3D static and time-dependent modelling of a dc transferred arc twin torch system

International audienceThe transferred arc plasma torch device consists of two electrodes generating a plasma arc sustained by means of an electric current flowing through the body of the discharge. Modeling works investigating of transferred electric arc discharges generated between two suspended metallic electrodes, in the so called twin torch configuration, are scarce. The discharge generated by this particular plasma source configuration is characterized by a complex shape and fluid dynamics and needs a 3D description in order to be realistically predicted. The extended discharge length that goes from the tungsten pencil cathode to the flat copper anode without any particular confinement wall and the fluid dynamics and magnetic forces acting on the arc may induce an unsteady behavior. In order to capture the dynamic behavior of a twin torch discharge, a 3D time dependent plasma arc model has been developed using a customized commercial code FLUENT form in both Local Thermodynamic Equilibrium (LTE) and non-LTE. A two temperature (2T) model has been developed taking into account only the thermal non-equilibrium effects in argon plasma. The main differences between LTE and 2T models results concern the increased extension of the horizontal section of the discharge and the predicted reduced (of about 60-80V) voltage drop between the electrodes when using a 2T model

Limpet Shells from the Aterian Level 8 of El Harhoura 2 Cave (Témara, Morocco): Preservation State of Crossed-Foliated Layers

International audienceThe exploitation of mollusks by the first anatomically modern humans is a central question for archaeologists. This paper focuses on level 8 (dated around * 100 ka BP) of El Har-houra 2 Cave, located along the coastline in the Rabat-Témara region (Morocco). The large quantity of Patella sp. shells found in this level highlights questions regarding their origin and preservation. This study presents an estimation of the preservation status of these shells. We focus here on the diagenetic evolution of both the microstructural patterns and organic components of crossed-foliated shell layers, in order to assess the viability of further investigations based on shell layer minor elements, isotopic or biochemical compositions. The results show that the shells seem to be well conserved, with microstructural patterns preserved down to sub-micrometric scales, and that some organic components are still present in situ. But faint taphonomic degradations affecting both mineral and organic components are nonetheless evidenced, such as the disappearance of organic envelopes surrounding crossed-foliated lamellae, combined with a partial recrystallization of the lamellae. Our results provide a solid case-study of the early stages of the diagenetic evolution of crossed-foliated shell layers. Moreover, they highlight the fact that extreme caution must be taken before using fossil shells for palaeoenvironmental or geochronological reconstructions. Without thorough investigation, the alteration patterns illustrated here would easily have gone unnoticed. However, these degradations are liable to bias any proxy based on the elemental, isotopic or biochemical composition of the shells. This study also provides significant data concerning human subsistence behavior: the presence of notches and the good preservation state of limpet shells (no dissolution/recrystallization, no bioerosion and no abrasion/fragmentation aspects) would attest that limpets were gathered alive with tools by Middle Palaeolithic (Aterian) populations in North Africa for consumption

The Skeletal Organic Matrix from Mediterranean Coral Balanophyllia europaea Influences Calcium Carbonate Precipitation

Scleractinian coral skeletons are made mainly of calcium carbonate in the form of aragonite. The mineral deposition occurs in a biological confined environment, but it is still a theme of discussion to what extent the calcification occurs under biological or environmental control. Hence, the shape, size and organization of skeletal crystals from the cellular level through the colony architecture, were attributed to factors as diverse as mineral supersaturation levels and organic mediation of crystal growth. The skeleton contains an intra-skeletal organic matrix (OM) of which only the water soluble component was chemically and physically characterized. In this work that OM from the skeleton of the Balanophyllia europaea, a solitary scleractinian coral endemic to the Mediterranean Sea, is studied in vitro with the aim of understanding its role in the mineralization of calcium carbonate. Mineralization of calcium carbonate was conducted by overgrowth experiments on coral skeleton and in calcium chloride solutions containing different ratios of water soluble and/or insoluble OM and of magnesium ions. The precipitates were characterized by diffractometric, spectroscopic and microscopic techniques. The results showed that both soluble and insoluble OM components influence calcium carbonate precipitation and that the effect is enhanced by their co-presence. The role of magnesium ions is also affected by the presence of the OM components. Thus, in vitro, OM influences calcium carbonate crystal morphology, aggregation and polymorphism as a function of its composition and of the content of magnesium ions in the precipitation media. This research, although does not resolve the controversy between environmental or biological control on the deposition of calcium carbonate in corals, sheds a light on the role of OM, which appears mediated by the presence of magnesium ions

