Auswahl der wissenschaftlichen Literatur zum Thema „Turbulent two-Phase flows“

An experimental method is proposed to study dispersed two-phase flows at an airwater interface, a family of flows of practical significance in environmental and industrial settings. The applicability of this technique is demonstrated through the study of a lightly-deformed turbulent free-surface laden with floating particles (`floaters'). A low-mean turbulent flow is generated in a turbulence box actuated by a 10×10 synthetic jet array. Using LEDs and a single camera, free-surface flow measurements are carried out by Particle Image Velocimetry (PIV) simultaneously with Lagrangian tracking of the floaters, allowing the potential to characterise the coupling between the floater dynamics and the (sub)surface flow. Discrimination of the dispersed and continuous phases is carried out based on size. Individual floaters and clusters of floaters are successfully tracked throughout the field of view while they navigate through elongated and circular regions of high and low vorticity, characteristic features typically observed when a subsurface turbulent flow interacts with a free surface. Preliminary results of the floater-fluid interactions are presented to highlight the potential of this technique to better our understanding of floaterladen turbulent free surfaces.

Pressure loss is one of the significant parameters in designing pipe bends. In this paper, the pressure distribution and pressure losses induced by turbulent flows in a circular cross-sectioned piping elbow with or without guide vane were simulated. The flow distribution in the piping elbow was simulated by the k- model using control volume method. The main objective of this study is to characterize the effect of changing the angle of pipe bend and Reynolds number on the flow separation of single-phase turbulent flow through numerical simulation. Results were validated by other experimental results and then loss coefficient was calculated in different angles from 45 to 135-degree pipe bend in various radius ratios with or without guide vane. Despite the fact that increasing pipe angle increased the pipe bend loss coefficient, using guide vane in the pipe elbow decreased this coefficient. In the radius ratio 1.5 with one guide vane, the loss coefficient of the pipe bends decreased by 50 percent in all degrees. Results revealed that the use of two vanes in pipe bend is more effective on the reduction of elbow pressure losses. Moreover, two guide vanes can decrease loss coefficient more than 50 percent. Also, the results indicated that loss coefficient decreased by increasing Reynolds number.

Two-phase pipe flow is a common occurrence in many industrial applications such as sewage, water, oil, and gas transportation. Accurate prediction of liquid velocity, holdup and pressure drop is of vast importance to ensure effective design and operation of fluid transport systems. This paper aimed at the simulation of a two-phase flow of air and sewage (water) using an open source software OpenFOAM. Numerical Simulations have been performed using varying dimensions of pipes as well as their inclinations. Specifically, a Standard k- turbulence model and the Volume of Fluid (VOF) free water surface model is used to solve the turbulent mixture flow of air and sewage (water). A two dimensional, 0.5m diameter pipe of 20m length is used for the CFD approach based on the Navier-Stokes equations. Results showed that the flow pattern behaviour is influenced by the pipe diameters as well as their inclination. It is concluded that the most effective way to optimize a sewer network system for Tororo Municipality conditions and other similar situations, is by adjusting sewer diameters and slope gradients and expanding the number of sewer network connections of household and industries from 535 (i.e., 31.2% of total) to at least 1,200 (70% of total).

Thermal analysis of high-temperature phase change materials (PCM) is conducted with the consideration of a 20% void and buoyancy-driven convection in a stainless-steel capsule. The effects of the thermal expansion and the volume expansion due to phase change on the energy storage and retrieval process are explored. The used water to fill the void between two different wax paraffin and stearic acid spheres is considered as a potential PCM for concentrated solar power. The charging/discharging process into and from the capsule wall is simulated under different boundary conditions for laminar and turbulent flows. Computational models are conducted by applying an enthalpy-porosity method and volume of fluid method to calculate the transport phenomena within the PCM capsule, including an internal air void. A simplified two-dimensional model of the PCM contained within the spheres is constructed and thermal analyses are performed for the transition from solid to liquid states. Simulated charging process modes are compared with the theory. According to experiments, the temperature distributions from 40-60 mm without and with 60 mm with copper fin have different behavior. The paraffin takes less time than stearic acid for total transformation at a rate of 0.5. The size of the sphere increases over the amount of time and the phase of the sphere to complete changes as stearic acid expands more than paraffin during the transition. Inserting a rectangular fin, that is made from copper into the ball reduces the cycle time and increases output.

