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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
401

Lattice-Boltzmann method and immiscible two-phase flow

Rannou, Guillaume 19 November 2008 (has links)
This thesis focuses on the lattice-Boltzmann method (LBM) and its ability to simulate immiscible two-phase flow. We introduce the main lattice-Boltzmann-based approaches for analyzing two-phase flow: the color-fluid model by Gunstensen, the interparticle-potential model by Shan and Chen, the free-energy model by Swift and Orlandini, and the mean-field model by He. The first objective is to assess the ability of these methods to maintain continuity at the interface of two fluids, especially when the two fluids have different viscosities or densities. Continuity issues have been mentioned in the literature but have never been quantified. This study presents a critical comparison of the four lattice-Boltzmann-based approaches for analyzing two-phase flow by analyzing the results of the two-phase Poiseuille flow for different viscosity ratios and density ratios. The second objective is to present the capability of the most recent version of the color-fluid model for simulating 3D flows. This model allows direct control over the surface tension at the interface. We demonstrate the ability of this model to simulate surface tension effects at the interface (Laplace bubble test), stratified two-phase flows Poiseuille two-phase flow), and bubble dynamics (the free rise of a bubble in a quiescent viscous fluid).
402

Caracterização experimental do escoamento intermitente líquido-gás em tubulações horizontais

Vicencio, Fernando Enrique Castillo 04 September 2013 (has links)
O presente estudo visa ampliar o conhecimento sobre a dinâmica do escoamento intermitente líquido-gás, caracterizando experimentalmente os principais parâmetros deste tipo de escoamento. Para este fim, é utilizada a bancada experimental que consta de uma linha bifásica horizontal de 9,2 m de comprimento e 25,8 mm de diâmetro. A monitoração das estruturas do escoamento bifásico foi realizada utilizando dois (02) sensores de malha de eletrodos de 12x12 nós separados 3,75 cm entre si. Os testes experimentais foram realizados para 47 condições de vazão de líquido e de gás do escoamento bifásico no padrão intermitente, levando em conta as limitações da bancada experimental. Após processar o sinal medido, é obtida a série temporal dos dados e os parâmetros característicos do escoamento intermitente em golfadas, como a velocidade da bolha alongada, frequência, comprimentos da bolha e do pistão de líquido e a fração de vazio. A distribuição de probabilidades de cada parâmetro foi aproximada com uma função probabilística, do tipo normal ou log-normal. Foram elaboradas relações funcionais para calcular os parâmetros característicos médios do escoamento em golfadas e seus respectivos desvios padrão a partir das condições de operação do escoamento (velocidades superficiais e propriedades dos fluidos). Essas correlações visam predizer o comportamento dos parâmetros do escoamento intermitente líquido-gás e podem ser utilizados como dados de entrada ou de fechamento dos modelos matemáticos existentes. Este estudo também pode servir de referência para futuros estudos e para o desenvolvimento e validação de modelos teóricos do escoamento em golfadas em dutos horizontais. / The goal of the present work is to broaden the knowledge on the dynamics of two-phase intermittent gas-liquid flows through the experimental characterization of their parameters. The experimental work was carried out at the LACIT-UTFPR labs and a two-phase flow apparatus consisting of a 25.8-mm ID, 9-m long transparent line was used. A pair of 12x12-nodes wire mesh sensors 3.75 cm apart was used to identify the existing phase at each node. A test grid comprehending a broad range of gas and liquid flow rates within the intermittent flow pattern region was chosen. The operational limitations of the experimental rig were taken into account on defining the boundaries of the aforementioned test grid. The temporal series of the data, as well as characteristic parameters of the intermittent flows such as frequency, void fraction, elongated bubble velocity and the lengths of the liquid slug and the elongated bubble were obtained after signal processing. The probability distribution of each parameter was found and approximated to a known statistical distribution such as the normal or the log-normal ones. Correlations to fit the averaged parameters and their standard deviations were proposed. The objective of such correlations is to predict the average values of the characteristic parameters of the intermittent flows so that such predicted values might be used as input data for the theoretical models developed. Finally, statistical distributions based on the empirical correlations and their normal and log-normal distributions were found. These distributions validated the aforementioned correlations, which can henceforth be used in future theoretical modelling in this field.
403

