Le phosphore (P) est un nutriment essentiel dans le contrôle de l'eutrophisation des eaux de surface. La majorité du P causant cette eutrophisation dans les pays occidentaux est aujourd'hui issu des sols agricoles, ce qui explique pourquoi les recherches actuelles sur l'eutrophisation se focalisent sur la compréhension des mécanismes par lequel le P est relargué de ces sols. Dans cette thèse, nous étudions ces mécanismes en nous focalisant sur la fraction dissoute (DP) du P, fraction la plus menaçante du point de vue de l'eutrophisation. Une double approche a été utilisée, combinant le suivi de la composition d'eaux du sol et d'eaux de ruisseau dans un petit bassin versant (BV) agricole représentatif (BV de Kervidy-Naizin, France) et des simulations expérimentales au laboratoire. Les suivis de terrain ont révélé que les zones humides ripariennes (RW) étaient les principales zones de relargage de DP dans le BV étudié, via deux mécanismes essentiellement déclenchée par les fluctuations des hauteurs de nappe, i) la réhumectation des sols (DRW) et ii) la dissolution réductrice des oxydes de fer du sol dans de périodes d'anoxie. Ces mêmes suivis ont révélé la présence de fortes variations spatiales de la nature chimique (inorganique et organique/colloïdale) du DP relargué. Les variations saisonnières et interannuelles de l'hydroclimat, combinées aux variations locales de topographie ont été démontrées être les deux facteurs principaux contrôlant i) la fréquence des épisodes DRW, et ii) la durée des périodes anoxiques, entraînant au final de fortes variations saisonnières et interannuelles de la dynamique de relargage du DP. Comme indiqué dans un modèle conceptuel général, la topographie est sans doute le facteur clé de contrôle des variations observées, en raison de son rôle sur i) le transfert de P à partir des parcelles agricoles amont, ii) le taux de minéralisation du P organique du sol P et iii), le déclenchement des deux mécanismes de relargage précités. Les expériences en laboratoire ont confirmé le rôle des événements DRW comme processus clé causant le relargage de DP dans les RWs. Les résultats ont démontré que le DP relargué consistait non seulement de "vrai" DP inorganique et organique, mais aussi de P colloïdal, le P colloïdal et le DP organique étant les plus réactifs aux événements DRW. Les données ont aussi révélées que ces différentes formes de P provenaient de différentes sources dans le sol (méso et macroporosité pour P colloïdal et le DP organique; microporosité pour DP inorganique), et que la quantité de P colloïdal relargué était positivement corrélée avec la teneur en matière organique et la taille de biomasse microbienne du sol. Ces mêmes expériences ont confirmé le rôle des conditions anoxiques comme conditions favorisant la libération de DP dans les RWs. La dissolution réductive de sol Fe-oxyhydroxydes n'est cependant pas le seul processus impliqué, un autre processus étant la hausse du pH causée par des réactions de réduction. Les résultats obtenus démontrent que l'augmentation de pH contrôle la libération de DP dans les sols riches en matière organique, alors que ce relargage est contrôlé principalement par la réduction des oxydes de fer dans les sols pauvres en matière organique. Les données expérimentales démontrent également que l'apport de sédiments issus des sols agricoles amont accroit le relargage de DP dans les RW, probablement en raison de la dissolution des oxydes de fer de ces mêmes sédiments par les bactéries ferroréductrices des Rws. Au final, cette thèse permet de mieux contraindre les mécanismes et facteurs responsables du relargage de DP dans les bassins versants agricoles. Une conséquence très pratique de ce travail est que la conception de stratégies pour limiter les fuites de DP dans ces bassins ne peut se faire sans une prise en compte des rôles de l'hydroclimat, de la topographie locale et des propriétés du sol sur ce relargage. / Phosphorus (P) is a key nutrient in controlling surface water eutrophication. Because of the decrease of urban and industrial P emissions, most of the P nowadays causing surface water eutrophication in western countries consists of P transferred from agricultural soils, explaining why current eutrophication research focused on understanding the mechanisms by which P is released from soils. In this thesis, we studied these release mechanisms for dissolved P (DP) – i.e. the most bioavailable P component for algae - using an approach combining field monitoring of soil and stream water compositions in a small, headwater catchment typical of western countries agricultural catchments (the Kervidy-Naizn catchment, France), and laboratory experimental simulations. Field monitoring data revealed that riparian wetlands (RW) are the main zones of DP release and DP production in the studied catchment, through essentially two mechanisms triggered by groundwater table fluctuations, namely i) rewetting of dry soils (DRW), and ii) reductive dissolution of soil Fe (hydr)oxides during anaerobic periods. Field monitoring data also revealed the presence of strong spatial variations in the chemical nature (inorganic vs. organic/colloidal) of the released DP, which was in relation to differences in soil properties and local topography. Seasonal and inter-annual hydroclimate variations, combined with variations in