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Ces travaux de thèse s’intègrent au sein du projet Biocarb lancé par le Commissariat à l’Énergie Atomique dont l’objectif est de développer des procédés de production de carburants liquides ou gazeux à partir de gaz de synthèse riche en H2 et CO obtenu par gazéification de la biomasse lignocellulosique. L’objectif de cette étude est d’étudier le comportement de particules de biomasse millimétriques lors de la pyrolyse dans des conditions types de gazéifieurs industriels tels que les réacteurs à lit fluidisé ou à flux entraîné, qui fonctionnent pour des flux de chaleur élevés (105 – 106 W⋅m-2) et pour de hautes températures (>800°C). Tout d’abord, des expériences de pyrolyse sont menées à 800 et 950°C dans un four à chute de laboratoire sur des particules de bois entre 350 et 800 μm. Les résultats montrent que dans les conditions de l’étude, l’augmentation de la taille de la particule augmente seulement la durée de la pyrolyse mais ne modifie pas les rendements ou la composition du solide et du gaz au cours de la pyrolyse. Par ailleurs, des mesures basées sur la technique de PTV (Particle Tracking Velocimetry) sont réalisées à température ambiante pour caractériser la taille et la densité des particules de bois brut et de résidu, et valider une corrélation donnant le coefficient de traînée qui sert à calculer le temps de séjour des particules dans le réacteur. On constate à la fin de la pyrolyse une diminution de la densité comprise entre 70 et 80% ainsi qu’une diminution de la taille des particules entre 25 et 40%. Les résultats montrent également que la vitesse de glissement de la particule et l’évolution de ses propriétés doivent être prises en compte lors du calcul de sa vitesse. Enfin, à partir des résultats expérimentaux, un modèle unidimensionnel à coeur rétrécissant est développé pour décrire le comportement d’une particule de bois lors de sa pyrolyse. Le modèle est capable de prévoir l’évolution du rendement en solide, en gaz total et en goudrons au cours de la pyrolyse ainsi que la vitesse de glissement de la particule et son temps de séjour dans le réacteur.L’analyse de sensibilité du modèle montre que même pour des particules millimétriques, une connaissance précise de la chaleur de réaction associée à la pyrolyse, de la densité du bois et de la conductivité thermique du résidu solide est essentielle / The present work is part of a project of the French energy research centre Commissariat à l’Energie Atomique. The goal of the project is to develop processes of production of gaseous or liquid fuel from synthesis gas obtained by gasification of lignocellulosic biomass. The objective of the present work is to study the pyrolysis behaviour of millimetric biomass particles under the operating conditions encountered in fluidized bed or entrained flow gasifiers, namely high external heat flux (105 – 106 W⋅m-2) and high temperature (> 800°C). First, pyrolysis experiments are conducted at 800 and 950°C in a lab-scale drop tube reactor on wood particles between 350 and 800 μm. The results show that under the explored conditions, the increase of the particle size only increases the time required for pyrolysis but does not affect the product distribution during pyrolysis. Since in the pyrolysis experiments, the particle residence time cannot be directly measured, PTV (Particle Tracking Velocimetry) measurements are performed at room temperature to characterize the evolution of the particle size and density along pyrolysis and to validate a drag coefficient correlation for the particle residence time calculation. The optical measurements show that at the end of pyrolysis there is a decrease of particle density of 70 – 80% and of particle size of 25 – 40%. It is also proven that the particle slip velocity cannot be neglected and that the change of these particle properties must be taken into account for the calculation of the particle slip velocity and residence time. Finally, based on these experimental results, a 1D shrinking-core model is developed that is able to predict the solid/gas/tar yields and the residence time of a single particle along pyrolysis in the drop tube reactor. It is validated on both the pyrolysis and optical experiments. The model sensitivity analysis shows that even for millimetric particles, the accurate knowledge of the heat of pyrolysis, of the wood density and of the char thermal conductivity is essential

