Adequate sealing in turbomachinery reduces secondary leakage and results in more efficient and stable systems. Labyrinth seals are most common, although brush seals are popular in specialized applications. The Hybrid Brush Seal (HBS) is a novel design that adds to the bristle brush matrix a number of cantilever pads that rest on the rotor surface. Upon shaft rotation the pads lift due to the generation of a hydrodynamic gas film while the brushes effectively seal an upstream pressure. Hence the HBS has no wear and no local thermal distortion effects. Measurements of leakage versus pressure differential are obtained in a three-teeth labyrinth, a conventional brush seal, and a hybrid brush seal for operation at high temperature (300ºC), with shaft surface speeds to 27 m/s, and at supply pressures to 3.5 bar. Flow measurements are presented in terms of a flow factor to remove dependency on the air temperature and supply pressure. The measurements demonstrate the HBS leaks less (~61%) than a standard brush seal and is significantly better (~38%) than a similarly sized labyrinth seal. Predictions of flow through a labyrinth seal predict well at supply pressures under 1.7 bar but overpredict by as much as 25% at high supply pressures. A porous medium fluid flow model predicts the flow through the HBS and brush seal. The model for the HBS and brush seal underpredicts the flow rate at low supply pressures but match well at high supply pressures. Measurements of the drag torque of the test seals show the HBS has a larger torque when pressurized compared to the brush seal and labyrinth seal. This indicates that the HBS experiences a larger degree of blow-down due to the pads decreasing the clearance. The mechanical parameters of the brush seal and HBS are found based upon the flexibility function from impact load tests. A combined structural and dry friction damping model represent well the measured flexibility. An equivalent damping is found based upon the energy dissipation. Based upon the damping ratio, the HBS has twice of the viscous damping as the brush seal at a supply pressure of 2.0 bar.

Advanced rotating shaft seals have the potential to significantly increase the efficiency and performance of steam and gas turbines. Two such seals, brush and leaf seals, rely on the use of thousands of flexible filaments to close clearances between rotating components and their static casings. The current life of the components is poor compared to the rest of the gas turbine, limiting the seals' deployment, particularly in the jet engine at high temperature and pressure. Poor understanding of the seal installation response to frictional heat generated at the point of filament-rotor contact during operation has limited the ability to predict engine closures and hence seal behaviour and life. The resulting temperature rises may compromise the mechanical integrity of the engine rotor in extremis leading to a shaft failure. This thesis considers the heat transfer mechanisms that govern frictional heating, of both the fluid and solid components in the vicinity of such seals, characterising the process both experimentally and using numerical models. Through the identification of key features of the heat transfer a simple numerical methodology is shown to predict the thermal behaviour of the seal installation sufficiently accurately for engine design purposes. A low order heat transfer model, using a simple electrical analogy for heat transfer is used to investigate frictional heat generation. When contact occurs between the rotor surface and the seal filaments, mechanical energy is dissipated as heat at the interface. This is conducted into the rotor and the seal filaments in proportions that depend on the heat transfer characteristics of both contacting bodies (thermal resistances). To calculate the heat partition ratio and the resulting contact temperature, the thermal resistances of both rotor and seal need to be known. To that end, a new test facility, the Seal Static Thermal Test Facility (SSTTF), is developed. This is first used to study the convective heat transfer occurring in the vicinity of the seal; heat transfer coefficients based on appropriate, scalable, gas reference temperatures are reported. Importantly the results show a larger area on the rotor surface affected by the presence of the seal than was assumed by previous workers. The test rig is further modified to generate heating in a static test rig equivalent to the frictional heating at the filament tips. The test rig allows the contact temperature between rotor and seal, a critical previously unknown parameter to be measured in a well-conditioned environment. The presence of many thousands of vanishingly small flow passages in filament seals makes their explicit modelling unfeasible for engine design purposes. Thus the results from the experimental campaign are used to develop a simple computational fluid dynamic model of the seal, including empirically derived frictional heating, and seal porosity models, to achieve similar leakage and surface heat transfer to the rotor as was seen in the static experiments. The low order CFD methodology presented in the thesis is finally employed to model the transient operation of a brush seal under engine representative rotor surface speeds and differential pressures. Experimental data were generated in the Oxford Engine Seal Test Facility for a typical brush seal rubbing against a high growth rotor. These experiments were modelled using CFD and finite element analysis using parameters derived from static tests for the porous modelling of the seal leakage. Comparison of results shows that, without further tuning, the thermal behaviour is captured well with a moderate conservative overestimation of rotor heating with increased differential pressure across the seal allowing the strategy to be used as an engine design tool.

