Etude de la transition choc-détonation du nitrométhane par spectrométrie d'émission

This study is part of a research program on the detonation of high explosives. The objective of this work is to clarify the mechanism of shock initiation of nitromethane (NM), by spectral analysis in the range 0.3-0.85 µm. NM is a liquid explosive, transparent in the visible and near-infrared range. The experiments consist in plane shock impact on explosive targets at 8.6 GPa. Time-resolved radiance profiles and intensity spectra show that chemical species have several optical characteristics depending on the phases of the shock to detonation transition. Shocked NM remains transparent and its temperature is higher than 2500 K because of local chemical reactions. The reactions products produced during the superdetonation formation are semi-transparent: they are optically thick for wavelengths higher than 0.6 µm. The study shows that detonation products behind the detonation wave do not behave like a blackbody in the visible range. Detonation products are semi-transparent and optically thick. A model for the absorption coefficient is proposed for a water vapour and carbon clusters mixture. It is based on a Rayleigh scattering regime and equation of radiative tranfer (ERT) for an emitting and absorbing medium. The resolution of the ERT for semi-transparent gases by an inversion method enables to determine the temperature profiles in the explosive. The method is tested on simple cases of detonation.Ce travail de thèse porte sur l'étude des mécanismes d'amorçage par choc du nitrométhane (NM), explosif liquide, transparent dans le domaine visible et dans le proche infrarouge. Afin de réaliser l'analyse spectrale des produits de réaction du NM en vue de déterminer les profils de température, un banc de spectrométrie d'émission a été mis au point dans le domaine UV-Visible 0,3-0,85 µm. Des essais d'impacts plans à 8,6GPa ont été réalisés. L'étude temporelle et spectrale de la luminance émise durant la TCD montre que les milieux en présence présentent différentes caractéristiques optiques. Le NM sous choc est transparent alors que la température de luminance mesurée est de 2500K, du fait de la présence de réactions chimiques localisées. Les espèces chimiques produites lors de la formation de l'onde de superdétonation constituent un milieu semi-transparent, milieu optiquement mince aux courtes longueurs d'onde, inférieures à 0,6 µm et optiquement épais pour les longueurs d'onde comprises entre 0,6 et 0,85 µm. En détonation quasi-stationnaire, l'étude spectrale dans le domaine visible montre que les produits de détonation n'ont pas le comportement d'un corps noir, mais celui d'un milieu semi-transparent et optiquement épais. Un modèle d'absorption est proposé sur la base d'un milieu constitué de vapeur d'eau et de particules de carbone qui suivent le régime de diffusion Rayleigh. A partir des mesures de luminance, l'inversion mathématique de l'équation de transfert radiatif d'un milieu semi-transparent homogène et non diffusant permet de retrouver les profils de température dans l'épaisseur d'explosif. Elle est validée sur des cas synthétiques de détonation

Instabilité de Rayleigh-Bénard sous champ magnétique : structures et transfert de chaleur

Nous étudions l’influence d’un champ magnétique constant et vertical sur les structures de l’instabilité de Rayleigh-Bénard, ainsi que son effet sur les transferts de chaleur en paroi. Proche de la stabilité marginale, l’écoulement développe des structures fines qui s’élargissent lorsqu’on s’éloigne de cette stabilité marginale. Localement, le transfert de chaleur est modifié, les zones de fort transfertcorrespondant aux zones où la vitesse dirigée selon la normale sortante de la paroi est importante. Le transfert global est légèrement diminué par rapport au cas non magnétique

Magnetic field effects on three-dimensional natural convection (S23)

We study the structure of a magnetoconvective flow. The magnetic field modifies the heat fluxes at the walls. It tends to destroy the three-dimensional patterns in favor of two-dimensional rolls. The last part of this paper consists in an extent to the marginal stability theory, thanks to a finite difference scheme. We also present the validation of the developments of the finite volume code Jadim, thanks to the simulation of magnetohydrodynamic (MHD) flows that have analytical solutions

Etude de la transition choc-détonation du nitrométhane par spectrométrie d'émission

Ce travail de thèse porte sur l'étude des mécanismes d'amorçage par choc du nitrométhane (NM), explosif liquide, transparent dans le domaine visible et dans le proche infrarouge. Afin de réaliser l'analyse spectrale des produits de réaction du NM en vue de déterminer les profils de température, un banc de spectrométrie d'émission a été mis au point dans le domaine UV-Visivle 0,3-0,85 ưm. Des essais d'impacts plans à 8,6 GPa ont été réalisés. L'étude temporelle et spectrale de la luminance émise durant la TCD montre que les milieux en présence présentent différentes caractéristiques optiques. Le NM sous choc est transparent alors que la température de luminance mesurée est de 2500 K, du fait de la présence de réactions chimiques localisées. Les espèces chimiques produites lors de la formation de l'onde de superdétonation constituent un milieu semi-transparent, milieu optiquement mince aux courtes longueurs d'onde, inférieures à 0,6 ưm et optiquement épais pour des longueurs d'onde comprises entre 0,6 et 0,85 ưm...NANTERRE-BU PARIS10 (920502102) / SudocSudocFranceF

: structures et transfert de chaleur

National audienceNous étudions l’influence d’un champ magnétique constant et vertical sur les structures de l’instabilité de Rayleigh-Bénard, ainsi que son effet sur les transferts de chaleur en paroi. Proche de la stabilité marginale, l’écoulement développe des structures fines qui s’élargissent lorsqu’on s’éloigne de cette stabilité marginale. Localement, le transfert de chaleur est modifié, les zones de fort transfertcorrespondant aux zones où la vitesse dirigée selon la normale sortante de la paroi est importante. Le transfert global est légèrement diminué par rapport au cas non magnétique