La turbulence est connue pour sa capacité à disperser efficacement de la matière, que ce soit des polluantes dans les océans ou du carburant dans les moteurs à combustion. Deux considérations essentielles s’imposent lorsqu’on considère de telles situations. Primo, l’écoulement sous-jacente pourrait avoir une influence non-négligeable sur le comportement des particules. Secundo, la concentration locale de la matière pourrait empêcher le transport ou l’augmenter. Pour répondre à ces deux problématiques distinctes, deux dispositifs expérimentaux ont été étudiés au cours de cette thèse. Un premier dispositif a été mis en place pour étudier l’écoulement de von Kàrmàn, qui consiste en une enceinte fermé avec de l’eau forcé par deux disques en contra-rotation. Cette écoulement est connu pour être très turbulent, inhomogène, et anisotrope. Deux caméras rapides ont facilité le suivi Lagrangien des particules isodenses avec l’eau et petites par rapport aux échelles de la turbulence. Ceci a permis une étude du bilan d’énergie cinétique turbulente qui est directement relié aux propriétés de transport. Des particules plus lourdes que l’eau ont aussi été étudiées et montrent le rôle de l’anisotropie de l’écoulement dans la dispersion des particules inertielles. Un deuxième dispositif, un écoulement de soufflerie ensemencé avec des gouttelettes d’eau micrométriques a permis une étude de l’effet de la concentration locale de l’eau sur la vitesse de chute des gouttelettes grâce à une montage préexistant. Un modèle basé sur des méthodes théorique d'écoulements multiphasiques a été élaboré enfin de prendre en compte les effets collectifs de ces particules sedimentant dans un écoulement turbulent. Les résultats théoriques et expérimentaux mettent en évidence le rôle de la polydispersité et du couplage entre les deux phases dans l’augmentation de la sédimentation des gouttelettes. / Turbulence is well known for its ability to efficiently disperse matter, whether it be atmospheric pollutants or gasoline in combustion motors. Two considerations are fundamental when considering such situations. First, the underlying flow may have a strong influence of the behavior of the dispersed particles. Second, the local concentration of particles may enhance or impede the transport properties of turbulence. This dissertation addresses these points separately through the experimental study of two different turbulent flows. The first experimental device used is the so-called von K\'arm\'an flow which consists of an enclosed vessel filled with water that is forced by two counter rotating disks creating a strongly inhomogeneous and anisotropic turbulence. Two high-speed cameras permitted the creation a trajectory data base particles that were both isodense and heavier than water but were smaller than the smallest turbulent scales. The trajectories of this data base permitted a study of the turbulent kinetic energy budget which was shown to directly related to the transport properties of the turbulent flow. The heavy particles illustrate the role of flow anisotropy in the dispersive dynamics of particles dominated by effects related to their inertia. The second flow studied was a wind tunnel seeded with micrometer sized water droplets which was used to study the effects of local concentration of the settling velocities of these particles. A model based on theoretical multi-phase methods was developed in order to take into account the role of collective effects on sedimentation in a turbulent flow. The theoretical results emphasize the role of coupling between the underlying flow and the dispersed phase.

Die Analyse von Kollisionen zwischen Partikeln und Blasen in einer turbulenten Strömung ist ein grundlegendes Problem von hoher technologischer Relevanz, z. B. für die Abtrennung wertvoller Mineralpartikel durch Schaumflotation. Dieser Relevanz steht ein Defizit an experimentellen Daten und Erkenntnissen über den Kollisionsprozess gegenüber. Ein Hauptproblem ist die geringe Anzahl der verfügbaren Messtechniken zur direkten Beobachtung der Kollisionen zwischen Partikeln und Blasen. Daher besteht das Ziel dieser Dissertation darin, neue Methoden zu entwickeln, um die Wechselwirkung zwischen Blasen und Partikeln unter definierten hydrodynamischen Bedingungen zu messen. Diese Methoden beruhen auf der Verfolgung von einzelnen Partikeln mit 4D Particle Tracking Velocimetry (PTV) und Positron Emission Particle Tracking (PEPT), um die Lagrangeschen Partikeltrajektorien in der Nähe einer Blase zu bestimmen und die kollidierenden Partikel zu klassifizieren. In zwei Versuchsaufbauten werden diese Messmethoden angewandt, um die Wechselwirkung zwischen Blasen und Partikeln in turbulenten Strömungen zu untersuchen. In einer Blasensäule wird die Turbulenz im Nachlauf einer frei aufsteigenden Blasenkette erzeugt, während in einem Wasserkanal die Turbulenz durch die Umströmung eines Gitters produziert wird. In beiden Fällen wird das vorhandene turbulente Strömungsfeld um die Blasen mittels Tomographic Particle Image Velocimetry (TPIV) charakterisiert. Zunächst