A continuous cloud chamber would be a valuable asset to laboratory work in nuclear and atomic physics. For this reason the construction and investigation of a continuously sensitive diffusion cloud chamber has been undertaken. It is the purpose of this paper to report the design and operating characteristics of such a chamber.

The most ubiquitous mode of frost formation on substrates is condensation frosting, where dew drops condense on a supercooled surface and subsequently freeze, and has been known since the time of Aristotle. The physics of frost incipience at a microscopic scale has, nevertheless, eluded researchers because of an unjustified ansatz regarding the primary mechanism of condensation frosting. It was widely assumed that during condensation frosting each supercooled droplet in the condensate population freezes in isolation by heterogeneous nucleation at the solid-liquid interface, quite analogous to the mechanism of icing. This assumption has very recently been invalidated with strong experimental evidence which shows that only a single droplet has to freeze by heterogeneous nucleation (typically by edge effects) in order to initiate condensation frosting in a supercooled condensate population. Once a droplet has frozen, it subsequently grows an ice bridge towards its nearest neighboring liquid droplet, freezing it in the process. Thus ensues a chain reaction of ice bridging where the newly frozen droplets grow ice bridges toward their nearest neighbor liquid droplets forming a percolating network of interconnected frozen droplets. Not always are these ice bridges successful in connecting to their adjacent liquid droplets. Sometimes the liquid droplet can completely evaporate before the ice bridges can connect, thus forming a local dry region in the vicinity of the ice bridge. In this work, we first formulate a thermodynamic framework in order to understand the localized vapor pressure gradients that emerge in mixed-mode phase-change systems and govern condensation and frost phenomena. Following this, we study droplet pair interactions between a frozen droplet and a liquid droplet to understand the physics behind the local ice bridge connections. We discuss the emergent scaling laws in ice bridging dynamics, their relative size dependencies, and growth rates. Thereafter, we show how with spatial control of interdroplet distances in a supercooled condensate and temporal control of the first freezing event, we can tune global frost propagation on a substrate and even cause a global failure of all ice bridges to create a dry zone. Subsequently, we perform a systematic study of dry zones and derive a scaling law for dry zones that collapses all of our experimental data spanning a wide parameter space. We then show that almost always the underlying mechanism behind the formation of dry zones around any hygroscopic droplet is inhibition of growth and not inhibition of nucleation. We end with a discussion and preliminary results of our proposed anti-frosting surface that uses ice itself to prevent frost. / Master of Science / In the movie Iron Man, during the very final battle sequence between our eponymous hero and Iron Monger, there is a moment when Tony Stark realizes that Iron Monger can fly. Immediately Iron Man shoots up into the sky. He tries to reach as high as he can. Iron Monger chases after him. Eventually, high up in the sky amidst the clouds, Iron Monger catches up. He grabs Iron Man in his enormous grip and punches him. It seems like there is no escape for Iron Man. But right then, writhing in Iron Monger’s grip, Tony Stark asks Iron Monger, ‘How’d you solve the icing problem?’ It is then revealed that Iron Monger’s suit has completely frozen over, whereas Iron Man’s suit has no ice whatsoever. Iron Monger shuts down and starts falling from the sky. The icing problem, which Tony Stark mentions, occurs when supercooled liquid water droplets impact on a chilled substrate and subsequently freeze. Another way of accretion of ice constitutes the frosting problem. The most common mode of frost formation on a surface is called condensation frosting, where the ambient water vapor first condenses on the chilled surface as dew drops, and these liquid droplets subsequently freeze. Until very recently, it was widely assumed that during condensation frosting all the dew droplets freeze in isolation at the solid-liquid interface, without interacting with each other. This assumption, however, is not true. It has recently been shown that in order to initiate condensate frosting, only a single droplet has to freeze by itself, at the solid-liquid interface. Thereafter, frost propagates by the formation of an inter-droplet ice bridge network, where the frozen droplets grow ice bridges toward their nearest neighbor liquid droplets. Interestingly, these ice bridges are not always successful in connecting to their adjacent liquid droplets. If the inter-droplet distance is too large, the liquid droplet can completely evaporate before the ice bridges can connect, thus forming a local dry region in the vicinity of the ice bridge. In this work, we do extensive experiments to investigate the underlying physics of frost incipience on a microscopic scale. We then derive scaling laws for successful ice connections, their growth rates and for dry zone formations, and end by discussing possible anti-frosting strategies. It appears that Tony Stark in his universe has solved the icing problem, and most probably also the frosting problem. In reality, however, on earth, we have not. Though anti-icing has received a lot of attention, the same cannot be said about the frosting problem. This work tries to take the first steps towards that. Quite ironically, it looks like ice itself might be the solution to the frosting problem, because of its ability to create dry zones.