: structures et transfert de chaleur

National audienceNous étudions l’influence d’un champ magnétique constant et vertical sur les structures de l’instabilité de Rayleigh-Bénard, ainsi que son effet sur les transferts de chaleur en paroi. Proche de la stabilité marginale, l’écoulement développe des structures fines qui s’élargissent lorsqu’on s’éloigne de cette stabilité marginale. Localement, le transfert de chaleur est modifié, les zones de fort transfertcorrespondant aux zones où la vitesse dirigée selon la normale sortante de la paroi est importante. Le transfert global est légèrement diminué par rapport au cas non magnétique

Influence of skin depth on convective heat transfer in induction heating

We investigate convection driven by induction heating of a horizontal fluid layer using direct numerical simulations (DNS). This problem is of particular interest in the context of nuclear severe accident mastering. In a real severe accident, the molten core is subjected to homogeneous internal sources resulting from nuclear disintegrations. This situation is mimicked in the laboratory using induction heating as the internal source. In induction heating, however, heat sources are localized in the skin layer. Consequently, this concentration of heat may modify the flow and wall heat transfer compared to the case of homogeneous internal sources. DNS are carried out for three typical skin depths and three total deposited powers. Skin depth variations show surprising results regarding flow structures and heat transfer. It is found that the heat sources' heterogeneity has a weak effect on flow patterns. Consequently, models of heat transfer in the case of homogeneous sources remain valid even with strong localized heating near the bottom

High-temperature ex-vessel corium spreading. Part 1: Experimental investigations on ceramic and sacrificial concrete substrates.

International audienceOptimizing melt spreading in the aftermath of a core disruptive accident is crucial for achieving sufficient melt cooling to maintain reactor containment integrity. Two $\approx$ 30 kg-scale experiments performed at the VULCANO facility explore the spreading of high-temperature molten corium-concrete mixtures over ceramic and sacrificial concrete substrates. Imaging of the melt front propagation revealed a 7% increase in spreading length and a 30% increase in maximum front velocity during spreading over sacrificial concrete, despite a reduced mass partaking in spreading due to holdup within the crucible. Infrared imaging of the melt indicated surface temperatures around 45 °C lower during spreading on sacrificial concrete, which is anticipated to result in a roughly three-fold increase in melt viscosity. The reduced mass and elevated viscosity during spreading on sacrificial concrete imply an enhanced spreadability on concrete greater than the observed 7% increase in spreading length. This enhanced spreadability on sacrificial concrete could be explained by the apparent gliding motion of the melt, consistent with reduced friction at the melt-substrate interface. Reduced friction at the melt-substrate interface is best explained by a diphasic film of molten concrete and gaseous concrete decomposition products acting as a lubricant between the melt and solid substrate

Shock-to-detonation transition of nitromethane: Time-resolved emission spectroscopy measurements

International audienceThe objective of this work is to improve the knowledge of the shock-to-detonation transition of nitromethane. The study is based on a spectral analysis in the range 0.3–0.85 µm, with a 28-nm resolution, during experiments of plane shock impacts on explosive targets at 8.6 GPa. The time-resolved radiant spectra show that the detonation front, the reaction products produced during the superdetonation, and the detonation products are semitransparent. The temperature and absorption coefficient profiles are determined from the measured spectra by a mathematical inversion method based on the equation of radiative transfer with Rayleigh scattering regime. Shocked nitromethane reaches at least 2500 K, showing the existence of local chemical reactions after shock entrance. Levels of temperature of superdetonation and steady-state detonation are also determined

MOnitoring Outbreak events for Disease surveillance in a data science context

International audienceClimate change, animal and human mobility, growing populations, and urbanization increase the risk of emergence and spread of new pathogens. It is crucial to rapidly detect hazard emergence and assess the risk to public health through all the available sources of data. The MOOD project aims to develop innovative tools and services to answer these needs and monitor current and future infectious disease threats in the context of global, environmental, and climatic change. Through big data and disease modelling innovations, the MOOD project is addressing the challenges of cross-sectoral data sharing and valorization in a One Health framework based on multi-disciplinary collaborations for the animal, human, and environmental health. Co-creation of tools and services with human and veterinary public-health agencies, responsible for designing and implementing strategies to mitigate the identified risks, is at the core of MOOD innovation: the process started by a participatory users needs assessment, further defined with potential users during the development of tools through case studies. We are currently developing a platform to provide access to epidemic intelligence and disease surveillance practitioners to the MOOD outputs. It will have three main modules; a covariate access module where users can visualize and download relevant standardized covariates related to infectious disease emergence in support of risk assessment and modeling. The second is an epidemiological data visualization module where users can visualize and access data on current disease outbreaks, extracted from online media news using text mining, as well as their own collected data, on a GDPR compliant and secure local version of the tool. The last module will provide risk maps and other modelled outputs, aiming at highlighting areas suitable for the emergence of infectious diseases in animals and humans, to support improved disease detection, monitoring, and surveillance. In parallel, an evaluation of the influence of the level of co-creation on the uptake and effectiveness of MOOD tools and services is conducted through an integrated epidemiological and socio-anthropological approach. The long-term sustainability and societal impact of MOOD will be achieved through the creation of the MOOD Epi-Platform International Non-Profit Association (INPA), which will perpetuate the work of the MOOD project