wird der Einfluss des Blasennachlaufs auf die Blasen-Partikel-Kollision für beide Versuchsaufbauten mit dem 4D-PTV-Verfahren analysiert. Es wird gezeigt, dass in beiden Versuchsanordnungen die Kollision von feinen Partikeln nicht nur an der Vorderseite, sondern auch an der Hinterseite der Blase stattfindet. Diese Ergebnisse werden mit der gemessenen turbulenten kinetischen Energie und der Dissipationsrate um die Blase korreliert. Anschließend werden die experimentell ermittelte turbulente kinetische Energie und Dissipationsrate genutzt, um die Kollisionsfrequenz vorherzusagen. Dafür werden bestehende Modelle angewendet und deren Vorhersagen den experimentellen Ergebnissen gegenübergestellt. Weiterhin wird der Wasserkanal genutzt, um den Einfluss der turbulenten Flüssigkeitsströmung auf die Kollision zwischen einer stagnierenden Blase und den Modellpartikeln zu verdeutlichen. Neben der Untersuchung in einer verdünnten Feststoffsuspension wird auch die Blasen-Partikel-Wechselwirkung in einer dichten Strömung mit dem PEPT-Verfahren untersucht. Das PEPT-Verfahren hat das Potenzial, Suspensionen mit einem hohen Feststoffanteil zu messen, was mit optischen Trackingverfahren, wie 4D-PTV, nicht möglich ist. Für den Nachweis einzelner Partikel mit dem PEPT-Verfahren wurden radioaktive Tracerpartikel entwickelt, welche repräsentativ für die Modellpartikeln sind. Die Trajektorien der markierten Partikel werden verwendet, um die durchschnittliche Partikelverteilung im turbulenten Feld zu bestimmen und die Blasen-Partikel-Wechselwirkung zu beschreiben. Insgesamt bieten die entwickelten Methoden eine Möglichkeit die Kollision zwischen Partikeln und Blasen in einer turbulenten Strömung direkt zu untersuchen. Die gewonnenen experimentellen Daten ermöglichen es, bestehende Kollisionsmodelle zu überprüfen und das Verständnis über die Rolle von Turbulenzen in der Schaumflotation zu verbessern. / The analysis of collisions between particles and bubbles in a turbulent flow is a fundamental problem of high technological relevance, e.g. for the separation of valuable mineral particles by froth flotation. That relevance contrasts with an apparent lack of experimental data and insights into this collision process. A major issue is the limitation of available measurement techniques to directly observe the collisions between particles and bubbles. In this dissertation, novel methodologies are developed to measure the interaction between bubbles and particles under defined hydrodynamic conditions. These methodologies comprise particle tracking techniques such as 4D PTV and PEPT to triangulate the Lagrangian particle trajectories in the vicinity of a bubble and classify those which are colliding. In two experimental setups, these techniques are applied to investigate the bubble-particle interaction in turbulent flows. In a bubble column, turbulence is generated in the wake of a freely rising bubble chain, whereas in a water channel, a fluid passing through grid produces a turbulent flow upstream of a stagnant bubble. Accordingly, the turbulent flow field around these bubbles is characterized by TPIV. Firstly, the influence of the bubble wake on the bubble-particle collision is analyzed for both experimental setups with 4D PTV. It is shown that the collision of fluorescent fine particles take place not only at the leading edge but also at the trailing edge of the bubble, independently of the experimental setup. These findings are correlated with the measured TKE and dissipation rates around the bubble and in the bubble wake. Subsequently, the experimental TKE and dissipation rates are applied to existing models for collision frequency, and their predictions are discussed. Secondly, the impact of the turbulent liquid flow on the collision between a stagnant bubble and model particles is studied for a range of turbulent length scales. Besides the investigation in a dilute solid suspension, the bubble-particle interaction is also examined in a dense flow with PEPT. PEPT has the potential to measure suspensions with a high solid fraction, which could not be achieved with optical particle tracking methods. For the detection of individual particles with PEPT, radioactive tracer particles were designed to represent the bulk particles. The trajectories of the labeled particles are used to determine the average particle distribution in the turbulent field and describe the bubble-particle interactions. Overall, the developed methodologies in this dissertation provide a framework to investigate directly the collision between particles and bubble in a turbulent flow. The gained experimental validation data allows to verify existing collision models and to advance our understanding of the role of turbulence in froth flotation.

