Ce travail de thèse s'inscrit dans le cadre d'un projet de recherche entre le CEA et ARMINES (Centre LGEI/ Ecole des Mines d'Alès). Il vise à améliorer la connaissance des mécanismes physiques se produisant lorsque qu’une nappe de liquide (inflammable et/ou toxique stocké à pression atmosphérique) s’évapore suite à la rupture de son stockage. La démarche expérimentale employée consiste à réaliser un plan d'expériences visant à exprimer le débit d'évaporation initial d’une nappe sous différentes conditions initiales de température de liquide et de sol, sous différentes vitesse d’écoulement, de température d’air et selon différentes épaisseurs initiales de liquide. Les différents flux thermiques échangés entre la nappe et son environnement, la température de la nappe et le débit d'évaporation sont mesurés et quantifiés.Les débits d'évaporation expérimentaux sont confrontés à ceux prédits par les différentes corrélations disponibles dans la littérature. Deux analyses de sensibilité sont également réalisées sur ces corrélations et les résultats confrontés à ceux du plan d'expériences afin de vérifier si les corrélations attribuent le même poids aux différents paramètres expérimentaux que le phénomène en lui-même.Les relevés de température dans l'épaisseur de la nappe mettant en évidence la présence de cellules de convection naturelle est également étudiée. Par ailleurs, la température moyenne de la surface est déterminée à partir des différents flux thermiques échangés entre la nappe et son environnement.A l'aide des résultats obtenus, l'étude de plusieurs éléments a été réalisée: l’écart de prédiction sur les résultats des équations bilan thermique et massique selon la température employée pour les incrémenter, la nette différence de température entre la surface et le coeur du liquide, rarement prise en compte dans les modèles théoriques, le rôle prépondérant de la convection naturelle dans le phénomène d'évaporation.Un dernier chapitre étudie la dispersion de la température de surface (phénomène peu étudié dans la littérature) à l'aide d'une caméra thermique. Des zones de températures homogènes apparaissent alors dans le cas de l'essai mettant en oeuvre un écoulement de cavité au-dessus du liquide. La présence de différentes zones de température implique que la cinétique d’évaporation n’est pas uniforme sur la surface de la nappe. A partir de ces résultats, le coefficient de transfert de matière est étudié en fonction de la régression du niveau de liquide dans le bac et conclut à une diminution non modélisée par les corrélations existantes. / This work belongs to a research project between CEA and ARMINE (LGEI center/ Ecole des Mines d’Alès). It aims at increasing comprehension of physical mechanism generating when a liquid pool (either flammable or toxic parked under atmospheric pressure) evaporates after loss of containment. An experimental design is realized in order to express some characteristics of evaporation phenomena (initial evaporation rate, steady evaporation rate and duration of unsteady evaporation rate) as a function of initial liquid and soil temperature, wind velocity, air temperature and initial liquid thickness. Heat fluxes exchanged between the pool and its environment are either measure or computed.Experimental evaporation rates are compared to those predicted by correlations available in the literature. Two sensitivity analyses are performed and their results are confronted to those from experimental design. It allows determining if the importance of the different experimental parameters is the same from the correlations to the phenomena itself.Temperature measurements in liquid thickness highlight the presence of natural convection cells. Besides, mean surface temperature is computed from measurements of heat fluxes exchanged between the pool and its environment. From the different results, several points are investigated: the shift between heat and mass balance equations according to the temperature employed to compute them the difference between the liquid bulk and liquid surface temperature, barely taken into account in correlations the noteworthy role of natural convection in the evaporation phenomena.A last chapter studies the surface temperature distribution thanks to an infrared thermometer. Homogeneous temperatures areas appear in the case of cavity flows. The presence of different temperature areas implies that evaporation kinematic in not uniform in the whole surface. From these result the mass transfer coefficient is studied as a function of the step height between the top of the cavity and the liquid surface. It concludes to a mass transfer coefficient decrease non modeled by the different correlations in the literature.

