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Les contraintes thermiques et mécaniques vues par les turbines haute pression ont conduit à la conception de turbines à aubes monocristallines. Ces aubes utilisent des nuances présentant d’excellentes propriétés en fluage selon les directions cristallographiques de type <001> et ont ainsi permis, en augmentant la température des gaz et la vitesse de rotation de la turbine, une amélioration des performances spécifiques des turbomachines. En utilisation, les aubes sont principalement soumises à l’accélération centrifuge induite par la rotation de la turbine autour de son axe. Ce chargement est cyclique avec des cycles qui correspondent au décollage/atterrissage, mais aussi à des vibrations aux fréquences plus hautes pendant la phase de vol. Le chargement de la pale est transmis au disque de turbine par une liaison de type pied de sapin, qui par nature contient un jeu. Au niveau du contact aube/disque les contraintes sont très élevées et des mouvements oscillatoires de faible amplitude apparaissent. Cette configuration de contact est appelée fretting et sa superposition avec la fatigue oligocyclique est appelée fretting fatigue. On rencontre le phénomène de fretting fatigue dans la plupart des contacts quasi-statiques subissant de la fatigue et des vibrations. C’est un phénomène qui réduit considérablement la durée de vie des assemblages. Les travaux de cette thèse se sont articulés autour de deux grands thèmes ; la connaissance du matériau et en particulier de la couche corticale (couche superficielle de la matière modifiée par le traitement de grenaillage) et l’étude des endommagements subits lors des différents chargements mécaniques élémentaire de l’attache aube/disque. Le grenaillage fait partie du processus de fabrication des pieds de pales. Il permet d’aplanir les rainures de rectification et de pré-contraindre localement le matériau en compression. Il a été montré par le passé qu’il augmentait la durée de vie moyenne en fatigue des éprouvettes technologiques représentatives de l’attache aube/disque. C’est donc un traitement mécanique fondamental. Sur un monocristal le grenaillage, en déformant plastiquement la couche corticale, modifie profondément la structure du matériau, et augmente significativement la concentration en dislocation. Ce nombre important de dislocations contribue à une rotation locale du réseau cristallographique. Ainsi la couche corticale ne peut plus être considérée comme orthotrope. Pour caractériser ces rotations, deux techniques ont été utilisées. Dans un premier temps, l’observation de la matrice γ a permis d’identifier la rotation du réseau cristallographique, ensuite une série de mesures par EBSD a permis de la quantifier. Par la suite, c’est la relaxation de la contrainte résiduelle introduite par le grenaillage dans un alliage polycristallin à base de nickel qui a été étudiée. Deux processus de relaxation ont été étudiés, le premier thermique et le second mécanique. Ces deux processus ont des cinétiques extrêmement importantes comparées à la durée de vie d’un moteur. Pour analyser le comportement en fissuration d’un plan monocristallin (MC2) soumis à une sollicitation de fretting imposée par un cylindre polycristallin (Udimet 720), trois orientations ont été sélectionnées parcourant l’ensemble des configurations rencontrées sur une aube. Les premiers résultats montrent que l’orientation du plan n’a pas d’influence sur le coefficient de frottement. Ensuite, le seuil d’amorçage a été déterminé, ce faisant les observations fractographiques des plans de MC2 ont mis en valeur l’importance de l’orientation relative des plans octaédriques et de la direction de fretting sur la direction de croissance des fissures. Ces constatations ont conduit à la réalisation de modèles éléments finis prenant en compte le caractère cubique des monocristaux de nickel et permettant d’évaluer l’amplitude du cisaillement résolu subi par un plan de fretting. [...] / No abstract

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In this thesis, secondary flow in a two stage, low aspect ratio turbine is investigated using CFD. A parameter study is carried out to investigate how the turbine performance is affected by the choice of aspect ratio. This is done in two steps, first by changing the blade height and then the blade size. The study shows that increasing the aspect ratio will lead to a significant increase of efficiency, but the effect diminishes for large aspect ratios, at which the efficiency moves towards an asymptotic value. Furthermore it is shown that increasing the aspect ratio to a certain value by changing the blade height results in a higher efficiency compared to changing the blade size, which is due to the difference in hub-to-tip ratio. An attempt to quantify the secondary losses is also made by looking at the radial kinetic energy at the outlet of a blade row. It turns out though, that the radial kinetic energy does not follow the same trend as the total pressure loss coefficient, which implies that it can not be used to quantify the secondary losses. Lastly, an effort to improve the method used for generating blade profiles is made, and the updated method is used to redesign rotor 2 to reduce losses.