This master’s thesis explores the possibility of calculating and using the pressure andvelocity fields responsible for acoustophoresis. The goal was to use simulated data,similar to two-dimensional particle tracks from a specific microfluidic platform, toestimate the force potential and solve the partial differential equation (PDE) thatgoverns the relationship between the force potential and the acoustic fields. Lastly,the possibility of identifying the mechanical properties of unknown particles, once theacoustic field was known, was investigated. The thesis found that solving the PDE usingthe finite difference method was likely not possible and an alternative method has beensuggested. It also found that the use of particle tracks to measure the compressibilityand density of cells as a biomarker is promising, as most simulated particles wereaccurately measured. / Denna uppsats utforskar möjligheten att bestämma och använda de tryck- och hastighetsfält som är ansvariga för akustofores. Målet var att använda simulerad data, likt två-dimensionella partikelbanor för en specifik mikrofluidisk plattform, för att uppskatta kraftpotentialen och lösa den partiella differentialekvation (PDE) som styr relationen mellan kraften och de akustiska fälten. Möjligheten att bestämma mekaniska egenskaper hos okända partiklar, när det akustiska fältet är känt, utreddes också. Slutsatsen är att det inte var möjligt att lösa PDEn med hjälp av finita differensmetoden och istället föreslås en alternativ metod. Preliminära simulerade tester att karaktärisera okända partiklar gav positiva resultat, givet att de akustiska fälten är kända i två dimensioner.

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Fluidized bed technology is used for thermal conversion of solid fuels (combustion and gasification) and is especially suitable for conversion of low-rank fuels such as biomass and waste. The performance of fluidized bed units depends on the fuel mixing and fuel-gas contact. Thus, it is important to understand these two phenomena in order to develop models for reliable design and scale up of fluidized bed units. This work investigates, under conditions representative for industrial fluidized bed units, the lateral fuel mixing (in a unit with a cross section of 1.44 m2 both at hot and cold conditions) and the bubble flow distribution (in a 1.2 m-wide 2-dimensional unit). The work confirms previous findings on the formation of preferred bubble paths and shows that these bubble paths are enhanced by lowering the fluidization velocity, increasing the dense bed height and reducing the pressure drop across the gas distributor. From the fuel mixing experiments, an estimation of the lateral effective dispersion coefficient to values in the order of 10-3 m2/s is obtained under both hot and cold conditions. The experiments under cold conditions give additional qualitative information on the fuel mixing patterns such as flotsam/jetsam tendencies. The camera probe developed for fuel tracking under hot conditions enables to study the fuel dispersion under real operation at relevant industrial scales. Based on the characteristics of the bubble path flow, a model for the horizontal fuel dispersion on a macroscopic scale is formulated and shown to be able to give a good description of the experimental data. As opposed to the commonly applied diffusion-type modeling of the lateral solids dispersion, the proposed model facilitates integration with models of the bubble flow. Thus, the present modeling work is a first step to provide a modeling of the fuel dispersion, which uses as inputs only the main operational parameters of the fluidized bed.