Análise do desempenho de condensadores de parade-aquecida usados em refrigeradores domésticos /

Lima, Roniel de Souza. January 2017 (has links)
Orientador: André Luiz Seixlack / Resumo: Neste trabalho apresenta-se um modelo distribuído para análise de condensadores do tipo parede aquecida, hot-wall, usados em refrigeradores domésticos. Nesse modelo considera-se o escoamento do fluido refrigerante no interior do tubo do condensador e a transferência de calor entre a placa de revestimento externo do refrigerador e o ar ambiente. No modelo, inclui-se também a transferência de calor ao longo da fita de alumínio, usada para fixação do tubo na placa de revestimento externo e a transferência de calor através do isolamento térmico para o interior do compartimento refrigerado. O escoamento no interior do tubo é considerado unidimensional e dividido em três regiões: região monofásica de vapor superaquecido, região bifásica líquido-vapor e região monofásica de líquido sub-resfriado. O escoamento bifásico é analisado segundo o modelo homogêneo, ou seja, considerando condições de equilíbrio térmico e hidrodinâmico entre as fases. Considera-se também a queda de pressão do escoamento no interior do tubo. A fita de alumínio e a placa de revestimento externo são consideradas como aletas com extremidades adiabáticas. As equações do escoamento do fluido refrigerante: conservação da massa, quantidade de movimento e conservação de energia, são resolvidas por integração numérica e a equação de conservação de energia ao longo da fita de alumínio é resolvida pelo método de Volumes Finitos. O sistema de equações algébricas, decorrente do processo de discretização das equações difere... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This work presents a distributed model for the analysis of hot-wall condensers used in domestic refrigerators. In this model, the flow of refrigerant inside the condenser tube and the heat transfer between the outer coating plate of the refrigerator and the ambient air are considered. Also included in this model the heat transfer along the aluminum tape used to hold the tube to the outer coating plate and the heat transfer through the thermal insulation into the refrigerated compartment. The flow inside the tube is taken as one-dimensional and divided into three regions: superheated vapor region, two-phase liquid-vapor region and sub-cooled liquid region. The two-phase flow is analyzed according to the homogeneous model, that is, considering conditions of thermal and hydrodynamic equilibrium between the phases. It is also considered the pressure drop of the flow inside the tube. The aluminum tape and the outer coating plate are considered as fins with adiabatic ends. The equations of the refrigerant flow: mass conservation, momentum and energy conservation, are solved by numerical integration and the energy conservation equation along the aluminum tape is solved by the Finite Volume method. The set of algebraic equations, resulting from the discretization process of the differential equations, is solved iteratively by successive iterations, point-to-point along the condenser tube and the aluminum tape. The results obtained are compared with experimental data available in the ... (Complete abstract click electronic access below) / Mestre
404

Analysis of homogeneous film flows on inclined surfaces and on corrugated sheet of packing using CFD