local topography were found to control the frequency of soil DRW events and duration of anaerobic periods, resulting in strong seasonal and inter-annual variations of DP release dynamics. As shown in a conceptual model, topography is likely to be the key driver of the observed spatial and temporal variations, because of its combined control on i) the transfer of P from upland fields to RW zones, ii) the mineralization rates of soil organic P and iii) the triggering of the above two release mechanisms. Laboratory leaching experiments on the same soils confirmed the role of DRW events as a major process causing DP release pulses in RWs. The data demonstrated that the released DP consisted not only of true dissolved inorganic and organic P but also of colloidal P, the latter phase being the most reactive to DRW events. The data also revealed that the different P forms came from different P sources in the soil (soil macro/mesopores for colloidal P and organic DP; soil micropores for inorganic DP) and that the amount of released colloidal P correlated positively with the organic matter contents and soil microbial biomass size of the soil. Anaerobic incubation experiments, on their hand, confirmed the role of anoxic conditions as conditions favoring the release of DP in RW. Reductive dissolution of soil Fe-oxyhydroxide was, however, not the sole process involved in that release, another process being the rise in pH caused by reduction reactions. Experimental data showed that the pH rise controlled the DP release in organic-rich soils, this release being on the contrary mainly controlled by soil Fe-oxyhydroxides reductive dissolution in organic-poor soils. Experimental data also showed that the input of soil sediments from upland fields enhanced the release of DP in RW, most likely due to the enhanced dissolution of sediment Fe-oxyhydroxides by RW Fe-reducing bacteria. Overall, this thesis allowed new constraints to be placed on the release mechanisms of DP in headwater agricultural catchments. One very practical output is that great care should be taken of hydroclimate variability, local topography, and soil property when designing and implementing management options to reduce DP release and transfer in agricultural catchments.

<p>  </p> <p>Heat pipes and vapor chambers are passive two-phase heat transport devices that are used for thermal management in electronics. The passive operation of a heat pipe is facilitated by capillary wicking of the working fluid through a porous wick, and thus is subject to an operational limit in terms of the maximum pressure head that the wick can provide. This operational limit, often termed as the capillary limit, is the maximum heat input at which the pressure drop in the wick is balanced by the maximum capillary pressure head; operating a heat pipe or a vapor chamber above the capillary limit at steady-state leads to dryout. It thus becomes important to predict the performance of heat pipes and vapor chambers and explore the parametric design space to provide guidelines for minimized thermal resistance while satisfying this capillary limit. An increasingly critical aspect is to predict the transient thermal response of vapor chambers. Moreover, heat pipes and vapor chambers are extensively being used in electronic systems where the power input is dictated by the end-user activity and is expected to even exceed the capillary limit for brief time intervals. Thus, it is imperative to understand the behavior of heat pipes and vapor chambers when operated at steady and transient heat loads above the capillary limit as dryout occurs. However, review of the literature on heat pipe performance characterization reveals that the regime of dryout operation has been virtually unexplored, and thus this thesis aims to fill this critical gap in understanding.</p> <p>The design for minimized thermal resistance of a vapor chamber or a heat pipe is guided by the relative contribution of thermal resistance due to conduction across the evaporator wick and the saturation temperature gradient in the vapor core. In the limit of very thin form factors, the contribution from the vapor core thermal resistance dominates the overall thermal resistance of the vapor chamber; recent work has focused on working fluid selection to minimize overall thermal resistance in this limit. However, the wick thermal resistance becomes increasingly significant as its thickness increases to support higher heat inputs while avoiding the capillary limit. A thermal resistance network model is thus utilized to investigate the importance of simultaneously considering the contributions of the wick and vapor core thermal resistances. A generalized approach is proposed for vapor chamber design which allows <em>simultaneous</em> selection of the working fluid and wick that provides minimum overall thermal resistance for a given geometry and operating condition. While the thermal resistance network model provides a convenient method for exploring the design space, it cannot be used to predict 3-D temperature fields in the vapor chamber. Moreover, such thermal resistance network models cannot predict transient performance and temperature evolution for a vapor chamber. Therefore, an easy-to-use approach is proposed for mapping of vapor chamber transport to the heat diffusion equation using a set of appropriately defined effective anisotropic thermophysical properties, thus allowing simulation of vapor chamber as a sold conduction block. This effective anisotropic properties approach is validated against a time-stepping analytical model and is shown to have good match for both spatial and transient temperature predictions.