Polyetheretherketone (PEEK) is a designation given to materials of the polyaryletherketone family having a characteristic distribution of ether and ketone groups in the polymer backbone. PEEK materials have high strength and chemical resistance as well as very high melting points and glass transition temperatures. Because of this combination of properties, PEEK materials find use for wear application in extreme environments where they provide a light-weight and corrosion resistant bearing material that often does not require lubrication. An initial study focused on determining the effects of supplier and molecular weight on the wear of particular PEEK materials, in addition to the effect of contact pressure. This work is significant because it highlights the fact that tribologically relevant polymers, such as PEEK materials, vary greatly in terms of their polymer morphology and processing history, and this variation must be recognized by investigators when reporting wear data. Because of their light weight, chemical resistance, and self-lubricating properties, polymers are used in applications ranging from biomedical to aerospace. Some polymers exhibit significant differences in wear resistance based on whether they are in unidirectional or multidirectional sliding. Shear induced polymer chain orientation is believed to be responsible for this behavior. Polyetheretherketone (PEEK) has excellent wear resistance, but its multidirectional sliding behavior has not been thoroughly investigated. A factorial multidirectional pin-on-plate wear study of PEEK was conducted with a focus on molecular weight and sliding path directionality. These factors were studied for their correlation to overall wear performance. Additionally, transfer film thickness was measured at locations along the wear path using white light interferometry. A result of this work has been a greater understanding of PEEK wear mechanisms in various sliding configurations and how they relate to transfer film formation. A major outcome was the development of a quantitative metric to describe transfer film thickness and continuity. It was found that thinner more continuous transfer films form under sliding conditions that change direction rather than overlapping along the same path. The thinner more continuous transfer film was found to also correspond with statistically lower wear behavior. Scanning electron microscope (SEM) investigation of the transfer film and pin wear surface confirmed the relationship between transfer film quality and wear.

The chemical nature of the Transfer Film (T.F.) or Third-Body Layer (T.B.L.) formed at the friction interface of an automotive friction brake during off-brake motoring has been studied using Energy Dispersive X-ray (E.D.X.) analysis and Scanning Electron Microscopy (S.E.M.). Although these third-body layers are deposited on both mating surfaces of the friction couple, special attention has been paid to those formed on the disc brake rotor surface. Concurrently, detailed investigations have been undertaken examining the temperature-dependent, physico-chemical interactions of friction material constituents with each other, atmospheric oxygen and countermember materials using X-Ray Diffraction (X.R.D.).Evidence is presented relating the tribological performance of the friction pair to both the transfer film thermochemistry and the friction material composition. Among those characteristics describing the tribological performance of the friction couple, particular attention has been applied to the generation of Disc Thickness Variations (D.T.V.) induced by Off-Brake or Non-Braking Wear (O.B.W. or N.B.W.). The critical role of solid lubricants and abrasive friction modifiers and their effectiveness over a range of contact pressures / temperatures has received particular attention. Information obtained using various surface analytical techniques combined with detailed dimensional assessments of the affected triboelements has been used to show the considerable significance of abrasive particle size in determining the overall tribological behaviour of the friction pair, especially with respect to the wear regime and extent encountered at the surface of the countermember during O.B.W.Wear mechanisms are described for the generation of off-brake wear, these varying with friction material formulation. Dynamic and temperature-dependent influences on the level of in-service disc brake rotor runout are named as causes for particular forms of disc thickness variation generated by aggressive friction materials. Keywords: Third-body layer; Transfer film; Tribochemistry; Automotive Friction Braking; Cold Judder; Disc Thickness Variation; Disc Brake; Friction Material.