La question de la qualité de l'air est un enjeu de santé publique crucial qui se pose dans les grandes métropoles, à mesure que les activités humaines deviennent de plus en plus polluantes. Selon l’OMS, en diminuant les niveaux de pollution atmosphérique, les pays peuvent réduire la charge de morbidité imputable aux accidents vasculaires cérébraux, aux cardiopathies, au cancer du poumon et aux infections respiratoires. Le transport ferroviaire est à juste titre considéré comme étant une solution de mobilité durable à faible impact en gaz à effet de serre et un faible contributeur aux émissions de polluants dans l'air. Néanmoins, plusieurs études mettent en évidence que les concentrations polluantes dans les enceintes ferroviaires souterraines doivent être considérées comme préoccupantes. Dans certains cas, les concentrations de particules fines peuvent être dix fois plus élevées en intérieur qu'en extérieur. Dans ce contexte, la réduction ou la mitigation des sources d’émissions liées au freinage, principal contributeur dans le secteur ferroviaire, représente un enjeu majeur pour la santé des personnes. Cette thèse prend place dans le cadre du projet BREAQ (BRaking Emissions characterisation and mitigation for Air Quality improvement), mené conjointement par ALSTOM, l’ADEME et plusieurs organismes de recherche. Ce projet a pour vocation de réduire les émissions de particules de freinage à la source et de prédire la diffusion de ces particules dans l’environnement pour développer des solutions de captation efficientes. Dans ce contexte, l’objectif de l’étude consiste à développer et mettre en œuvre une méthode numérique CFD capable de modéliser des écoulements chargés en particules issues du freinage ferroviaire afin de prédire la diffusion de ces particules dans l’environnement proche
The issue of air quality is a crucial public health issue that arises in large cities, as human activities become increasingly polluting. According to WHO, by reducing air pollution levels, countries can reduce the burden of disease from stroke, heart disease, lung cancer and respiratory infections. Rail transport is rightly considered to be a sustainable mobility solution with low greenhouse gas impact and a low contributor to air pollutant emissions. However, several studies highlight that pollutant concentrations in underground railway enclosures must be considered as worrying. In some cases, concentrations of fine particles can be ten times higher indoors than outdoors. In this context, reducing or mitigating sources of emissions linked to braking, the main contributor in the railway sector, represents a major challenge for people's health. This thesis is part of the BREAQ (BRaking Emissions characterization and mitigation for Air Quality improvement) project, jointly conducted by ALSTOM, ADEME and several research organizations. The aim of this project is to reduce braking particle emissions at source and to predict the diffusion of these particles in the environment in order to develop efficient capture solutions. In this context, the objective of the study is to develop and implement a numerical CFD method for modeling particulate flows from railway braking in order to predict the diffusion of these particles in the nearby environment

