The dynamic of cavitation in hydraulic components cannot be computed accurately yet and therefore cavitation is hard to predict. The cavitation phenomenon can be divided in three sub-phenomenona: Pseudo-cavitation, Gas-cavitation and Vapour-cavitation. Pseudo-cavitation discribes the enlargement of an air bubble due to a pressure drop. Gas-cavitation refers to bubble growth which is driven by diffusion of dissolved air from the surrounding fluid into the bubble, when the solubility of air in the fluid is lowered by a pressure drop. Vapor-cavitation is the evaporation of the liquid phase on the bubble surface. Usually all three sub-phenomenona occur simultaneously when the pressure decreases and are summarised as cavitation in general. To implement the physics of gas-cavitation in a dynamic mathematical model it is necessary to know the diffusion coefficient of air in the hydraulic liquid and the maximum amount of air which can be dissolved in the liquid. The calculation can be accomplished by using the Bunsen coefficient. In this paper both coefficients for three different hydraulic oils are calculated based on experimental results.

High velocity channels are typically designed to discharge surplus water during severe flood events, and these types of flow are distinguished by high velocity, usually supercritical. A major challenge in high velocity channel design is to predict the free surface flow. Being able to predict the free surface flow profile beforehand can assist in selecting the best design for the channel as a whole. When the flow encounters a bridge pier, the streamline of the flow is separated and pressure may drop to a minimum; in contrast, velocity rises to its maximum value. As a result, cavitation damage may occur. The present study has used the computational fluid dynamics code ANSYS-CFX to investigate a full scale, three-dimensional engineering flow simulation of high velocity channels with different cross sections. The simulations were carried out on a high performance computing HPC cluster with 32 nodes. The code is based on the finite volume method and the Volume of Fluid (VOF) method was used to predict the position of the free surface profile. The impact of variation of the following parameters was investigated in terms of the free surface flow profile, both along the centreline and the wall of the channel: the minimum cavity index, and maximum shear stress on both bed and wall of the channel and on bridge pier; aspect ratio (channel bed width/flow depth), bed and side slopes of the channel, different discharges, which are represented by Froude numbers; the length and thickness of the bridge pier. First, the code sensitivity tools for convergence were examined. For this purpose, cases with different mesh sizes were examined and the best size chosen, depending on computation expense and convergence. Then, different turbulence models, such as the standard k-ε, RNG k-ε, and SST turbulence models were tested. The results show that the standard k-ε gives satisfactory results. Next, efforts were made to establish whether the flow achieved steady state conditions. This involved simulating two cases, one with steady state and the other with a transient state. Comparison of the two results shows that the flow properties do not change after three seconds and stay stable thereafter, so the flow can be considered as attaining a steady state. Finally, symmetry within the model geometry was tested, as this would allow a reduction in computation time, with only one side of the symmetrical model needing to be simulated. Two cases were investigated: firstly a simulation of only half of the channel geometry, and secondly a full geometry simulation. A comparison of the results of each case showed that the flow can be considered symmetrical along the centreline of the channel. Next, the code was validated against both numerical and experimental published results. For the free surface flow profile and velocity distribution the published experimental and numerical work of Stockstill (1996) was used; the ANSYS-CFX code results agree more closely with Stockstill’s experimental data than Stockstill’s numerical data. To test for shear stress distribution on the wall, uniform flow within a trapezoidal cross section channel was investigated and the results compared with those presented in the literature. The comparison shows good agreement between the ANSYS-CFX and published experimental works, for the predicted shear stress distributions on the walls and the bed of the channel. In total, sixty cases were simulated in order to investigate the impact of variations in the aforementioned parameters on maximum flow depth (both along the centreline and the wall of the channel) minimum cavity index, and maximum shear stress on both bed and wall of the channel and on bridge pier. Finally, non-dimensional curves are provided in addition to formulae derived from the data regression, which are intended to provide useful guidelines for designers.