Full-coverage or effusion cooling is commonly used in the thermal management of gas turbine combustion systems. The combustor environment is characterised by highly turbulent free-stream conditions and relatively large turbulent length scales. This turbulent flow field is predominantly created by the upstream fuel injector for lean burn systems. In rich burn systems the turbulent flow field is augmented further by the addition of dilution ports. The available evidence suggests that large energetic eddies interact strongly with the injected coolant fluid and may have a significant impact on the film-cooling performance. The desire to create compact low-emission combustion systems with improved specific fuel consumption, has given rise to a desire to reduce the quantity of air used in wall cooling, and has led to the need for improved cooling correlations and validated computational methods. In order to establish a greater understanding of effusion cooling under conditions of very high free-stream turbulence, a new laboratory test facility has been created that is capable of simulating representative combustor flow conditions, and that allows for a systematic investigation of cooling performance over a range of free-stream turbulence conditions (up to 25% intensity, integral length scale-to-coolant hole diameter ratios of 26) and coolant to mainstream density ratios (??_c/??_??? ???2). This thesis describes this new test facility, including the method for generating combustor relevant flow conditions. The hot side film cooling performance of cylindrical and fanned hole effusion has been evaluated in terms of adiabatic film-cooling effectiveness and normalised heat transfer coefficient (HTC) and heat flux reduction (HFR). Infrared thermography was employed to produce spatial resolved surface temperature distributions of the effusion surface. The analysis of this data is supported by fluid temperature field measurements. The interpretation of the data has established the impact of turbulence intensity, integral length scale and density ratio on the mixing processes between free-stream and coolant flows. Elevated levels of free-stream turbulence increase the rate of mixing and degrade the cooling effectiveness at low blowing ratios whereas at high blowing ratios, where the coolant detaches from the surface, a modest increase has been observed under certain conditions; this is due to the turbulent transport of the detached coolant fluid back towards the wall. For angled cylindrical hole injection the impact of density ratio as an independent parameter was found to be relatively weak. Adiabatic effectiveness data gathered at DR's of 1 - 1.4 scaled reasonable well when plotted against momentum flux ratio. This suggests data collected at low DR's can be scaled to engine representative DR's. The investigation of shaped cooling holes found fanned effusion has the potential to dramatically improve film effectiveness. The diffusion of the flow through a fanned exit prevented jet detachment at blowing ratios up to 5, increasing spatially averaged effectiveness by 89%.

Considerable reductions in gas turbine weight and fuel consumption can be achieved by operating at a higher turbine entry temperature. The move to lean combustors with flatter outlet temperature profiles will increase temperatures on the turbine endwalls. This work will study methods to improve endwall film cooling, to allow these advances. Turbine secondary flows are caused by a deficit in near-wall momentum. These flow features redistribute near-wall flows and make it difficult to film-cool endwalls. In this work, endwall film cooling was studied by CFD and validated by experimental measurements in a linear cascade. This study will add to the growing body of evidence that injection of high momentum coolant into the upstream boundary layer can suppress secondary flows by increasing near-wall momentum. The reduction of secondary flows allows for effective cooling of the endwall. It is also noted that excess near-wall momentum is undesirable. This leads to upwash on the vane, driving coolant away from the endwall. A passive-scalar tracking method has been devised to isolate the contribution of individual film cooling holes to cooling effectiveness. This method was used to systematically optimize endwall cooling systems. Designs are presented which use half the coolant mass flow compared to a baseline design, while maintaining similar cooling effectiveness levels on the critical trailing endwall. By studying the effect of coolant injection on vane inlet total pressure profile, secondary flows were suppressed and upwash on the vane was reduced. The methods and insight obtained from this study were applied to a high pressure nozzle guide vane endwall from a current engine. The optimized cooling system developed offers significant improvement over the baseline.

Heat management is an essential part of combustor design, as operating temperatures within the combustor generally exceed safe working temperatures of the materials employed in its construction. Two principal methods used to manage this heat are impingement and film cooling. Impingement heat transfer refers to jets of impinging fluid delivered by orifices integrated into internal structures in order to remove undesired heat. This mode of heat transfer has a relatively high effectiveness, making it an attractive method of heat management. As such, a considerable number of studies have been done on the subject providing a substantial body of useful knowledge. However, there are innovative cooling configurations being used in gas turbines which generate compressibility and temperature ratio effects on heat transfer which are currently unexplored. Presented here are data showing that these effects have a significant impact on heat transfer and new correlations are presented to account for temperature ratio and Mach number effects for a range of conditions. These findings are significant and can be applied to impinging flows in other areas of a gas turbine engine such as turbine blades and vanes. Film cooling refers to the injection of coolant onto a surface through an array of sharply angled holes. This is done in a manner that allows the coolant to remain close to the surface where it provides an insulating layer between the hot gas freestream and the cooler surface. In order to improve turbine efficiency, research efforts in film cooling are directed at reducing film cooling flow without decreasing turbine inlet temperatures. Both impingement cooling and film cooling are heavily utilized in combustor liners. Frequently, cooling air first impinges against the back side of the liner, then the spent impingement fluid passes through film cooling holes. This arrangement combines the convective heat transfer of the impinging jets convection as the coolant passes through the film cooling holes and the benefits that come from having a thin film of cool air between the combustor wall and the combustion products. In order to improve the understanding of internal cooling in gas turbine engines, the influence of previously unexplored physical parameters such as compressible flow effects and temperature ratio in impingement flows and variable blowing ratio in a film cooling array must be examined. Prior to this work, there existed in the available literature only an extremely limited exploration of compressibility effects in impingement heat transfer and the results of separately examining the effects of Mach number and Reynolds number. The film cooling literature provides no information for a full array of film cooling holes along a contraction at high blowing ratios. Exploring these effects and conditions adds to the body of available data and allows the validation of numerical predictions.

