Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2003. / Thesis Advisor(s): Terry R. McNelley. Includes bibliographical references (p. 63-65) Also available online.

Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis (Ph. D.)--Aerospace Engineering, Georgia Institute of Technology, 2008. / Ruzzene, Massimo, Committee Chair ; Bottasso, Carlo L., Committee Member ; Costello, Mark, Committee Member ; Hodges, Dewey H., Committee Member ; Weston, Neil, Committee Member.

In this dissertation the power draw, the mass transfer and the liquid mixing behaviour of boiling and hot sparged stirred tank reactors (STRs) were investigated. The power draw characteristics of six different impellers were studied under varying operating conditions and expressed in terms of the relative power draw, RPD. Generally the impellers showed considerably higher power draw in hot gassed than in comparable cold conditions. The main phenomenon in hot systems, the evaporation of liquid into a gas bubble suddenly exposed to it, was investigated experimentally. The growth of nitrogen and air bubbles in hot water was found to be very fast and be completed typically within milliseconds after their exposure to the liquid. This was confirmed by studies applying acoustic bubble sizing techniques which were carried out together with Richard Manasseh from CSIRO, Melbourne, Australia. Experimental studies of the gas-liquid mass transfer in hot sparged STRs were carried out using a tank of 450 mm in diameter agitated by a 180 mm Rushton turbine. Experiments involving the temperature kinetics in an air-water system and absorption and desorption of ammonia were used to determine gas and also some liquid side mass transfer coefficients. The gas side coefficients kga were found to be typically in the order of 0.01 s-1. An enhancement of the liquid side mass transfer was also observed. Both Fick and Stefan-Maxwell models were employed to express gas side mass transfer coefficients. The models predict there will be a decrease of the coefficients at high concentrations. In the hot sparged tank the temperature is essentially uniform throughout the STR. Liquid mixing times have been measured for various boil-off and sparging rates. No significant deviations from the single phase or the cold gassed mixing times in this equipment were recorded.

It is believed, that due to the large surface areas provided by the nano scale materials, various composite properties could be enhanced when such particles are incorporated into a polymer matrix. There is also a trend of utilizing natural resources or reusing and recycling materials that are already available for the fabrication of the new composite materials. Cellulose is the most abundant natural polymer on the planet, and therefore it is not surprising to be of interest for composite fabrication. Basic structures of cellulose, comprised of long polysaccharide chains, are the building blocks of cellulose nano fibers. Nano fibers are further bound into micro fibrils and macro fibers. Theoretically pure cellulose nano fibers have tremendous strengths, and therefore are some of the most sought after nano particles. The fiber extraction however is a complex task. The ultrasound, which creates pressure variation in the medium, was employed to extract nano-size cellulose particles from microcrystalline cellulose (MCC). The length and the intensity of the cavitations were evaluated. Electron microscopy studies revealed that cellulose nanoparticles were successfully obtained from the MCC after ultrasound treatment of just 30 minutes. Structure of the fractionated cellulose was also analyzed with the help of X-ray diffraction, and its thermal properties were evaluated with the help of differential scanning calorimetry (DSC). Ultrasound treatment performed on the wheat straw, kenaf, and miscanthus particles altered fiber structure as a result of the cavitation. The micro fibers were generated from these materials after they were subjected to NaOH treatment followed by the ultrasound processing. The potential of larger than nano-sized natural fibers to be used for composite fabrication was also explored. The agricultural byproducts, such as wheat or rice straw, as well as other fast growing crops as miscanthus or kenaf, are comprised of three basic polymers. Just like in wood the polymers are: cellulose, hemicelluloses, and lignin. When subjected to elevated pressures and temperatures, we are able to get access to some of these natural polymers and use them as a matrix material for composite fabrication. Therefore, fabrication of composite materials without addition of synthetic polymers is possible. Thermal and mechanical properties of such composites are evaluated with the help of electron microscopy, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and flexural strength measurements. The chemical changes in the composites are also probed with the help of Fourier transform infrared (FTIR) analysis. Various additives introduced into composite materials provide different properties. The addition of small amounts of synthetic polymers further enhances the properties of natural fiber composites and do not require high fabrication pressures. Calcium sulfite crystals, which are one of the coal combustion products, were combined with the natural fibers and recycled HDPE polymer to form wood substitute composites. The introduction of these additives resulted in composites with the properties similar to those of the natural wood. Coal combustion products, often used in composite material fabrication, contain mercury which may be rereleased during composite fabrication. Mercury behavior under composite fabrication conditions, such as elevated pressures and temperatures were evaluated. Sulfite rich scrubber material, generated during the flue gas desulphurization process was the main target of the study. It was observed that the release of the mercury is highly dependent on the composite fabrication pressure as well as the temperature.

Made available in DSpace on 2015-07-13T12:10:22Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-11-22. Added 1 bitstream(s) on 2015-07-13T12:25:20Z : No. of bitstreams: 1 000836546.pdf: 6808625 bytes, checksum: 764bf6b60c1ced9815654e070e7b2d47 (MD5) / No atual cenário energético, torna-se cada vez mais imperiosa a necessidade de produzir energia com maior eficiência. Dessa forma, o ganho de eficiência através de processos que permitam um maior tempo entre as manutenções, gera também uma redução nos custos destinados à produção de energia. Entretanto, um dos grandes problemas enfrentados pelas Usinas Hidrelétricas, é o fenômeno da cavitação, que gera como consequência a erosão de partes da turbina, com perda de material e desempenho. Os estudos indicam como forma de minimizar essa situação, um melhor projeto hidrodinâmico das turbinas e a utilização de materiais mais nobres e resistentes a erosão cavitacional. Em turbinas novas, a melhoria do projeto e a utilização de materiais mais nobres, como o ASTM A743 Gr CA6- MN, reduzem os efeitos da cavitação e prolongam a necessidade de recuperação das máquinas. Entretanto, para atenuar esses efeitos nas turbinas que já se encontram em operação, principalmente as com mais de 30 anos de vida, é necessário a aplicação de revestimentos com características que permitam a atenuação desse fenômeno. Nesse sentido, ocorreu o desenvolvimento das ligas austeníticas com cobalto, que em função das dificuldades operacionais existentes na época, inicialmente se mostraram ineficientes para o aumento da resistência a cavitação e dos intervalos entre as recuperações. Visando garantir a operacionalidade dessas ligas foi desenvolvido em 2000, pelo LABSOLDA, o processo MIG/MAG com pulsação térmica, ou bi-pulsado, que proporcionou, junto às usinas hidrelétricas, uma popularização dessa liga na recuperação de erosão cavitacional e uma efetiva melhoria da resistência a cavitação das áreas revestidas das turbinas hidráulicas, como consequência a redução no período de parada e de deposição de material. Entretanto, nos últimos 10 anos, houve uma significativa... / In the current energy scenario, it becomes increasingly imperative to produce energy more efficiently. Thus, the gain in efficiency through processes that allow a greater time between maintenance, also generates a reduction in costs for the production of energy. However, a major problem faced by hydropower plants, is the phenomenon of cavitation, which generates as a result of erosion turbine parts, with loss of material and performance. Studies indicate as to minimize this situation, a better hydrodynamic design of the turbines and the use of materials noblest and resistant to cavitation erosion. In the new turbines, design improvement and the use of noble materials such as ASTM A743 Gr CA6-MN, reduce the effects of cavitation and prolong the need for recovery of machinery. However, to mitigate these effects in turbines that are already in operation, especially with more than 30 years, is necessary the application of coatings with features that enable mitigation of this phenomenon. In that sense, occurred the development of austenitic alloys with cobalt, which due to operational difficulties that existing at the time, initially proved ineffective for increasing the resistance against cavitation and the intervals between recoveries. To ensure the operability of these alloys was developed in 2000 by LABSOLDA, MIG / MAG welding with thermal pulse, or bi-pulsed, which provided, along with hydroelectric plants, one popularization of this alloy in the recovery of cavitation erosion and an effective improvement cavitation resistance of coated areas of hydraulic turbines, the effect of reducing the period of arrest and deposition of material. However, in the last 10 years, significant developments in conventional soldering machines, including the incorporation of electronic controls welding, which also reduces interference welder during welding. In this study, a comparison was made of an stainless steel ...

Orientador: Ruis Camargo Tokimatsu / Banca: Vicente Afonso Ventrella / Banca: Cassius Olivio Figueiredo Terra Ruchert / Resumo: No atual cenário energético, torna-se cada vez mais imperiosa a necessidade de produzir energia com maior eficiência. Dessa forma, o ganho de eficiência através de processos que permitam um maior tempo entre as manutenções, gera também uma redução nos custos destinados à produção de energia. Entretanto, um dos grandes problemas enfrentados pelas Usinas Hidrelétricas, é o fenômeno da cavitação, que gera como consequência a erosão de partes da turbina, com perda de material e desempenho. Os estudos indicam como forma de minimizar essa situação, um melhor projeto hidrodinâmico das turbinas e a utilização de materiais mais nobres e resistentes a erosão cavitacional. Em turbinas novas, a melhoria do projeto e a utilização de materiais mais nobres, como o ASTM A743 Gr CA6- MN, reduzem os efeitos da cavitação e prolongam a necessidade de recuperação das máquinas. Entretanto, para atenuar esses efeitos nas turbinas que já se encontram em operação, principalmente as com mais de 30 anos de vida, é necessário a aplicação de revestimentos com características que permitam a atenuação desse fenômeno. Nesse sentido, ocorreu o desenvolvimento das ligas austeníticas com cobalto, que em função das dificuldades operacionais existentes na época, inicialmente se mostraram ineficientes para o aumento da resistência a cavitação e dos intervalos entre as recuperações. Visando garantir a operacionalidade dessas ligas foi desenvolvido em 2000, pelo LABSOLDA, o processo MIG/MAG com pulsação térmica, ou bi-pulsado, que proporcionou, junto às usinas hidrelétricas, uma popularização dessa liga na recuperação de erosão cavitacional e uma efetiva melhoria da resistência a cavitação das áreas revestidas das turbinas hidráulicas, como consequência a redução no período de parada e de deposição de material. Entretanto, nos últimos 10 anos, houve uma significativa... / Abstract: In the current energy scenario, it becomes increasingly imperative to produce energy more efficiently. Thus, the gain in efficiency through processes that allow a greater time between maintenance, also generates a reduction in costs for the production of energy. However, a major problem faced by hydropower plants, is the phenomenon of cavitation, which generates as a result of erosion turbine parts, with loss of material and performance. Studies indicate as to minimize this situation, a better hydrodynamic design of the turbines and the use of materials noblest and resistant to cavitation erosion. In the new turbines, design improvement and the use of noble materials such as ASTM A743 Gr CA6-MN, reduce the effects of cavitation and prolong the need for recovery of machinery. However, to mitigate these effects in turbines that are already in operation, especially with more than 30 years, is necessary the application of coatings with features that enable mitigation of this phenomenon. In that sense, occurred the development of austenitic alloys with cobalt, which due to operational difficulties that existing at the time, initially proved ineffective for increasing the resistance against cavitation and the intervals between recoveries. To ensure the operability of these alloys was developed in 2000 by LABSOLDA, MIG / MAG welding with thermal pulse, or bi-pulsed, which provided, along with hydroelectric plants, one popularization of this alloy in the recovery of cavitation erosion and an effective improvement cavitation resistance of coated areas of hydraulic turbines, the effect of reducing the period of arrest and deposition of material. However, in the last 10 years, significant developments in conventional soldering machines, including the incorporation of electronic controls welding, which also reduces interference welder during welding. In this study, a comparison was made of an stainless steel ... / Mestre

La cavitation est la transformation d'un liquide en vapeur qui est causée par une chute de pression en dessous de la pression de saturation vapeur. Ce phénomène se manifeste le plus souvent dans les turbomachines qui sont en interaction avec des liquides. On peut citer les pompes hydrauliques, les injecteurs, les inducteurs ou encore les hélices de bateaux. Vue les effets néfastes qu'elle engendre : bruit, vibrations, détérioration du métal et baisse des performances (chute des rendements et pertes de charges), sa prise en compte est indispensable dans le design des turbomachines. Cette thèse a pour objectif de modéliser ce phénomène de manière à reproduire la nucléation, la convection et l'implosion des bulles de cavitation. Nous nous basons sur un modèle à interface diffuse (le modèle d'équilibre homogène) sur lequel nous greffons un modèle de tension de surface basé sur les équations de Navier Stokes & Korteweg compressibles. Nous réalisons en somme une étude sur l'influence de la tension de surface sur le phénomène de collapse. Nous utilisons un code de volumes finis dont la discrétisation spatiale est assurée par méthode des moindres carrés mobiles. Combinée à un solveur de Riemann de type SLAU, le modèle numérique permet d'outre passer les difficultés liés à la nature du phénomène de cavitation qui sont principalement les forts gradients qui subsistent à travers l'interface liquide-vapeur. L'autre point traité dans la thèse est la détermination d'un coefficient capillaire numérique qui correspond à une tension de surface réelle en fonction de l'épaisseur de l'interface artificiellement élargie pour un maillage donné. / Cavitation is the transformation of a liquid into vapor which is caused by a pressure drop below the vapor saturation pressure. This phenomenon usually occurs in turbine engines that interact with liquids like: hydraulic pumps, injectors, inductors or boat propellers. View its negative effects: noise, vibrations, damage to the metal and decreased performance, it should be included in the design of turbomachinery The main objective of this thesis is to model this phenomenon so as to reproduce the nucleation, convection and the implosion of cavitation bubbles. We rely on a diffuse interface model (the homogeneous equilibrium model) on which we graft a surface tension model based on compressible Navier Stokes & Korteweg equations. We study the influence of surface tension on the bubble collapse. We used a finite volume approach whose spatial discretization is made by moving least squared method. Coupled with a Riemann solver called SLAU, the numerical model can go further difficulties related to the nature of the cavitation phenomenon which is mainly the strong gradients that remain through the liquid-vapor interface. Another issue addressed in this thesis is the determination of a numerical capillary coefficient which corresponds to a real surface tension in function of the thickness of the artificially extended interface for a given mesh.

Bibliography: pages 68-70. / An investigation involving the vibratory cavitation erosion of WC-Co alloys was undertaken in order to determine the mechanisms of material removal. Nineteen grades of WC-Co alloys were studied. These alloys had been previously characterised according to microstructural and mechanical properties. Further characterisation by way of Young's modulus and density of the materials was undertaken. An investigation of the i nfluence of various parameters on cavitation erosion established a binder content dependence on erosion. For two grain sizes, erosion was found to increase to a maximum at 12 vo1-% binder content (1.8 μm grain size) and 23 vol-% binder (2.8 μm grain size). The main mode of material removal was found to be cobalt removal followed by WC grain pull-out. In high binder content alloys, cobalt removal was predominant with little loss of WC grains. X-ray diffraction showed that the allotropic phase transformation of the binder under cavitational attack was beneficial to the erosion resistance of these alloys. The erosion of low binder content alloys was controlled by the contiguity of the WC skeleton. Maximum erosion occurred at binder contents which corresponded to the combination of a fragile WC skeleton and a small volume of available cobalt for strain induced transformation.

This Master’s thesis is dedicated to cavitation erosion of a material surface. First part of thesis describes basic principle of cavitation and its erosion consequences of solid objects. There are mentioned basic cavitation models used in CFD software. Second part incorporate concept design and numerical calculation of critical shape in reference to highest intensity of cavitation erosion at flow area. Last part of thesis is experimental and it is focused on cavitation erosion at flow area due to critical shapes from numerical calculation.