The thermal velocity fluctuations of microscopic particles mediate the transition from microscopic statistical mechanics to macroscopic long-time diffusion. Prior to this work, detection methods lacked the sensitivity necessary to resolve motion at the length and time scales at which thermal velocity fluctuations occur. This dissertation details two experiments which resulted in velocity measurement of the thermal motion of dielectric microspheres suspended by an optical trap in gases and liquids. First, optical tweezers were used to trap glass microspheres in air over a wide range of pressures and a detection system was developed to track the trapped microspheres' trajectories with MHz bandwidth and <100 fm/rt(Hz) position sensitivity. Low-noise trajectory measurements allowed for observation of fluctuations in the instantaneous velocity of a trapped particle with a signal to noise ratio (SNR) of 26 dB, and provided direct verification of the equipartition theorem and of the Maxwell-Boltzmann velocity distribution for a single Brownian particle. Next, the detection technology was further optimized and used to track optically trapped silica and barium titanate glass microspheres in water and acetone with >50 MHz bandwidth and <3 fm/rt(Hz) sensitivity. Brownian motion in a liquid is influenced by hydrodynamic, time-retarded coupling between the particle and the fluid flow its motion generates. Our measurements allowed for instantaneous velocity measurement with an SNR of up to 16 dB and confirmed the Maxwell Boltzmann distribution for Brownian motion in a liquid. The measurements also revealed several unusual features predicted for Brownian motion in the regime of hydrodynamic coupling, including faster-than-exponential decay of the velocity autocorrelation function, correlation of the thermal force and non-zero cross-correlation between the particle's velocity and the thermal force preceding it. / text

Inclined negatively buoyant jets are commonly used to dispose brine effluent produced by desalination plants. Desalination and associated research has expanded in recent years due to the continued depletion and degradation of natural potable water sources. Desalination plants are the preferred option for meeting water demand deficits in many countries around the world. Inclined negatively buoyant jets are produced when the brine is discharged at an upward inclined angle via an offshore pipeline and diffuser system. Previous experimental studies have focused on the rapid mixing and dilution achieved by these discharges, as well as geometric parameters. Dilution measurements between these experimental studies vary significantly, which is possibly due to variations in the location of a lower boundary on observed flow behaviour. In the present study, velocity field information is experimentally measured for inclined negatively buoyant jets and compared to integral model predictions. Experiments are conducted with and without a lower boundary influencing observed flow behaviour, thus allowing the effects of a lower boundary to be determined. The particle tracking velocimetry experimental technique is employed to measure near field velocities of these discharges. Firstly, discharges with source angles between 15\degree and 75\degree are investigated without boundary influence in stationary ambient conditions. The source was a minimum of 655 mm above the bottom of the experimental tank to ensure there was no lower boundary influence on observed behaviour. Time-averaged and fluctuating data are extracted along the trajectory of discharges. All non-dimensionalised geometric and centreline velocity parameters are found to collapse. Empirical coefficients are compared to previous experimental studies and integral model predictions. A new detrainment model is developed to predict the behaviour of inclined negatively buoyant jets without boundary influence. The model further develops recent attempts to allow for buoyancy flux reduction along the flow path. The reduction in buoyancy flux is dependent on the local parameters of the flow and simulates experimentally observed detrainment. Dilution, geometric, and velocity predictions are found to be improved over previous models when compared to experimental data. Finally, a raised platform was placed inside the experimental tank to determine the influence of a lower boundary on inclined negatively buoyant jets. Source angles of 30\degree, 45\degree, and 60\degree are investigated at three different non-dimensional source heights. The lower boundary is horizontal and ambient conditions are again stationary. Discharges impinge the lower boundary before forming a radially spreading layer along the boundary. Geometric and velocity data are compared to the first set of experiments in this study to determine the influence of the lower boundary on observed flow behaviour. Empirical coefficients at maximum height are similar with and without the influence of the boundary, whereas coefficients are substantially influenced at the return point when the boundary is present.

Tsunami are a very dangerous natural hazard, as highlighted in recent years by the Indian Ocean Tsunami of 2004 and the Japan Tsunami of 2011. In the last decade, tsunami have claimed hundreds of thousands of lives, and caused billions of dollars in damage around the world. The hazard posed to coastal communities by tsunami is expected to increase in the future, due to population growth, intensification of coastal development and sea level rise due to climate change. Tsunami may be generated by a number of different source mechanisms. One such source mechanism is a submarine landslide, which can occur in a number of marine environments containing significant sediment accumulation on a sloping seafloor. The high amplitudes and rapid celerities of landslide-generated tsunami make them very dangerous to communities in the vicinity of the landslide, although these waves do not possess the potential for transoceanic devastation. The objectives of this research project are to carry out a series of two-dimensional physical experiments investigating the waves generated by a rigid block landslide moving along a horizontal boundary. The use of a horizontal boundary has the advantage that waves propagating in the offshore and onshore directions may be measured (unlike previous studies using sloping boundaries). The landslide motion is provided by a mechanical system, allowing testing of a broad range of motion, and isolation of the wavemaking properties of different phases of landslide motion. Experiments are carried out in a 14.66 m long flume, with width 0.25 m and working depth 0.50 m. A false floor installed in the flume provides the sliding surface for the landslide motion, and houses the mechanical system. A series of preliminary particle tracking velocimetry experiments confirm the ability of the mechanical system to achieve its velocity targets to within 5% or better, depending on the parameters of the landslide motion. Full spatial and temporal resolution of the wave field is achieved using a laser-induced fluorescence technique to identify the air-water interface to sub-pixel accuracy. The measurements obtained using laser-induced fluorescence are validated against measurements from a resistance wave gauge, with sub-millimetre agreement. In an additional experiment, the particle tracking velocimetry technique provides measurements of the subsurface velocity field. The landslide motion during all experiments consists of an initial period of constant acceleration, followed by a period of constant velocity, followed by a deceleration to rest (at the same rate as the initial acceleration). The landslide acceleration generates two dispersive packets of waves, travelling in the offshore and onshore directions. The offshore-propagating wave packet contains a leading crest and the onshore-propagating wave packet contains a leading trough, with both waves approaching the shallow water limit. A free surface depression forms above the landslide during its constant-velocity motion, and its amplitude may be predicted to within approximately 20% using standard hydraulic theory (considering a frame of reference moving with the landslide). The offshore-propagating waves passing over the landslide cause the amplitude of this depression to fluctuate over time. The deceleration of the landslide generates two additional packets of waves with the opposite polarity to the waves generated by the landslide acceleration. The full spatial and temporal resolution of the generated wave field allows the calculation of the potential energy within the wave field. Additionally, the energy (and mass) within the onshore- and offshore-propagating wave packets may be estimated by calculating these quantities within the onshore and offshore regions of the experimental domain. The wave packets generated by the initial landslide acceleration transport positive mass in the offshore direction, and negative mass in the onshore direction. This mass transport is balanced by the waves generated during the deceleration of the landslide. The nondimensional landslide acceleration, landslide Froude number and submergence depth are varied during the physical experiments. The landslide Froude number has the greatest effect on the behaviour of the generated wave field. At low Froude numbers, the wave field is dominated by the waves generated by the acceleration and deceleration of the landslide. As the Froude number increases, the onshore-propagating waves become negligible in amplitude compared to the offshore-propagating waves. Additionally, the free surface depression increases in amplitude and a group of short-wavelength waves become trapped behind the landslide. These waves exhibit highly nonlinear behaviour at landslide Froude numbers greater than 0.5. The simple experimental geometry allows comparison between the measured wave fields with the predictions of three mathematical models. Two inviscid-irrotational models, differing in their treatment of the bottom boundary condition, provide comparisons over the entire parameter space. These models under-predict the amplitudes of the generated waves, and fail to correctly predict the ongoing interaction between the landslide and the offshore-propagating waves. The inclusion of bottom boundary nonlinearity improves the predictions of the amplitude of the leading waves, and the potential energy within the wave field. However, both of the inviscid models do not predict the extent of wave trapping behaviour behind the landslide observed in the experiments. A viscous model, formulated in the DNS solver Gerris, improves the predictions of wave trapping (and amplitude in general) in one experiment. Although the model still under-predicts the amplitudes of the generated waves, it correctly predicts the amplification of the waves behind the landslide during its constant-velocity motion. The failure of the inviscid models to predict the amplitudes of these waves can be mostly attributed to the linearised free surface condition used by both models. The presence of the turbulent wake may also have a secondary effect on these predictions. An extension of the linear inviscid-irrotational model to three dimensions allows the effect of the landslide width on the amplitudes of the generated waves to be determined. As the width increases, the behaviour of the waves approaches the two-dimensional limiting case.