L’habitat sain est le thème central des réflexions contemporaines du domaine du bâtiment élargies à l’environnement. Il comporte des préoccupations notables en matière de santé, de consommation énergétique (la ventilation, le chauffage, la climatisation et l’eau chaude), d’impacts environnementaux et de durabilité des matériaux de construction. Le choix préliminaire des matériaux utilisés pour la construction joue un rôle important dans la réussite d’un projet HQE (Haute Qualité Environnementale). Dans ce contexte, la problématique de prévision des champs de température et d’humidité demeure essentielle à l’intérieur des matériaux poreux de construction, où les matériaux biosourcés font l'objet d'un fort intérêt vu leurs qualités environnementales. Les matériaux biosourcés, étant hygroscopiques, ont tendance à absorber ou à restituer l’humidité, ce qui génère respectivement un gonflement ou un retrait. A l’échelle microscopique, l’humidité prend place soit par l’absorption de l’eau liée par les fibres, soit par l’existence d’eau libre dans les pores. Cette complexité des phénomènes microscopiques dans les matériaux biosourcés mène à une forte interaction entre l’aspect mécanique et les aspects de transferts de masse et de chaleur. L’existence de ce couplage est susceptible de modifier sensiblement les performances thermiques du bâtiment, et même sa durabilité. L’objectif visé par ce travail de thèse est l’étude et l’analyse microscopique du comportement hygrique des matériaux poreux de construction. L’aspect mécanique couplé à l’aspect hygrique est abordé en prenant en considération les déformations locales de gonflement - retrait, et leur impact sur l’hystérésis de teneur en eau. La maîtrise de ce couplage est primordiale tant sur le plan de la prédiction de la qualité des ambiances habitables que sur l’évaluation de la durabilité de ces structures. Le projet de thèse consiste à travailler à la fois sur les aspects modélisation, caractérisation et mesure des transferts hygriques. La quantification de ces phénomènes est réalisée à travers des campagnes de mesures expérimentales basées sur des techniques d’imagerie 3D (micro-tomographie aux rayons X). Le recours à la diffraction aux rayons X (DRX), à la corrélation d’images volumique, ainsi qu’à la résonance magnétique nucléaire (RMN) permet d’avoir une meilleure compréhension des échanges entre la matrice solide et l’eau liée et/ou libre. Tous ces travaux ont mené à une meilleure caractérisation de la morphologie du bois d’épicéa à l’échelle microscopique, ainsi qu’à une meilleure estimation des diverses variations dimensionnelles (gonflement) à l’échelle des parois cellulaires et de leurs constituants chimiques. Les résultats numériques obtenus sur la structure réelle 3D du matériau ont été couplés aux mesures expérimentales à travers la corrélation d’images volumiques (micro-tomographie aux rayons X) afin d’identifier les propriétés intrinsèques des phénomènes et du matériau. Ces travaux de thèse constitueront une base scientifique permettant une meilleure modélisation du couplage mécanique avec les transferts de chaleur et de masse dans les matériaux biosourcés. / Healthy living is a main contemporary concern of the construction field, extended to the environment. It has significant concerns about health, energy consumption, environmental impact and sustainability of building materials. The preliminary selection of materials used for construction plays an important role in the success of high environmental quality projects. In this context, it remains essential to predict the temperature and humidity fields inside porous building materials, where bio-based materials are subject to a strong interest due to their environmental qualities.As bio-based materials are hygroscopic, they tend to absorb or restore moisture, which respectively generates swelling or shrinkage. At the microscopic scale, moisture takes place either by absorption of bound water by the fibers, or by the existence of free water in the pores. The complexity of microscopic phenomena in bio-based materials will lead to strong interactions between the mechanical aspect on one side and heat and mass transfers’ aspects on the other side. The existence of this coupling may significantly alter the building's thermal performance, as well as its durability.The objective of this thesis work is to study the microscopic hygric behavior of porous building materials. The mechanical aspect coupled to the hygric one is studied, taking into consideration the local swelling and shrinkage strains, and their impact on the hysteresis phenomenon. Understanding this coupling is very important in order to improve the quality of habitat and evaluate the durability of these structures.The PhD project consists on working on all aspects, modeling, characterization and measurement of hygric transfers. Quantification of these phenomena is achieved through experimental campaigns based on 3D imaging techniques (X-ray micro-tomography). The use of X-ray diffraction (XRD), digital volume correlation, as well as nuclear magnetic resonance (NMR) allows a better understanding of the interactions between the solid matrix and bound and/or free water. The corresponding results have led to a microscopic morphological characterization of spruce wood, as well as to a better estimation of the various dimensional variations of the cell walls, and their chemical components.The numerical results achieved on the real 3D structure of the material have been coupled to the experimental ones, using digital volume correlation technique (X-ray tomography), in order to identify the intrinsic properties of the material.These thesis works provide a scientific basis allowing the improvement of modeling of the mechanical coupling with heat and mass transfers in bio-based materials.