Subramanian, Kumar 02 June 2015 (has links) (PDF)
The key to success in separation of liquid mixtures is the efficient creation and utilization of vapour-liquid contact area. By packing the column with gas-liquid contact devices such as structured packing, the vapour-liquid contact area can be increased. However, the efficiency of these packed columns depends strongly on the local flow behaviour of the liquid and vapour phase inside the packing. The aim of this work was to develop three-dimensional CFD models to study the hydrodynamic behaviour on the corrugated sheets of packing. Different approaches are possible to simplify the problem and to extend it for more complex flow scenarios. In this work, three-dimensional CFD simulations were performed to study the complete fluid-dynamic behaviour. This was performed in two steps. As a first step, the developed model was validated with experimental studies using a simplified geometry i.e., an inclined plate. The three-dimensional Volume-of-Fluid (VOF) model was utilized to study the flow behaviour of the gas-liquid countercurrent flow. The influence of the liquid surface tension was taken into consideration using the Continuum Surface Force (CSF) model. The wetting characteristics of liquids with different viscosity (1 and 5 mPas) and contact angle (70° and 7°) were studied for different flow rates. Three different mixtures (water, water-glycerol (45 wt. %) and silicon-oil (DC5)) were considered. Initially, the rivulet width of experiments and simulations were compared and an error of 5 % maximum was determined. The results were also in good agreement with earlier studies. The percentage of wetting due to changes in flow rate, viscosity and contact angle was compared and discussed. For all tested systems, excellent agreement between the experiments and simulation studies was found. In addition, profiles of the velocity in the film at film flow conditions over a smooth inclined plate obtained from simulations were compared with experimental profiles obtained using a μPIV technique. A detailed sensitivity study was also performed in order to understand the changes in the velocity profiles due to small change in liquid flow rate, temperature and inclination angle. As a next step, the developed model was extended to geometries resembling real corrugated sheets of packing used in industrial applications. In earlier numerical studies of structured packing, geometries were simplified to enable easy meshing and faster computation. In this work, the geometries of corrugated sheets of packing were developed without any simplification and the flow behaviour was studied using the model validated in the first step. The flow behaviour on sheets with different geometrical modifications such as smooth and triangular crimp surfaces as well as perforations on the sheets were numerically studied and quantitatively compared with experimental studies for the three different fluid test systems. The agreement between the simulations and experiments was within an acceptable range for all system. The difference in the interfacial area between the corrugated sheets of a packing with and without perforation was analyzed and the prediction ability of different empirical correlations for the interfacial area available in literature was also compared and discussed. Furthermore, the numerical study was extended to understand the influence of the second corrugated sheet. Studying the flow behaviour between two sheets experimentally is very challenging, especially inside opaque packing. The model proved to be a very suitable tool to study the hold-up of the liquid between two sheets, the change in wetting behaviour due to small change in liquid inlet position. The results are also in good agreement with the earlier experimental studies, where researchers measured the liquid hold-up mainly in the region where two corrugated sheets touch each other. The three-dimensional CFD model was validated to study the flow behaviour on corrugated sheets of packing. The results from the simulations agree very well with findings from the experimental studies in terms of wetting and hold-up.
405

Modelování dynamiky ledových měsíců s kapalnou fází / Dynamics of icy satelites with a liquid phase

Kalousová, Klára January 2015 (has links)
Title: Dynamics of icy satellites with a liquid phase Author: Klára Kalousová Department: Department of Geophysics Department: Laboratoire de Planétologie et Géodynamique de Nantes Supervisors: Doc. RNDr. Ondřej Čadek, CSc. upervis ors: Dr. Gaël Choblet Abstract: Jupiter's moon Europa has a young surface with a plethora of unique terrains that indicate recent endogenic activity. Morphological models and spectral observations suggest that it possesses an internal ocean as well as shallow pockets of liquid water within its outer ice shell. Presence of water in a chemically rich environment and a longterm energy source ensured by tidal heating, make Europa one of the best candidates for habitability, for which the material exchange between the surface and the ocean is critical. In this thesis, we investigate internal melting and subsequent meltwater evolution within Europa's ice shell by using a two-phase formalism developed for this context. Results of a parametric study for a temperate ice shell indicate that the time scale of water transport by porous flow is governed by the ice permeability, while the ice viscosity affects the solution wavelength. We then consider a polythermal ice shell with two melting scenarios: (i) In a tidally-heated convecting ice shell, melting occurs mainly in its lower half and...
406

Interaction corium-béton : étude du transfert de chaleur en écoulement diphasique / Molten corium core interaction : investigation of heat transfer in two-phase flow

