Modélisation du changement de phase d'une goutte liquide entourée d'un gaz multi-constituant

Les écoulements constitués de gouttelettes liquides dispersées au sein d'un gaz multi-constituant en présence d'évaporation et de condensation apparaissent dans de nombreuses applications liées aux écoulements diphasiques. Cependant, les modèles existants traitent essentiellement le cas de l'évaporation dans le cadre de la combustion de sprays. Cela signifie que l'énergie est transférée du gaz chaud au liquide pour produire son changement de phase. Cette approche est non-symétrique car dans certaines situations, l'énergie est déjà stockée dans la phase liquide et une évaporation explosive (flashing) se produit comme conséquence de la goutte en surpression. Dans notre approche, un modèle de transfert de masse dans le cadre de gouttelettes est présenté et validé dans chacune des situations : évaporation, flashing et condensation. Il tient compte : - du couplage des diffusions de masse et de chaleur dans la phase gazeuse, - de la thermodynamique du gaz multi-constituant. - de la diffusion de chaleur à l'intérieur de la gouttelette liquide, permettant la considération du chauffage et du refroidissement de la goutte. Ces effets sont importants dans des situations telles que l'évaporation et le flashing. Le modèle résultant est un système non-linéaire algébrique constitué de trois équations. Il fournit ainsi la température d'interface, le débit massique de gaz et la fraction massique de vapeur d'eau dans le gaz déterminée à l'interface. Ces variables d'interface permettent le calcul des taux de transfert de masse des espèces, de mouvement et d'énergie apparaissant dans les modèles d'écoulements diphasiques moyennés en volume. Le couplage est réalisé entre le sous-modèle algébrique de transfert de masse et un modèle d'écoulement diphasique moyenné de type Baer et Nunziato (1986). Des résultats numériques sont présentés

A SAXS outlook on disordered carbonaceous materials for electrochemical energy storage

Ordered and disordered carbonaceous materials cover a wide range of the energy storage materials market. In this work a thorough analysis of the Small Angle X-ray Scattering (SAXS) patterns of a number of carbon samples for energy storage (including graphite, soft carbon, hard carbon, activated carbon, glassy carbon and carbide-derived carbon) is shown. To do so, innovative geometrical models to describe carbon X-ray scattering have been built to refine the experimental SAXS data. The results obtained provide a full description of the atomic and pore structures of these carbons that in some cases challenge more traditional models. The correlative analysis of the descriptors here used provide novel insight into disordered carbons and can be used to shed light in charge storage mechanisms and to design improved carbonaceous materials

Non-linear effects and Joule heating in I-V curves in manganites

We study the influence of the Joule effect on the non-linear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 by using the crystalline unit cell parameters as an internal thermometer in X-ray and neutron diffraction. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath are at the origin of the non-linearity observed in the I-V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.Comment: accepted in Journal of Applied Physic

Field dependence of the electronic phase separation in Pr0.67Ca0.33MnO3 by small angle magnetic neutron scattering

We have studied by small angle neutron scattering the evolution induced by the application of magnetic field of the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in a crystal of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$. The results are compared to magnetic measurements which provide the evolution of the ferromagnetic fraction. These results show that the growth of the ferromagnetic phase corresponds to an increase of the thickness of the ferromagnetic ''cabbage'' sheets

Assessing the reactivity of hard carbon anodes : linking material properties with electrochemical response upon sodium‐ and lithium‐ion storage

Hard carbon (HC) is the negative electrode (anode) material of choice for sodium‐ion batteries (SIBs). Despite its advantages in terms of cost and sustainability, a comprehensive understanding of its microstructure is not complete yet, thus hindering a rational design of high‐performance HC electrodes. In this study, rather than investigating how the precursor and synthesis method influence on the electrochemical properties of HC anodes, we examine the microstructure and surface chemistry of three optimized HC anodes obtained from different precursors by using different synthesis routes. The main goal is to evaluate the influence of the final materials properties (in their optimized state) on the electrochemical reactivity in lithium and sodium cells after a comprehensive structural characterization performed by means of X‐ray photoelectron spectroscopy (XPS), wide‐angle X‐ray scattering (WAXS), Raman spectroscopy, scanning electron microscopy (SEM), and gas sorption measurements. The different electrochemical performance observed in terms of cycling stability and rate capability, and the stability of the solid electrolyte interphase (SEI) formed on the various HCs have been comprehensively investigated. A correlation of the material properties with their electrochemical response upon sodium and lithium uptake and release is clarified. By comparing the Na‐ and Li‐ion storage behavior, a structure‐function relation is identified

Magnetic states and spin-glass properties in Bi0.67Ca0.33MnO3: macroscopic ac measurements and neutron scattering

We report on the magnetic properties of the manganite Bi_{1-x}Ca_{x}MnO_3 (x=0.33) at low temperature. The analysis of the field expansion of the ac susceptibility and the observation of aging properties make clear that a spin glass phase appears below T = 39K, in the presence of magnetic order. Neutron scattering shows both magnetic Bragg scattering and magnetic diffusion at small angles, and confirms this coexistence. In contrast to Pr_{1-x}Ca_{x}MnO_3 (x=0.3-0.33) which exhibits a mesoscopic phase separation responsible for a field driven percolation, the glassy and short range ferromagnetic order observed here does not cause colossal magnetoresistance (CMR).Comment: accepted in Phys Rev

Phase 3, Randomized, 20-Month Study of the Efficacy and Safety of Bimatoprost Implant in Patients with Open-Angle Glaucoma and Ocular Hypertension (ARTEMIS 2)

