Les systèmes hétérogènes « eau–zéolithe hydrophobe »: de nouveaux ressorts moléculaires

Pressure–volume isotherms have been determined for three heterogeneous ‘water–zeolite’ systems. The first two concern hydrophobic purely siliceous zeolites: silicalite-1 (F⁻) and zeolite β (F⁻); the third comprises a more hydrophilic commercial zeolite of the type ZSM-5. The P–V diagram for the water–silicalite-1 (F⁻) system is characterized by a plateau corresponding to the intrusion of water inside the pores of the solid. During the release the phenomenon is reversible. This system, which is able to accumulate and restore superficial energy, constitutes a molecular spring. For zeolite β, the P–V curve displays a plateau during the compression, but during the release, the phenomenon is not reversible. In that case, the system absorbs mechanical energy and acts as a bumper. The third system, based on the more hydrophilic ZSM-5 zeolite shows a linear isotherm without any plateau. These results open new applications perspectives in the field of the energetics for hydrophobic zeolites in contact with water. To cite this article: V. Eroshenko et al., C. R. Physique 3 (2002) 111–119     

De la microstructure du polymère à ses propriétés rhéologiques

We extend the previous work by Benallal et al. on the relationship between structure and rheological properties of linear polymer melts. The aim of this paper is to quantify the effect of the chemical structure on the viscoelastic properties. We show that these properties are governed by the monomer dimensions and the interaction energy. To cite this article: A. Allal et al., C. R. Physique 3 (2002) 1451–1458.     

Le pouvoir résolvant du système „objectif-émulsion“

Sans résumé

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Polarisation de la lumière diffusée par des particules agrégées fractales

Some of the dust particles present in the solar system are likely to be aggregates. The study of polarization of the light scattered by such aggregated particles is performed for the six main formation processes. The analysis of the results shows that an irregular and fluffy structure of the particles allows a better understanding of the light scattering observations.     

Influence des phénomènes d'hystérésis sur les propriétés hydriques de matériaux poreux: Comparaison de deux modèles de simulation du comportement thermohydrique de parois de bâtiment

Porous materials hysteretic moisture characteristics : influence on water content distributions in buildings walls. Comparison of two simulation models. Two models for determining the temperature and moisture distributions within a wall are presented. The first one is a direct application of the classical Philip and de Vries model. The second one, using the suction potential, is derived from Milly's researches and is more efficient because it allows the hysteresis phenomena to be taken into account. First, the different equilibrium and transfer parameters used in the models are reviewed. Indications are given on how they can be experimentally determined and results for two cement pastes and a cellular concrete are illustrated. Then, calculation results of water content distributions for the two models are presented for an homogeneous cellular concrete wall in steady environmental conditions. In that particular case, the two models estimations are in good agreement despite different numerical treatments and experimental procedures. Using Milly's model with cyclic boundaries conditions, a significant influence of hysteresis phenomena is clearly shown.     

MODICS. Généralisation de la méthode d'optimisation systémique aux systèmes thermiques avec échangeurs imposés

We present here the generalization of the systemic optimization method for complex thermal installations, based on the thermal integration, that was the subject of a previous publication in the October 1995 (n^o 406) issue of the Revue Générale de Thermique. The proposed method allows to distinguish component irreversibilities, that are linked to the internal functioning of a component alone, independently of the rest of the considered system, and systemic irreversibilities that can only be modified by changing the configuration of the system. It constitutes a tool for design engineers, the originality and the power of which are to give them systemic insights on the installation under study.The general method MODICS is obtained by mixing on the one hand the previously described method and on the other hand an analogous approach adapted to the case when heat exchangers are imposed. The article details the totality of procedures to follow to implement this method and gives various examples.     

Polarisabilités optiques moléculaires de dérivés benzèniques en solution déterminées par des mesures d'effet Kerr statique

Une expression théorique simplifiée de la constante de Kerr permet une comparaison cohérente des mesures d'effet Kerr statique et de diffusion Rayleigh dépolarisée. Cette comparaison ainsi que des mesures de moments dipolaires sont effectuées sur une série de composés benzèniques en solution diluée dans le tétrachlorure de carbone. La méthode d'exploitation des résultats expérimentaux conduit à la détermination du tenseur des polarisabilités optiques moléculaires α de ces composés. Un accord satisfaisant est obtenu en comparant la constante spécifique de Kerr S _k et l'anisotropie optique moléculaire γ², calculées à partir de ces polarisabilités, à leurs valeurs expérimentales.     