Amižić, Milan 14 March 2014 (has links)
Dans le cadre de la recherche sur les accidents graves pour la deuxième et la troisième génération de réacteurs nucléaires, certains aspects de l'ablation de béton dans le puits de cuve au cours de l'interaction corium-béton (ICB) restent encore inexpliquées. La détermination d'échange de chaleur le long de la région interfaciale entre un bain de corium et un béton est importante pour l'évaluation de la progression d'ablation du béton et, éventuellement, la percée de fondation. Le projet CLARA s'inscrit une recherche expérimentale sur la thermohydraulique au sein d'un bain de liquide agitée par des bulles de gaz. Les essais CLARA sont réalisés avec des matériaux simulants. Ils permettent de mettre en évidence l'influence de la vitesse superficielle du gaz, de la viscosité du liquide et de la géométrie sur le coefficient d'échange de chaleur entre le bain de liquide chauffé et les parois verticales et horizontales de la piscine qui sont maintenues à une température uniforme. La première campagne d'essais a été réalisée avec la configuration du bain de petite taille (50 cm × 25 cm × 25 cm). Les essais ont été réalisés avec des liquides couvrant un large éventail de viscosité dynamique, d'environ 1 mPa s à 10000 mPa s. La vitesse superficielle du gaz est modifiée jusqu'à 8 cm/s. Cette thèse comporte une brève description de la phénoménologie de l'ICB, une synthèse bibliographique sur les corrélations d'échange de chaleur existantes pour l'écoulement diphasique et le taux de vide, une description de l'installation CLARA, les résultats des essais et leur interprétation. Les résultats expérimentaux sont comparés avec les modèles existants et certains nouveaux modèles pour l'évaluation du coefficient d'échange de chaleur dans un écoulement diphasique. / In the context of severe accident research for the second and the third generation of nuclear power plants, there are still open issues concerning some aspects of the concrete cavity ablation during the molten corium - concrete interaction (MCCI). The determination of heat transfer along the interfacial region between the molten corium pool and the ablating basemat concrete is crucial for the assessment of concrete ablation progression and eventually the basemat meltthrough. For the purpose of experimental investigation of thermalhydraulics inside a liquid pool agitated by gas bubbles, the CLARA project has been launched. The CLARA experiments are performed using simulant materials and they reveal the influence of superficial gas velocity, liquid viscosity and pool geometry on the heat transfer coefficient between the internally heated liquid pool and vertical and horizontal pool walls maintained at uniform temperature. The first test campaign has been conducted with the small pool configuration (50 cm × 25 cm × 25 cm). The tests have been performed with liquids covering a wide range of dynamic viscosity from approximately 1 mPa s to 10000 mPa s and the superficial gas velocity is varied up to 8 cm/s. This thesis comprises a brief description of MCCI phenomenology, literature reviews on the existing heat transfer correlations for twophase flow and the void fraction, a description of CLARA setup, experimental results and their interpretation. The experimental results are compared with existing models and some new models for the assessment of heat transfer coefficient in two-phase flow. / U kontekstu istraživanja teških nesre´ca u nuklearnim elektranamadruge i tre´ce generacije, neka pitanja vezana za ablaciju temelja kontejnmentatijekom interakcije rastaljenog korijuma i betona i dalje ostajuotvorena. Odred¯ivanje prijenosa topline u površinskom podrucˇjuizmed¯u bazena rastaljenog korijuma i betona kljucˇno je za odred¯ivanjenapredovanja ablacije i u konaˇcnici procjene vremena rastapanjatemelja kontejnmenta. U svrhu eksperimentalnog istraživanja prijenosatopline u tek´cinama miješanima ubrizgavanjem zraka, pokrenutje projekt nazvan CLARA.CLARA eksperimenti izvode se koriste´ci imitacijske materijale i otkrivajuutjecaj fiktivne brzine plina, viskoznosti teku´cine i geometrijebazena na koeficijent prijenosa topline izmed¯u grijanog bazena te njegovihvetrikalnih i horizontalnih stijenki ˇcija se temperatura održavana konstantnoj temperaturi. Prva serija eksperimenata provedena je sbazenom male konfiguracije (50 cm × 25 cm × 25 cm). Eksperimentisu izvedeni s teku´cinama dinamiˇcke viskoznosti od približno 1 mPas do 10000 mPa s, dok je maksimalna fiktivna brzina plina 8 cm/s.Ova disertacija sadrži kratak opis fenomenologije procesa interakcijerastaljenog korijuma i betona, pregled postoje´cih korelacija zaviprijenos topline u dvofaznom toku i korelacija za poroznost, opisCLARA eksperimentalne postave, rezultate eksperimenta i njihovuinterpretaciju. Rezultati eksperimenta su uspored¯eni s predvid¯anjimaprema postojec´im modelima. Predloženi su takod¯er i neke nove korelacijeza odred¯ivanje koeficijenta prijenosa topline u dvofaznom toku.
407

