Beim Stoffübergang einer grenzflächenaktiven Substanz in einem flüssigen Zweiphasensystem kann solutale Marangoni-Instabilität einsetzen. Die weitere nichtlineare Entwicklung der Marangoni-Instabilität geht mit einer enormen Vielfalt von Strömungsmustern einher. In der Literatur wird dieser Aspekt häufig unter dem unscharfen Ausdruck „Grenzflächenturbulenz“ zusammengefasst. Diese Arbeit stellt heraus, dass drei grundlegende Strukturformen existieren: Rollzellen, Relaxationsoszillationen und Relaxationsoszillationswellen. Ein großer Teil der Komplexität der Strömungsmuster ist dadurch begründet, dass die Grundstrukturen unterschiedliche Hierarchieebenen aufweisen. Es werden die zugrunde liegenden Bedingungen für das Auftreten der jeweiligen Strukturtypen, ihre transiente Natur und die Bildung der hierarchischen Strömungsmuster untersucht. Des Weiteren betrachtet diese Arbeit die Wechselwirkungen mit Dichteeffekten, die sowohl die Charakteristik der Strukturen als auch ihre zeitliche Entwicklung beeinflussen.

Es werden die Gleichgewichtskonstanten der Zinkextraktion ermittelt. Das Wilson-Modell und das erweiterte Debye-Hückel-Gesetz werden zur Beschreibung der Aktivitäten verwendet. Die tensiometrischen Untersuchungen erfolgen am hängenden Tropfen. Die Modellauswahl zur Beschreibung der Gleichgewichtsgrenzflächenspannung erfolgt im submizellaren Konzentrationsbereich. Die pseudo-nichtionische Modellierung auf der Basis der Langmuir-Isothermen der Mehrkomponentenadsorption bei Verwendung der Stern-Isothermen für die Gegenionenanreicherung liefert die beste Datenanpassung. Durch ein einfaches Modell zur Mizell- und Aggregatbildung gelingt die Modellerweiterung. Die gemessenen dynamischen Grenzflächenspannungskurven werden sorptionskinetisch und durch diffusive Approximationen angepasst. Es zeigt sich, dass der Stofftransport diffusionsdirigiert ist. Am oszillierenden Tropfen werden die Ergebnisse bestätigt. Für das Kationenaustauscheranion wird die Gültigkeit des Maxwell-Modells zur Beschreibung der Grenzflächendilatationsrheologie nachgewiesen.

<p>With the extreme increases in power density of electronic devices, the contact thermal resistance imposed at interfaces between mating solids becomes a major challenge in thermal management. This contact thermal resistance is mainly caused by micro-scale surface asperities (roughness) and wavy profile of surface (nonflatness) which severely reduce the contact area available for heat conduction. High contact pressures (1~100 MPa) can be used to deform the surface asperities to increase contact area. Besides, a variety of conventional thermal interface materials (TIM), such as greases and pastes, are used to improve the contact thermal conductance by filling the remaining air gaps. However, there are still some applications where such TIMs are disallowed for reworkability concerns. For example, heat must be transferred across dry interfaces to a heat sink in pluggable opto-electronic transceivers which needs to repeatedly slide into / out of contact with the heat sink. Dry contact and low contact pressures are required for this sliding application.</p> <p>This dissertation presents a metallized micro-spring array as a surface coating to enhance dry contact thermal conductance under ultra-low interfacial contact pressure. The shape of the micro-springs is designed to be mechanically compliant to achieve conformal contact between nonflat surfaces. The polymer scaffolds of the micro-structured TIMs are fabricated by using a custom projection micro-stereolithography (μSL) system. By applying the projection scheme, this method is more cost-effective and high-throughput than other 3D micro-fabrication methods using a scanning scheme. The thermal conductance of polymer micro-springs is further enhanced by metallization using plating and surface polishing on their top surfaces. The measured mechanical compliance of TIMs indicates that they can deform ~10s μm under ~10s kPa contact pressures over their footprint area, which is large enough to accommodate most of surface nonflatness of electronic packages. The measured thermal resistances of the TIM at different fabrication stages confirms the enhanced thermal conductance by applying metallization and surface polishing. Thermal resistances of the TIMs are compared to direct metal-to-metal contact thermal resistance for flat and nonflat mating surfaces, which confirms that the TIM outperforms direct contact. A thin layer of soft polymer is coated on the top surfaces of the TIMs to accommodate surface roughness that has a smaller spatial period than the micro-springs. For rough surfaces, the polymer-coated TIM has reduced thermal resistance which is comparable to a benchmark case where the top surfaces of the TIM are glued to the mating surface. A polymer base is designed under the micro-spring array which can provide the advantages for handling as a standalone material or integration convenience, at the toll of an increased insertion resistance. Through-holes are designed in the base layer and coated with thermally conductive metal after metallization to enhance thermal conductance of the base layer; a thin layer of epoxy is applied between the base layer and the working surface to reduce contact thermal resistance exposed on the base layer. Cycling tests are conducted on the TIMs; the results show good early-stage reliability of the TIM under normal pressure, sliding contact, and temperature cycles. The TIM is thermally demonstrated on a pluggable application, namely, a CFP4 module, which shows enhanced thermal conductance by applying the TIM. </p> To further enhance the potential mechanical compliance of microstructured surfaces, a stable double curved beam structure with near-zero stiffness composed of intrinsic negative and positive stiffness elastic elements is designed and fabricated by introducing residual stresses. Stiffness measurements shows that the positive-stiffness single curved beam, which is the same as the top beam in the double curved beam, is stiffer than the double curved beam, which confirms the negative stiffness of the bottom beam in the double curved beam. Layered near zero-stiffness materials made of these structures are built to demonstrate the scalability of the zero-stiffness zone.

