En raison de sa nature hautement inflammable, l’hydrogène constitue un risque technologique important et son utilisation nécessite un très haut niveau de sûreté. Le travail de thèse présenté dans ce mémoire a été réalisé en collaboration avec EDF et s’inscrit dans le cadre des études de sécurité liées à la libération d’hydrogène dans un des locaux de l’îlot nucléaire. Le dégagement d’une fuite peut, en effet, entraîner la formation d’une atmosphère inflammable, qui peut exploser et provoquer des graves dégâts.Cette étude vise ainsi à apporter une meilleure compréhension des phénomènes de dispersion et de déflagration à l’issue d’une fuite d’hydrogène. Les résultats expérimentaux obtenus à échelle de laboratoire sont comparés aux simulations numériques obtenues via le logiciel FLACS. Un plan détaillé a donc été élaboré, en divisant le mémoire en deux parties : dispersion et déflagration. Pour chaque partie, un point bibliographique est proposé, ainsi qu’une description des dispositifs expérimentaux utilisés. Une partie expérimentale et numérique est présentée pour chacun des deux phénomènes. Les travaux réalisés ici ont donc permis d’obtenir une analyse réelle et complète du phénomène de rejet d’hydrogène en milieu fermé et obstrué et d’explosion de mélange hétérogène hydrogène-air à petite et grande échelle. Les cas les plus pénalisants en termes de concentration maximale et gradient de concentration ont été identifiés à travers une étude paramétrique sur l’influence du débit et de la position du rejet sur la dispersion. Les conséquences lors de la déflagration de ces mélanges hydrogène-air ont ensuite été étudiées à travers l’analyse de l’onde de pression et de la propagation de la flamme. / The highly combustible nature of hydrogen poses a great hazard and its use imposes an accurate analysis of risk characterization and consequences to protect the installation and to reduce the potential risk. This thesis has been done in collaboration with EDF and it is included in the context of the explosion risks of an air hydrogen mixture in a room of the nuclear facility where there is a risk of accidental release. Indeed, the hydrogen can disperse quickly and burn easily in the presence of an ignition source causing heavy damage. The goal of this study is to provide a better understanding of the phenomena of dispersion and deflagration after an accidental release of hydrogen. Experimental results obtained at small scale are compared with numerical simulations obtained using FLACS code. A detailed plan has been drawn up, dividing the thesis into two parts: dispersal and deflagration. For each part, a bibliographic point is proposed, as well as a description of the experimental devices used. An experimental and numerical part is presented for each of the two phenomena. The studies carried out here have thus enabled us to obtain a real and complete analysis of the phenomenon of closed and clogged hydrogen discharge and small-scale and large-scale heterogeneous hydrogen-air mixture explosion. The most penalizing cases in terms of maximum concentration and concentration gradient were identified through a parametric study on the influence of the flow rate and the position of the rejection on the dispersion. The consequences during the explosion of these hydrogen-air mixtures were then studied through the analysis of the pressure wave and the propagation of the flame.

This work determines a suitable value for a dispersion coefficient to be used in the One-Dimensional Advection-Dispersion equation to model dispersion within sewer collection systems. Dispersion coefficients for sewer systems have only recently begun to be studied, and there is not yet an established value that is commonly accepted. The work described in this paper aimed, through observational study, to find a suitable value to be used. Salt tracers were placed in two separate reaches of sewer line. The first line studied was a straight, linear reach of sewer that included three manholes. The tracer was placed in the first manhole and the conductivity was measured at the two consecutive manholes downstream. These measurements were compared to a model developed using the 1D Advection-Dispersion Equation. The flow information and sewer network geometry was used in the model and the dispersion coefficient was adjusted to find a best fit. It was found that a value of 0.18 m2/s for the dispersion coefficient provided the best statistical match. The next reach of sewer that was studied was a reach with a 90 degree angle. This section was chosen to observe the effect that mixing has on dispersion, because of the change in direction of flow. The same procedure was applied, and an optimal dispersion coefficient of 0.22 m2/s was found. These values represent optimal dispersion coefficients under a specific set of conditions. It should not be assumed that they will provide accurate results in all circumstances, but are rather a base point for average flows under dry, stable conditions. Using these values inferences can begin to be made about dispersion characteristics throughout the entire sewer network. This can lead to specific engineering applications, and well as applications in other fields of study.

First demonstrated by Isaac Newton in his prism experiment, angular dispersion (AD) is a prevalent effect in optics where each wavelength in a pulsed field propagates at a different angle. Angular dispersion occurs after a collimated pulse traverses a diffractive or dispersive device and, as a result, helps modify the group velocity of a pulse in free space and introduces group-velocity dispersion into the freely propagating wave packet. These are essential ingredients in group-velocity matching and dispersion cancellation in various optical settings. With 300 years of development, it was only recently that a new class of angular dispersion materialized as non-differentiable AD. This non-differentiable AD has also been studied under the moniker space-time wave packets (STWP) and has shown to be propagation-invariant and possess arbitrary group velocity. In this dissertation, I will study (1) the underpinning theory of how non-differentiable AD allows for an optical field to break the pre-conceived notions of group velocity, group velocity dispersion (GVD), and pulse front tilt for on-axis propagation through analytical and experimental demonstrations. From these developments, I will (2) apply these concepts of non-differentiable AD to dispersive materials. I will validate these analytical predictions through experiments showing that propagation-invariant wave packets can also be supported in normal and anomalous media. Moreover, I will prove, through the use of non-differentiable AD, that the dispersive properties of a material can be overwritten to produce arbitrary group velocity and GVD characteristics. With this new information on propagation-invariant fields in dispersive materials, I will (3) exhibit new classes of optical fields that were previously theorized but never synthesized in dispersive materials, such as the X- to O- transition in anomalous GVD materials, which will be connected to the de-Broglie-Mackinnon wave packet and particle wave packets. To address the propagation invariance of non-differentiable AD, I will (4) demonstrate the STWP propagation throughout a kilometer in a turbulent environment and develop a new Rayleigh length for the STWP. Finally, I will (5) establish the consequences of discretization on the non-differentiable AD and produce a new form of the Talbot effect in which the temporal and spatial degrees of freedom are interlocked along with independent spatial and temporal Talbot effects in free space.

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

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