In the power generation sector, more specifically, the gas turbine industry, competition has forced the lead time-to-market for product advancements to be more important than ever. For design engineers, this means that product design iterations and final product development must be completed within both critical time windows and budgetary constraints. Therefore, two areas that have received significant attention in the research and in practice are: (1) rapid prototyping technology development, and (2) rapid prototyping technology selection. Rapid prototyping technology selection is the focus of this research. In practice, selecting the rapid prototyping method that is acceptable for a specific design application is a daunting task. With technological advancements in both rapid prototyping and conventional machining methods, it is difficult for both a novice design engineer as well as an experienced design engineer to decide not only what rapid prototyping method could be applicable, but also if a rapid prototyping method would even be advantageous over a more conventional machining method and where in the manufacturing process any of these processes would be utilized. This research proposes an expert system that assists a design engineer through the decision process relating to the investment casting of a superalloy gas turbine engine component. Investment casting is a well-known technique for the production of many superalloy gas turbine parts such as gas turbine blades and vanes. In fact, investment-cast turbine blades remain the state of the art in gas turbine blade design. The proposed automated expert system allows the engineer to effectively assess rapid prototyping iii opportunities for desired gas turbine blade application. The system serves as a starting point in presenting an engineer with commercially-available state-of-the-art rapid prototyping options, brief explanations of each option and the advantages and disadvantages of each option. It is not intended to suggest an optimal solution as there is not only one unique answer. For instance, cost and time factors vary depending upon the individual needs of a company at any particular time as well as existing strategic partnerships with particular foundries and vendors. The performance of the proposed expert system is assessed using two real-world case studies. The first case study shows how the expert system can advise the design engineer when suggesting rapid manufacturing in place of investment casting. The second case study shows how rapid prototyping can be used for creating part patterns for use within the investment casting process. The results from these case studies are telling in that their implementations potentially result in an 82 to 94% reduction in design decision lead time and a 92 to 97% cost savings.

Clearances between gas turbine casings and rotating blades is of quite importance on turbo machines since a significant loss of efficiency can occur if the clearances are not predicted accordingly. The radial thermal growths of the blade may be over or under predicted if poor assumptions are made on calculating the metal temperatures of the surfaces exposed to the fluid. The external surface of the blade is exposed to hot gas temperatures and it is internally cooled with air coming from the compressor. This cold air enters the radial channels at the root of the blade and then exists at the tip. To obtain close to realistic metal temperatures on the blade, the Conjugate Heat Transfer (CHT) approach would be utilized in this research. The radial thermal growth of the blade would be then compared to the initial guess. This work focuses on the interaction between the external boundary conditions obtained from the commercial Computational Fluid Dynamics software package CFX, the internal boundary conditions along the channels from a 1D flow solver proprietary to Siemens Energy, and the 3D metal temperatures and deformation of the blade predicted using the commercial Solid Mechanics software package ANSYS. An iterative technique to solve CHT problems is demonstrated and discussed. The results of this work help to highlight the importance of CHT in predicting metal temperatures and the implications it has in other aspect of the gas turbine design such as the tip clearances.

This study presents an attempt to reduce the losses produced by the purge flow in a turbine stage by incorporating circumferential platform contouring. Two contours are proposed and compared against a baseline at different levels of swirl. The computational simulations were performed using a RANS three-dimensional Computational Fluid Dynamics code with the Shear Stress Transport turbulence model. The results of steady simulations demonstrate that for the first contour, when the flow is swirled to 50% of the rim speed, the purge flow exits the cavity with less cross flow. This in turn reduces the strength of the passage vortex. However, at swirl extremes of 0% and 100% the baseline has the best performance. The results show that a carefully designed platform has the potential to reduce losses when the operating condition is in the proximity of 50% swirl.

The erosion damage to stator and rotor blades associated with flow containing particulates in turbines is investigated. The main parameters studied are blade leading edge thickness, blade turning angle, turbine inlet temperature, particle size, and particle densities. The computer programs used in the investigation are based on inviscid flow theory. Flow velocities relative to blades ranged up to sonic values. Results predict that decreasing flow turning angles and increasing blade leading edge thicknesses are the most effective ways to reduce erosion damage caused by impacting particles. Decreasing particle sizes and densities can also significantly reduce erosion rates. The erosion model uses the brittle and ductile mode response exhibited by materials subjected to particle impacts to predict the total erosion damage. The accuracy for small (1 micron) particles is questionable since some of these particles will have long residence times in the boundary layers, causing deposition rather than erosion. / Master of Science

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