Film cooling is vital for the cooling of the blades and vanes in the high temperature environment of a jet engine high pressure turbine stage. Previous research into film cooling has typically concentrated on its time-mean performance. However, results from other studies upon more simplified geometries, suggest that coherent unsteadiness is likely to also be present in film cooling flows. The research presented in this thesis, therefore, aims to characterise what coherent unsteadiness, if any, is present within film cooling flows. Cylindrical and shaped cooling holes, located upon the pressure surface of a turbine blade within a large scale linear cascade, have been investigated. A blowing ratio range of 0.5 to 2.0 has been investigated, with either a plenum or perpendicular crossflow at the cooling hole inlet. Particle Image Velocimetry, high speed photography and Hot Wire Anemometry have been used to investigate the jet downstream of both cooling holes. The impact of crossflow at the hole inlet upon the flowfield inside both cooling holes has been investigated using Hot Wire Anemometry and a further numerical model solved by applying TBLOCK. The results presented in the current thesis, show the existence of two coherent unsteady structures in the jet downstream of both the cylindrical and the shaped holes. These structures are called shear layer vortices and hairpin vortices, and their formation is dependent on the velocity difference across the jet shear layer. Inside the cooling hole coherent hairpin vortices also appear to occur, with their formation dependent on the direction and magnitude of the crossflow at the hole inlet. The coherent unsteadiness presented here is shown for the first time for film cooling flows, and recommendations to build on the current study, in what is potentially an interesting research area, are made at the end of this thesis.

Les polymères sont de plus en plus répandus dans différents secteurs industriels comme une alternative aux métaux. En effet, les composants en polymère permettent une réduction accrue du poids et une meilleure inertie chimique dans les structures où ils sont utilisés. Ils permettent également une réduction du frottement sans recourir à des systèmes de lubrification externe. Parmi ces polymères, le PTFE - connu sous le nom du Teflon et découvert en 1938 - se caractérise par une morphologie semi-cristalline particulière où les molécules de PTFE forment des superstructures dites « à bandes ». Il possède d’excellentes propriétés thermiques, un très faible coefficient de frottement et une très bonne inertie chimique justifiant sa vaste utilisation dans différentes applications : comme renfort de type lubrifiant solide, revêtement antiadhésif, isolant électrique des câbles dans le domaine de l’aéronautique, récipients pour des produits chimiques réactifs. Ce travail de thèse s’inscrit dans ce contexte et adopte une démarche tribologique globale. Il étudie les réponses tribologiques d’un contact PTFE/Ti-6Al-4V sollicité en fretting-usure alternatif - notamment en mode glissement total et reciprocating - dans une configuration cylindre/plan. Les paramètres étudiés sont : nombre de cycles, vitesse de glissement, force normale et propriétés thermomécaniques et surfaciques des matériaux. Cette étude propose de nouvelles formalisations analytiques basées sur l’approche d’Archard et établit des lois de frottement et d’usure qui intègrent les effets de ces paramètres. Le rôle joué par le film de transfert dans la détermination des réponses tribologiques est également mis en évidence. / The tribology of thermoplastic polymers is nowadays one of the major issues in several engineering fields. These non-metallic materials increasingly provide a useful alternative to metals that are in contact with a harder counterface. Areas of application are various and include offshore oil-drilling, aeronautics and the automotive industry and biomedical applications such as prosthesis designs. The aim of polymer-metal, and especially PTFE/Ti-6Al-4V, designs is to reduce wear and friction. Polytetrafluoroethylene (PTFE) is one such interesting thermoplastic polymer that has been closely studied since it was discovered in 1938. It is a high-performance engineering polymer characterized especially by a high melting point, chemical inertness and low coefficient of friction due to its banded structure. PTFE is a tribologically attractive material, widely used especially as a solid lubricant in a variety of dry sliding tribosystems. The purpose of this study is to evaluate friction and wear rate evolution for gross slip fretting-reciprocating sliding conditions of PTFE/Ti-6Al-4V interface in a cylinder/plane configuration. Various investigations have been made to study the influence of wear test conditions: number of cycles, sliding speed, normal force, materials thermomechanical and surface properties. Analysis aimed to determine tribological processes as well as formulation of experimental evolution of friction and wear rates regarding selected parameters. Finally, characterization of the transfer film formed on the counterpart under repeated cyclic sliding is undertaken to determine its role and its interference with wear and friction response.