The design of last stage low pressure steam (LP) turbines has become increasingly complicated as turbine manufacturers have pushed for larger and more efficient turbines. The tip sections of these LP turbines encounter condensing wet steam at high velocities resulting in increased losses. These losses are difficult to predict with computational fluid dynamic models. To study these losses and improve the design of LP turbines a study was commissioned to determine the feasibility and cost of a steam cascade facility for measuring low pressure turbine blade tip section aerodynamic and thermodynamic performance. This study focused on two objectives: 1) design a steam production facility capable of simulating actual LP turbine operating conditions, and 2) design an instrumentation system to measure blade performance in wet steam. The steam production facility was designed to allow the test section size to be selected later. A computer code was developed to model the facility cycle and provide equipment requirements. Equipment to meet these requirements, vendors to provide it, and costs were found for a range of test section sizes. A method to control the test section conditions was also developed. To design the instrumentation system two methods of measuring blade losses through entropy generation were proposed. The first method uses existing total pressure probe techniques. The second method uses advanced particle imaging velocimetry techniques possibly for the first time in wet steam. A new method is then proposed to modify the two techniques to take measurements at non-equilibrium states. Finally accuracy issues are discussed and the challenges associated with achieving periodic flow in this facility are investigated. / Master of Science

Il est probable qu'à terme les émissions de nanoparticules soient réglementées et ce sont donc les concentrations en nombre qui seront considérées. Il convient donc d'adapter les modèles afin de pouvoir simuler correctement les concentrations en nombre, dans les ambiances confinées comme dans l'atmosphère. Un modèle de dynamique des particules capable de suivre avec autant de précision la concentration en nombre que la concentration en masse, avec un temps de calcul optimal, a été développé. La dynamique des particules dépend de divers processus, les plus importants étant la condensation/évaporation, suivie par la nucléation, la coagulation, et les phénomènes de dépôts. Ces processus sont bien connus pour les particules fines et grossières, mais dans le cas des nanoparticules, certains phénomènes additionnels doivent être pris en compte, notamment l'effet Kelvin pour la condensation/ évaporation et les forces de van der Waals pour la coagulation. Le travail a tout d'abord porté sur le processus de condensation/évaporation, qui s'avère être le plus compliqué numériquement. Les particules sont présumées sphériques. L'effet Kelvin est pris en compte car il devient considérable pour les particules de diamètre inférieur à 50 nm. Les schémas numériques utilisés reposent sur une approche sectionnelle : l'échelle granulométrique des particules est discrétisée en sections, caractérisées par un diamètre représentatif. Un algorithme de répartition des particules est utilisé, après condensation/évaporation, afin de conserver les diamètres représentatifs à l'intérieur de leurs sections respectives. Cette redistribution peut se faire en terme de masse ou de nombre. Un des points clé de l'algorithme est de savoir quelle quantité, de la masse ou du nombre, doit être redistribuée. Une approche hybride consistant à répartir la quantité dominante dans la section de taille considérée (le nombre pour les nanoparticules et la masse pour les particules fines et grossières) a été mise en place et a permis d'obtenir une amélioration de la précision du modèle par rapport aux algorithmes existants, pour un large choix de conditions. Le processus de coagulation pour les nanoparticules a aussi été résolu avec une approche sectionnelle. La coagulation est régie par le mouvement brownien des nanoparticules. Pour cette approche, il a été constaté qu'il est plus efficace de calculer le noyau de coagulation en utilisant le diamètre représentatif de la section plutôt que de l'intégrer sur la section entière. Les simulations ont aussi pu montrer que les interactions de van der Waals amplifient fortement le taux de coagulation pour les nanoparticules. La nucléation a été intégrée au modèle nouvellement développé en incorporant un terme source de nanoparticules dans la première section, commençant à un nanomètre. La formulation de ce taux de nucléation correspond à celle de l'acide sulfurique mais le traitement des interactions numériques entre nucléation, coagulation et condensation/évaporation est générique. Différentes stratégies de couplage visant à résoudre séparément ou en même temps les trois processus sont discutées. Afin de pouvoir proposer des recommandations, différentes méthodes numériques de couplage ont été développées puis évaluées par rapport au temps de calcul et à la