Multiphase flows in chemical reactors with micro- and millimeter-size channel structures such as monolith froth reactors, compact heat exchangers and fuel cells have received great attention in the last years. They are considered as a promising alternative to conventional reactors, such as fixed bed reactors and bubble columns which are mainly used for gas absorption, catalytic hydrogenation and biochemical conversions. Slug or Taylor bubble flow is a desired operating state for this type of contactors due to the frequent change of efficient gas-liquid contacting in the film around the bubbles and the enhanced mixing in the liquid slugs behind the bubbles. Consequently, capillary Taylor flow is currently a target of intensive investigations. However, a full understanding of design parameters and optimum operating conditions are still lacking. For milli- and microreactors mass transfer between gas and liquid phases depends upon various parameters such as bubble shape, relative velocity between the two phases, degree of liquid contamination and many more. To further advance the fundamental understanding of micro- and milli-channel reactors with Taylor flow, main design parameters and operating conditions were investigated, which include (a) the effect of bubble size, channel diameter and cross sectional shape of channel on the mass transfer coefficient of dissolving bubbles, (b) the influence of the presence of surface active agents on the bubble shape, velocity and also on the mass transfer rate of bubbles and (c) the intensification effect of oscillation of channels on the mass transfer performance of Taylor bubbles. For the study of gas-liquid mass transfer high-resolution X-ray radiography and tomography were used as measurement techniques. The X-ray imaging methods were chosen as their accuracy is less affected by changes in the refractive index, as it is the case for conventional optical methods. The mass transfer was calculated by measuring the changes in the size of the bubbles at constant pressure. The utilization of X-ray visualization enabled the acquisition of a series of radiographic images of bubbles. The images gave the volume, interfacial area and length of the bubble with high accuracy as a function of time and were used to evaluate the mass transfer coefficient using the mass conservation equations. In case of circular channels, the results show that Sherwood numbers have a large dependency on the bubble length and also equivalent diameter which is in accordance with previous results for larger channel diameters. However, the values of measured Sherwood numbers could not be predicted by available correlations which are valid only for larger pipes. As a result, a new mass transfer correlation in the form of Sherwood number as a function of Peclet number as well as bubble size ratio was derived. The proposed correlation is applicable for a large range of bubble sizes with high accuracy. The comparison of the results for the square and circular channels showed that despite the fact that the rise velocity of bubbles in the square channel is about three times higher than in the circular channel, the mass transfer coefficient is about the same. Furthermore, the results show that in square channels the dissolution curves are relatively even, while the dissolution curves of circular channels exhibit some distinguishable change in the slope. In addition, the results show that the calculated mass transfer coefficient based on the measured data show good agreement with the data predicted by the penetration theory. Regarding the influence of surfactants on the mass transfer in small channels with Taylor flow, it was shown that a small amount of surfactant reduces the mass transfer and its impact is more pronounced on small bubbles. Furthermore, it was demonstrated that the presence of surfactants causes the change of the bubble shape and leads to a slight increase of the liquid film thickness around the bubble and as a result the elongation of contaminated bubbles. Intensification of mass transfer in small channels with Taylor bubbles was investigated by measuring the motion, shape and dissolution rate of individual elongated Taylor bubbles of air and CO2 in water. The comparison of the results for the stationary and oscillating channel showed that mechanical vibration of the channel is able to enhance the mass transfer coefficient from 80% to 186%. Moreover, the mass transfer rate positively correlates with frequency and amplitude of oscillation, which is more pronounced at higher amplitudes. In addition, it was shown that the intensification of mass transfer with increase of amplitude/frequency of vibration is mainly attributed to the increase of bubble surface wave oscillations that causes an enlargement of contact area between the phases and also a reduction of mass transfer resistance in the liquid-side boundary layer.