La réussite du développement de vols spatiaux de longue durée, ainsi que de l’établissement de stations permanentes nécessite des systèmes fermés autonomes bien contrôlés. L’optimisation d’une boucle fermée d’un système support vie bio-régénératif, impose le contrôle de l’hydrodynamique et des transferts de chaleur et de masse couplés qui se développent au sein d’un habitacle spatial comprenant des hommes ou des plantes. Un protocole expérimental (expérience terrestre et méthode de mesure) a été conçu pour quantifier les vitesses de l’air et les transferts hétérogènes qui se développent par condensation d’air humide sur une surface plate horizontale de petite taille (25 cm2), en conditions contrôlées (régime d’écoulement, hygrométrie, température). Une surface active était maintenue isotherme sous le point de rosée par thermoélectricité et le flux de masse était mesuré par pesée. Un tunnel climatique a été utilisé pour générer des écoulements laminaires ou faiblement turbulents. Environ 70 expériences de condensation ont été réalisées à température ambiante (19-23°C) avec une humidité relative de 35-65 % et pour des vitesses comprises entre 1.0 et 3.0 m/s. Le dispositif de condensation a un comportement de type profil épais pour l’écoulement et les coefficients de transferts de masse ont été évalués. L’augmentation de l’intensité de l’écoulement se traduit par une dépendance du nombre de Sherwood en Re2/3. Une relation empirique est proposée pour estimer la température de la surface. Le comportement de l’écoulement au sein de la couche limite et de la croissance des gouttes sur la surface de la plaque sont discutées. Ce travail expérimental sera utile pour le développement de modèles théoriques adaptés à d’autres géométries. / For the development of successful long term space flights, and the establishment of permanent bases in space, a well controlled self sustained closed environment is required. In order to optimize a closed-loop bio-regenerative life support system, it is necessary to control the hydrodynamics and the coupled heat and mass transfer, which develop in a space habitat concerned with humans and plants. We have designed a ground based experimental setup and protocol to measure the air flow velocities and concomitant mass transfer by condensation of water vapour from humid air on a horizontal flat plate of small size (area 25 cm2), in a controlled air flow conditions (flow regime, hygrometry, temperature). An active isothermal surface was kept below the dew point, by using thermoelectricity, and precise weighing of the condensate in order to evaluate the rate of mass flux. An air-conditioned closed circuit wind tunnel has been used to produce laminar to weakly turbulent flows. Almost 70 condensation experiments have been performed at an ambient temperature (19-23 °C) for a relative humidity between 35-65 %, and for the velocity range 1.0-3.0 m/s. The condensing unit behaves as a blunt-faced body and mass transfer coefficients were deduced. When increasing the flow intensity it was found that the Sherwood number had a dependence on Re2/3. An empirical relation was proposed to estimate the surface temperature. The flow behaviour within the boundary layer and the analysis of the drop growth on the flat plate surface under weakly turbulent flows has been discussed. This experimental work will be helpful to develop theoretical models for further studies with other geometries.

This thesis focuses on solute transport upscaling. Upscaling of solute transport is usually required to obtain computationally efficient numerical models in many field applications such as, remediation of aquifers, environmental risk to groundwater resources or the design of underground repositories of nuclear waste. The non-Fickian behavior observed in the field, and manifested by peaked concentration profiles with pronounced tailing, has questioned the use of the classical advection-dispersion equation to simulate solute transport at field scale using numerical models with discretizations that cannot capture the field heterogeneity. In this context, we have investigated the use of the advection-dispersion equation with mass transfer as a tool for upscaling solute transport in a general numerical modeling framework. Solute transport by groundwater is very much affected by the presence of high and low water velocity zones, where the contaminant can be channelized or stagnant. These contrasting water velocity zones disappear in the upscaled model as soon as the scale of discretization is larger that the size of these zones. We propose, for the modeling solute transport at large scales, a phenomenological model based on the concept of memory functions, which are used to represent the unresolved processes taking place within each homogenized block in the numerical models. We propose a new method to estimate equivalent blocks, for which transport and mass transfer parameters have to be provided. The new upscaling technique consists in replacing each heterogeneous block by a homogeneous one in which the parameters associated to a memory functions are used to represent the unresolved mass exchange between highly mobile and less mobile zones occurring within the block. Flow upscaling is based on the Simple Laplacian with skin, whereas transport upscaling is based in the estimation of macrodispersion and mass transfer parameters as a result of the interpretation of the r / Llerar Meza, G. (2009). Upscaling nonreactive solute transport [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/5848 / Palancia

Understanding thermal transport across length scales lays the foundation to developing high-performance electronic devices. Although many experiments and models of the past few decades have explored the physics of heat transfer at nanoscale, there are still open questions regarding the impact of periodic nanostructuring and coherent phonon effects, as well as the interaction of strain and thermal transport. Thermomechanical effects, as well as strains applied in flexible electronic devices, impact the thermal transport. In the simplest kinetic theory models, thermal conductivity is proportional to the phonon group velocity, heat capacity, and scattering times. Periodic porous nanostructures impact the phonon dispersion relationship (group velocity) and the boundaries of the pores increase the scattering times. Strain, on the other hand, affects the crystal structure of the lattice and slightly increases the thermal conductivity of the material under compression. Intriguingly, applying strain combined with the periodic porous structures is expected to influence both the dispersion relation and scattering rates and yield the ability to tune thermal transport actively. But often these interrelated effects are simplified in models.<br><br>This work evaluates the combination of structure and strain on thermal conductivity by revisiting some of the essential methods used to predict thermal transport for a single layer of graphene with a periodic porous lattice structure with and without applied strain. First, we use the highest fidelity method of Non-Equilibrium Molecular Dynamics (NEMD) simulations to estimate the thermal conductivity which considers the impact of the lattice structure, strain state, and phononic band structure together. Next, the impact of the geometry of the slots within the lattice is interrogated with Boltzmann Transport Equation (BTE) models under a Relaxation Time Approximation. A Monte Carlo based Boltzmann Transport Equation (BTE) solver is also used to estimate the thermal conductivity of phononic crystals with varying pore geometry. Dispersion relations calculated from continuum mechanics are used as input here. This method which utilizes a simplified pore geometry only partially accounts for the effects of scattering on the pore boundaries. Finally, a continuum level model is also used to predict the thermal conductivity and its variations under applied strain. As acoustic phonon branches tend to carry the most heat within the lattice, these continuum models and other simple kinetic theories only consider their group velocities to estimate their impact on phonon thermal conductivity. As such, they do not take into account the details of phonon transport across all wavelengths.<br><br>By comparing the results from these different methods, each of which has different assumptions and simplifications, the current work aims to understand the effects of changes to the dispersion relationship based on strain and the periodic nanostructures on the thermal conductivity. We evaluate the accuracy of the kinetic theory, ray tracing, and BTE models in comparison to the MD results to offer a perspective of the reliability of each method of thermal conductivity estimation. In addition, the effect of strain on each phononic crystal with different pore geometry is also predicted in terms of change to their in-plane thermal anisotropy values. To summarize, this deeper understanding of the nanoscale thermal transport and the interrelated effects of geometry, strain, and phonon band structure on thermal conductivity can aid in developing lattices specifically designed to achieve the required dynamic thermal response for future nano-scale thermoelectric applications.

Diplomová práce se zabývá membránovou destilací, především z matematické perspektivy. Jedná se o tepelně poháněný separační proces, ve kterém se pro rozdělení kapalné a plynné fáze používá porézní membrána. Kapalina se vypařuje a její plynná fáze prochází přes póry v membráně. Během tohoto procesu dochází k tepelné i látkové výměně, které jsou popsány systémem parciálních diferenciálnich rovnic. Další model je založen na analogii s elektrickými obvody, zákonu zachování energie, hmotnostní bilanci a empirických vztazích. Je ověřen s experimentálně naměřenými daty z nové alternativní destilační jednotky používající membránu a kondenzátor z polymerních dutých vláken. Výkon a účinnost jednotky jsou vyhodnoceny. Další možná vylepšení jsou navržena.