L'objectif principal de cette thèse a été de déterminer le rôle joué par l'architecture macromoléculaire du réseau sur les propriétés élastiques non linéaires, la résistance à la rupture et la résistance à la cavitation sous chargement hydrostatique. Nous avons synthétisé, dans des conditions contrôlées, trois réseaux élastomères dits « modèles » de Polyuréthanne (PU), à partir d'un triisocyanate et de polyether diols isomoléculaires (PPG). Une caractérisation physico-chimique fine des réactifs et des réseaux a été réalisée en utilisant des techniques telles que : RMN, FTIR et fractions solubles. Les propriétés élastiques non linéaires, viscoélastiques linéaires et la résistance à la rupture en mode I des trois réseaux modèles ont été caractérisées. Les essais de cavitation ont été effectués sur un dispositif expérimental développé pour cette étude, permettant de suivre les mécanismes de formation de cavités, à la résolution optique près, en temps réel. En menant une analyse systématique des conditions de cavitation, en fonction de la vitesse de déformation et de la température, il est apparu que, contrairement au modèle d'instabilité élastique communément utilisé, l'expansion critique de la cavité n'est pas uniquement pilotée par le module élastique; mais dépend fortement de l'énergie de rupture, GC et de l'extensibilité limite du réseau. Par ailleurs, nous avons observé l'apparition de cavités pré-critiques avant la fracture catastrophique ; ce qui met en évidence l'existence de deux critères : l'un, propre au processus de nucléation, principalement piloté par des mécanismes statistiques et activés thermiquement (distribution de défauts, temps, température, etc.) ; et l'autre, lié à la croissance de la cavité en milieu confiné contrôlé par GIC, et par le comportement aux grandes déformations. Enfin, la présence d'enchevêtrements dans l'architecture du réseau macromoléculaire s'est avérée clairement bénéfique pour stabiliser la croissance de cavités et donc pour renforcer la résistance à la cavitation.

Un modèle de cavitation à poche partielle a été développé et implémenté dans un code potentiel bidimensionnel stationnaire puis dans un code potentiel tridimensionnel instationnaire. L'originalité de la méthode est d'utiliser la technique des vitesses de transpiration pour simuler la présence de la poche de cavitation. Cette méthode permet une fermeture naturelle de la poche sans avoir à imposer un quelconque artifice dans le modèle ou dans la méthode de calcul. Adaptable à tout solveur de modèle d'écoulement, le modèle couplé aux codes potentiels permet, grâce à cette technique, une simulation raisonnablement rapide de l'écoulement cavitant sur un hydrofoil ou une hélice en régime d'écoulement stationnaire ou instationnaire. Le modèle a été validé par comparaison avec des essais en régime stationnaire sur un profil bidimensionnel. On retrouve numériquement les longueurs de poche observées lors des essais ainsi que les pressions mesurées. Quant à l'écoulement tridimensionnel, des résultats disponibles dans la littérature n'ont permis que la validation qualitative au niveau de la longueur de poche. Une campagne d'essais en collaboration avec le BSHC pour mesurer la portance et la traînée d'un hydrofoil a alors été effectuée avec pour objectif la validation tridimensionnelle quantitative. Les résultats obtenus montrent une bonne concordance entre les mesures expérimentales et les résultats numériques, ce qui permet de conclure de façon satisfaisante quant à la validation du modèle. Finalement, des résultats de simulations instationnaires sur des hélices en régime cavitant sont présentés. Les résultats numériques montrent que la cavitation à poche partielle affecte très peu les performances hydrodynamiques de l'hélice.

This thesis investigates the underlying causes of friction and ine ciency within an internal combustion engine, focusing on the ring-liner conjunction in the vicinity of the power-stroke top dead centre reversal. In such lubricated contacts, friction is the result of the interplay between numerous kinetics, with those at micro- and nano-scale interactions being signi cantly di erent than the ones at larger scales. A modi ed Elrod's cavitation algorithm is developed to determine the microscopic tribological characteristics of the piston ring-liner contact. Predicting lubricant tran- sient behaviour is critical when the inlet reversal leads to thin lms and inherent metal-to-metal interaction. The model clearly shows that cavitation at the trailing edge of the ring-liner contact generated pre-reversal, persists after reversal and pro- motes starvation and depletion of the oil lm. Hence, this will lead to boundary friction. A fractal based boundary friction model is developed for lightly loaded asperity con- tacts, separated by diminishing small lms, usually wetted by a layer of molecules adsorbed to the tips of the asperities. In nano-scale conjunctions, a lubricant layering e ect often takes place due to the smoothness of surfaces, which is governed by the surface and lubricant properties. A molecularly thin layer of lubricant molecules can adhere to the asperities, being the last barrier against direct surface contact. As a result, boundary friction (prevailing in such diminishing gaps) is actually determined by a combination of shearing of a thin adsorbed lm, adhesion of approaching as- perities and their plastic deformation. A model for physio-chemical hydrodynamic mechanism is successfully established, describing the formation of thin adsorbed lms between asperities. This model is e ectively integrated with separately devel- oped models that predict the adhesive and plastic contact of asperities.

No description available.