précision obtenue en terme de concentration massique et numérique / It is necessary to adapt existing models in order to simulate the number concentration, and correctly account for nanoparticles, in both free and confined atmospheres. A model of particle dynamics capable of following accurately the number as well as the mass concentration of particles, with an optimal calculation time, has been developed. The dynamics of particles depends on various processes, the most important ones being condensation/evaporation, followed by nucleation, coagulation, and deposition phenomena. These processes are well-known for fine and coarse particles, but some additional phenomena must be taken into account when applied to nanoparticles, such as the Kelvin effect for condensation/evaporation and the van der Waals forces for coagulation. This work focused first on condensation/evaporation, which is the most numerically challenging process. Particles were assumed to be of spherical shape. The Kelvin effect has been taken into account as it becomes significant for particles with diameter below 50 nm. The numerical schemes are based on a sectional approach : the particle size range is discretized in sections characterized by a representative diameter. A redistribution algorithm is used, after condensation/ evaporation occurred, in order to keep the representative diameter between the boundaries of the section. The redistribution can be conducted in terms of mass or number. The key point in such algorithms is to choose which quantity has to be redistributed over the fixed sections. We have developed a hybrid algorithm that redistributes the relevant quantity for each section. This new approach has been tested and shows significant improvements with respect to most existing models over a wide range of conditions. The process of coagulation for nanoparticles has also been solved with a sectional approach. Coagulation is monitored by the brownian motion of nanoparticles. This approach is shown to be more efficient if the coagulation rate is evaluated using the representative diameter of the section, rather than being integrated over the whole section. Simulations also reveal that the van derWaals interactions greatly enhance coagulation of nanoparticles. Nucleation has been incorporated into the newly developed model through a direct source of nanoparticles in the first size section, beginning at one nanometer. The formulation of this rate of nucleation corresponds to that of sulfuric acid but the treatment of the numerical interactions between nucleation, coagulation and condensation/evaporation is generic. Various strategies aiming to solve separately or jointly these three processes are discussed. In order to provide recommendations, several numerical splitting methods have been implemented and evaluated regarding their CPU times and their accuracy in terms of number and mass concentrations

L'objectif de cette étude est d'examiner les écoulements diphasiques liquide/gaz dans les canaux des plaques bipolaires des piles à combustible afin de comprendre et de trouver des solutions au problème d'engorgement. L'influence de la section du canal et du matériau utilisé sur les pertes de charge (PDC) et sur la structure de l'écoulement d'eau liquide est plus particulièrement étudiée dans une expérience hors pile. Les mesures des PDC ont permis de mettre en évidence des séquences de bouchages et de débouchages, la fréquence de ces dernières augmentant avec le débit. Par ailleurs il est montré que le rapport des PDC diphasiques moyennées par les PDC en air sec décroit avec le débit et ceci indépendamment de la dimension du canal et qu'il est d'autant plus faible que le revêtement est hydrophile. Enfin différents régimes d'écoulements diphasiques (stratifié et de gouttes) ont pu être mis en évidence dans les différentes zones du canal et un modèle pour chacun d'eux a été établi, confirmant les résultats expérimentaux / This work focuses on the gas-liquid flows in the cathode plate, with the objective to observe their patterns, to understand their behavior, to estimate the pressure drops (PD) and eventually, to reduce clogging and its possible consequences in term of oxygen starvation downstream. A special emphasis is put on the effect of the channel section (typically between 0.5 and 1 mm²) and on the surface properties of the flow field plate materials. The experiments are performed ex-situ. The PD is measured locally along the channel as well as globally between the inlet and outlet, which put forward the existence of clogging/unclogging sequences. The characteristic frequency of these sequences increases with the air flow rate. The results show that the ratio of PD in two-phase flow to PD in dry flow decreases with the air flow rate while it does not seem to depend on the channel size (within the tested range). Moreover this ratio is lower with hydrophilic coating. Finally two main flow patterns (slug and annular flow) have been observed depending on the distance from the inlet and they have been simulated