<p>Multiphase flows, such as liquid/gas and solid/gas, dominate many different areas of life, including the medical, agricultural, propulsion, and chemical industries. Gaining insight into the dynamic processes that drive these multiphase flows can therefore have far-reaching impact in many sectors of scientific research. Of key interest is the non-invasive tracking of important state properties such as the mass and temperature distributions in high optical depth multiphase flows. To accomplish this, X-ray diagnostic approaches are utilized due to their ability to probe complex phenomena without being hampered by multiple scattering that arise from complex interactions at the flow surface boundaries.</p> <p>This work accomplishes the measurement of mass distribution through time-resolved tomographic reconstructions of the liquid mass distributions in fuel sprays within liquid/gas flows. The developed diagnostic tool shown here uses a novel multiple line of sight tube source tomography setup to obtain simultaneous time-resolved two-dimensional radiographs of different spray geometries at various perspectives. Through tomographic reconstruction, these radiographs are converted into volumetric reconstructions to give a true sense of mass distribution—where exactly is the liquid mass located in the <em>x</em>, <em>y</em>, <em>z</em> spatial extents at a specific moment in time <em>t</em>? This technique is first showcased in a simple spray as a feasibility test and later applied to a more complex spray geometry and compared against other state-of- the-art diagnostics for a full quantitative understanding of the developed technique. Outside of tomography, improvements in decreasing the uncertainties in line of sight averaged mass distribution measurements in radiography imaging experiments are also showcased through source characterization efforts both for tube source and synchrotron source experiments.</p> <p>Efforts in ascertaining the temperature distributions in liquid/gas flows is done through an application of wide angle X-ray scattering, a technique that is commonly used in the materials, chemistry, and biology sciences but has yet to be widely used in the propul- sion community. These newly developed X-ray scattering measurements are accomplished through the use of a focused monochromatic beam available at the Advanced Photon Source synchrotron facility, and is applied first in calibration jets and later towards more complex dynamic sprays and multi-species liquid solutions.</p>

Nukleare Brennelemente werden nach ihrem Betrieb mehrere Jahre in Nasslagerbecken gelagert, wo ihre Nachzerfallswärme durch elektrisch betriebene Kühlsysteme abgeführt wird. Bei Ausfall der Stromversorgung droht eine Überhitzung der Brennelemente und im schlimmsten Fall die Schädigung der Brennstabhüllen und der Austritt von radioaktivem Material in die Umwelt. Im Mittelpunkt der vorliegenden Dissertation steht die Untersuchung des komplexen Zusammenspiels von Strömung und Wärmetransport bei solch einem angenommenen Unfall, der zu teilweise freigelegten Brennelementen führt. Eine Auswertung des aktuellen Forschungsstandes verdeutlicht, dass die zugrundeliegenden physikalischen Prozesse zwar theoretisch verstanden sind, aber bisher keine speziellen Simulationsprogramme zur präzisen Vorhersage der Temperaturverteilung für mögliche Unfallszenarien existieren. Für die detaillierte Analyse der Vorgänge werden deshalb erstmals numerische Strömungssimulationen unter Berücksichtigung der exakten Geometrie und aller relevanten Wärmetransportmechanismen für ein teilweise freigelegtes Brennelement durchgeführt. Zur Gewährleistung eines praktikablen Rechenaufwands wird der instationäre Verdampfungsvorgang in mehrere, eigenständige Simulationen mit stationären Randbedingungen und jeweils konstantem Füllstand unterteilt. Die Validierung mit experimentellen Daten zeigt, dass dieser Ansatz bei niedriger Nachzerfallsleistung geeignet ist, um die Stabtemperaturen mit ausreichender Genauigkeit vorherzusagen. Durch eine umfassende Sensitivitätsanalyse wird darüber hinaus der Einfluss zahlreicher unsicherer Faktoren auf die Temperaturverteilung und Zusammensetzung im Brennelement untersucht, der sich rein auf Grundlage des Experiments nicht beurteilen lässt. Die Simulationsergebnisse zeigen, dass die maximale Stabtemperatur hauptsächlich vom Füllstand und der Leistung der Brennstäbe abhängt. Eine horizontal gerichtete Luftströmung oberhalb des Brennelements führt insgesamt zu einem Temperaturgefälle in Strömungsrichtung innerhalb des Brennelements. Die Ursache dafür ist ein charakteristisches Strömungsfeld, bei dem kaltes Gas an der stromabwärts gelegenen Wand des Brennelements nach unten und heißes Gas an der stromaufwärts gelegenen Wand nach oben befördert wird. Die alleinige Variation der Geschwindigkeit der Luftströmung bewirkt jedoch keine nennenswerte Änderung der maximalen Stabtemperatur. Erst durch die Verwendung realitätsnaher Randbedingungen für Geschwindigkeit, Temperatur und Zusammensetzung, die aus großskaligen Simulationen des gesamten Lagerbeckens gewonnen wurden, wird der Einfluss der Querströmung auf die Temperaturverteilung im Brennelement deutlich. Bedingt durch das Verhältnis aus Auftriebs- zu Trägheitskräften, steigt die Temperatur im Brennelement bei einer Kombination aus geringer Temperatur, geringem Dampfmassenanteil und hoher Geschwindigkeit der Querströmung signifikant an. Diese Ergebnisse ermöglichen die Ableitung gezielter Beladungsstrategien von Lagerbecken, sofern die Randbedingungen oberhalb der Brennelemente hinreichend genau bekannt sind bzw. vorhergesagt werden