Several types of reactors are encountered in industry where reagents in a gas and a liquid phase need to be catalysed by a solid catalyst. Common reactors that are used to this end, are trickle-bed reactors, where gas and liquid flow cocurrently down a packed bed of catalyst. Apart from the catalytic process itself, several mass transfer steps can influence the rate and/or selectivity of a solid catalysed gas-liquid reaction. In trickle-bed reactors, flow morphology can have a major effect on these mass transfer steps. This study investigates the interaction between liquid flow morphology and mass transfer in trickle-bed reactors from three different angles. The primary focus is on liquid-solid mass transfer and internal diffusion as affected by the contacting between the liquid and the catalyst. First, the contacting between the liquid and the solid in trickleflow, or wetting efficiency, is characterised using colorimetry. Though this investigation is limited to the flow of nitrogen and water over a packed bed at ambient conditions, it provides useful information regarding liquid flow multiplicity behaviour and its influence on the distribution of fractional wetting on a particle scale. The colorimetric study also provides descriptions of the geometry of the liquid-solid contacting on partially wetted particles. These are used in a second investigation, for the numerical simulation of reaction and diffusion in partially wetted catalysts. This second investigation uses numerical simulations to evaluate and develop simple theoretical descriptions of liquid-solid contacting effects on catalyst particle efficiency. Special attention is given to the case where external and intraparticle mass transfer rates of both a volatile and non-volatile reagent affect the overall rate of reaction. Also, since these are not often considered in theoretical studies, some suggestions are made for the evaluation of the particle efficiency of eggshell catalyst. Finally, liquid-solid contacting is investigated in a high-pressure pilot reactor. Wetting efficiency is measured with a useful technique that does not rely on descriptions of particle kinetics or liquid-solid mass transfer rates. Liquid-solid mass transfer coefficients are also measured and results agree well with the colorimetric investigation, suggesting the existence of different types of flow within in the hydrodynamic multiplicity envelope of trickle-flow. Since it consists of different investigations of liquid-solid contacting from different angles, the study highlights several aspects of liquid-solid contacting and how it can be expected to influence trickle-bed reactor performance. / Thesis (PhD)--University of Pretoria, 2009. / Chemical Engineering / unrestricted