</p> <p>Moving the focus from steady-state and transient operation of vapor chambers, a comprehensive characterization of heat pipe operation above capillary limit is performed. Different user needs and device workloads can lead to highly transient heat loads which could exceed the notional capillary limit for brief time intervals. Experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that exceed the capillary limit. Transient dryout events due to a wick pressure drop exceeding the maximum available capillary pressure can be detected from an analysis of the measured temperature signatures. It is discovered that under such transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. If the heating pulse is sufficiently long as to induce transient dryout, the heat pipe may experience an elevated steady-state temperature even after the heat load is reduced back to a level lower than the capillary limit. The steady-state heat load must then be reduced to a level much below the capillary limit to fully recover the original thermal resistance of the heat pipe. The recovery process of heat pipes is further investigated, and a mechanism is proposed for the thermal hysteresis observed in heat pipe performance after dryout. A model for <em>steady-state</em> heat pipe transport is developed based on the proposed mechanism to predict the parametric trends of thermal resistance following recovery from dryout-induced thermal hysteresis, and the model is mechanistically validated against experiments. The experimental characterization of the recovery process demonstrates the existence of a maximum hysteresis curve, which serves as the worst-case scenario for thermal hysteresis in heat pipe after dryout. Based on the learnings from the experimental characterization, a new procedure is introduced to experimentally characterize the steady-state dryout performance of a heat pipe.</p> <p>To recover the heat pipe performance under steady-state, it has been shown that the heat input needs to be lowered down or <em>throttled</em> significantly below the capillary limit. However, due to the highly transient nature of power dissipation from electronic devices, it becomes imperative to characterize heat pipe recovery from dryout under transient operations. Hence, power-throttling assisted recovery of heat pipe from dryout has been characterized under transient conditions. A minimum throttling time interval, defined as time-to-rewet, is identified to eliminate dryout induced thermal hysteresis using power throttling. Dependence of time-to-rewet on throttling power is explored, and guidelines are presented to advise the throttling need and choice of throttling power under transient conditions. </p> <p>The experimental characterization of heat pipe operation at pulse loads above the capillary limit and power throttling following the pulse load helped define the dryout and recovery performance of a heat pipe. Next, a physics-based model is developed to predict the heat pipe <em>transient</em> thermal response under dryout-inducing pulse load and power throttling assisted recovery. This novel model considers wick as a partially saturated media with spatially and temporally varying liquid saturation, and accounts for the effect of wick partial saturation in heat pipe transport. The model prediction are validated against experiments with commercial heat pipe samples, and it is shown that the model can accurately predict dryout and recovery characteristics, namely time-to-dryout, time-to-rewet, and dryout-induced thermal hysteresis, for heat pipes with a range of wick types, heat pipe lengths and pulse loads above the capillary limit. </p> <p>The work discussed in this thesis opens certain questions that are expected to guide further research in this area. First, the thermal hysteresis mechanism proposed could be further validated with direct visualization of the liquid in a vapor chamber. To achieve this, X-ray microscopy is proposed as a viable option for the imaging <em>in situ</em> wetting dynamics in a vapor chamber. Second, the model developed to predict the dryout and recovery characteristics of the heat pipe can be used to design heat pipe with improved performance under pulse loads and power throttling. Third, novel wick designs can be explored that utilize the understanding developed of governing mechanisms for recovery from dryout, and can eliminate thermal hysteresis at powers closer to capillary limit. Fourth, the modeling approach can be extended to predict dryout and recovery trends in vapor chamber since the heat transfer pathways in a vapor chamber are different than those of a heat pipe. Fifth, and lastly it was observed several times during experiments that some of the heat pipe samples would exhibit complete dryout (sudden catastrophic rise in temperature and thermal resistance at the point of dryout) whereas other samples would exhibit partial dryout (noticeable but small increase in thermal resistance at dryout) at operating powers just above the capillary limit. Exploring and explaining the cause of complete dryout, in particular, would be an extremely valuable contribution to the heat pipe research. </p> <p>The work discussed in this thesis has led to the comprehensive development of a functional and mechanistic understanding of heat pipe operation above the notional capillary limit. The experimental procedures developed in this work are utilized to characterize a heat pipe performance under dryout and recovery. The models based on the mechanistic understanding developed from experimental characterization of dryout and recovery operation of a heat pipe have been experimentally validated and are useful for predicting heat pipe performance under dryout-inducing pulse loads and power-throttling.   </p>