können. Im letzten Schritt wird eine Methode zur skalenübergreifenden Modellierung eines Lagerbeckenbereichs vorgestellt. Durch die Kopplung zweier Modellierungsansätze wird eine teilweise geometrieauflösende Simulation ermöglicht, bei der das zentrale Brennelement geometrisch aufgelöst und die benachbarten Brennelemente als poröse Körper modelliert werden. Diese Vorgehensweise verbessert die Übertragbarkeit der Ergebnisse auf ein ganzes Lagerbecken, weil die Auswertung im geometrisch aufgelösten Brennelement unabhängiger von den mit Unsicherheit behafteten Randbedingungen wird.:1 Einleitung 1 1.1 Chancen und Risiken der Kernenergienutzung 1 1.2 Randbedingungen für den Wärme- und Stofftransport im Lagerbecken 3 1.2.1 Zerfallsleistung 3 1.2.2 Brennelement-Typ und Aufbau 4 1.2.3 Wärmetransportmechanismen 6 1.2.4 Verdampfungsrate 8 1.2.5 Grenztemperaturen 9 1.3 Simulation des Wärme- und Stofftransports im Lagerbecken 10 1.3.1 Das Lagerbecken als Multiskalenproblem 10 1.3.2 Systemcodes und Codes für schwere Störfälle 12 1.3.3 CFD-Simulation mit Brennelementen als poröse Körper 13 1.3.4 Geometrieauflösende CFD-Simulation 15 1.4 Zielstellung und Aufbau der Arbeit 16 2 Modell für ein ausdampfendes Brennelement 19 2.1 Vorbetrachtungen 19 2.1.1 Strömungsform 19 2.1.2 Form des Wärmeübergangs 22 2.2 Physikalische Modellierung 23 2.2.1 Simulationsstrategie 23 2.2.2 Physikalische Modellgleichungen 24 2.2.3 Rechengebiet und Randbedingungen 27 2.3 Numerische Modellierung 32 2.3.1 Örtliche Diskretisierung 32 2.3.2 Zeitliche Diskretisierung 34 3 Sensitivitätsanalyse für ein ausdampfendes Brennelement 37 3.1 Vorgehensweise 37 3.2 Einfluss der Strahlungsmodellierung 39 3.2.1 Motivation 39 3.2.2 Bestimmung des Absorptionskoeffzienten 40 3.2.3 Einfluss der Gasstrahlung 41 3.2.4 Einfluss der numerischen Parameter 44 3.3 Einfluss unsicherer Randbedingungen 46 3.3.1 Wärmeverlust über die Isolierschicht 46 3.3.2 Verteilung des Dampfmassenstroms an der Wasseroberfläche 51 3.4 Einfluss der effektiv freigelegten Länge der Heizstäbe 56 3.5 Einfluss der Stableistung 58 4 Wechselwirkung zwischen Querüberströmung und Wärmetransport im Brennelement 63 4.1 Rechengebiet und Randbedingungen 63 4.2 Physikalische und numerische Modellierung 65 4.2.1 Physikalische Modellierung 65 4.2.2 Numerische Einstellungen 67 4.3 Ergebnisse und Diskussion 67 4.3.1 Generelles Vorgehen 67 4.3.2 Temperaturentwicklung und Strömung im Stabbereich 69 4.3.3 Temperatur und Strömung im Überströmkanal 75 5 Ansätze zur skalenübergreifenden Modellierung eines Lagerbeckens 81 5.1 Einordnung 81 5.2 Co-Simulation des Wärme- und Stoffaustauschs zwischen Einzelbrennelement und Lagerbeckenatmosphäre 81 5.2.1 Konfiguration 81 5.2.2 Einfluss der Konvektionsströmung oberhalb der Brennelemente 86 5.3 Gekoppelte Simulation eines Lagerbeckenbereichs 92 5.3.1 Motivation 92 5.3.2 Parametrierung des porösen Körpers 92 5.3.3 Vergleich der Simulationsansätze 94 5.3.4 Simulation der Brennelement-Gruppe 96 6 Zusammenfassung und Ausblick 101 Literaturverzeichnis 115 Symbol- und Abkürzungsverzeichnis 119 / After their operation, spent nuclear fuel assemblies are stored for several years in wet storage pools, where their decay heat is removed by electrically operated cooling systems. If the power supply fails, this poses the risk of overheating of the fuel assemblies and, in the worst case, damage to the fuel rod cladding and the release of radioactive material into the environment. This dissertation focuses on the investigation of the complex interaction of flow and heat transport in such an assumed accident, which leads to partially uncovered fuel assemblies. A review of the current state of research illustrates that although the underlying physical processes are theoretically understood, no specific simulation programmes exist to date to accurately predict the temperature distribution for possible accident scenarios. For the detailed analysis of the processes, numerical flow simulations taking into account the exact geometry and all relevant heat transport mechanisms are therefore carried out for a partially uncovered fuel assembly for the first time. To ensure a manageable computational effort, the transient evaporation process is subdivided into several, independent simulations with steady boundary conditions and a constant water level in each case. The validation with experimental data shows that this approach is suitable for predicting the rod temperatures with sufficient accuracy for low decay heat. A comprehensive sensitivity analysis also identifies the influence of numerous uncertain factors on the temperature distribution and composition in the fuel assembly, which cannot be assessed purely on the basis of the experiment. The simulation results show that the maximum rod temperature depends mainly on the water level and the power of the fuel rods. A horizontally directed air flow above the fuel assembly leads to an overall temperature gradient in the flow direction within the fuel assembly. This is caused by a characteristic flow field in which cold gas is transported down the downstream wall of the fuel assembly and hot gas is transported up the upstream wall. However, varying the velocity of the airflow alone does not cause a significant change in the maximum rod temperature. The influence of the crossflow on the temperature distribution in the fuel assembly only becomes clear by using realistic boundary conditions for velocity, temperature and composition, obtained from large-scale simulations of the entire storage pool. Determined by the ratio of buoyant to inertial forces, the temperature in the fuel assembly increases