La régulation des polluants a mené à la création de nouveaux systèmes de combustion. Le carburant étant stocké sous forme liquide, sa transformation jusqu’à sa combustion est complexe. La capacité de la Simulation aux grandes échelles à simuler des écoulements turbulents réactifs a été montrée sur des cas académiques comme sur des configurations industrielles, tout en prenant en compte les phénomènes multiphysiques intervenant dans ces configurations, mais les études sur la structure de flamme diphasique sont encore trop peu nombreuses. La présence de deux solveurs pour la simulation d’une phase liquide étant disponible dans le code AVBP, leur utilisation permet une comparaison et une compréhension des phénomènes en jeu combinant dispersion, évaporation, et combustion. La première partie de l’étude relate la validation du modèle d’injection FIM-UR. Ce modèle est capable de reconstruire les profils de vitesses et de granulométrie à l’injecteur sans avoir à simuler les phénomènes d’atomisation primaire et secondaire. Une validation en régime turbulent avait déjà été réalisée, et on propose ici de valider le modèle dans un cas laminaire. Des comparaisons entre simulations monodisperses et polydisperse et des expériences sont effectuées. La simulation monodisperse Lagrangienne donne une bonne structure globale mais la simulation polydisperse Lagrangienne permet de retrouver le comportement au centre du cône avec la présence des petites gouttes et à la périphérie du cône par la présence des grosses gouttes. De plus, des améliorations sont apportées au modèle pour le formalisme Eulérien et montrent de bons résultats. La partie suivante s’intéresse à caractériser un spray polydisperse par une distribution monodisperse. En effet, au cas où une approche polydisperse n’est pas possible, le choix du diamètre moyen à prendre pour une simulation monodisperse est délicat. On propose donc d’analyser le comportement d’un spray polydisperse en le comparant à ceux de sprays monodisperses. Deux configurations académiques sont choisies : des cas de Turbulence Homogène Isotrope chargée en particules pour étudier la dynamique, et des calculs d’évaporation 0D. Trois paramètres sont étudiés pour la dynamique : la concentration préférentielle (ou ségrégation), la traînée moyenne et la traînée réduite moyenne. Cette dernière et la ségrégation de la distribution polydisperse semblent affectées par les tailles de goutte les plus faibles, et la concentration préférentielle apparait alors comme la moyenne des ségrégations des classes qui la composent pondérées par l’inverse du nombre de Stokes associé à chacune de ces classes. La traînée moyenne de la simulation polydisperse possède un comportement proche des diamètres moyens D10 et D20. Ces analyses nous poussent donc à choisir le D10 pour caractériser la dynamique d’un spray polydisperse. Les calculs d’évaporation 0D ne permettent pas dans un premier temps de caractériser efficacement la masse évaporée d’un spray polydisperse par celle d’un spray monodisperse équivalent, mais la définition de nouveaux diamètres issus de la littérature des lits fluidisés comme le D50% le permet, ce qui le place autour du D32. On propose donc de caractériser l’évaporation d’un spray polydisperse par ce diamètre. Enfin, la dernière partie étudie la structure de flamme diphasique dans la chambre MERCATO, à l’aide du formalisme Lagrangien, monodisperse et polydisperse, mais aussi en utilisant le formalisme Eulérien. La validation du modèle FIM-UR du premier chapitre et ses améliorations sont utilisées pour représenter les conditions d’injection liquide. En plus d’un calcul polydisperse, deux simulations monodisperses Lagrangiennes sont réalisées en prenant les diamètres moyens D10 et D32, suite à la partie précédente. Des comparaisons qualitatives et des validations sont réalisées, en comparant des profils de vitesses gazeuses axiale et fluctuante et vitesse axiale liquide issus de l’expérience
Regulations on pollutants have led to the creation of new combustion systems. Giving that fuel is stored in a liquid form, its evolution until combustion is complex. The ability of Large Eddy Simulation has been demonstrated on academic cases, as well as on industrial configurations, by taking into account the multi-physics phenomena, but there is a lack of studies about two-phase flow flame structures. Two solvers for the simulation of two-phase flows are available in the AVBP code, hence both simulations are performed to compare and increase understanding of the phenomena involved such as dispersion, evaporation and combustion. The first part of the study focuses on the validation of the FIM-UR injection model. This model is able to build velocity and droplet profiles at the injector, without simulating primary and secondary break up. A validation in a turbulent case has already been done, and this study validates the model in a laminar case. Comparisons between monodisperse and polydisperse simulations, and experiments are performed. The monodisperse Lagrangian simulation shows good results but the polydisperse simulation is able to represent profiles in the center of the cone by small droplets and at the peripheral part of the cone, by big ones. Moreover, improvements in the Eulerian model exhibit good results. The next section tries to evaluate the impact of polydispersion. Indeed, when a polydisperse approach is not available, choosing the mean diameter can be tricky. A comparison between the behavior of polydisperse spray and monodisperse sprays ones is realised. Two academic cases are studied: Homogeneous Isotropic Turbulence with particles to analyze the dynamics, and 0D evaporation cases. For the dynamics, preferential concentration, mean drag and reduced mean drag are studied. The latter and preferential concentration are affected by small droplets, and the preferential concentration of a polydisperse spray is equivalent to the average of preferential concentration of classes, extracted from the polydisperse distribution, weighted by the inverse of the Stokes number of each class. The mean drag behaves like the D10 and D20 mean drags. This analysis allows us to choose the D10 to characterize a polydisperse distribution for the dynamics. Zero-D evaporation simulations cannot characterize the polydisperse spray evaporated mass by the evaporated mass of monodisperses sprays. New definitions of diameters from fluidized bed literature enable the use of D50%, which is close to D32. We propose to use this diameter to characterize the evaporation of a polydisperse spray. Finally, the last section studies the structure of two-phase flames in the MERCATO bench, using the Lagrangian formalism, monodisperse and polydisperse but also using the Eulerian formalism. The validation of FIM-UR model and improvements from the first section are used to represent liquid injection conditions. A polydisperse simulation is realized and two monodisperse simulations are computed using mean diameters D10 and D32, thanks to the previous section. Qualitative comparisons and validations are realized, comparing gaseous velocity profiles and liquid velocity profiles. Good agreements are found and the mean diameter D32 seems to be close to the polydisperse spray. A comparison between mean flames is done with an Abel transform of the flame from the experiments. The flame has an "M shape", anchored by small recirculation zones out of the swirler, and by a point at the tip of the central recirculation zone. Then, the impact of droplet distributions is analyzed. Even if few bigger droplets from the polydisperse distribution are convected in the hot gases due to bigger particular time and evaporation time, two-phase flow flame structures are equivalent. Different combustion regimes appeared with premixed flames and pockets of fuel burning in the hot gases