L’étude de l’ébullition transitoire est un enjeu important pour la sureté nucléaire. Un tel phénomène peut se produire lors d’un accident de type RIA (Reactivity Initiated Accident)dans un réacteur nucléaire où le pic de puissance au niveau d’un crayon de combustible peut déclencher une ébullition transitoire conduisant à une forte augmentation de la température de la gaine et à un risque de rupture. Plusieurs études en conditions réacteurs ont permis d’obtenir des courbes d’ébullition transitoires mais la modélisation qui en découle manque encore de fiabilité. Dans le cadre d’une collaboration avec l’Institut de Radioprotection et de Sûreté Nucléaire (IRSN), une expérience modèle a été construite à l’Institut de Mécanique des Fluides de Toulouse (IMFT). Elle génère un écoulement de réfrigérant HFE7000 dans un canal de section semi-annulaire, simulant l’écoulement autour d’un crayon de combustible, dont la partie intérieure, composée d’une feuille de métal, est chauffée rapidement par effet Joule, simulant l’échauffement de la gaine du crayon. La thermographie infra-rouge permet de mesurer la température de la paroi externe du métal. L’application d’une peinture noire sur le métal augmente son émissivité mais aussi la résistance thermique de la paroi. La précision de la mesure de la température d’intérêt a été optimisée en fonction de l’épaisseur de peinture et une correction sur le bilan d’énergie prend en compte ce paramètre. Ces mesures sont couplées avec une caméra rapide qui permet de visualiser les régimes d’ébullition et d’obtenir des tailles de bulles à l’aide de la mise en place d’algorithmes de traitement d’image. On représente sur un diagramme flux-température les transferts thermiques lors des différents régimes en stationnaire et en transitoire. Chaque régime d’ébullition, en conditions stationnaire ou transitoire, est alors passé en revue : la convection, le déclenchement de l’ébullition, l’ébullition nucléée, la crise d’ébullition, l’ébullition en film et le remouillage. Les régimes stationnaires sont correctement modélisés par des corrélations usuelles. La convection transitoire est caractérisée sur toute la paroi et son évolution se rapproche de la solution quasistationnaire. Il est montré que les transferts thermiques lors du passage vers l’ébullition nucléée sont dépendants de la formation d’une importante poche de vapeur qui se propage sur la paroi. Une étude locale de cette propagation est alors nécessaire. Afin de simuler des transitoires de température durant l’ébullition nucléée, un système d’asservissement de type P.I.D. permet d’imposer des créneaux ou des rampes de températures (de 5 à 500 K.s 1 ). Les résultats en ébullition nucléée sont conformes avec ceux de la littérature, tant en conditions stationnaire que transitoire. L’expérience permet d’étudier le transfert de chaleur lorsqu’un film de vapeur se forme et isole la paroi. Ce régime d’ébullition en film, pendant la chauffe ou le refroidissement de la paroi peut ainsi être stabilisée pendant plusieurs secondes avec ce système. On caractérise ainsi les conditions de déclenchement de l’ébullition en film, la dynamique de sa propagation et les transferts une fois établi. Enfin, l’implémentation des caractéristiques physiques de notre expérience dans le code SCANAIR de l’IRSN, permet de commencer à calculer et comparer nos résultats expérimentaux avec les simulations numériques. Des calculs de conduction instationnaire sont notamment considérés en imposant la température mesurée pour analyser nos résultats lors du régime de convection et après le déclenchement de l’ébullition. / The study of rapid transient boiling is an important issue in the nuclear safety. Such a phenomenon may occur in the case of a RIA (Reactivity Initiated Accident) in the core of a nuclear reactor powerplant, where a power excursion can trigger the formation of a vapour film around the fuel rod, leading to an important rise of the rod temperature and a risk of failure. Some studies in reactor conditions provided transient boiling curves but the modeling lacks of reliability. In collaboration with the IRSN (Institut de Radioprotection et de Sûreté Nucléaire), an experiment model was built at the Institute of Fluid Mechanics of Toulouse. It generates the flow of a refrigerant, HFE7000, in a semi-annular section channel, whose inner wall is made of a metal foil rapidly heated by Joule effect, simulating the heating of a fuel rod. Infrared thermography is used to measure the temperature of the metal foil, painted with a black paint to increase its