significantly with a combination of low temperature, low steam mass fraction and high velocity of the crossflow. These results provide information on how to best arrange fuel assemblies in spent fuel pools, provided that the boundary conditions above the fuel assemblies are known or can be predicted with sufficient accuracy. Finally, a method for modelling a larger part of the spent fuel pool is presented. The combination of two modelling approaches enables a partially geometry-resolving simulation in which the central fuel assembly is geometrically resolved and the neighbouring fuel assemblies are modelled as porous bodies. This approach improves the transferability of the results to an entire spent fuel pool, because the evaluation in the geometrically resolved fuel assembly becomes more independent from the uncertain boundary conditions.:1 Einleitung 1 1.1 Chancen und Risiken der Kernenergienutzung 1 1.2 Randbedingungen für den Wärme- und Stofftransport im Lagerbecken 3 1.2.1 Zerfallsleistung 3 1.2.2 Brennelement-Typ und Aufbau 4 1.2.3 Wärmetransportmechanismen 6 1.2.4 Verdampfungsrate 8 1.2.5 Grenztemperaturen 9 1.3 Simulation des Wärme- und Stofftransports im Lagerbecken 10 1.3.1 Das Lagerbecken als Multiskalenproblem 10 1.3.2 Systemcodes und Codes für schwere Störfälle 12 1.3.3 CFD-Simulation mit Brennelementen als poröse Körper 13 1.3.4 Geometrieauflösende CFD-Simulation 15 1.4 Zielstellung und Aufbau der Arbeit 16 2 Modell für ein ausdampfendes Brennelement 19 2.1 Vorbetrachtungen 19 2.1.1 Strömungsform 19 2.1.2 Form des Wärmeübergangs 22 2.2 Physikalische Modellierung 23 2.2.1 Simulationsstrategie 23 2.2.2 Physikalische Modellgleichungen 24 2.2.3 Rechengebiet und Randbedingungen 27 2.3 Numerische Modellierung 32 2.3.1 Örtliche Diskretisierung 32 2.3.2 Zeitliche Diskretisierung 34 3 Sensitivitätsanalyse für ein ausdampfendes Brennelement 37 3.1 Vorgehensweise 37 3.2 Einfluss der Strahlungsmodellierung 39 3.2.1 Motivation 39 3.2.2 Bestimmung des Absorptionskoeffzienten 40 3.2.3 Einfluss der Gasstrahlung 41 3.2.4 Einfluss der numerischen Parameter 44 3.3 Einfluss unsicherer Randbedingungen 46 3.3.1 Wärmeverlust über die Isolierschicht 46 3.3.2 Verteilung des Dampfmassenstroms an der Wasseroberfläche 51 3.4 Einfluss der effektiv freigelegten Länge der Heizstäbe 56 3.5 Einfluss der Stableistung 58 4 Wechselwirkung zwischen Querüberströmung und Wärmetransport im Brennelement 63 4.1 Rechengebiet und Randbedingungen 63 4.2 Physikalische und numerische Modellierung 65 4.2.1 Physikalische Modellierung 65 4.2.2 Numerische Einstellungen 67 4.3 Ergebnisse und Diskussion 67 4.3.1 Generelles Vorgehen 67 4.3.2 Temperaturentwicklung und Strömung im Stabbereich 69 4.3.3 Temperatur und Strömung im Überströmkanal 75 5 Ansätze zur skalenübergreifenden Modellierung eines Lagerbeckens 81 5.1 Einordnung 81 5.2 Co-Simulation des Wärme- und Stoffaustauschs zwischen Einzelbrennelement und Lagerbeckenatmosphäre 81 5.2.1 Konfiguration 81 5.2.2 Einfluss der Konvektionsströmung oberhalb der Brennelemente 86 5.3 Gekoppelte Simulation eines Lagerbeckenbereichs 92 5.3.1 Motivation 92 5.3.2 Parametrierung des porösen Körpers 92 5.3.3 Vergleich der Simulationsansätze 94 5.3.4 Simulation der Brennelement-Gruppe 96 6 Zusammenfassung und Ausblick 101 Literaturverzeichnis 115 Symbol- und Abkürzungsverzeichnis 119

Biofuel production via fermentation is produced primarily by fermentation of simple sugars. Besides the sugar fermentation route, there exists a promising alternative process that uses syngas (CO, H2, CO2) produced from biomass as building blocks for biofuels. Although syngas fermentation has many benefits, there are several challenges that still need to be addressed in order for syngas fermentation to become a viable process for producing biofuels on a large scale. One challenge is mass transfer limitations due to low solubilities of syngas species. The hollow fiber reactor (HFR) is one type of reactor that has the potential for achieving high mass transfer rates for biofuels production. However, a better understanding of mass transfer limitations in HFRs is still needed. In addition there have been relatively few studies performing actual fermentations in an HFR to assess whether high mass transfer rates equate to better fermentation results. Besides mass transfer, one other difficulty with syngas fermentation is understanding the role that CO and H2 play as electron donors and how different CO and H2 ratios effect syngas fermentation. In addition to electrons from CO and H2, electrodes can also be used to augment the supply of electrons or provide the only source of electrons for syngas fermentation. This work performed an in depth reactor comparison that compared mass transfer rates and fermentation abilities. The HFR achieved the highest oxygen mass transfer coefficient (1062 h-1) compared to other reactors. In fermentations, the HFR showed very high production rates (5.3 mMc/hr) and ethanol to acetic acid ratios (13) compared to other common reactors. This work also analyzed the use of electrons from H2 and CO by C. ragsdalei and to study the effects of these two different electron sources on product formation and cell growth. This study showed that cell growth is not largely effected by CO composition although there must be at least some minimum amount of CO present (between 5-20%). Interestingly, H2 composition has no effect on cell growth. Also, more electron equivalents will lead to higher product formation rates. Following Acetyl-CoA formation, H2 is only used for product formation but not cell growth. In addition to these studies on electrons from H2 and CO, this work also assessed the redox states of methyl viologen (MV) for use as an artificial electron carrier in applications such as syngas fermentation. A validated thermodynamic model was presented in order to illustrate the most likely redox state of MV depending on the system setup. Variable MV extinction coefficients and standard redox potentials reported in literature were assessed to provide recommended values for modeling and analysis. Model results showed that there are narrow potential ranges in which MV can change from one redox state to another, thus affecting the potential use as an artificial electron carrier.