The thesis is focused on determining the energy flows and the creation of an internal microclimate swimming pool halls. It describes the problems of design, the potential risks associated with the use of pools and thermodynamic processes heat and mass transfer. For this purpose was developed simulation program, which examines the behavior of heat and humidity in the room depending on external factors. The room can be simulated for any time period and geographic location. The outputs of the simulation are heat and mass balance, which will be covered air condition units. Three variants were designed and optimized solutions comparing claims on their operations. The experimental part is focused on the evaporation of the water. Measurement examines dependence of the speed of evaporation of water at different boundary conditions. Was investigated temperature dependence of water, temperature and relative humidity and air velocity above the water surface.

Heat transfer and solidification models were developed for use in a numerical model of a continuous caster to provide a means of predicting how the developing shell would react under variable operating conditions. Measurement data of the operating conditions leading up to a breakout occurrence were provided by an industrial collaborator and were used to define the model boundary conditions. Steady-state and transient simulations were conducted, using boundary conditions defined from time-averaged measurement data. The predicted shell profiles demonstrated good agreement with thickness measurements of a breakout shell segment – recovered from the quarter-width location. Further examination of the results with measurement data suggests pseudo-steady assumption may be inadequate for modeling shell and flow field transition period following sudden changes in casting speed. An adaptive mesh refinement procedure was established to increase refinement in areas of predicted shell growth and to remove excess refinement from regions containing only liquid. A control algorithm was developed and employed to automate the refinement procedure in a proof-of-concept simulation. The use of adaptive mesh refinement was found to decrease the total simulation time by approximately 11% from the control simulation – using a static mesh.

This work aims at modelling and evaluating a new type of photocatalytic reactors, named fixed-bed photocatalytic membrane reactor (FPMR). Such reactors are based on the deposition of a thin layer of photocatalysts on a permeable substrate by filtration. This layer serves as a photocatalytic membrane, named fixed-bed photocatalytic mem-brane (FPM), which is perpendicularly passed by the reactant solution and illuminated by a suitable light source. One advantage of FPMs is their renewability. The model, which was developed for this reactor, relates the overall reaction rate in the FPM with the intrinsic reaction kinetic at the catalyst surface and accounts for light intensity, structural and optical layer properties as well as the mass transfer in the pores. The concept of FPMR was realised by using a flat sheet membrane cell. It facilitated principal investigations into the reactor performance and the validity of the model. For this purpose, the photocatalytic degradation of organic compounds, such as meth-ylene blue and diclofenac sodium, was conducted at varying conditions. Pyrogenic ti-tania was used as a photocatalyst. The experimental data support the developed mod-el. They also indicate a significant impact of the flow conditions on the overall photo-catalytic activity, even though the Reynolds number in the FPM was very small; the to-tal mass transfer rate in the FPM amounted to more than 1.0 s−1. The experiments also showed a sufficient structural strength of the FPM and photocatalytic stability. In addi-tion, the renewal and regeneration of FPMs was successfully demonstrated. Furthermore, another FPMR was designed by means of submerged ceramic mem-branes. This reactor was mainly used to assess the effectiveness and efficiency of FPMRs at the example of the photocatalytic degradation of oxalic acid. The correspond-ing reactor was run closed loop and in continuous mode. The effectiveness of the reac-tor was evaluated based on common descriptors, such as apparent quantum yield, photocatalytic space-time yield and light energy consumption. The results showed that the FPMR based on submerged ceramic membrane had a higher efficiency than other reported photocatalytic reactors. The comparison of the different modes of operation revealed that the closed loop FPMR is most efficient with regard to light energy con-sumption. Finally, this