Mechanical effect of absorption Carbon sequestration and swelling of coal

International audienceMechanical effect of absorption Carbon sequestration and swelling of coa

Comparison between classical potentials and ab initio for silicon under large shear

The homogeneous shear of the {111} planes along the <110> direction of bulk silicon has been investigated using ab initio techniques, to better understand the strain properties of both shuffle and glide set planes. Similar calculations have been done with three empirical potentials, Stillinger-Weber, Tersoff and EDIP, in order to find the one giving the best results under large shear strains. The generalized stacking fault energies have also been calculated with these potentials to complement this study. It turns out that the Stillinger-Weber potential better reproduces the ab initio results, for the smoothness and the amplitude of the energy variation as well as the localization of shear in the shuffle set

Dislocation formation from a surface step in semiconductors: an ab initio study

The role of a simple surface defect, such as a step, for relaxing the stress applied to a semiconductor, has been investigated by means of large scale first principles calculations. Our results indicate that the step is the privileged site for initiating plasticity, with the formation and glide of 60$^\circ$ dislocations for both tensile and compressive deformations. We have also examined the effect of surface and step termination on the plastic mechanisms

Poromechanics of Microporous Carbons: Application to Coal Swelling during Carbon Storage

International audienceCoal seams are naturally filled with natural gas. Enhanced Coal Bed Methane recovery (ECBM) is a technique which consists in injecting carbon dioxide (CO2) in coal seams in order to enhance the recovery of the methane (CH4) present in the coal seams. A major issue for the industrial development of this technique is the loss of permeability of the reservoirs during injection. In a coal bed, most of the transport of fluids occurs in a network of natural frac- tures. The loss of permeability is attributed to the closure of the fractures induced by the swelling of the coal ma- trix during the progressive replacement of CH4 by CO2. Since both fluids are mostly adsorbed in the microporous matrix of coal, this particular problem raises the funda- mental question of how adsorption impacts the mechanics of a microporous solid. In this work, we present a porome- chanical modeling valid for microporous solids under ad- sorption and we apply this modeling to the specific case of ECBM. The first section presents the theoretical derivation of general constitutive equations of poromechanics which are valid for generic pore sizes and morphologies. In the second section, we apply this general poromechanics to the specific case of CH4 adsorption in coal. We use molecu- lar simulations to calibrate the derived constitutive laws. In the third section we validate this calibration by analyz- ing results of adsorption experiments in unjacketed condi- tions. The fourth section is dedicated to the case of CO2 adsorption in coal. Finally in the last section, we use this modeling to predict the swelling of coal in the context of ECBM

Variations du volume pulmonaire au cours de la ventilation mécanique (modes ventilatoires et manœuvres positionnelles)

Le Syndrome de détresse respiratoire aiguë (SDRA) est une pathologie fréquente et grave. Son traitement fait appel à la ventilation mécanique qui est indispensable pour maintenir une oxygénation suffisante mais elle peut induire des lésions pulmonaires responsables d'une morbidité importante. Le volume pulmonaire est diminué au cours du SDRA ; sa mesure a longtemps été du domaine de la recherche clinique mais une technique de mesure au lit du patient a récemment été proposée : le lavage de l'azote. Ce travail avait pour but de tester la mesure du volume pulmonaire par la technique du lavage de l'azote dans des conditions d'utilisation comparables à celles du SDRA c'est à dire avec des niveaux de pression expiratoire positive (PEP) et de FiO2 élevés. Une fois cette étape préliminaire validée, nous avons utilisé la mesure du volume pulmonaire pour évaluer le recrutement induit par des réglages de PEP différents et lors de changements de position.Nous avons montré dans l'étude n1 que les mesures étaient suffisamment précises et reproductibles pour une utilisation clinique. La PEP et le niveau d'oxygénation influençaient peu les mesures. Dans l'étude n2, le recrutement induit par une PEP de type recrutement maximal réglée pour obtenir une pression de plateau (Pplat) entre 28 et 32 cmH2O, a été estimé à partir des mesures de volume pulmonaire comparées à l'augmentation minimale prédite du volume pulmonaire. Une bonne corrélation avec le recrutement mesuré par la technique des courbes pression - volume était trouvée sous réserve d'une élimination des mesures incohérentes.Parallèlement, la mesure du volume pulmonaire et l'oxygénation de patients en SDRA et Acute Lung Injury (ALI) ont été analysées lors de changements de position (Position demi-assise puis assise). Cette étude montre que le volume pulmonaire est augmenté lors de la verticalisation et particulièrement chez les patients augmentant leur oxygénation > 20%. Les patients ne répondant pas au positionnement avaient des volumes pulmonaires plus élevés et qui variaient peu.Les différents travaux réalisés ont permis de montrer la simplicité d'utilisation de la technique. Ceci offre des perspectives de recherche au lit du patient plus accessibles qu'avec les techniques de référence (scanner et dilution de l'hélium), et également des perspectives cliniques. Une approche de la déformation pulmonaire induite par la ventilation (strain) et potentiellement des lésions induites par la ventilation mécanique est rendue possible par la mise à disposition en clinique de cette technique.The acute respiratory distress syndrome (ARDS) is a frequent and severe form of acute respiratory failure. Mechanical ventilation is the cornerstone of treatment but it may induce a specific form of lung injury (Ventilator induced Lung Injury) responsible for superimposed morbidity and mortality. Lung volume is dramatically decreased during ARDS. Lung volume measurements remained limited to clinical research until recently when the nitrogen washout/washin technique has been adapted for bedside use and implemented in an intensive care ventilator. The aim of this work was to test the nitrogen washout/washin method in clinical conditions of ARDS treatment with high Positive End Expiratory Pressure (PEEP) and high oxygen fraction (FiO2). Once this preliminary validation study was realised, we used the technique to assess the amount of lung recruitment induced by PEEP and positioning.We showed in the first study that accuracy and reproducibility of the technique were acceptable. PEEP and FiO2 had a minor influence on measurements. In the second study, the recruitment induced by a maximal recruitment PEEP set to obtain a plateau pressure between 28-32 cmH2O has been estimated using end-expiratory lung volume (EELV) measurements. A significant correlation was found between the recruitment measured on Pressure/Volume curves and the recruitment estimated comparing the predicted minimal increase in lung volume and the true increase in EELV.In a third study, we have evaluated the concomitant effects of verticalization on EELV and oxygenation following a change from supine to semi recumbent, seated and back to supine position. In this third study, verticalization (seated position) resulted in a significant concomitant increase in lung volume and oxygenation. Interestingly, patients responding to verticalization had lower EELV at baseline than non-responders. Only the group of patients increasing their PaO2/FiO2>20% during verticalization had a significant increase in their EELV compared to non-responders.These three studies confirmed the feasibility of the technique, easier than the gold standard techniques (helium dilution or CT scan), and offering both research and clinical perspectives. This technique should also allow an easier approach of the strain induced by ventilation and assess the risk of ventilation induced lung injury.PARIS-EST-Université (770839901) / SudocPARIS12-Bib. électronique (940280011) / SudocSudocFranceF