Objective- To evaluate the intraocular pressure (IOP)-lowering efficacy and safety of 10 and 15 µg bimatoprost implant in patients with open-angle glaucoma (OAG) or ocular hypertension (OHT). Methods- This randomized, 20-month, multicenter, masked, parallel-group, phase 3 trial enrolled 528 patients with OAG or OHT and an open iridocorneal angle inferiorly in the study eye. Study eyes were administered 10 or 15 µg bimatoprost implant on day 1, week 16, and week 32, or twice-daily topical timolol maleate 0.5%. Primary endpoints were IOP and IOP change from baseline through week 12. Safety measures included treatment-emergent adverse events (TEAEs) and corneal endothelial cell density (CECD). Results- Both 10 and 15 µg bimatoprost implant met the primary endpoint of noninferiority to timolol in IOP lowering through 12 weeks. Mean IOP reductions from baseline ranged from 6.2–7.4, 6.5–7.8, and 6.1–6.7 mmHg through week 12 in the 10 µg implant, 15 µg implant, and timolol groups, respectively. IOP lowering was similar after the second and third implant administrations. Probabilities of requiring no IOP-lowering treatment for 1 year after the third administration were 77.5% (10 µg implant) and 79.0% (15 µg implant). The most common TEAE was conjunctival hyperemia, typically temporally associated with the administration procedure. Corneal TEAEs of interest (primarily corneal endothelial cell loss, corneal edema, and corneal touch) were more frequent with the 15 than the 10 µg implant and generally were reported after repeated administrations. Loss in mean CECD from baseline to month 20 was ~ 5% in 10 µg implant-treated eyes and ~ 1% in topical timolol-treated eyes. Visual field progression (change in the mean deviation from baseline) was reduced in the 10 µg implant group compared with the timolol group. Conclusions- The results corroborated the previous phase 3 study of the bimatoprost implant. The bimatoprost implant met the primary endpoint and effectively lowered IOP. The majority of patients required no additional treatment for 12 months after the third administration. The benefit-risk assessment favored the 10 over the 15 µg implant. Studies evaluating other administration regimens with reduced risk of corneal events are ongoing. The bimatoprost implant has the potential to improve adherence and reduce treatment burden in glaucoma

Etude de la séparation de phase magnétique dans les manganites à effet CMR par diffusion de neutrons aux petits angles

This manuscript presents the study by small angles neutron scattering of the magnetic inhomogeneities from nanometric to mesoscopic scales at low temperature in CMR manganite compounds of the series Pr1-xCaxMnO3, x close to 1/3, and its evolution under applied magnetic field.These systems show a large scale phase separation between a ferromagnetic insulating (FI) phase and an antiferromagnetic insulating (AFI) one, corresponding to two different crystalline structures. They are transformed into a third crystalline phase, ferromagnetic metallic (FM), under an external magnetic field. We have tried to understand the mechanism of this transformation.We have shown the existence of magnetic nanometric inhomogeneities in both FI and AFI phases. Our study under field reveals the apparition of a strong SANS signal due to a nucleation of mesoscopic clusters (several hundreds of nanometers) of FM phase and a consequent removing of the SANS intensity of the nanometric objects. Consequently we can assert that the CMR effect is not due to a nucleation and growth of nanometric conducting clusters, but mesoscopic ones.Ce manuscrit présente l?étude par diffusion de neutrons aux petits angles des inhomogénéités magnétiques de l?échelle nanométrique à l?échelle mésoscopique à basse température dans les composés manganites à effet CMR de la série Pr1-xCaxMnO3, x proche de 1/3, et son évolution sous champ magnétique appliqué. Ces systèmes montrent une séparation de phase à grande échelle entre une phase ferromagnétique isolante (FI) et une phase antiferromagnétique isolante (AFI) correspondant à deux phases cristallines distinctes. Ils se transforment en une troisième phase cristalline, ferromagnétique métallique (FM), sous l?effet du champ magnétique. Nous avons tenté de comprendre par quel mécanisme. Nous avons mis en évidence l?existence d?inhomogénéités magnétiques nanométriques dans chacune des phases FI et AFI. Notre étude sous champ révèle l?apparition d?un fort signal de diffusion dû à une nucléation de clusters de phase FM mésoscopiques (quelques centaines de nanomètres) lors de la transition I-M induite par le champ, faisant ainsi disparaître la diffusion par les objets nanométriques. L?effet CMR n?est donc pas dû à une nucléation à l?échelle nanométrique mais mésoscopique

A simple HLLC-type Riemann solver for compressible non-equilibrium two-phase flows

International audienceA simple, robust and accurate HLLC-type Riemann solver for two-phase 7-equation type models is built. It involves 4 waves per phase, i.e. the three conventional right- and left-facing and contact waves, augmented by an extra “interfacial” wave. Inspired by the Discrete Equations Method (Abgrall and Saurel, 2003), this wave speed (uIuI) is assumed function only of the piecewise constant initial data. Therefore it is computed easily from these initial states. The same is done for the interfacial pressure PIPI. Interfacial variables uIuI and PIPI are thus local constants in the Riemann problem. Thanks to this property there is no difficulty to express the non-conservative system of partial differential equations in local conservative form. With the conventional HLLC wave speed estimates and the extra interfacial speed uIuI, the four-waves Riemann problem for each phase is solved following the same strategy as in Toro et al. (1994) for the Euler equations. As uIuI and PIPI are functions only of the Riemann problem initial data, the two-phase Riemann problem consists in two independent Riemann problems with 4 waves only. Moreover, it is shown that these solvers are entropy producing. The method is easy to code and very robust. Its accuracy is validated against exact solutions as well as experimental data

Etude de la séparation de phase magnétique dans les manganites à effet CMR par diffusion de neutrons aux petits angles

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF