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<p dir="ltr">This thesis delves into the dynamics and driving factors of thermal transport via natural convection, the onset and severity of quiescent cavitation and its impact of auto-injector device performance, and the controlled release of rapidly swelling pharmaceutical tablets. In each of these instances showcases how variations in external conditions or the introduction of new variables can disrupt the equilibrium of fluid systems, leading to complex behaviors. Vertical thermal convection illustrates how temperature gradients induce fluid movement and patterns; cavitation inception focuses on the formation of vapor cavities due to pressure drops within a fluid; and rapid tablet swelling explores the interaction between solid materials and liquids, leading to significant changes in concentration and mass transfer. These studies collectively enhance our understanding of transport dynamics, highlighting pathways to achieve optimal transport and delivery conditions for various industrial and pharmaceutical processes.</p>

This research project was carried out at the von Karman Institute for Fluid Dynamics (VKI), in Belgium, in collaboration and with the funding of Centre Technique des Industries Mécaniques (CETIM) in France.<p>The flow of a mixture of two fluids in pipes can be frequently encountered in nuclear, chemical or mechanical engineering, where gas-liquid eactors, boilers, condensers, evaporators and combustion systems can be used. The presence of section changes or more generally geometrical singularities in pipes may affect significantly the behavior of twophase flow and subsequently the resulting pressure drop and mass flow rate. Therefore, it is an important subject of investigation in particular when the application concerns industrial safety valves.<p>This thesis is intended to provide a thorough research on two-phase (air-water) flow phenomena under various circumstances. The project is split in the following steps. At first, experiments are carried out in simple geometries such as smooth and sudden divergence and convergence singularities. Two experimental facilities are built; one in smaller scale in von Karman Institute and one in larger scale in CETIM. During the first part of the study, relatively simple geometrical discontinuities are investigated. The characterization and modeling of contraction and expansion nozzles (sudden and smooth change of section) is carried out. The pressure evolution is measured and pressure drop correlations are deduced. Flow visualization is also performed with a high-speed camera; the different flow patterns are identified and flow regime maps are established for a specific configuration.<p>A dual optical probe is used to determine the void fraction, bubble size and velocity upstream and downstream the singularities.<p>In the second part of the project, a more complex device, i.e. a Safety Relief Valve (SRV), mainly used in nuclear and chemistry industry, is thoroughly studied. A transparent model of a specific type of safety valve (1 1/2" G 3") is built and investigated in terms of pressure evolution. Additionally, flow rate measurements for several volumetric qualities and valve openings are carried out for air, water and two-phase mixtures. Full optical access allowed identification of the structure of the flow. The results are compared with measurements performed at the original industrial valve. Flowforce analysis is performed revealing that compressible and incompressible flowforces in SRV are inversed above a certain value of valve lift. This value varies with critical pressure ratio, therefore is directly linked to the position at which chocked flow occurs during air valve operation. In two-phase flow, for volumetric quality of air=20%, pure compressible flow behavior, in terms of flowforce, is remarked at full lift. Numerical simulations with commercial CFD code are carried out for air and water in axisymmetric 2D model of the valve in order to verify experimental findings.<p>The subject of modeling the discharge through a throttling device in two-phase flow is an important industrial problem. The proper design and sizing of this apparatus is a crucial issue which would prevent its wrong function or accidental operation failure that could cause a hazardous situation. So far reliability of existing models predicting the pressure drop and flow discharge in two-phase flow through the valve for various flow conditions is questionable. Nowadays, a common practice is widely adopted (standard ISO 4126-10 (2010), API RP 520 (2000)); the Homogeneous Equilibrium Method with the so-called !-method, although it still needs further validation. Additionally, based on !-methodology, Homogeneous Non-Equilibrium model has been proposed by Diener and Schmidt (2004) (HNE-DS), introducing a boiling delay coefficient. The accuracy of the aforementioned models is checked against experimental data both for transparent model and industrial SRV. The HNE-DS methodology is proved to be the most precise among the others. Finally, after application of HNE-DS method for air-water flow with cavitation, it is concluded that the behavior of flashing liquid is simulated in such case. Hence, for the specific tested conditions, this type of flow can be modeled with modified method of Diener and Schmidt (CF-HNE-DS) although further validation of this observation is required. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Chute aerators are widely used in spillways to avoid cavitation damage. When the water flow passes the aerator, two jets form – upper and lower jet. The purpose of this thesis has been to study the effects from the aerator by conducting experiments in a model with a flow depth large enough to ensure that the upper and lower jet remain separated. This means that the effects from the self-aeration at the upper surface has no effect on the process in the lower jet, thus making it possible to quantify the effects from the aerator. This thesis has also provided information of the bubble formation in the lower jet to aid in the ongoing research at Sichuan University. The following questions were set up for this thesis: • What is cavitation and how is it harmful? • What is the working principle of an aerator? • How is air concentration and bubble frequency distributed in the flow? • How well do the experimental results coincide with theoretical calculations? • How are air bubbles formed and transported within the flow? The effects from the aerator have been quantified by measuring the air concentration and bubble frequency throughout the cavity zone. The model was modified and the velocity was varied between the experiments to study how different parameters effected the aeration. The results indicate that much air is being entrapped in the lower surface, but only a small amount of the entrapped air is being entrained into the flow and that the bubble frequency increases with both distance from the aerator and with an increased flow velocity. No difference in behaviour was noticed between the different modifications of the model. The bubble formation was studied by recording the flow with a high-speed camera. These recordings were used to obtain data about important parameters for the ongoing research at Sichuan University.

For delivery, dosing and pressure control of fluids in mobile and stationary applications electromagnetically operated piston pumps are an established solution. The volume per stroke is exactly defined by the geometry. Nevertheless cavitation, more likely with the new fuel blends containing a high proportion of ethanol /1/, deteriorates the dosing precision of the liquid portion. One important criterion of precise metering is the transport of the liquids through the reciprocating piston pump without transferring bubbles. Especially, pumping in the range of vapour pressure of gasoline fuels implies challenges for precision. The objective of this work is revealing potential sources of reduced cavitation by optimising the design. For doing so, optical investigations have been applied. In addition to this, cavitation can be diminished controlling the piston’s travel externally. The second important item covers pumping of degenerated fluids even without negative effects on the pump’s performance. Up to now, wide, inefficient gaps or high force surplus are necessary. A new helix-design /2/ has been investigated and built up in order to reduce the described effort. The effects coming with the helix allow a permanent rinsing of the stressed surfaces, leading to lubrication and lower temperature loads. The results are shown in simulation, fundamental tests and is validated in practical pump operation.

The disintegration of a liquid jet emerging from a nozzle has been under investigation for several decades. A direct consequence of the liquid jet disintegration process is droplet formation. The breakup of a liquid jet into discrete droplets can be brought about by the use of a diverse forcing mechanism. Cavitation has been thought to assist the atomization process. Previous experimental studies, however, have dealt with cavitation as a secondary phenomenon assisting the primary atomization mechanism. In this dissertation, the role of the energy created by the collapse of cavitation bubbles, together with the liquid pressure perturbation is explicitly configured as a principal mechanism for the disintegration of the liquid jet. A prototype of an atomizer that uses this concept as a primary atomization mechanism was developed and experimentally tested using water as working fluid. The atomizer fabrication process and the experimental characterization results are presented. The parameters tested include liquid injection pressure, ultrasonic horn tip frequency, and the liquid flow rate. The experimental results obtained demonstrate improvement in the atomization of water. To fully characterize the new atomizer, a novel infrared thermography-based technique for the characterization and visualization of liquid sprays was developed. The technique was tested on the new atomizer and two automotive paint applicators. The technique uses an infrared thermography-based measurement in which a uniformly heated background acts as a thermal radiation source, and an infrared camera as the receiver. The infrared energy emitted by the source in traveling through the spray is attenuated by the presence of the droplets. The infrared intensity is captured by the receiver showing the attenuation in the image as a result of the presence of the spray. The captured thermal image is used to study detailed macroscopic features of the spray flow field and the evolution of the droplets as they are transferred from the applicator to the target surface. In addition, the thermal image is post-processed using theoretical and empirical equations to extract information from which the liquid volume fraction and number density within the spray are estimated.

Froth flotation is a widely used, cost effective particle separation process. However, its high performance is limited to a narrow particle size range, e.g., between 50 µm and 600 µm for coal and between 10 µm and 100 µm for minerals. Outside this range, the efficiency of froth flotation decreases significantly, especially for difficult-to-float particles of weak hydrophobicity (e.g., oxidized coal). Nanobubbles integrated into a specially designed column flotation expanded the particle size range for efficient froth flotation as a result of increased probabilities of particle- bubble collision and attachment and reduced probability of detachment. The major advantages of nanobubble enhanced flotation include lower collector and frother dosages since nanobubbles that are mostly smaller than 1 µm can be formed selectively on hydrophobic coal particles from dissolved air in coal slurry. Nanobubbles act as a secondary collector on particle surfaces, thereby resulting in considerably lower operating costs. A systematic parametric investigation of the proposed technology was performed to understand the effects of process variables on separation performance with a typical coal sample using a specially designed flotation column and three 10-liters conventional flotation cells. Results indicate that the combustible recovery of a -150 µm coal increased by 5-50% in the presence of nanobubbles, depending on process operating conditions. Nanobubbles also significantly improved process separation efficiency and increased the flotation rate constant by more than 40%. Theoretical evaluation of the innovative flotation technology was employed using specially designed apparatus to study the nanobubbles stability and the roles of nanobubbles on particle-bubble interactions, froth stability, and surface area flux. In addition, a detailed technical performance and economic evaluation was performed.

Orientador: Mauricio Araújo Zanardi / Coorientador: Teófilo Miguel de Souza / Banca: José Elias Tomazini / Banca: José Antonio Perrella Balestieri / Banca: Humberto Araújo Machado / Banca: Newton Galvão de Campos Leite / Resumo: A busca por melhores desempenhos em motores de combustão interna, máquinas elétricas e sistemas mecânicos que utilizam mancais hidrodinâmicos tem motivado os recentes estudos na área de lubrificação com a aplicação da dinâmica de fluidos computacional. A modelagem numérica tem se mostrado bastante eficaz em comparação com modelos físicos, com baixo custo. Dessa forma, o presente trabalho estuda a dinâmica de alguns fluidos não-Newtonianos em mancais radiais curtos, utilizando os modelos Herschel-Bulkley, Bingham e Pseudoplástico, nos quais os resultados foram comparados com os dos fluidos Newtonianos. Aqui, o objetivo principal é comparar a distribuição de pressão do fluido, a ocorrência de cavitação e os parâmetros de desempenho, tais como capacidade de carga, força de atrito e taxa de vazamento lateral. O modelo matemático utilizado foi baseado nas equações de Reynolds com o procedimento proposto por Payvar-Salant acoplado ao modelo de cavitação de Elrod e as condições de contorno de Jakobsson, Floberg e Olsson (JFO). As equações dos mancais foram resolvidas em um programa escrito em linguagem MATLAB com discretização pelo Método de Volumes Finitos e a solução através do Algoritmo de Matriz Tri-Diagonal cíclica (TDMA). Os resultados apontaram algumas vantagens em todos os parâmetros de desempenho quando os fluidos não Newtonianos de Herschel-Bulkley ou Bingham são comparados com fluidos Newtonianos / Abstract: The search for better performance in internal combustion engines, electrical machines and mechanical systems which use hydrodynamic journal bearings has motivated the recent studies in the field of lubrication with the application of computational fluid dynamics. Numerical modeling has been shown to be quite effective compared to physical models with low cost. In this way, the present work studies the dynamics of some non-Newtonian fluids in short journal bearings, using the Herschel-Bulkley, Bingham and Pseudoplastic models and the results were compared to those for Newtonian fluids. Here, the main objective is to compare fluid pressure distribution, cavitation occurrence and performance parameters, such as load carrying capacity, friction force and side leakage rate. The used mathematical model was based on the Reynolds equations with the procedure proposed by Payvar-Salant coupled with the Elrod cavitation model and the Jakobsson, Floberg and Olsson (JFO) boundary conditions. The bearing equations were solved in a program written in MATLAB language with discretization by the finite volumes method and a cyclic Tri-Diagonal Matrix Algorithm solver. The results pointed out some advantages in all performance parameters when HerschelBulkley or Bingham non-Newtonian fluids are compared to Newtonian fluids / Doutor

Esta dissertação trata da aeração induzida em vertedores com objetivo de mitigar os efeitos da cavitação sobre os mesmos. A análise bibliográfica do tema está calcada na introdução ao mecanismo da cavitação e no levantamento do estado da arte a partir dos trabalhos pioneiros, nos clássicos e nos recentes. Com base nos princípios da física: Conservação de Massa e Primeira Lei da Termodinâmica, foi desenvolvido um modelo matemático para aeração induzida em vertedores. O modelo proposto, após ser analisado do ponto de vista de sua consistência, foi avaliado em comparações com resultados fornecidos por outros pesquisadores, obtidos através de modelos físicos reduzidos. Os resultados obtidos pelo modelo matemático proposto correspondem a boas estimativas das grandezas envolvidas na aeração induzida em vertedores, o que credencia o modelo proposto como uma ferramenta apropriada para projetos de engenharia hidráulica e futuros desenvolvimentos científicos. / This study adress prompt aeration in spillways reaching to reduce the cavitation effects on them. References were based on introduction to cavitation machanism and, state of art survey, to leave on not only earlier studies, but also on classical and most recent ones. Based on principles of Physics, such as Mass Conservation and the First Law of Thermodynamics, a mathematical model has been developed as an example of prompt aeration in spillways. After extensive consistency analyses the proposed model has been validated by comparing different results furnished by studies on physical reduced models by other researchers. Results obtained from the mathematical model proposed here correspond to good estimates of greatnesses involved in prompt aeration is spillways and that turns the proposed model into an adequate tool for Hydraulic Engineering projects and for future scientifics stydies.

Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Ocean Engineering. / Microfiche copy available in Archives and Engineering. / Includes bibliographical references. / M.S.