Search

Mass transfer occurrence is investigated in the current manuscript. At the beginning the geometry that is used for the simulation of the current study is explained and shown. The effect of turbulence behavior on mass transfer is investigated and explained.

International audience; Secondary refrigeration is one of the currently available means for reducing refrigerant leaks in large cooling units. Compared to the well-known ice slurry, hydrate slurries present new features, especially when they involve a gaseous phase, the pressure of which influences the fusion temperature. The clathrate-hydrate system considered herein is the mixed hydrate of CO2 plus tetra-n-butyl-phosphonium-bromide (TBPB) melting around 12°C. The concern of energy efficiency focuses attention on the total energy consumption of the process (herein air-conditioning), and therefore includes the primary cooling unit into the analysis. The present issue is: is very slow phase-change kinetics (compared to the breakthrough time of the slurry in the heat exchangers) problematic with respect to the energy consumption of the whole process? The equations of phase change in the heat exchangers are modified for including a first-order kinetics law characterized by a time constant and using the temperature shift from equilibrium as the driving force. The mixed hydrate is compared to the ice slurry. The results are unexpected but fully explained by a thermodynamic analysis.

International audience; We report a new synthesis route towards polycrystalline Ba(2)FePnSe(5) (Pn = Sb, Bi) chalcogenides based on ball milling and subsequent spark plasma sintering. The polycrystalline samples were characterized by transmission electron microscopy and powder X-ray diffraction. We discovered that both compounds undergo rapid, radiation induced phase changes between a crystalline and an amorphous phase. Thin layers of amorphous Ba(2)FePnSe(5) (Pn = Sb, Bi) were obtained by applying 0.3 ms laser pulses (lambda = 1064 nm) on the surface of polycrystalline samples. By using a series of subsequent laser pulses at room temperature the surface of Ba2FeBiSe5 can be reversibly changed between amorphous and crystalline states. The thermal conductivity can be used to follow the amorphization and recrystallization processes as a function of laser pulses. Resistivity measurements and calculations of the electronic structure revealed band gaps of 1.04 eV (Pn = Bi) and 1.14 eV (Pn = Sb). We found anomalies in the thermal conductivity of Ba(2)FePnSe(5) (Pn = Sb, Bi) at the Neel temperature due to the antiferromagnetic ordering of the Fe spins. We also show that the distortion of the PnSe(6) octahedra is caused by stereoactive 5s(2) and 6s(2) lone pairs of Sb and Bi. (C) 2017 Elsevier B.V. Ail rights reserved.

Below the melting temperature Tm, crystals are the stable phase of typical elemental or molecular systems. However, cooling down a liquid below Tm, crystallization is anything but inevitable. The liquid can be supercooled, eventually forming a glass below the glass transition temperature Tg. Despite their long lifetimes and the presence of strong barriers that produces an apparent stability, supercooled liquids and glasses remain intrinsically a metastable state and thermodynamically unstable towards the crystal. Here we investigated the isothermal crystallization kinetics of the prototypical strong glassformer GeO2 in the deep supercooled liquid at 1100 K, about half-way between Tm and Tg. The crystallization process has been observed through time-resolved neutron diffraction for about three days. Data show a continuous reorganization of the amorphous structure towards the alpha-quartz phase with the final material composed by crystalline domains plunged into a low-density, residual amorphous matrix. A quantitative analysis of the diffraction patterns allows determining the time evolution of the relative fractions of crystal and amorphous, that was interpreted through an empirical model for the crystallization kinetics. This approach provides a very good description of the experimental data and identifies a predator-prey-like mechanism between crystal and amorphous, where the density variation acts as a blocking barrier.

We propose a level-set model of phase change and apply it to the study of the Leidenfrost effect. The new ingredients used in this model are twofold: first we enforce by penalization the droplet temperature to the saturation temperature in order to ensure a correct mass transfer at interface, and second we propose a careful differentiation of the capillary interface with respect to a moving interface with phase change. We perform some numerical tests in the axisymmetric case and show that our numerical method, while not avoiding well known numerical caveats of diffuse interface methods, behave quite well in the limit of numerical interface width going to zero in comparison to an analytical formula.

Poly(lactide), a bio-based aliphatic polyester, is a subject to large research effort. One point of optimization is the acceleration of its crystallization kinetics to promote crystallinity under nonisothermal polymer processing conditions by means of compounding with nucleating agents and plasticizers. The nonisothermal crystallization kinetics of neat and formulated poly(L,D-lactide) (PDLLA) from the melt with talc and polyethylene glycol (PEG) or acetyl tributyl citrate (ATBC) were studied with the help of the AvramiJeziorny and LiuMo analysis. Talc showed to be a moderately efficient nucleating agent, as it causes only small increase of crystallization kinetics and shows no effect on the crystallization activation energy. A synergistic effect with plasticizers was observed, expanding the crystallization window significantly. PEG was found to be a more efficient plasticizer than ATBC but causes large decrease in the molecular weight average of PDLLA upon thermal treatment. The talc/ATBC system is efficient starting with an ATBC concentration of 9 wt%. The acceleration observed was a crystallization half-time decrease of 30% compared to neat PLA and reaching maximum crystallization enthalpies even at cooling rate of 25 degrees C min1. The ATBC/talc system can be recommended as an efficient system for acceleration of nonisothermal crystallization kinetics of PDLLA. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers

Modeling heat and mass transfer in two-phase flows with phase-change is crucial in many industrial studies including nuclear safety. Only averaged two-phase flow models can simulate such complex flows. Their accuracy depends in particular on closure laws for interfacial mass, momentum, and energy transfers that often rely on experimental correlations. Supporting averaged models, the goal of this thesis is to bring local closure information from finer simulations to consider 3D-effects and interfacial transfers more accurately. In this prospect, as direct resolution of the local balance equations is too expensive, we seek for a two-phase equivalent of Large Eddy Simulation (LES) in order to tackle simulations with enough bubbles to extract statistics needed in averaged models. Applying a spatial filter, we aim at modeling subgrid turbulence and interfacial transfers. The largest turbulent scales and interface deformations are captured because the filter size is chosen in-between the Kolmogorov scale and the bubble size. Because of the importance of the phase-change phenomena, this thesis extend the Interfaces and Subgrid Scales (ISS) model proposed by Toutant et al. (2009a) to non-material interfaces, i. e., with phase-change. Explicit interface tracking is valuable to accurately estimate interfacial transfers such as the phase-change rate. In the first part of this document, we establish a smeared-interface description of two-phase flows. Sub-grid transfers and interfacial deformations are modeled using the modified Bardina et al. (1983) scale-similarity hypothesis. Main modeling issues comes from (i) the specific interfacial velocity, (ii) the velocity and the temperature gradient discontinuities at the interface and (iii) the saturation condition of the interface. Models are validated using reference data from DNS. In the second up-scaling step, we transform interfacial subgrid models into source terms in the jump conditions in order to establish an equivalent discontinuous model thus benefitting from the knowledge acquired in DNS numerical methods. Transfers between phases are modified and the interfacial velocity is redefined considering the time evolution of curvature and the velocity jump at the interface. As a result, the normal momentum jump is modified. A tangential velocity jump is also introduced to cancel out the sub-resolution of the boundary layer. From a thermal point of view, the classical saturation condition is recovered ; the phase-change rate not only depends on the conduction heat flux but a contribution from the subgrid velocity and temperature correlations must be added to account for the poor resolution of the thermal boundary layer. As for single-phase LES, ISS modeling enables local-scale simulations of industrial configurations. It is the first step of a multi-scale approach towards turbulent bubbly flows. In this thesis, we illustrates how to bridge the gap between DNS and averaged descriptions from reference results obtained on condensing bubbles in a pseudo-turbulent subcooled liquid. Averaged quantities are compared with correlations for the condensation sink term used in the two-fluid model. We are able to underline the phase-change enhancement with increasing void fraction. It shows that this path could be used to improve the understanding of the strong two-way coupling between flow dynamics and interfacial heat transfers., La modélisation des transferts thermiques en écoulements diphasiques est l'une des pierres angulaires de l'étude de la sûreté des réacteurs nucléaires. À l'échelle du réacteur, elle repose sur des corrélations expérimentales. L'utilisation croissante de la mécanique des fluides numérique pour les études de sûreté renforce la demande d'expertise dans les outils de simulation, en particulier du point de vue de la modélisation. En soutien aux modèles moyennés à deux fluides, nous souhaitons apporter des informations de fermetures locales pour considérer la physique des transferts interfaciaux et les effets 3D. Pour cela, comme la résolution directe des équations de bilan locales par SND est trop coûteuse, nous souhaitons développer un outil de SGE diphasique pour modéliser les petites échelles turbulentes et les petites déformations interfaciales. Comme le changement de phase est à l'origine de l'écoulement diphasique pour les applications visées, nous étendons dans ce mémoire le modèle Interfaces and Subgrid-Scales (ISS, Toutant et al., 2009a) aux interfaces avec changement de phase, pour lesquelles l'hypothèse de continuité de la vitesse à l'interface n'est plus valable. Le suivi explicite des interfaces permet d'évaluer précisément les transferts comme le taux de transfert de masse. Dans un premier temps, nous établissons une description mésoscopique du problème où l'interface est diffuse en filtrant les équations locales instantanées et en modélisant les transferts sous-filtres aux interfaces. Les principales difficultés de modélisations proviennent (i) de la détermination de la vitesse de l'interface, (ii) de l'effet de la discontinuité des vitesses sur les modèles sous-maille, (iii) de la discontinuité du flux et (iv) de la condition de saturation de l'interface. Les modèles proposés sont qualifiés a priori en observant leur prédiction par filtrage explicite de solutions de SND. Dans un deuxième temps, nous établissons un système macroscopique discontinu équivalent au problème diffus pour bénéficier de l'expertise acquise pour les méthodes numériques de SND. Aux interfaces, les modèles sous-maille sont concentrés pour modifier les conditions de raccord entre les phases. Les conditions de saut ainsi déterminées montrent que la vitesse de l'interface est affectée par la courbure et par le saut de vitesse. Un saut de vitesse tangentielle est introduit pour modéliser la couche limite dynamique. Sur le plan thermique, nous retrouvons la condition de saturation caractéristique du changement de phase ; le taux de changement de phase ne dépend plus uniquement du saut de flux conductif mais, pour pallier la sous-résolution de la couche limite thermique au voisinage de l'interface, nous proposons de lui ajouter la contribution sous-maille des corrélations vitesse/température. Comme en SGE monophasique, le gain apporté par la modélisation ISS permet d'envisager l'utilisation de simulations fines pour des problèmes appliqués. C'est la première étape d'une démarche multi-échelle pour fournir des fermetures aux modèles moyennés à deux fluides. Nous illustrons son potentiel sur une SND multi-bulles complexe.

International audience; Zn(II) L- and DL-pidolates diaqua complexes dehydrate at around 407 K, leading to amorphous anhydrous complexes. Only amorphous anhydrous Zn(II) L-pidolate was found to crystallize on heating into a crystalline anhydrous phase whose crystal structure was solved from a high-resolution X-ray powder diffraction pattern. Orthorhombic anhydrous Zn(II) L-pidolate exhibits a structure in which (4 + 2)-coordinated Zn atoms (with four usual and two additional Zn-O distances of 2.53 and 2.69 Å) and L-pidolate ligands alternate so as to form a three-dimensional polymerized network. Room-temperature rehydration processes under saturating water vapor for amorphous and crystalline anhydrous complexes were found to be different from each other, although they all led back to crystalline diaqua complexes whose relative stabilities were inferred from measurements of their solubilities in water.

International audience; This paper treats a one dimensional phase-change problem, 'ice melting', by a vertex-centered finite volume method. Numerical solutions are obtained by using two approaches where the first one is based on the heat conduction equation with the fixed grid, latent heat source approach (LHA), while the second uses the equivalent thermodynamic parameters defined by considering the apparent heat capacity method (AHC). A comparison between the two approaches is presented, furthermore the accuracy and flexibility of the numerical methods are verified by comparing the results with existing analytical solutions. Results indicate that one phase-change problems can be handled easily with excellent accuracies by using the AHC method.

Funding-Acknowledgement = CNM Barcelona (Spain) ; Franck Torregrossa from IBS-Rousset (France) Funding-Text = We greatly thank Philippe Godignon from CNM Barcelona (Spain) and Franck Torregrossa from IBS-Rousset (France) for providing us with the implanted samples investigated in this work. We also acknowledge Pierre Valvin and Philippe Arcade from GES for expert assistance with the irradiation and measurement procedures. Finally we acknowledge Marcin Zielinski and Servane Blanque from GES for performing SIMS and Raman measurements. One of us (D.J.B.) also acknowledges the CNRS and NRF for supporting grants during his stays in Montpellier; International audience; We report on a quantitative analysis of the effect of visible light laser irradiation (VLLI) on hexagonal (alpha) silicon carbide implanted with nitrogen and aluminum. In both cases of 4H and 6H polytypes we show that a short, but intense, irradiation with the 532 nm wavelength of a frequency-doubled neodymium: ytterbium aluminum garnet (Nd:YAG) laser results in a substantial reduction in the damage level produced by room temperature ion implantation. Up to now the recovery could not be made complete but, in the best conditions, it could reach similar to 80\% of the initial damage value. This is not enough to qualify VLLI as a full activation step but, rather, suggests to use it as a new processing tool in order to lower the constraints of high temperature ion implantation or, after implantation performed at room temperature, to reduce the total budget for high temperature annealing and activation steps.

http://link.aip.org/link/?APCPCS/712/39/1; International audience; The development of finite element simulation of material forming processes started about 30 years ago in academic laboratories, while the introduction of the corresponding commercial computer codes in industry is less than twenty years old. The main mechanical integral formulations for solid or viscous liquids are briefly recalled: classical Eulerian, Eulerian with a characteristic function, updated Lagrangian and arbitrary Euler Lagrange, with some comments on the finite element discretization using a mixed formulation and mini tetrahedral elements. The crucial remeshing issues are analyzed for non steady-state processes with different levels of sophistication: Updated Lagrangian for solids, or Euler and a characteristic function, possibly combined with error estimation and adaptivity. As real problems are usually very complex in industry, we must consider different levels of coupling such as thermal and mechanical coupling with the tools in forging and gas - liquid - solid coupling as in polymer foaming. In an attempt to model microstructure evolution of the work-piece during the different stages of forming, three approaches are reviewed. In the first example the physical evolution of metal is described by macroscopic parameters and their laws of evolution, while the second one is based on a finite element modeling of the two-phase material at the microscopic level. Finally the third case is the presentation of a new approach of polymer crystallization during injection molding and its introduction in a computer code