University of Cambridge
Effects of blade damage on low pressure ratio fan windmill aerodynamics
Abstract
dc:description.abstractThe windmill condition can occur after the in-flight shutdown of a turbofan, when the fan continues to spin due to the aircraft forwards speed. This condition leads to high levels of drag, and the fan operates at very low rotational speeds. Windmill often follows damage to the fan, which will lead to large out-of-balance centrifugal loads in the engine. In combination with the low rotational speed, such loads, may lead to harmful vibrations interacting with the airframe modes. Therefore, airframe designers need to know the windmill drag and rotational speed. This becomes particularly important for future low fan pressure ratio designs with increased fan diameters and reduced rotational speeds. A low speed rig has been designed, matched a future low pressure ratio fan at windmill, to take measurements of the undamaged fan, as well as fans with different types of axisymmetric and non-axisymmetric damage. Steady pressure measurements are used to characterise the windmill operating point, and hot-wire measurements are used to resolve the rotor blade-to-blade flow field and investigate the effects of non-axisymmetric damage. RANS simulations have been performed to obtain details of the 3D flow features. The windmill condition is characterised by compressor work near the hub and turbine work near the casing, leading to zero overall work output. The low fan rotational speed leads to high flow coefficients of φ > 2 for the undamaged fan. Blade speed is set by the zero-work radius velocity triangle, and it scales with mass flow rate such that a windmill characteristic can be found based on the flow coefficient. The rotor and stator operate at highly off-design incidence such that the flow separates on the pressure side. In the case of the rotor, the separated flow forms a streamwise vortex near the casing which reduces the work output and increases loss. The stator pressure side is fully separated, which dominates the stage loss. Axisymmetric fan tip damage shifts the work distribution down the span, which leads to a reduction in blade speed, and increase in flow coefficient to φ > 5. A streamwise vortex forms at the rotor damaged blade tip, and there is flow redistribution towards the casing such that high velocity axial flow entrains the blade tip vortex. Non-axisymmetric tip damage leads to a smaller reduction in rotor blade speed compared to axisymmetric damage. For small damage pitch, the blades adjacent to the damaged region still turn the flow. As flow redistributes to the damaged tip region, the blades are turning higher velocity flow, increasing turbine work output. This leads to the fan rotational speed being similar to that of the undamaged case, with the flow coefficient varying from φ = 2.73 to 3.24 for the different amounts and distribution of damage. A fan blade-off case was also studied, which showed that there is flow redistribution towards the missing blade passage, but this has little impact on the rotor work distribution and rotational speed. The rotor loss depends on the damage distribution, where an undamaged blade next to a damaged tip region leads to increased loss due to shear. Finally, it was shown that a model for predicting the undamaged fan windmill rotational speed based on blade geometry and flow velocity can be improved to within 2% accuracy by capturing the effect of the casing vortex on the velocity profile. In the case of damage, a prediction accuracy of 8% can be obtained by modelling the deviation from experimental results.
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
- 2024
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Medina Cassillas, Sofia
- Advisor dc:contributor.advisor
-
- Hall, Cesare A
Subjects
dc:subject × 5Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.117986
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/383731