[en] NUMERICAL SIMULATION OF A GAS-LIQUID MIXER FOR PUMPING STATIONS / [pt] SIMULAÇÃO NUMÉRICA DE UM MISTURADOR GÁS-LÍQUIDO PARA ESTAÇÕES DE BOMBEIO

LEONARDO CHAGAS CARBONE 26 January 2016 (has links)
[pt] A produção de petróleo offshore tem requerido cada vez mais a utilização de equipamentos de bombeio no leito marinho para a produção de poços a longas distâncias das unidades de produção. A grande maioria destes equipamentos tem dificuldade de operar com a presença de gás, ainda mais em condições de golfadas, onde a fração de vazio na sucção pode variar significativamente. Isto limita enormemente a janela operacional dos equipamentos. Uma forma de minimizar os efeitos das golfadas e, assim, ampliar a janela de operação destes equipamentos é a instalação de misturadores de fluxo na sucção das bombas, que atuam de forma a garantir uma maior sobrevida ao equipamento. Portanto, o projeto e a previsão do comportamento deste tipo de equipamento são fundamentais para bom funcionamento dos sistemas de bombeamento submarino. O presente trabalho investiga numericamente, com o modelo de dois fluidos, a eficiência de um tipo de misturador em uniformizar as frações de vazio. Diferentes configurações geométricas são analisadas e a influência das vazões de admissão de ambas as fases é discutida. A metodologia é validada através da comparação da queda de pressão através do misturador com dados experimentais disponíveis, onde se observou boa concordância. O modelo numérico mostra ainda que uma reserva de líquido é mantida no interior do equipamento e, o gás é forçado a se dispersar na corrente de líquido para sair do misturador. O comportamento do misturador frente à presença de golfadas também é investigado. Ao final do trabalho é proposta uma nova geometria, que se mostra mais tolerante a intermitências no escoamento. / [en] The offshore oil production is increasingly using pumping equipment at the seabed for production of wells at long distances from production units. Most of these devices have difficulty to operate in the presence of gas, especially in slug flow conditions, where the pump suction s void fraction can vary significantly. This problem limits the equipment s range of operation. One way to minimize the effects of slugging and thus enlarge the operation range of these devices is to use flow mixers at the pump s suction, which helps to ensure the operation even in the presence of large gas bubbles. Therefore, the design and prediction of the behavior of this type of equipment are essential for proper operation of subsea pumping systems. At the present work, the efficiency of a type of flow mixer to provide uniform void fractions at its outlet is numerically investigated, with the two fluid model. Different geometric configurations are analyzed and the influence of intake flow of both phases is discussed. The methodology is validated through comparison of the pressure drop through the flow mixer with available experimental data, where good agreement was observed. The numerical model also shows that a buffer of liquid is maintained inside the equipment and the gas is forced to disperse in the liquid stream to exit the mixer. The mixer s behavior in the presence of slugs is also investigated. Finally, a new geometry is proposed, which seems to be more efficient to avoid flow s intermittencies.
408

Simulation numérique du mouvement et de la déformation des neutrophiles : influence de la rhéologie et du type d'écoulement / Numerical simulation of neutrophils motion and deformation : influence of rheology and flow type