Biodiesel and hydrotreated vegetable oil (HVO) are currently viewed by the transportation sector as the most viable alternative fuels to replace petroleum-based fuels. The use of biodiesel has, however, been limited by the deteriorative effect of biodiesel on rubber parts in automobile fuel systems. This work therefore aimed at investigating the degradation of acrylonitrile butadiene rubber (NBR) and fluoroelastomers (FKM) on exposure to biodiesel and HVO at different temperatures and oxygen concentrations in an automated ageing equipment and a high-pressure autoclave. The oxidation of biodiesel at 80 °C was promoted by an increase in the oxygen partial pressure, resulting in the formation of larger amounts of hydroperoxides and acids in the fuel. The fatty acid methyl esters of the biodiesel oxidized less at 150 °C on autoclave aging, because the termination reactions between alkyl and alkylperoxyl radicals dominated over the initiation reactions. HVO consists of saturated hydrocarbons, and remained intact during the exposure. The NBR absorbed a large amount of biodiesel due to fuel-driven internal cavitation in the rubber, and the uptake increased with increasing oxygen partial pressure due to the increase in concentration of oxidation products of the biodiesel. The absence of a tan δ peak (dynamical mechanical measurements) of the bound rubber and the appearance of carbon black particles devoid of rubber suggested that the cavitation was caused by the detachment of bound rubber from particle surfaces. A significant decrease in the strain-at-break and in the Payne-effect amplitude of NBR exposed to biodiesel was explained as being due to the damage caused by biodiesel to the rubber-carbon-black network. During the high-temperature autoclave ageing, the NBR swelled less in biodiesel, and showed a small decrease in the strain-at-break due to the cleavage of rubber chains. The degradation of NBR in the absence of carbon black was due only to biodiesel-promoted oxidative crosslinking. The zinc cations released by the dissolution of zinc oxide particles in biodiesel promoted reduction reactions in the acrylonitrile part of the NBR. Heat-treated star-shaped ZnO particles dissolved more slowly in biodiesel than the commercial ZnO nanoparticles due to the elimination of inter-particle porosity by heat treatment. The fuel sorption was hindered in HVO-exposed NBR by the steric constraints of the bulky HVO molecules. The extensibility of NBR decreased only slightly after exposure to HVO, due to the migration of plasticizer from the rubber. The bisphenol-cured FKM co- and terpolymer swelled more than the peroxide-cured GFLT-type FKM in biodiesel due to the chain cleavage caused by the attack of biodiesel on the double bonds formed during the bisphenol curing. The FKM rubbers absorbed biodiesel faster, and to a greater extent, with increasing oxygen concentration. It is suggested that the extensive biodiesel uptake and the decrease in the strain-at-break and Young’s modulus of the FKM terpolymer was due to dehydrofluorination of the rubber by the coordination complexes of biodiesel and magnesium oxide and calcium hydroxide particles. An increase in the CH2-concentration of the extracted FKM rubbers suggested that biodiesel was grafted onto the FKM at the unsaturated sites resulting from dehydrofluorination. / <p>QC 20170227</p>

A presente dissertação investiga o comportamento do escoamento em um túnel de cavitação, através de simulações fluido-dinâmicas computacionais, excluindo a bomba de circulação. Para tanto um extenso estudo buscou selecionar a abordagem numérica e configurações de simulação mais adequadas, de uma forma a reproduzir as características hidrodinâmica inerentes à operação do túnel. Portanto, as informações referentes à perda de carga, uniformidade no perfil de velocidade, tendência de cavitação, descolamento e altura da camada limite foram apreciadas. Esse estudo foi direcionado por dados provenientes de métodos empíricos da literatura, resultados experimentais do próprio túnel pesquisado e de outros túneis. Em geral, os resultados foram satisfatórios, pois a perda de carga estimada foi semelhante ao obtido experimentalmente. Além de serem identificados os trechos com efeito desfavorável na uniformidade do escoamento, foram determinadas a distribuição de pressão nas aletas e o perfil de velocidade incidente na bomba. / This study deals with a cavitation tunnel flow (except the pump region) through computational fluid dynamic simulations. Therefore, previously one made a literature survey to aid the selection of the most appropriate techniques and simulation settings to simulate the hydrodynamic characteristics of the tunnel. The pressure drop, uniformity of the velocity profile, existence of cavitation, detachment and height of the boundary layer were evaluated. This study used empirical methods and also experimental results. The results were satisfactory, as long as the estimated pressure drop was close to the one obtained experimentally. In addition, it was forecasted low flow uniformity in some parts of the tunnel, the pressure distribution on the fins and the incident velocity profile at the pump.