This research work is focused on a conceptual combination of membrane-based energy recovery ventilator (ERV) and phase change material (PCM) to provide energy savings in building heating, ventilation & air-conditioning (HVAC) systems. An ERV can recover thermal energy and moisture between the outside fresh air (OFA) entering into the building and the exhaust air (EA) leaving from the building thus reducing the energy consumption of the HVAC system for cooling and heating the spaces inside the building. The membranes were stacked parallel to each other forming adjacent channels in a counter-flow arrangement for OFA and EA streams. Heat and moisture is diffused through the membrane core. Flat-plate encapsulated PCM is arranged in OFA duct upstream/downstream of the ERV thereby allowing for further reduction in temperature by virtue of free cooling. Paraffin-based PCMs with a melting point of 24°C and 31°C is used in two different configurations where the PCM is added either before or after the ERV. Computational fluid dynamics (CFD), and heat and mass transfer modeling is employed using COMSOL Multiphysics v5.3 to perform the heat and mass transfer analysis for the membrane-based ERV and flat-plate PCMs. An 8-story office building was considered to perform building energy simulation using eQUEST v3.65 from Department of Energy (DOE). Based on the heat and mass transfer analysis, it is found that the sensible effectiveness (heat recovery) stood in the range of 65%-97% while the latent effectiveness (moisture recovery) stood at 55%-80%. Also, the highest annual energy savings achieved were 72,700 kWh in electricity consumption and 358.45 MBtu in gas consumption.

<div>Thermal management in next generation power electronic devices, radar applications and semiconductor packaging architectures is becoming increasingly challenging due to the need to reject localized high heat fluxes as well as large total powers. Air cooling has been considered as a simple and reliable method for thermal management compared to architectures that incorporate liquid cooling. However, air-cooled heat sinks typically require effective heat spreading to provide the requisite level of area enhancement to dissipate high heat fluxes. Compared to solid metallic heat spreaders, advanced heat sinks that incorporate two-phase heat transfer devices such as vapor chambers can significantly enhance the power dissipation capabilities in such configurations. Vapor chambers are devices that utilize evaporation/boiling processes within a sealed cavity to achieve efficient heat spreading. In high-heat-flux applications, boiling can occur within the internal wick structure of the vapor chamber at the location of the heat input (i.e., the evaporator). The maximum dryout heat flux and thermal resistance of the device is dictated by the resulting two-phase flow and heat transfer in the porous evaporator due to boiling. While various works in the literature have introduced new evaporator wick designs to improve the dryout heat flux during boiling, the enhancement is limited to small, millimeter scale hotspots or at a very high thermal resistance. In additixon, the effective design of such evaporator systems requires mechanistic models that can accurately predict the dryout limit and thermal performance. </div><div> This thesis first explores the usage of a novel ‘two-layer’ evaporator wick for passive high heat flux dissipation over large heater areas at a low thermal resistance. Moreover, a new mechanistic (first principles based) model framework is introduced for dryout limit and thermal performance prediction during boiling in capillary fed evaporators, by considering the resulting simultaneous flow of two phases (liquid and vapor) within the microscale porous media.</div><div> The novel two-layer wick concept uses a thick ‘cap’ layer of porous material to feed liquid to a thin ‘base’ layer through an array of vertical liquid-feeding ‘posts’. Vapor ‘vents’ in the cap layer allow for vapor formed during the boiling process (which is constrained to the base layer) to escape out of the wick. This two-layer structure decouples the functions of liquid resupply and capillary-fed boiling heat transfer, making the design realize high heat flux dissipation greater than 500 W/cm2 over large heat input areas of ~1 cm2. A reduced-order model is first developed to demonstrate the performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The fabrication of the proposed two-layer wick is then presented. The thermal performance of the fabricated wicks is characterized using a boiling test facility that utilizes high speed visualization to identify the characteristic regimes of boiling operation in the wicks. The performance is also benchmarked to conventional single-layer wicks. </div><div> It is observed that single-layer wicks exhibit an unfavorable boiling regime where the center of the heater area dries out locally, leading to a high value of thermal resistance. The two-layer wicks avoid local dryout due to the distributed feeding provided by the posts and enhance the dryout heat flux significantly compared to single-layer wicks. A two-layer design that consists of a 10 × 10 array of liquid feeding posts provided a 400% improvement in the dryout heat flux. Following a parametric analysis of the effect of particle size, two-layer wicks composed of 180 – 212 µm particles and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ~0.052 K/W. The effect of vapor venting and liquid-feeding areas is also experimentally studied. By understanding these effects, a parametrically optimized design is fabricated and shown to demonstrate an extremely high dryout limit of 512 W/cm2. We identify that the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than was previously possible in the literature.</div><div> The results from the characterization of two-layer wicks revealed that the overall performance of the design was limited by the boiling process in the thin base wick layer. A fundamental model-based understanding of the resulting two-phase flow and heat transfer process in such thin capillary-fed porous media was still lacking. This lack of a mechanistic model precluded the accurate prediction of dryout heat flux and thermal performance of the two-layer wick. Moreover, such an understanding is needed for the optimal design of advanced hybrid evaporator wicks that leverage capillary-fed boiling. Despite the existence of various experimental works, there are currently no mechanistic approaches that model this behavior. To fill this unmet need, this thesis presents a new semi-empirical model for prediction of dryout and thermal resistance of capillary-fed evaporator systems. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses. This physics–based modeling approach is then implemented into a vapor chamber model to predict the thermal performance limits of air-cooled heat sinks with embedded vapor chambers. The governing energy and momentum equations of a low-cost analytical vapor chamber modeling approach is coupled with the evaporator model to capture the effect of boiling in the evaporator wick. An example case study illustrating the usage of the model is demonstrated and compared to a purely evaporation-based modeling approach, for quantifying the differences in dryout limit prediction, signifying the need to account for boiling in the evaporator wick. </div><div> The understanding gained from this thesis can be utilized for the prediction of dryout and thermal performance during boiling in capillary limited evaporator systems. The work also suggests the usage of a universal relative permeability correlation for the two-phase flow configuration studied herein for capillary-fed boiling, based on a wide calibration to experimental data. The modeling framework can also be readily leveraged to find novel and unexplored designs of advanced evaporator wicks. From an application standpoint, the new vapor chamber model developed here can be used for the improved estimation of performance limits specifically when high heat fluxes are encountered by the device. This will enable better and informed design of air-cooled heat sink architectures with embedded vapor chambers for high performance applications. </div><div><br></div>