Over the last decades, it has been progressively acknowledged that reducing the specific fuel consumption and the emission of pollutants as well as improving the thrust-to-weight ratio involves extensive research on advanced sealing technologies. Amongst these, brush seals are particularly well considered for their excellent leakage performance, their low friction properties, and their ability to cope with inevitable rotor excursions during flights. This thesis presents the experimental work that has been carried on in order to characterize carbon brush seals performance in function of the bristle pack geometry and the operating conditions. The analyzed parameters are the bristle free length, the density, and the inter-platedistance. The work, performed by the ULB in collaboration with French engine manufacturer Safran Aircraft Engines, provides first, a description of the test installation that reproduces accurately the severe working conditions encountered in a bearing chamber. A total of eight samples were submitted to extensive testing, and allowed to perform a qualitative analysis of the main performance indicators of a brush seal: the leakage flow, and the seal torque. Complex phenomena acting on the bristle pack were put in evidence under the effect of differential pressure androtation speed, and oil, which fundamentally deteriorate the leakage performance of a brush seal. Subsequently, performance models were developed through empirical correlations, based on the experimental data. They predict the leakage flow and the seal torque as a function of the geometrical parameters and operating conditions. In addition, hysteresis issues were also addressed, and an IR camera helped investigating the heat generation properties of a brush seal.Brush seal samples were submitted to endurance testing, in order to highlight wear mechanisms, and study the performance degradation with the operating time. Oil plays a major part in extending brush seals operating life, despite the leakage performance degradation. Finally, the correlations developed throughout the PhD thesis were used to develop an optimization process in function of the operating conditions of a modern aero-engine. Ultimately, large savings in air consumption were put in evidence when replacing labyrinth seals by brush seals. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Les joints à brosse sont des joints compliants utilisés dans les turbomachines. Ils sont constitués d'une brosse de fils très fins compactés entre deux plaques, de telle manière que l'ensemble comble l'écart séparant le stator du rotor. Les fils sont le plus souvent constitués d'un alliage à base de cobalt appelé Haynes 25 (d'autres matériaux peuvent être utilisés, tels que le Kevlar). Le comportement de ces joints présente un fort couplage entre l'écoulement du fluide et la déformation de la brosse. Ce type de joints peut être modélisé en considérant la brosse comme un milieu poreux. Cette méthode présente l'avantage de permettre un calcul relativement simple de l'écoulement, mais elle demande l'usage de données expérimentales afin de calibrer ses perméabilités (i.e. sa capacité à laisser un fluide s'écouler à travers lui). L'objectif de cette thèse est de proposer une modélisation par milieu poreux indépendante des données expérimentales. Une simulation itérative entre le calcul de la déformation des fils sous l'action des efforts de pression et la résolution de l'écoulement dans le milieu poreux formé par la brosse déformée a été mise au point. Elle repose sur le calcul numérique des perméabilités obtenues à partir de la perte de charge générée par un réseau de cylindres représentatifs de la brosse.Des joints de faible diamètre en Haynes 25 ont été testés avec de l'air afin de vérifier la validité des résultats donnés par la simulation. L'évolution du débit avec la différence de pression et avec l'interférence des joints avec le rotor a été étudiée. Les déplacements du rotor et la température des joints ont été suivis durant les essais. / Brush seals are compliant dynamic seals that are used in the turbomachines. They are made of a brush of fine bristles sandwiched between two plates, in such a way that the assembly fills the gap between the rotor and the stator. The bristles are usually made from a cobalt based alloy called Haynes 25 (other materials can be used, such as Kevlar). The behaviour of these seals shows a strong coupling between the fluid flow and the brush deformation.This kind of seal can be modelled by considering the brush as a porous medium. This method presents the benefit of a rather simple calculation of the flow, but it requires the use of experimental data in order to calibrate its permeabilities (i.e. its ability to let the fluid flow through it). The aim of this thesis is to propose a model using the porous medium approach but without the need of preliminary experimental data. An iterative simulation between the calculation of the bristles' deformation under the influence of the pressure forces and the solving of the flow in the porous medium formed by the deformed brush was developed. It relies on the numerical calculation of the permeabilities obtained from the pressure drop generated by an array of cylinders representative of the brush.Low diameter Haynes 25 seals were tested with air in order to verify the validity of the results given by the simulation. The evolution of the mass flow rate with the pressure difference and the interference between the seal and the rotor was studied. Displacements of the rotor and temperature of the seals were also monitored during the tests.