<p>  </p> <p>Acoustic streaming is a phenomenon which occurs when acoustic excitations interact with a fluid (stationary or non-stationary). Exploitation of this phenomenon has the potential to open doors to new methods of flow control through the enhancement or diminishment of the present flow instabilities. A particular use of acoustic streaming shown by previous numerical studies is the enhancement of heat transfer in violation of the Reynold’s Analogy within a small range of Mach numbers and frequencies of periodic excitation. The focus of this thesis is to experimentally assess the usage of a Rossiter cavity in generating periodic acoustic excitations and its effects on the shear stress and heat transfer. </p> <p>In the present research, two large models are tested using a blow-down facility. The models are made of aluminum and Teflon and were developed to ensure optical access for infrared thermography. The geometries are tested at Mach number ranging from 0.373 to 0. 866. The target Mach number-frequency pair where significant heat transfer enhancement is a free stream Mach number at the cavity, Mc, of 0.75 and the frequency, fc, of 7.5 kHz. The cavity is tuned using the Rossiter equation with Rossiter constants k = 0.66 and y = 0.25. The heat transfer and skin friction enhancement are measured immediately upstream and downstream of the cavity and compared to the previous numerical studies.</p> <p>When testing the Teflon model with an ambient back pressure and 11 lb/s mass flow, a frequency of 7.8 kHz was generated by the cavity. For the aluminum model tested at a high vacuum and 3 lb/s mass flow, frequencies near 7, 10, and 20 kHz were generated by the cavity with 10 and 20 kHz appearing most often. High speed schlieren imaging was used to confirm the flow structures being generated in the flow. There was good agreement with the Rossiter modes at lower Mach numbers and moderate agreement at transonic Mach numbers. A correlation is presented which defines a band of Mach number-Reynolds number pairs which present with a discontinuous frequency behavior during operation of the wind tunnel. Measurable effects on both skin friction and heat transfer between tests with comparable operating conditions to a reference were observed and are presented.</p>

<p>Droplet dispersions with adsorbed exotic surface active species (proteins, fatty alcohol, fatty acids, solid particulates, lipids, or polymers) find an immense number of applications in the field of engineering and bioscience. Interfacial rheology plays an essential role in the dynamics of many of these systems, yet little is understood about how these effects alter droplet dynamics. Most surfactants studied historically have been simple enough that the droplet dynamics can be described by Marangoni effects (surfactant concentration gradients), surface dilution, and adsorption/desorption kinetics without including the intrinsic surface rheology. One of the challenges in examining droplet systems with complex interfaces is that the intrinsic rheological effects are strongly coupled with surfactant transport effects (surface convection, diffusion, dilution and adsorption/desorption). The surface rheology can impact the ability of surfactant to transport along the surface, while surfactant transport can alter the surface rheology by changing the surface concentration. In this work, we develop axisymmetric boundary-integral simulations that allow us to quantitatively explore the combined effect of intrinsic surface rheology and surfactant transport on droplet dynamics in the Stokes flow limit. We assume that the droplet interface is predominantly viscous and that the Boussinesq Scriven constitutive relationship describes the properties of the viscous membrane. The key questions that we address in this work are:</p> <p><br></p> <ul> <li>How do viscous membranes impact droplet deformation, breakup and relaxation?      </li> </ul> <p>     When a droplet is placed under external flow, it can either attain a stable shape under flow or stretch indefinitely above a critical flow rate and break apart. In this topic, we first discuss the breakup conditions for a droplet suspended in an unbounded immiscible fluid under a general linear flow field using perturbation theories for surface viscosity in the limit of small droplet deformation. We neglect the inhomogeneity in surfactant concentration and surface tension for this part. We find that the surface shear/dilational viscosity increases/decreases the critical capillary number for droplet breakup compared to a clean droplet at the same capillary number and droplet viscosity ratio value. In the second part of this topic, we solve the problem using boundary integral simulations for the case of axisymmetric extensional flow. Numerically solving this problem allows us to examine the effect of Marangoni stresses, pressure thickening/thinning surface viscosities, and stronger flows. We compare the droplet breakup results from our simulations to results from second-order perturbation theories. We present the physical mechanism behind our observations using traction arguments from interfacial viscosities. We conclude this topic by examining the combined role of surface viscosity and surfactant transport on the relaxation of an initially extended droplet in a quiescent external fluid.</p> <p><br></p> <ul> <li>How do viscous membranes alter droplet sedimentation?</li> </ul> <p>      When an initially deformed droplet sediment under gravity, it can either revert to a spherical shape or undergo instability where the droplet develops a long tail or cavity at its rear end. Here, we use numerical simulations to discuss how interfacial viscosity alters the breakup criterion and the formation of threads/cavities under gravity. We examine the combined influence of intrinsic surface viscosity and surfactant transport on droplet stability by assuming a linear dependence of surface tension on surfactant concentration and an exponential dependence of interfacial viscosities on surface pressure. We find that surface shear viscosity inhibits the tail/cavity growth at the droplet’s rear end and increases the critical capillary number compared to a clean droplet. In contrast, surface dilational viscosity promotes tail/cavity growth and lowers the critical capillary number compared to a clean droplet.</p> <p><br></p> <ul> <li>How do viscous membranes affect droplet coalescence?</li> </ul> <p>      When two droplets approach under external flow, a thin film is formed between the two droplets. Here, we develop numerical simulations to model the full coalescence process from the collision of two droplets under uniaxial compressional flow to the point where the film approaches rupture. We investigate the role of interfacial viscosity on the film profiles and drainage time. We observe that both surface shear and dilational viscosity significantly delay the film drainage time relative to a clean droplet. Interestingly, we find that the film drainage behaviour of a droplet with surface viscosity is not altered by the relative ratio of shear to dilational viscosity but rather depends on the sum of shear and dilational Boussinesq numbers. This is in contrast to the effect of surface viscosity observed in the previous processes (droplet breakup and sedimentation), where surface shear viscosity increases the critical capillary number compared to a clean droplet, while surface dilatational viscosity has the opposite effect.</p>

A mathematical model for the drying of particulate solids in a continuous vibrated fluidised bed dryer was developed and applied to the drying of grain wetted with a single liquid and porous particles containing multicomponent liquid mixtures. Simple equipment and material models were applied to describe the process. In the plug-flow equipment model, a thin layer of particles moving forward and well mixed in the direction of the gas flow was regarded; thus, only the longitudinal changes of particle moisture content and composition as well as temperature along the dryer were considered. Concerning the material model, mass and heat transfer in a single isolated particle was studied. For grain wetted with a single liquid, mass and heat transfer within the particles was described by effective transfer coefficients. Assuming a constant effective mass transport coefficient and effective thermal conductivity of the wet particles, analytical solutions of the mass and energy balances were obtained. The variation of both transport coefficients along the dryer was taken into account by a stepwise application of the analytical solution in space intervals with non-uniform inlet conditions and averaged coefficients from previous locations in the dryer. Calculation results were verified by comparison with experimental data from the literature. There was fairly good agreement between experimental data and simulation but the results depend strongly on the correlation used to calculate heat and mass transfer coefficients.   For the case of particles containing a multicomponent liquid mixture dried in the vibrated fluidised bed dryer, interactive diffusion and heat conduction were considered the main mechanisms for mass and heat transfer within the particles. Assuming a constant matrix of effective multicomponent diffusion coefficients and thermal conductivity of the wet particles, analytical solutions of the diffusion and conduction equations were obtained. The equations for mass transfer were decoupled by a similarity transformation and solved simultaneously with conduction equation by the variable separation method. Simulations gave a good insight into the selectivity of the drying process and can be used to find conditions to improve aroma retention during drying.   Also, analytical solutions of the diffusion and conduction equations applied to liquid-side-controlled convective drying of a multicomponent liquid film were developed. Assuming constant physical properties of the liquid, the equations describing interactive mass transfer are decoupled by a similarity transformation and solved simultaneously with conduction equation and the ordinary differential equation that describes the changes in the liquid film thickness. Variations of physical properties along the process trajectory were taken into account as in the previous cases. Simulation results were compared with experimental data from the literature and a fairly good agreement was obtained. Simulations performed with ternary liquid mixtures containing only volatile components and ternary mixtures containing components of negligible volatility showed that it is difficult to obtain an evaporation process that is completely controlled by the liquid-side mass transfer. This occurs irrespective of the initial drying conditions.   Despite simplifications, the analytical solution of the material model gives a good insight into the selectivity of the drying process and is computationally fast. The solution can be a useful tool for process exploration and optimisation. It can also be used to accelerate convergence and reduce tedious and time-consuming calculations when more rigorous models are solved numerically. / QC 20110614