work discusses the up-scaling of FPMRs for industrial applications and proposes a solution, which can e.g. be employed for wastewater treatment or CO2 conversion.:Abstract iii Kurzfassung v Acknowledgment vii Contents ix Nomenclature xiii 1 Introduction 1 1.1 Motivation 1 1.2 Aim and objectives of the work 3 1.3 Thesis outline 3 2 Heterogeneous photocatalytic reactors 5 2.1 Introduction to photocatalysis 5 2.2 Processes in heterogeneous photocatalysis 6 2.2.1 Optical phenomena 7 2.2.2 Mass transfer 8 2.2.3 Adsorption and desorption 9 2.2.4 Photocatalytic reactions 10 2.2.5 Factors affecting heterogeneous photocatalysis 12 2.3 Photocatalytic reactor systems towards water treatment 16 2.3.1 Introduction to photocatalytic reactors 16 2.3.2 Development of photocatalytic reactor designs 17 2.3.3 Quantitative criteria for evaluating photocatalytic reactor designs 21 2.4 Cake layer formation in membrane microfiltration 22 2.4.1 Suspension preparation 22 2.4.2 Cake layer formation 23 2.5 Fluid flow through a fixed bed of particles 25 2.5.1 Pressure drop through a fixed-bed 25 2.5.2 Liquid-solid mass transfer correlation in fixed-bed 25 3 Concept and mathematical modelling of FPMRs 29 3.1 Concept of fixed-bed photocatalytic membrane reactors 29 3.2 Modelling of fixed-bed photocatalytic membrane reactors 31 3.3 Model sensitivity analysis 37 3.4 Chapter summary 39 4 FPMR realised with flat sheet polymeric membrane 41 4.1 Introduction 41 4.2 Materials and set-up 41 4.2.1 Materials 41 4.2.2 Experimental set-up 43 4.3 Experiments and methods 48 4.3.1 Formation of fixed-bed photocatalytic membrane 48 4.3.2 Reactor performance 50 4.3.3 Parameters study and model verification 53 4.3.4 Catalyst layer characterisation 56 4.3.5 Measurement and evaluation of photocatalytic activity of FPM 59 4.4 Results and model verification 60 4.4.1 Reactor performance 60 4.4.2 Influence parameters 71 4.4.3 Model verification 79 5 FPMR realised with submerged ceramic membrane 92 5.1 Introduction 92 5.2 Materials and reactor set-up 93 5.2.1 Reactor set-up 93 5.2.2 Chemicals 97 5.3 Experiments and methods 97 5.3.1 Formation of fixed-bed photocatalytic membranes 97 5.3.2 Photocatalytic performance 97 5.3.3 Parameter study 98 5.3.4 Reactor model for calculating reaction rate constant of FPM 99 5.3.5 Comparison of different reactor schemes 102 5.4 Results and discussions 105 5.4.1 Reactor performance 105 5.4.2 Consistency of CPMR and LPMR data 107 5.4.3 Influence of catalyst loading 108 5.4.4 Influence of permeate flux and light intensity 109 5.4.5 Reactor efficiency 111 5.4.6 Comparison of different reactor schemes 113 5.5 Proposed up-scaled FPMR systems 113 5.6 Concluding remarks 116 6 Conclusion and outlook 118 6.1 Summary of thesis contributions 118 6.2 Discussion and outlook 120 References 122 List of Figures 134 List of Tables 138 Appendix A Calibration 139 A.1 Distribution of light intensity on the surface of catalyst layer 139 A.2 Concentration and absorbance of diclofenac 141 A.3 TOC concentration and electrical conductivity of oxalic acid 141 A.4 Concentration and absorbance of methylene blue 142 Appendix B Mathematical modelling 143 B.1 Influence of axial dispersion on the reaction rate 143 B.2 Special case 146 Appendix C Comparison the photocatalytic activity of TiO2 and ZnO 147 Appendix D Mathematical validation of model for LPMR and CPMR 148 D.1 Model for LPMR (cf. Eq. (5 12)):148 D.2 Model for CPMR (cf. Eq. (5 17)) 149 Appendix E Particle size distribution 151

The assessment of performance for PEM Fuel Cells (PEMFC) at the stack, Single Repeating Unit (SRU), and Membrane Electrode Assembly (MEA) level is dominated by the evaluation of polarization curves. However, polarization curves do not provide adequate detail as to the origin of the inefficiencies of the fuel cell performance and information on these sources of origin are critical to understand and address topics such as material selection, optimal operating conditions, and overall robust and reliable cell and stack design characteristics. To the purpose of understanding the origin of the inefficiencies underlying the fuel cell polarization curve a series of additional experimental and analysis techniques must be applied and from the resultant data the origin of the inefficiencies can then be assigned to kinetic, ohmic, and mass transport loss categorizations. Further, through a combination of the diagnostic methods further resolution can be implied down to the contribution of the individual components to the relative voltage loss categories. In this topic, a methodology will be presented and discussed that achieves and demonstrates this process.