Le polyamide 66 est un polycondensat formé par condensation de l'acide adipique et de l'hexaméthylène diamine. La réaction a lieu sans catalyse. Cependant, elle peut être accélérée en présence de catalyseurs, les plus répandus étant les acides phosphorés organiques et minéraux. Aujourd'hui, on connaît peu de choses sur le mécanisme d'activation de la réaction. Dans un souci d'innovation et de compréhension, les recherches en catalyse de polycondensation du polyamide 66 se sont intensifiées ces dernières années. C'est dans cet optique que le travail de thèse a été réalisé. Les objectifs étaient de proposer et tester de nouveaux systèmes catalytiques et d'engager une recherche permettant de mieux comprendre le mécanisme de catalyse de polycondensation du polyamide 66. Nous avons mis en évidence une réelle spécificité du phosphore en catalyse. Des acides alkylphosphoniques porteurs de fonctions polaires se sont révélées particulièrement actifs en synthèse et en post condensation liquide. Grâce à des études sur des réactions modèles, l'hypothèse du passage par un intermédiaire réactionnel de type anhydride mixte, formé entre le catalyseur et l'acide adipique, a pu être confortée / Polyamide 6,6 is prepared by polycondensation of hexamethylene diamine with adipic acid The reaction occurs without catalyst. It can be accelerated using a catalyst, the most usual activators are organic and inorganic phosphorous acids. Today, we know little about the mechanism of activation of the reaction. To innovate and understand, researches in catalysis of polyamide 66 polycondensation have intensified in recent years. It is the reason for why thesis was done. The goal of the thesis was to propose and test new catalytic systems and to initiate a work to get a better understanding of the mechanism of polycondensation catalysis of polyamide 66. We have highlighted a real specificity of phosphorus in catalysis. Alkylphosphonic acids with polar functions were found to be particularly active in synthesis and post condensation liquid. Studies on model compounds have supported that the polycondensation may proceed through a reaction-typed mixed anhydride formed between the catalyst and adipic acid

<p>Part I of this thesis is a theoretical background to parts II and III.</p><p>Part II treats the phenomenon of decreased visibility through a glazing due to external water condensation, dew, on the external surface. Some simulations are presented where it is shown that under certain circumstances condensation can be expected. A combination of coatings on the external surface is suggested to overcome the problem of external condesation. It consists of both a coating which decreases the emissivity of the surface and a hydrophilic coating which reduces the detrimental effects to the view through the window.</p><p>Fresnel calculations of the optical properties are used to discuss the feasibility of using different coatings. A new test box was used to verify that the proposed window coatings perform as expected.</p><p>Part III is a study on the angular dependence of solar absorptance in windows. Optical properties vary with the angle of incidence of the incoming light. The variation is different from one window pane to another. </p><p>A model is proposed to approximate the angular variation of the solar absorptance in window panes. The model is semi-empirical and involves dividing the wide range of windows into nine groups. To which group a window belongs, depends on how many panes it has and on the features of the outer pane. The strength of the model is that it can be used without knowing the exact optical properties of each pane of the window. This makes it useful in the many cases when these data are not given by the manufacturer and Fresnel calculations to get the optical properties of the window are not feasible. The model is simple and can be added as an appendix to existing standards for measuring optical properties of windows.</p>

