Abstract
Shaft seals control the leakage of fluid between areas of high and low pressure around rotating components inside turbomachinery. The leakage and rotordynamic characteristics are of great importance to engine designers to ensure that the turbines are operating at maximum efficiencies, and do not cause instabilities. The aim of adaptive seals is to reduce leakage flows whilst minimising wear due to rubs caused by eccentric shaft movement. The aim of this PhD was to determine a fundamental understanding of the fluid mechanics and stability characteristics of a new shaft seal design called the Film Riding Pressure Actuated Leaf Seal (FRPALS).Experimental leakage and rotordynamic measurements for different seal designs were collected up to 3.5 bar pressure difference, between 15 and 150 Hz excitation frequency, with rotational speeds up to 15,000 rpm. The test seals were; four brush seal with different bristle packs and back plate designs, and two iterations of prototype FRPALS. The experimental characteristics of brush seals were used as an industry standard baseline from which to compare the performance of the FRPALS. Leakage results have been presented in terms of effective clearance to allow the comparison between different seal designs and geometries. The rotordynamic behaviour was presented in terms of direct and cross-coupled stiffness, damping, and inertial mass coefficients.
Numerical modelling techniques were also implemented to characterise an early design of the FRPALS in terms of linear and angular translation towards the rotor. The model utilised a modified version of the Reynolds equation to predict the stiffness and damping characteristics of a fluid film. Preliminary experiments were conducted to validate the numerical results. Upon validation, the predictive model was used in a Design of Experiments to re-design the FRPALS. The new prototype also incorporated secondary design elements to improve leakage performance and ensure ease of manufacture.
Comparing the effective clearances showed that the new prototype improved the leakage performance relative to previous FRPALS designs, but was not as effective at restricting leakage flow as brush seals. The dynamic coefficients of the FRPALS were comparable with the brush seals. The direct stiffness and damping increased with pressure, and the cross-coupled stiffness marginally increased with rotational speed. The damping coefficient of the FRPALS was independent of excitation frequency, whereas the brush seal damping decreased. These findings show that the FRPALS prototype offered favourable dynamic characteristics, and that the seal could be a promising technology with additional design development to further reduce the leakage.
| Date of Award | 28 Mar 2023 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Cross Manufacturing Co (1938) Ltd |
| Supervisor | James Scobie (Supervisor), Carl Sangan (Supervisor) & Patrick Keogh (Supervisor) |