emissivity, causing also an increase of the wall thermal resistance. The measurement accuracy of the interest temperature has been optimized according to the paint thickness and a correction on the energy balance takes account this parameter. These measurements are coupled with a high-speed camera that allows visualizing the boiling regimes and get bubble sizes using image processing algorithms. On a flux-temperature diagram, the heat transfers are represented both for steady and transient regimes. Each boiling regime is then reviewed : convection, onset of nucleate boiling, nucleate boiling, boiling crisis, film boiling and rewetting. Steady regimes are correctly modeled by usual correlations. Transient convection is characterized over the whole wall and its evolution is closed to the quasi-steady solution. It is shown that heat transfer during the transition to nucleate boiling are strongly related to the formation of a large vapor phase that spreads on the wall. A local study of this propagation is then necessary. In order to simulate and control transient temperature during nucleate boiling, a P.I.D. is implemented to impose a steady or ramps temperature (from 5 to 500 K.s 1 ). The results in nucleate boiling make it possible to recover the results of the literature in both steady and transient conditions. The experiment allows to study the heat transfer when a vapor film is formed and insulates the wall. The film boiling regime during heating or the cooling of the wall can thus be stabilized for several seconds with this system. The conditions for triggering of film boiling are thus characterized, as its spread dynamic and its transfers once established. Finally, the implementation of the physical characteristics of our experience in IRSN’s SCANAIR code allows us to begin to calculate and compare our experimental results with numerical simulations. Unsteady conduction calculations are applied to the measured temperature to analyze our results during the convection regime and after the onset of boiling.

No description available.

Northern Canadian peatlands contain vast deposits of carbon. It is with growing urgency that we seek a better understanding of their assimilative capacity. Assimilative capacity and peat accumulation in raised bogs are linked to primary productivity of resident Sphagnum species. Understanding moisture-mediated photosynthesis of Sphagnum spp. is central to understanding peat production rates. The relationship between depth to water table fluctuation and spectral reflectance of Sphagnum moss was investigated using multispectral imaging at a recovering raised bog on the southwest coast of British Columbia, Canada. Burns Bog is a temperate oceanic ombrotrophic bog. Three ecohydrological indicator species of moss were chosen for monitoring: S. capillifolium, S. papillosum, and S. cuspidatum. Three spectral vegetation indices (SVIs) were used to characterize Sphagnum productivity: the normalized difference vegetation index 660, the chlorophyll index, and the photochemical reflectance index. In terms of spectral sensitivity and the appropriateness of SVIs to species and field setting, we found better performance for the normalized difference vegetation index 660 in the discrimination of moisture mediated species-specific reflectance signals. The role that spatiotemporal scale and spectral mixing can have on reflectance signal fidelity was tested. We were specifically interested in the relationship between changes in the local water table and Sphagnum reflectance response, and whether shifting between close spatial scales can affect the statistical strength of this relationship. We found a loss of statistical significance when shifting from the species-specific cm2 scale to the spectrally mixed dm2 scale. This spatiospectral uncoupling of the moisture mediated reflectance signal has implications for the accuracy and reliability of upscaling from plot based measurements. In terms of species-specific moisture mediated reflectance signals, we were able to effectively discriminate between the three indicator species of Sphagnum along the hummock-to-hollow gradient. We were also able to confirm Sphagnum productivity and growth outside of the vascular growing season, establishing clear patterns of reflectance correlated with changes in the local moisture regime. The strongest relationships for moisture mediated Sphagnum productivity were found in the hummock forming species S. capillifolium. Each indicator Sphagnum spp. of peat has distinct functional traits adapted to its preferred position along the ecohydrological gradient. We also discovered moisture mediated and species-specific reflectance phenologies. These phenospectral characteristics of Sphagnum can inform future monitoring work, including the creation of a regionally specific phenospectral library. It’s recommended that further close scale multispectral monitoring be carried out incorporating more species of moss, as well as invasive and upland species of concern. Pervasive vascular reflectance bias in remote sensing products has implications for the reliability of peatland modelling. Avoiding vascular bias, targeted spectral monitoring of Sphagnum indicator species provides a more reliable measure for the modelling of peatland productivity and carbon assimilation estimates. / Graduate