Part I of this thesis is a theoretical background to parts II and III. Part II treats the phenomenon of decreased visibility through a glazing due to external water condensation, dew, on the external surface. Some simulations are presented where it is shown that under certain circumstances condensation can be expected. A combination of coatings on the external surface is suggested to overcome the problem of external condesation. It consists of both a coating which decreases the emissivity of the surface and a hydrophilic coating which reduces the detrimental effects to the view through the window. Fresnel calculations of the optical properties are used to discuss the feasibility of using different coatings. A new test box was used to verify that the proposed window coatings perform as expected. Part III is a study on the angular dependence of solar absorptance in windows. Optical properties vary with the angle of incidence of the incoming light. The variation is different from one window pane to another. A model is proposed to approximate the angular variation of the solar absorptance in window panes. The model is semi-empirical and involves dividing the wide range of windows into nine groups. To which group a window belongs, depends on how many panes it has and on the features of the outer pane. The strength of the model is that it can be used without knowing the exact optical properties of each pane of the window. This makes it useful in the many cases when these data are not given by the manufacturer and Fresnel calculations to get the optical properties of the window are not feasible. The model is simple and can be added as an appendix to existing standards for measuring optical properties of windows.

Heat transfer and pressure drop during condensation of propane flowing through minichannels is investigated in this study. Studies of condensation of hydrocarbons are important for applications in the petrochemical industry. Insights into the mechanisms of propane condensation are required for accurate design of heat transfer equipment for use in hydrocarbon processing. At present, there is very little research on vertical condensation, especially of hydrocarbons, for the tube sizes and flow conditions of interest to the present study. An experimental facility was designed and fabricated to measure the frictional pressure drop and heat transfer coefficients during condensation of propane in plain tubes with an inner diameter of 1.93 mm. Measurements were taken across the vapor-liquid dome in nominal quality increments of 0.25 for two saturation temperatures (47°C and 74°C) and four mass flux conditions (75 – 150 kg m‾² s‾¹). The data were compared to the predictions of relevant correlations in the literature. The data from this study were also used to develop models for the frictional pressure drop and heat transfer coefficient based on the measurements and the underlying condensation mechanisms. These results and the corresponding correlations contribute to the understanding of condensation of hydrocarbons in vertical minichannels.

Une pile à membrane échangeuse de protons (PEMFC) est un dispositif convertissant l’hydrogène en électricité grâce à une réaction électrochimique appelé électrolyse inverse. Comme chaque pile à combustible ou batterie, les PEMFC sont composées d’une série de couches. Nous nous intéressons à la couche de diffusion (GDL) du côté de la cathode. La GDL est constituée de fibres de carbone traitées pour être hydrophobes. Elle peut être vue comme un milieu poreux mince avec une taille moyenne de pores de quelques dizaines de microns. Une question clé dans ce système est la gestion de l'eau produite par la réaction. Dans ce contexte, le principal objectif de la thèse est le développement d'un outil numérique visant à simuler la formation de l'eau liquide dans la GDL. Une approche réseau de pores est utilisée. Nous nous concentrons sur un scénario où l’eau liquide se forme par condensation dans la GDL. Les comparaisons entre simulations et expériences effectuées grâce à un dispositif microfluidique bidimensionnel, sont d'abord présentées pour différentes conditions de mouillabilité, de distributions de température et d'humidité relative à l’entrée, afin de valider le modèle. Une étude de sensibilité est alors effectuée afin de mieux caractériser les paramètres contrôlant l'invasion de l'eau. Enfin, les simulations sont comparées à des distributions d’eau obtenues in-situ par micro-tomographie à rayons X, ainsi que des distributions expérimentales de la littérature obtenues par imagerie neutronique. / A Proton Exchange Membrane Fuel Cell (PEMFC) is a device converting hydrogen into electricity thanks to an electrochemical reaction called reverse electrolysis. Like every fuel cell or battery, PEMFCs are made of a series of layers. We are interested in the gas diffusion layer (GDL) on the cathode side. The GDL is made of carbon fibers treated hydrophobic. It can be seen as a thin porous medium with a mean pore size of few tens of microns. A key question in this system is the management of the water produced by the reaction. In this context, the main objective of the thesis is the development of a numerical tool aiming at simulating the liquid water formation within the GDL. A pore network approach is used. We concentrate on a scenario where liquid water forms in the GDL by condensation. Comparisons between simulations and experiments performed with a two-dimensional microfluidic device are first presented for different wettability conditions, temperature distributions and inlet relative humidity in order to validate the model. A sensitivity study is then performed to better characterize the parameters controlling the water invasion. Finally, simulations are compared with in situ experimental water distributions obtained by X-ray micro-tomography as well as with experimental distributions from the literature obtained by neutron imaging.