Energy recovery ventilation (ERV) systems are designed to decrease the energy consumed by building HVAC systems. ERV’s scavenge sensible and latent energy from the exhaust air leaving a building or space and recycle this energy content to pre-condition the entering outdoor air. A few studies found in the open literature are dedicated to developing detailed numerical models to predict or simulate the performance of energy recovery wheels and desiccant wheels. However, the models are often computationally intensive, requiring a lot of time to perform parametric studies. For example, if the physical characteristics of a study target change (e.g., wheel diameter or depth) or if the system runs at different operating conditions (e.g., wheel rotation speed or airflow rate), the model parameters need to be recalculated. Hence, developing a mapping method with better computational efficiency, which will enable the opportunity to conduct extensive parametric or optimal design studies for different wheels is the goal of this research. In this work, finite difference method (FDM) numerical models of energy recovery wheels and desiccant wheels are established and validated with laboratory test results. The FDM models are then used to provide data for the development of performance mapping methods for an energy wheel or a desiccant wheel. After validating these new mapping approaches, they are employed using independent data sets from different laboratories and other sources available in the literature to identify their universality. One significant characteristic of the proposed mapping methods that makes the contribution unique is that once the models are trained, they can be used to predict performance for other wheels with different physical geometries or different operating conditions if the desiccant material is identical. The methods provide a computationally efficient performance prediction tool; therefore, they are ideal to integrate with transient building energy simulation software to conduct performance evaluations or optimizations of energy recovery/ desiccant wheel integrated HVAC systems.