Comportement thermique d'un matériau de référence poreux soumis à des atmosphères de chauffe différentes

This study presents an analysis of the temperature evolution in a porous sample. Various situations have been examined: the porous solid is initially dry or water saturated, and the heating fluid is dry air or steam. This analysis showed experimentally the optimal phases of using steam to heat a material. Moreover, as humidity in the solid considerably modifies the heating kinetics, the study has also been carried out on a non-porous solid. The treatment of the temperature curves under an adimensionnal form shows a singular behaviour of those temperatures following the heating fluid nature and its temperature. Equations giving temperature versus time have been proposed. They are based on the definition of a time lag and a time constant, both being defined as a function of the single fluid temperature parameter. Finally, in the particular case of the non-porous solid heated by dry air, with additional approximations, it has been possible to check that the experimental time constant is very close to a fundamental time constant.     

Théorème du viriel et dynamique des supernovae

The use of the transformation groups leads to dynamical solutions of the nonlinear gravitational hydrodynamical equations. These solutions, combined with the virial theorem, generalise the Chandrasekhar's static relation between the total and the internal energies. The new relations describe a collapse, an expansion or the sequence collapse/expansion of the external layers of a star (case of a supernova, for example).     

Effet isotopique en relaxation magnétique nucléaire. Cas du système AX

La comparaison des vitesses de relaxation du proton dans les couples de molécules isotopomères ¹³C-¹HBr₃ et ¹²C-¹HBr₃, ²⁹Si-¹HCl₃ et ²⁸Si-¹HCl₃, C₆H₅ ⁷⁷Se-¹H et C₆H₅ ⁷⁶Se-¹H permet de déterminer indirectement le temps de relaxation spin-milieu des noyaux de ¹³C, ²⁹Si et ⁷⁷Se. Dans le cas de spins ½ couplés (A-X), l'analyse des courbes d'évolution des raies permet de préciser les mécanismes de relaxation qui interviennent. La méthode présente sur l'observation directe, l'avantage de la sensibilité du proton mais l'évolution des raies au cours du temps est assez compliquée. Toutefois, nous avons vérifié que les deux méthodes concordaient bien dans le cas du carbone 13 du bromoforme.     

Quelques problèmes posés par les découvertes de Pluton et de Neptune

This Note reviews the history of the discovery of the planets Pluto and Neptune. To cite this article: V. Kourganoff, C. R. Physique 4 (2003).     

Relaxation nucléaire de l'azote et du proton dans le cyanoacétylène liquide

On a étudié les temps de relaxation spin-réseau de l'azote et du proton du cyanoacétylène en phase liquide à différentes températures. La vitesse de relaxation du proton est décomposée en différentes contributions : spin-rotation, intra et intermoléculaire dipole-dipole. Les mouvements moléculaires de translation et de réorientation sont analysés en utilisant différents modèles (conditions aux limites de ‘glissement total’ et de ‘frottement total’, correction de Perrin …).     

Etude du désenfumage par extraction mécanique dans un local siège d'un feu: expériences en grandeur et simulations numériques par modèles de zones et de champs

This study regards conditions of control of fire-induced smoke in a 7-m × 7-m rectangular cell, for an ‘intermediate configuration’ of a volume larger than that of an apartment room but smaller than that of an entrance hall, of a fire source a little less powerful than ‘design fires’ considered in standard approaches, of a fan-powered exhaust rate a little higher than rates typically required in regulations.From case to case were experimentally modified the following parameters: ceiling height (3 m, or 6.4 m), source type and power (between 100 and 500 kW), exhaust volume flow rate (from a few tenths of m³.s⁻¹ to a few m³.s⁻¹). Each experiment was supposed to reach a steady-state configuration, which has proved more or less true in practice. Smoke stratification within the cell has proved less sharp for a fire source non located in the centre of the cell, and the mean temperature of gases in the exhaust inlet has proved lower than under the cell ceiling, which suggested a phenomenon of ‘plugholing’ (air mixing upstream). In spite of the weaknesses of the computer models, and of a certain unaccuracy of measurements, plugholing phenomenon could be recognized in simulation results yielded by the zone model (which were in a better accordance with test results for higher exhaust rates), since the field model gave rise to a better agreement between computed and measured results for lower heat release rates and/or higher rates of smoke exhaust.     