In verfahrenstechnischen Anlagen der Flüssig-Flüssig-Stoffübertragung kommt es an der Phasengrenze zwischen den nicht mischbaren Flüssigphasen häufig zur Ausbildung hydrodynamischer Instabilitäten. Sie sind mit komplexen Geschwindigkeitsfeldern in den Flüssigphasen, insbesondere in den grenzschichtnahen Regionen verbunden und führen zu einem starken Anstieg der pro Zeiteinheit übertragenen Stoffmenge. Die Lösung der Diffusionsgleichung reicht in diesem Fall zur Vorausberechnung des für Auslegungszwecke bedeutsamen Stoffdurchgangskoeffizienten nicht mehr aus. Chemische Reaktionen stellen Quellen oder Senken von Wärme und Stoff dar, die das Auftreten von Instabilitäten begünstigen und die mathematische Beschreibung zusätzlich erschweren. Im Rahmen der vorliegenden Arbeit wurden experimentelle und numerische Untersuchungen zum Flüssig-Flüssig-Stoffübergang in einem vertikalen Kapillarspalt durchgeführt. Reaktionsfreie Stoffübergänge und solche mit einer exothermen chemischen Reaktion an der Phasengrenze zeigten eine große Vielfalt von Konvektionsstrukturen, wie Rollzellen, Thermiken und das doppeldiffusive Fingerregime. Die Visualisierung der Transportvorgänge erfolgte durch das Schattenschlierenverfahren. Die Beobachtungen wurden hinsichtlich geometrischer Eigenschaften von Konvektionsstrukturen sowie deren zeitlicher Änderung ausgewertet. Dazu zählten insbesondere das Längenwachstum von Thermiken und horizontale Wellenlängen von Fingerstrukturen. Zur mathematischen Beschreibung der Phänomene im Kapillarspalt wurde ein Modell entwickelt, welches auf den gekoppelten, zweidimensionalen Transportgleichungen von Impuls, Wärme und Stoff beruht. Es berücksichtigt dichte- und grenzflächenspannungsgetriebene Instabilitätsmechanismen sowie die besonderen Durchströmungseigenschaften des Kapillarspalts. Die Phasengrenze wurde als eben angenommen. Die Lösung der Modellgleichungen erfolgt auf numerischem Wege durch ein Computerprogramm. Das Modell ist in der Lage, die beobachteten Instabilitätsphänomene qualitativ richtig wiederzugeben. Mit Hilfe von Simulationsrechnungen konnte der Mechanismus aufgeklärt werden, der zum schnelleren Rückgang des Stoffdurchgangskoeffizienten im Rollzellenregime der rein grenzflächenspannungsgetrieben Instabilität im Vergleich zum Vorgang mit überlagerter Dichtekonvektion führt. Des Weiteren gelang der Nachweis des doppeldiffusiven Fingerregimes beim Stoffübergang mit exothermer Grenzflächenreaktion. Die berechnete Erhöhung des Stoffdurchgangskoeffizienten stimmt mit Angaben in experimentellen Arbeiten anderer Autoren überein.