Bench and pilot scale determinations of the volumetric oxygen transfer coefficient, K_La, were performed on an improved A²/O biological nutrient removal (BNR) pilot plant. Effluent from a full scale primary clarifier, used as pilot plant influent, was found to have an alpha (ratio of process to clean water K_La) of 0.71 as determined in a 21 liter bench scale reactor and an alpha of 0.332 as determined in a 0.45 m³ aeration basin of the 2.4 m³ pilot plant. Alpha of a 1:1 mixture of primary clarifier effluent with pilot plant return activated sludge was determined to be 0.94 at bench scale and 0.71 at pilot scale. An assay of alphas through the initial non aerated treatment zones of the pilot plant using the bench scale reactor indicated that alphas peaked in the effluent of the first anaerobic zone (alpha equal to 1.01) and were lower in the second anaerobic zone and first anoxic zone. An assay of alphas in the three pilot plant series sideline aeration basins indicated that alpha was maximum in the first aeration basin (alpha equal to 0.905) and were lower in the second and third aeration basins (0.716 and 0.661 respectively). A consistent increase in average surface tension was noted from the first to second to third aeration basins, however the differences were not statistically significant. A comparison of pilot plant alphas determined in the first aeration basin following anaerobic nominal hydraulic retention times of 0.0, 0.21, 0.43, and 0.64 hours yielded alpha values of 0.71, 0.94, 0.64, and 0.74 respectively. Like the assay using the bench scale reactor, the alpha values at pilot scale peaked following treatment in only one anaerobic zone (nominal HRT of 0.21 hours). The study concludes that short exposures in an initial anaerobic reactor as required for biological phosphorus removal may benefit oxygen transfer efficiency through increased alphas, however the benefits of long periods of anaerobic reaction time (over 0.43 hours) are uncertain. / Master of Science

La friture profonde de type batch a été étudiée dans le projet FUI Fry’In (Réf. AAP17, 2014-2018) dans le but de proposer des innovations de rupture pour des friteuses batch domestiques et professionnelles. La thèse a appuyé le projet sur la maîtrise de deux effets négatifs de la friture : i) la thermo-oxydation de l’huile responsable des mauvaises odeurs et produits de dégradation ainsi que ii) la prise d’huile généralement favorisée au détriment de son égouttage. L’étude a été réalisée en combinant des mesures directes (spectroscopie et imagerie infrarouges en mode ATR, photo-ionisation, mesures DSC, imagerie rapide…) et modélisation multi-échelle (écoulement de l’huile et égouttage lors du retrait, description lagrangienne des réactions en présence d’un écoulement, couplage avec les ciné-tiques de dissolution de l’oxygène). La complexité du processus de thermo-oxydation a été réduite en considérant les hydroperoxydes comme une forme de stockage organique de l’oxygène, qui propage l’oxydation dans des régions en anoxie. Leur décomposition produit de nombreux composés de scission, dont la nature est influencée par les conditions locales de température et de concentration en oxygène. La prise d’huile a été décrite comme le bilan net entre l’huile charriée au moment du retrait et l’huile égouttée. L’égouttage a été étudié sur des barreaux métalliques et des produits réels. Il se conduit à la formation de quatre à huit gouttes en quelques secondes. Les cinétiques de drainage anisothermes ont été prédites par un modèle mécanistique. Le mécanisme spécifique de prise d’huile en cours de friture a été aussi analysé ; il se produit uniquement dans le cas des produits préfrits congelés. / Batch deep-frying has been investigated within the collaborative project FUI Fry’In (ref. AAP17, 2014-2018) with the aim of proposing breakthrough innovations for small and medium size appliances. The PhD thesis was part of the project and focused on two specific adverse effects of deep-frying on food products: oil thermo-oxidation responsible for break-down products and off-flavors, and oil pickup process usually favored relatively to oil dripping. The work was carried out by combing direct measurements (FTIR-ATR spectroscopy and imaging, photoionization, DSC measurements, fast imaging…) and multiscale modeling (oil flow and oil dripping during product re-moval, Lagrangian description of reactions in aniso-thermal flows, coupling with oxygen dissolution kinetics). The complex problem of thermo-oxidation was split into simpler mechanisms by noticing that hydroperoxides are a kind of long-lived form of or-ganic oxygen, which trigger propagation in deep re-gions under anoxia. Their decomposition lead to various scission products, which were shown to be in-fluenced by both local temperature and oxygen con-centration. Oil uptake was described as the net balance between the amount of dragged oil during product removal and oil dripping at the tips of the product. The dripping process studied on both metal-lic sticks and real products occurs in less than few seconds and leads to a formation of four to eight drop-lets. The detailed drainage kinetics in anisothermal conditions were captured and predicted with the pro-posed mechanistic models. The specific mechanism of oil uptake during the immersion stage was eluci-dated and was shown to occur only in parfried frozen products.

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.