceptable level of 2% for the engine in this study. Significant hysteresis found during reverse thrust recovery, where the variable pitch fan can be stalled and unstalled by opening and closing the blades respectively, aligns with NASA's experimental campaigns and is captured computationally for the first time. The computations also contribute towards the subject by showing that stalling of the fan is driven by the extent of shock-induced flow separation on the suction surface and unstalling of the fan is driven by the extent of flow reattachment along the span. Based on the computational results, in order to operate a variable pitch fan with inlet distortion in reverse thrust, the rotor blades have to be closed to clear stall and the blades should then be opened to increase reverse thrust.
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
- 2022
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Ma, Kwun Yeung
- Advisor dc:contributor.advisor
-
- Hall, Cesare
Subjects
dc:subject × 7Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.104567
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/362293