Le Roux, Mélanie 07 November 2012 (has links)
La faible déformabilité et l'accumulation des neutrophiles (globules blancs) dans les capillaires pulmonaires peuvent entraîner des syndromes de détresse respiratoire. Nous étudions le comportement des neutrophiles grâce à un code numérique diphasique de type Volume Of Fluid dans différentes configurations confinées. Les cellules sont représentées successivement par quatre modèles différents : dans un premier temps, nous les modélisons par un fluide newtonien caractérisé par sa viscosité et sa tension de surface. Le noyau très visqueux qu'elles contiennent est pris en compte dans un second temps sous forme d'un solide non-déformable grâce à une méthode de frontières immergées. Puis les effets élastiques sont considérés en modélisant le cytoplasme de la cellule par un fluide visco-élastique d'Oldroyd-B. Enfin, une membrane élastique est introduite autour du cytoplasme afin de séparer le mouvement du cytoplasme de celui du plasma. Nous examinons le comportement des cellules ainsi modélisées dans trois configurations : une géométrie de canaux en croix générant en son centre des écoulements linéaires, comme le dispositif des rouleaux de Taylor, une contraction isolée et un réseau périodique dans lequel la cellule traverse plusieurs contractions successives. Alors que la première configuration permet de placer les cellules dans des écoulements de déformation ou de cisaillement pur, les deux autres géométries se rapprochent davantage des configurations réelles. Les résultats mettent en évidence les différences de comportement de la cellule selon le modèle choisi. L'introduction d'un fluide visco-élastique diminue la viscosité effective des cellules et facilite ainsi leurs déformations et leur entrée dans les contractions géométriques. La membrane en revanche modifie de manière notable la forme des cellules et diminue leurs déformations. / The low deformability of neutrophils (white blood cells) and their accumulation in pulmonary capillaries may cause acute respiratory distress syndrome. A computational fluid dynamic approach using a Volume Of Fluid method is adapted to simulate the behavior of neutrophils in different confined flow configurations. The white blood cells are successively represented by four different models: in a first time, the cell is modeled as a Newtonian fluid, characterized by a viscosity and a surface tension. In a second step, a non deformable core is added using an immersed boundary method to improve representativity of the model. Then, the cytoplasm of the cell is modeled by a viscoelastic fluid. Finally, the effects of an elastic membrane surrounding the cytoplasm are taken into account so as to separate the motion of the plasma from that of the cell. We discuss the behavior of the cell in three different configurations: an equivalent of the 4-roll mill device obtained with a suitable arrangement of micro-channels joining at right angle, an isolated contraction and a periodic network in which the cell goes through several successive contractions. The results shed light on the differences of the cell behaviors obtained with the various models. The introduction of a viscoelastic fluid in the cytoplasm decreases the effective viscosity of the cell and increases its deformability, allowing an easier entrance in a contraction, whereas the membrane affects the cell shape and decreases its deformation.
409

Mise en évidence expérimentale et modélisation des régimes de combustion diphasique présents dans les foyers aéronautiques / Experimental highlighting and modelling of spray combustion regimes present in gas turbine combustors

Vicentini, Maxime 03 June 2016 (has links)
De nos jours, la combustion d'hydrocarbures est largement répandue dans de nombreuses applications, notamment la propulsion aéronautique. Toutefois, les turbomachines produisent des niveaux d'émissions d'espèces polluantes qui ne sont plus acceptés. C'est pourquoi, la compréhension des phénomènes physiques mis en jeu dans les chambres de combustion est essentielle pour aider au développement de moteurs plus propres. Dans de tels foyers, le carburant est injecté sous la forme d'un brouillard de gouttes, ce qui génère de fortes interactions avec l'écoulement d'air turbulent et la flamme. L'objectif de cette thèse est de contribuer au développement de modèles en combustion diphasique en vue d'améliorer la capacité prédictive des outils de simulation numérique. Pour cela, un nouveau moyen d'essais dédié à l'étude des flammes diphasiques turbulentes a été conçu et une base de données expérimentales a été constituée (conditions inertes et réactives). Des visualisations simultanées de la diffusion de Mie des gouttes et du taux de dégagement de chaleur ont permis de mettre en évidence une structure de flamme complexe ainsi que l'existence de différents régimes de combustion. Un autre point important de ce travail a été d'analyser statistiquement la distribution spatiale de gouttes en conditions réactives à l'aide d'une méthode de mesure originale. Cette analyse a permis de quantifier les distances inter-gouttes (plus proches voisines) en différents points de l'écoulement et d'estimer les erreurs liées au traitement des données via une approche numérique. En outre, il apparaît que la distribution spatiale des gouttes s'apparente à une loi aléatoire uniforme alors que les modèles de combustion de gouttes s'appuient souvent sur une loi régulière. / Nowadays, combustion of hydrocarbons is widespread in many engineering applications, including aeronautical propulsion. However, gas turbines produce pollutant emission levels that are no longer accepted. Therefore, understanding physical phenomena involved in combustion chambers is a major issue to help to the development of more eco-friendly engines. In aero-engine combustors, fuel is injected as a spray of droplets which generates a strong interaction with the turbulent air flow and the flame. This thesis aims at contributing to the development of two-phase combustion models to improve the predictive ability of numerical simulation tools. To do so, a new test setup dedicated to the study of two-phase turbulent flames has been designed and an experimental database has been built (non-reactive and reactive conditions). Simultaneous visualizations of Mie scattering droplets and heat release rate have highlighted a complex flame structure and the existence of different spray combustion regimes. Another important point of this work was to perform a statistical analysis of the spatial distribution of droplets under reactive conditions using an original measurement method. This analysis permitted to measure the inter-droplet distances (nearest neighbor) in different points of the flow and to assess the errors related to the processing of data through a numerical approach. It further appears that the spatial distribution of droplets is similar to an uniform random law while droplet combustion models are often based on a regular law.
410