Étude de l'oxydation homogène du cyclohexane en phase gazeuse. II. Mécanisme réactionnel et modélisation

Après avoir effectué une étude expérimentale détaillée de la réaction d'oxydation en phase gazeuse homogène du cyclohexane décrite dans un premier mémoire (première partie), il a été construit un schéma réactionnel détaillé s'appuyant sur les résultats expérimentaux obtenus et permettant de rendre compte des produits formés. Pour simplifier ce mécanisme, les constantes de vitesse des processus élémentaires ont été évaluées et, par comparaison de la répartition des produits et des précisions du modèle, un mécanisme réactionnel simplifié (une trentaine de processus élémentaires) a été obtenu. Une première simulation à partir de ce modèle a montré de sensibles divergences entre points expérimentaux et courbes calculées. Ceci a conduit, par des méthodes d'analyse de sensibilité, à identifier les processus déterminants, à ajuster les paramètres cinétiques et à compléter de manière limitée le mécanisme proposé. Le mécanisme final retenu et les valeurs ajustées des paramètres cinétiques ont conduit à un accord satisfaisant entre simulation et expérience, accord souvent meilleur que celui obtenu par d'autres auteurs dans le cas d'oxydation d'alcanes. Les possibilités et limites de cette méthode sont évoquées, en particulier l'utilisation de techniques de lumping en vue d'une simplification supplémentaire du mécanisme

Effets de parois dans la pyrolyse du methyl tert-butyl éther (MTBE)

L’étude de la pyrolyse du méthyl tert-butyl éther (méthyl-2, méthoxy-2 propane) vers 350-400 °C présente de substantielles difficultés, irreproductibilité en particulier, attribuées à d’importants effets de parois. Ceux-ci ont été mis en évidence par l’utilisation de plusieurs types de réacteurs (vides et garnis, traités et non traités). Pour pouvoir accéder à la part homogène de la réaction, il a été nécessaire de relever les températures d’expérimentation au-dessus de 425 °C et jusqu’à 490 °C et de former un dépôt sur les parois par pyrolyse de bromure d’allyle selon la procédure déjà proposée par Daly et Wentrup.Contrairement à ce qui pouvait être attendu, la pyrolyse homogène n’a pas lieu par un mécanisme radicalaire en chaînes, mais par une réaction d’élimination à 4 centres, en accord avec la formation largement prédominante de méthanol et d’isobutène. Les paramètres cinétiques de cette réaction ont été déterminés et comparés à ceux d’autres auteurs

Study of the oxidation of unsymmetrical ethers (ETBE, TAME) and tentative interpretation of their high octane numbers

This work aims at a better understanding, from the reaction mechanism point of view, of the antiknock effect of two unsymmetrical ethers: tert-amyl methyl ether (TAME) and ethyl tert-butyl ether (ETBE).An experimental investigation of the reactions of equimolecular mixtures of these ethers in the presence of oxygen shows that these ethers oxidize less easily than the alkanes of similar structures.Taking into account the distribution of the primary reaction products at ca. 300°C and by using the techniques of Thermochemical Kinetics due to BENSON et al. for the estimation of kinetic parameters, it has been shown that these ethers do not oxidize according to a “low” temperature scheme, like the neighbouring alkanes. Their reaction with O2 seems to have the characteristics of a thermal degradation and not those of a genuine oxidation. In line with the experimental results, it appears that in the co-oxidation of one of the above-mentioned ethers and a hydrocarbon, the effect of the ether is that of a “negative catalyst” transforming reactive radicals, chain carriers of the hydrocarbon oxidation, through a rather complex reaction scheme into unreactive ones, unable to propagate the reaction at a sufficient rate to trigger the auto-ignition reactions, responsible for the occurrence of knock phenomena

Lettre à la rédaction

Le propène inhibe fortement la pyrolyse de l'éthane : (A) C2H6 = C2H4 + H2Simultanément, le propène fait apparaître deux co-réactions :(B) C2H6 + C3H6 = 2 C2H4 + CH4(C) C2H6 + C3H6 = C2H4 + C3H8qui impliquent l'addition d'un atome libre H. sur le propène et des processus subséquents des nouveaux radicaux libres C3H7. ainsi formés. Il apparaît, en outre, de petites quantités de butène-1 et de pentènes-1 et 2, dues à la formation de radicaux libres C5H11 . (par addition de C2H5. sur C3H6) et à des processus radicalaires subséquents