Phénomènes de glissement à l'interface liquide–solide

The problem of liquid–solid slip is described here, in a simplified manner. Today, several experiments have shown that substantial slip appears when a non-wetting liquid flows along a surface which is smooth on an atomic scale. This phenomena is characterised by a length, called the slip length, or Navier length, generally denoted by L_s. A number of experiments indicate that this quantity may be as large as several hundreds of nanometers. Numerical simulations also show the existence of slip in non-wetting conditions, but the corresponding lengths found here are much smaller than those found experimentally. A theory, based on the existence of a gas film of nanometre thickness has been proposed, but has not yet been experimentally confirmed. Experiments on this are difficult, and sometimes controversial. To cite this article: P. Tabeling, C. R. Physique 5 (2004).     

La filière thorium,une option intéressante pour le nucléaire du futur

To satisfy a significant part of the primary energy demand at the world level by 2050, fission nuclear energy production has to grow by a large factor (more than 10). Due to the natural limitations of Uranium reserves (at a reasonable price) such a development makes necessary the use of breeder reactors. A comparative study of such breeder reactors using Uranium or Thorium based fuels shows that the use of Thorium based fuel has clear advantages in terms of flexibility, of minor actinides contents in the fuel cycle as well as for the inventory and disposal radiotoxicities. To cite this article: J.-M. Loiseaux et al., C. R. Physique 3 (2002) 1023–1034.     

Étude des phénomènes de transferts thermiques dans les stérilisateurs à vapeur

Study of heat transfer phenomena in a vapor sterilizer. Steam sterilization occupies a preferential place in the hospital hygiene. Hospital infection designates any sickness contracted in a hospital, allocated to microorganisms which affected sick people or hospital staff. The research work aims at studying heat transfer phenomena in a vapor sterilizer. The interactions between the different modes of heat transfer confer great complexity to this problem. The development of calculation method, zonal method, allowed to predict the field of temperature and heat transfer to optimise vapor sterilizers. This work presents the results of experiments carried out on a cycle of instrument sterilization. The main motivations of this study are the droplets of water which remain pasted to the load at the end of the drying process.     

Étude numérique du modèle de Boussinesq de convection naturelle laminaire axisymétrique permanente dans un espace annulaire compris entre deux sphères

Authors study numerically the axisymetric steady natural convection in the annular space between two vertically eccentric spheres applying the Boussinesq approximation and an integro-interpolation finite volumes method. In the case of two isothermal concentric spheres their results agree with those of the literature. Also, they obtain correlations between the average Nusselt number and the Rayleigh number quasi identical to those publish by Chiu and Scanlan. When the internal sphere is heated by the application of a constant heat flux and eccentric as compared to the supposed external sphere isothermal, they propose correlations between the average Nusselt number and the Rayleigh number that depend on the eccentricity and on the Prandtl number. According to initialising conditions of calculation, the external sphere being isothermal, authors show that there exists a critical Rayleigh number beyond which the flow can be found unicellular or bicellular. This critical Rayleigh number depends on others parameters of the system as the eccentricity and the aspect ratio when the internal sphere is isothermal as well as subjected to a uniform constant heat flux density.     

Application de la méthode des ordonnées discrètes au transfert radiatif dans un milieu bidimensionnel gris á géométrie complexe

Application of the discrete method to the radiative heat transfer in a two-dimensional grey medium of complex geometry. This paper describes a new approach in determining the radiative intensity and the temperature fields in a semi-transparent medium enclosed in a two-dimensional cavity the boundary surfaces of which are uniformly grey and purely isotropic diffuse reflectors, with the help of a new combination of ray tracing, finite volumes and discrete ordinates method. Since the grid used can be unstructured, the technique is applicable to the calculation of radiative transfer in enclosures of complex geometry. The basic equations are given, followed by results for cases of simple geometry compared with the exact solutions and the treatment of other cases of more complex geometry. The method eliminates oscillations in the intensity field and yields accurate results.