Etude expérimentale de l'amélioration de la distribution diphasique dans un échangeur thermique à l'aide d'ultrasons / Experimental study of the enhancement of the two-phase distribution in a heat exchanger using ultrasound

Tingaud, Florian 18 December 2012 (has links)
Les préoccupations relatives à la consommation de l’énergie et notamment des pertes provoquent une demande d’optimisation toujours plus forte des procédés. La recherche de l’efficacité maximale dans les échangeurs de chaleur est d’autant plus forte lorsque ceux-ci ne fonctionnent pas au régime nominal. L’étude présentée dans ce mémoire se concentre sur une problématique de l’efficacité des échangeurs thermiques : la distribution diphasique. Le but de ce travail a été de montrer la faisabilité d’un dispositif améliorant la répartition des deux phases dans un échangeur à mini canaux. La méthode choisie a été l’introduction d’ultrasons par le biais de générateurs placés dans le distributeur produisant une fontaine. Cette technologie présente l’avantage d’être facilement modulable en ne changeant que la tension d’entrée des appareils. Ceci permet donc de pouvoir adapter le dispositif aux différentes conditions opératoires. Des essais expérimentaux ont été réalisés dans différentes conditions, en changeant notamment le débit de chaque phase. La densité de flux massique a été variée de 60 kg.m-2.s-1 à 450 kg.m-2.s-1 et le titre massique de moins de 1% à plus de 23% en entrée de section d’essais. Les comparaisons entre les différents essais se sont faite par la mesure des débits de chaque phase sortant des canaux. L’introduction des ultrasons a également été étudiée en jouant sur le placement et le nombre des générateurs d’ultrasons. Cette technologie s’est alors montré viable car la distribution diphasique est améliorée dans une grande majorité des cas testés. Des phénomènes intéressant ont même été observés, permettant lors d’une prochaine étude, des approches différentes mais complémentaires. / Concerns about energy consumption including thermal losses cause a need in greater optimization of the processes. The goal of the maximum efficiency in heat exchangers is even stronger when they do not work at nominal conditions. The study presented in this paper focuses on the two-phase flow distribution problematic. The aim of this work was to demonstrate the feasibility of a device that can improve the distribution of the two phases in a mini-channel heat exchanger. The method chosen was the introduction of ultrasound through generators placed in the distributor where they can produce an ultrasonic fountain. This technology has the advantage of being easily adjustable by changing the input voltage of the devices. This therefore allows the device to adapt to different operating conditions. Experimental tests have been done under different conditions, in particular by changing the flow rate of each phase. The mass flow density was varied from 60 kg.m-2.s-1 to 450 kg.m-2.s-1 and the mass quality of less than 1% to over 23% at the tests section inlet. Comparisons between different tests were made by measuring flow rates of each phase at the outlet of the channels. The introduction of ultrasound was also studied by varying the placement and number of the generators of ultrasound. This technology has been shown as viable two-phase distribution enhancement in a majority of cases tested. Interesting phenomena were even observed, which allows, for a future study, different but complementary approaches.

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