O presente trabalho consiste na avaliação de um novo modelo de reator de cavitação hidrodinâmica pertencente à classe dos homogeneizadores de alta velocidade, comercialmente conhecidos por High Speed Homogeneizers. É sabido que esse tipo de equipamento possui um amplo leque de aplicações industriais, sendo muitas vezes constituintes de operações unitárias para emulsificação, homogeneização, síntese química, dentre outras. Neste trabalho fez-se um estudo da eficiência deste novo modelo para duas aplicações, o aquecimento controlado de líquidos e a produção de emulsões de óleo em água com diâmetros de partículas em escala sub-micrométricas. As duas aplicações foram estudadas em uma planta piloto, de onde foi possível constatar o potencial de aplicação desse novo modelo de reator, especialmente devido ao baixo custo operacional e praticidade de operação. / This work evaluates a new model of hydrodynamic cavitation reactor, which belongs to the class of high-speed homogenizers. This type of equipment has a wide range of industrial applications and it is often constituent of unit operations for emulsification, homogenization, chemical synthesis, among others. In this master thesis a study regarding the efficiency of this new model for two applications was made: the controlled heating of fluid and the production of oil in water emulsions with particle diameters in the sub-micrometer scale. The two applications were studied in a pilot plant, where it was possible to see the potential of applying this new model of reactor, especially due to the low operating costs and practicality of operation.

Neste trabalho pretende-se estabelecer relações entre a micro e a nanoestrutura e a resistência à erosão-cavitação em aços inoxidáveis com alto teor de nitrogênio. Estas relações serão determinadas através do estudo dos mecanismos de deformação plástica e degradação observado nas superfícies e na micro e nanoestrutura durante ensaios de cavitação vibracional. As amostras do aço inoxidável duplex UNS S31803, com aproximadamente 0,9% N em peso, foram obtidas por nitretação gasosa em alta temperatura, divididas em amostras com nitrogênio em solução sólida e solubilizadas e amostras com nitrogênio em solução sólida e encruadas. As amostras de ensaio foram submetidas à cavitação vibratória em água destilada. Os mecanismos de dano que operam nas diferentes etapas do desgaste foram analisados utilizando microscópio eletrônico de varredura (MEV) para diferentes tempos de ensaio, analisando sempre o mesmo local através da perda de massa. A partir dos resultados obtidos nesse trabalho, observa-se que as amostras com nitrogênio em solução sólida e solubilizadas (318HTGN+Sol), e as laminadas (318HTGN+Enc), apresentaram taxa de desgaste 16 e 172 vezes menores, respectivamente, comparadas com o aço AISI 304L solubilizado. Por outro lado, a amostra do aço nitretado, com nitrogênio em solução sólida e solubilizado (318HTGN+Sol), e as laminadas (318HTGN+Enc), apresentaram taxa de desgaste 2 e 17 vezes menores, respectivamente, comparadas com o aço AISI 304L encruado. Foi possível constatar um aumento considerável do tempo de incubação, o que permitiu, de forma detalhada, acompanhar os mecanismos de deformação e evidenciar nos primeiros estágios da cavitação a formação de bandas de escorregamento. Foi possível, também, observar como os contornos de grão ou de macla, locais preferenciais de nucleação do dano, atuam quando comparados com danos no interior dos grãos. / This work aims to establish relationships between micro and nanostructure and cavitation-erosion resistance in high nitrogen stainless steels. These relations will be determined through the study of plastic deformation mechanisms and surface damage, as well as micro and nanostructure during vibratory cavitation testing. An UNS S 31803 samples with 0.9 N (%wt) were obtained by high temperature gas nitriding. Two sample groups were obtained: solubilized in solid solution and solubilized and work hardened. Vibratory cavitation tests were performed in distilled water. The damage mechanisms operating in different stages of wear were analyzed for different times in the same location using scanning electron microscopy (SEM). Mass losses were periodically measured during the cavitation-erosion tests. From the results, it was observed that the (318HTGN+Sol) samples, with nitrogen in solid solution showed wear rates 16.1 times smaller compared to those of the solubilized UNS S30403 steel. The work-hardened (318HTGN+Enc) samples showed wear rates 172.4 times smaller than UNS S30403specimens. The comparison of the 318HTGN+Sol samples with the work-hardened 304L+Enc specimens showed wear rates 2 times smaller, while the 318HTGN+Enc samples showed a wear rate 17 times smaller. Finally, a considerable increase of incubation time was readily observed, which allowed the detailed observation of the deformation mechanisms and the identification of the slip bands formation during the early stages of cavitation erosion. Additionally it was possible to observe the way the grain boundaries and twin boundaries are damaged, acting as preferential sites where the damage initially occurs.