This thesis presents a joint computational and experimental investigation into the heat transfer to unshrouded turbine blade tips suitable for use in high bypass ratio, large civil aviation turbofan engines. Both the heat transfer to the blade tip and the over-tip leakage flow over the blade tip are characterised, as each has a profound influence on overall engine efficiency. The study is divided into two sections; in the first, computational simulations of a very large scale, low speed linear cascade with a flat blade tip were conducted. These simulations were validated against experimental data collected by Palafox (2006). A thorough assessment of turbulence models and minimum meshing requirements was performed. The standard k-ω and standard k-ϵ turbulence models significantly overpredicted the turbulence levels within the tip gap. The other models were very similar in performance; the SST k-ω and realisable k-ϵ models were found to be the most suitable for the flow environment. The second section documents the development and testing of a novel hybrid blade tip design, the squealet tip, which seeks to combine the known benefits of winglet and double squealer tips. The development of the external geometry was performed primarily through engine-representative CFD simulations at a range of tip gaps from 0.45% to 1.34% blade chord. The squealet tip was found to have a similar aerodynamic sensitivity to tip clearance as a baseline double squealer tip, with a tip gap efficiency exchange rate of 2.03, although this was 18% greater than the alternative winglet tip. The squealet tip displayed higher predicted stage efficiency than the winglet tip over the majority of the range of tip clearances investigated, however. The overall heat load was reduced by 14% compared with the winglet tip but increased by 28% over the double squealer tip, primarily due to the change in wetted surface area. The predicted local heat transfer coefficients were similar across all geometries. A realistic internal cooling plenum and an array of blade tip cooling holes were subsequently added to the squealet tip geometry and the cooling configuration refined by the selective sealing of cooling holes. Film cooling performance was largely assessed by the predicted adiabatic wall temperature distributions. A viable cooling scheme which reduced the cooling air requirement by 38% was achieved, compared to the initial case which had all cooling holes open. This was associated with just a 7% increase in blade tip heat flux and no penalty in peak temperature on the blade tip. Film cooling air ejected from holes on the blade suction side was swept away from the blade tip region, making the squealet rim at the crown of the blade particularly challenging to cool. It was demonstrated that this region could be cooled effectively by ballistic cooling from holes located on the blade tip cavity floor, although this was expensive in terms of the mass flow rate of cooling air required. The computational results were reinforced with experimental data collected in a transonic linear cascade. Downstream aerodynamic loss measurements were taken for a linearised version of the squealet tip design without cooling at nominal tip gaps of 0.45%, 0.89% and 1.34% blade chord, which was compared to similar data taken by O’Dowd (2010) for flat and winglet tips. The squealet was seen to have a similar aerodynamic loss to the flat tip and a reduced loss compared with the winglet tip. Full surface heat transfer measurements were taken for the uncooled squealet tip, at tip gaps of 0.89% and 1.34% blade chord, and for two configurations of the cooled squealet tip, at a tip clearance of 0.89% blade chord. The qualitative similarity between the measured heat transfer distributions and the those predicted by the engine-representative CFD simulations was good. A CFD simulation of the uncooled linear cascade environment at the 1.34% blade chord tip clearance was performed using a single blade with translationally periodic boundary conditions. The predicted size of the over-tip leakage vortex was smaller than had been measured, resulting in a large underprediction in the magnitude of the downstream area-averaged aerodynamic loss. The magnitudes of the predicted blade tip Nusselt number distribution were similar to those produced by the engine-representative CFD simulations and lower than that measured experimentally. Differences in the shape of the Nusselt number distribution were observed in the vicinity of regions of separated and reattaching flow, but other salient features were replicated in the computational data. The squealet tip has been shown to be a promising, viable unshrouded blade tip design with an aerodynamic performance similar to the double squealer tip but is more amenable to film cooling. It is significantly lighter than a winglet tip and incurs a reduced thermal load. The squealet tip design can now be developed into a blade tip geometry for use in real engines to provide an alternative to shrouded turbine blades and current unshrouded blade tip designs. A commercial CFD solver, Fluent 6.3, was shown to capture blade tip heat transfer and over-tip leakage flow sufficiently well to be a useful design guide. However, the sensitivity of the flow structure (and hence, heat transfer) in the forward part of the blade tip cavity suggests that physical testing cannot be eliminated from the design process entirely.