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Due to the ever increasing concerns about pollutants and contaminants found in water, new treatment technologies have been developed. Ozonation is one of such technologies. It has been widely applied in the treatment of pollutants in water and wastewater treatment processes. Ozone has many applications such as oxidation of organic components, mineral matter, inactivation of viruses, cysts, bacteria, removal of trace pollutants like pesticides and solvents, and removal of tastes and odours. Ozone is the strongest conventional oxidant that can result in complete mineralisation of the organic pollutants to carbon dioxide and water. Because ozone is unstable, it is generally produced onsite in gas mixtures and is immediately introduced to water using gas/liquid type reactors (e.g. bubble columns). The ozone reactions are hence of the type gas liquid reactions, which are complex to model since they involve both chemical reactions, which occur in the liquid phase, and mass transfer from the gas to the liquid phase. This study focuses on two aspects: mass transfer and chemical reactions in multicomponent systems. The mass transfer parameters were determined by experiments under different conditions and the chemical reactions were studied using single component and multicomponent systems. Two models obtained from the literature were adapted to the systems used in this study. Mass transfer parameters in the semi-batch reactor were determined using oxygen and ozone at different flow rates in the presence and absence of t-butanol. t-Butanol is used as a radical scavenger in ozonation studies and it has been found to affect the gas¿liquid mass transfer rates. An experimental study was carried out to investigate the effects of t-butanol concentrations on the physical properties of aqueous solutions, including surface tension and viscosity. It was found that t-butanol reduced both properties by 4% for surface tension and by a surprising 30% for viscosity. These reductions in the solution physical properties were correlated to enhancement in the mass transfer coefficient, kL. The mass transfer coefficient increased by about 60% for oxygen and by almost 50% for ozone. The hydrodynamic behaviour of the system used in this work was characterised by a homogeneous bubbling regime. It was also found that the gas holdup was significantly enhanced by the addition of t-butanol. Moreover, the addition of t-butanol was found to significantly reduce the size of gas bubbles, leading to enhancement in the volumetric mass transfer coefficient, kLa. The multicomponent ozonation was studied with two systems, slow reactions when alcohols were used and fast reactions when endocrine disrupting compounds were used. ii These experiments were simulated by mathematical models. The alcohols were selected depending on their volatilization at different initial concentrations and different gas flow rates. The degradation of n-propanol as a single compound was studied at the lowest flow rate of 200 mL/min. It was found that the degradation of n-propanol reached almost 60% within 4 hours. The degradation of the mixture was enhanced with an increase in the number of components in the mixture. It was found that the degradation of the mixture as three compounds reached almost 80% within four hours while the mixture as two compounds reached almost 70%. The effect of pH was studied and it was found that an increase in pH showed slight increase in the reaction. Fast reactions were also investigated by reacting endocrine disrupting chemicals with ozone. The ozone reactions with the endocrine disrupters were studied at different gas flow rates, initial concentrations, ozone concentrations and pH. The degradation of 17¿-estradiol (E2) as a single compound was the fastest, reaching about 90% removal in almost 5 minutes. However estrone (E1) degradation was the lowest reaching about 70% removal at the same time. The degradation of mixtures of the endocrine disruptors was found to proceed to lower percentages than individual components under the same conditions. During the multicomponent ozonation of the endocrine disruptors, it was found that 17¿-estradiol (E2) converted to estrone (E1) at the beginning of the reaction. A MATLAB code was developed to predict the ozone water reactions for single component and multicomponent systems. Two models were used to simulate the experimental results for single component and multicomponent systems. In the case of single component system, good simulation of both reactions (slow and fast) by model 1 was obtained. However, model 2 gave good agreement with experimental results only in the case of fast reactions. In addition, model 1 was applied for multicomponent reactions (both cases of slow and fast reaction). In the multicomponent reactions by model 1, good agreement with the experimental results was also obtained for both cases of slow and fast reactions. / Ministry of Higher Education in Libya and the Libyan Cultural Centre and Educational Bureau in London.