Lye, Ryan
(2024)
Characterising the influence of material condition and in-service property evolution of an AlSi-PES abradable on blade-casing interactions.
PhD thesis, University of Nottingham.
Abstract
Environmental concerns are pushing manufacturers to improve the efficiency of their aero-engines. A reduction in compressor blade tip clearance can lead to an increase in the overall pressure ratio and thermal efficiency; however, this reduced clearance can lead to more frequent blade-casing interaction (rub) events. These clearance change types can be separated into two categories, the first of which being an axisymmetric clearance change which is uniform around the entire casing and can be caused by centrifugal loading of the blades or differences in rates of thermal expansion between components. The second type is asymmetric clearance changes which occur when eccentricity between the blades and casing exists in the form of either an off-centred rotor or casing ovalisation, often caused by hard landings, turbulence, or gyroscopic events during manoeuvres.
In order to reduce the severity of any ensuing rubs an abradable coating is often atmospherically plasma sprayed onto the internal casing surfaces, acting as a sacrificial layer that wears preferentially to the blades in addition to reducing the contact forces. There are several types of abradable used throughout the compressor, with their selection being determined by the normal operating temperatures seen in-service and the blade material for a given compressor stage. In this work an aluminium-silicon-polyester~(AlSi-PES) is studied, which is used in temperatures up to 345\textdegree{C} with titanium blades. The AlSi phase creates a coherent matrix, the PES is a dislocator phase that promotes crack initiation, and there are also porosities that ensure a fine debris is created when the abradable is worn away preventing issues downstream such as blocking turbine cooling holes.
In practice abradables experience several interaction mechanisms such as adhesion to the blade tip, blade melting and smearing, or abrasive blade wear. Abradables are typically characterised by their superficial Rockwell hardness~(HR15Y) which must fall within a relatively wide range to be deemed suitable. However, the observed interaction mechanisms are known to vary with hardness, incursion rate, and blade speed. Variations in hardness are also accompanied by changes in the constituent phase volume fractions, with softer abradables having more porosities and PES. Previous studies have been phenomenological in nature, and little work has been conducted to characterise the influence of material variability on the mechanical response of the abradable.
The present work aims to describe the influence of abradable condition on the interaction mechanisms, and how these affect the dynamic blade response. To do this experimental analyses that include high-rate and quasi static mechanical testing, thermal characterisation, and blade-casing rub testing have been conducted. These tests used a range of abradable conditions representative of those throughout an in-service lifecycle. These conditions were as-sprayed, thermally aged, compacted, and both thermally aged and compacted. Numerical rub models were also ran to confirm the experimental observations and to highlight the influence of abradable properties on the rubbing characteristics.
Experiential testing has shown that the PES crystallinity and level of compaction had a strong link with the interactions mechanisms during rubbing. Higher crystallinities led to reduced adhesive transfer while compaction had the opposite effect. The transition from cutting to adhesion during rubbing was found to affect the blade response and contact forces. Therefore, by controlling the abradable condition the interaction mechanism variability can be reduced. Finally, HR15Y testing was found to be ineffective for detecting changes in the PES state which had a strong influence on the rubs, and hence additional methods such as nano hardness testing and x-ray computed tomography are suggested to better characterise an abradable. Additionally, blade rub simulations have demonstrated that changes in the abradable mechanical properties due to its condition have an influence on the rubbing forces and blade response.
Future work based on the outcomes of this thesis have been recommended. This work has improved the understanding of blade-casing interactions with a conditioned AlSi-PES abradable. However, the relative influence of the each phases condition when present in differing quantities is unknown. For example, in this work the mechanical properties and rub characteristics were strongly affected by the PES phase. Though how this influence changes when the PES is present in smaller quantities with respect to the blade response and interaction mechanisms requires further study. There is also a need to conduct more testing with representative blade and abradable geometries over a range of conditions. Changes in the rub mechanism during testing have been linked to variations in the excited blade modes, though the role of abradable condition remains unknown. Another key area for development is to increase the fidelity of blade rub numerical models by including abradable condition, removal, and adhesion which are all known to play a key role in determining the interaction mechanisms but are not yet captured.
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