La zone Segment-Piston-Chemise (SPC) d’un moteur thermique est une source importante de dissipation d’énergie liée au frottement sévère. Il est important d’optimiser les lubrifiants agissant dans cette zone car ils ont un impact essentiel sur la durée de vie des pièces mécaniques et la réduction de la consommation énergétique des véhicules. Le dépôt de revêtements sur certaines pièces soumises à des frottements sévères est également une alternative intéressante, en particulier les revêtements carbonés de type DLC et diamant nanocristallin (NCD). La formulation des nouveaux lubrifiants moteurs doit tenir compte de la présence éventuelle de ces nouveaux matériaux. Dans ce travail de doctorat, l’action tribochimique de nouveaux additifs à base de molybdène, couplés à un additif anti-usure de type ZnDTP et à d’autres additifs de type modificateur de frottement, a été étudiée. Ces derniers sont depuis longtemps connus pour leur grande capacité à réduire le frottement grâce à la formation du composé lamellaire di-sulfure de molybdène (MoS2), notamment les additifs MoDTC dimer. Cependant la quantité importante de soufre qu’ils contiennent reste problématique du fait de son impact néfaste sur l’environnement. L’effet synergique d’un additif au molybdène purement organique, appelé Mo-organique, en combinaison avec un additif ZnDTP et d’une triamine grasse, a été découvert. Ce nouveau mélange ternaire permet de réduire jusqu’à 20% la contenance en soufre d’une formulation lubrifiante globale tout en améliorant les performances tribologiques par rapport à celles du MoDTC classique. De ce fait, une réduction du coefficient de frottement atteignant 50% a été observée. Une caractérisation physico-chimique multi-échelles des tribofilms binaires et ternaires dérivés du Mo-organique a été réalisée en utilisant une approche multi-techniques (XPS, ToF-SIMS, FIB/HRTEM). Un mécanisme réactionnel hypothétique conduisant à la formation du MoS2, passant par un composé intermédiaire de type « thiomolybdate » a été proposé. / The friction in the Piston-ring area is a significant cause of the energy waste. It is important to optimize the lubricants acting in this zone because they have an essential impact on the service life of the mechanical parts and the reduction of the energy consumption of vehicles. The coating on relevant part is also an interesting alternative, such as the carbon coating of the DLC type or a nano-crystalline diamond (NCD) coating. If such coating materials are used, the composition of new lubricants has to be adapted correspondingly. This doctoral work-studies the tribo-chemical action of new additives with molybdenum, coupled with an anti-wear additive of the ZnDTP type as well as with some other friction-modifying additives. Some of these additives, especially the MoDTC dimer, are known to reduce the friction through formation of the lamellar di-sulfur composite of molybdenum (MoS2). However, due to high sulfur content these additives produce significant adverse environmental effects. A synergy effect has been proven of a purely organic molybdenum additive, called Moorganic, combined with a ZnDTP additive and from a fatty triamine. This new ternary mixture allows reducing up to 20% the sulfur content in the lubricant’s global formula and improves the tribological properties in comparison with the classical MoDTC. Consequently, the observed reduction of friction coefficient reached 50%. Using the multi-technic approach (XPS, ToF-SIMS, FIB/HRTEM) we realized a multi-scale physicochemical characterization of the binary and ternary tribo-layers that derived from the Mo-organic. A reactional mechanism that leads to the MoS2 formation has been proposed; it goes through the intermediate composite of the « thiomolybdate » type.

No description available.