Etude de l'énergie de désorption des substances polaires

Polar substance energy desorption study. This study suggests an extension of the Dubinin-Radushkevich and Gibbs-Helmholtz equations, by using adsorption forces in microporous composites. These forces, mainly those of Van der Waals, have been set up in three groups of components according to their effects :

• •unspecified forces,
• •dispersion-perturbation forces,
• •quadrupole and dipole interaction forces.

The suggested models take into account the different forces and present a good coherence with the experiment.     

Identification des propriétés thermiques de composites fibres de verre/résines thermodurcissables: Application à l'optimisation des procédés de moulage

Identification of glass fiber/thermosetting resins composites thermal properties. Application to the optimization of molding processes. A procedure has been developed to optimize molding processes of thermosetting composite materials. Three stages have been distinguished. In the first one, some thermal and kinetic properties have been measured by differential scanning calorimetry. Thermal conductivities have been identified afterwards in thick instrumented pieces, placed in a thermally regulated press. High dependences of thermal conductivities on temperature and transformation degree have been shown. Secondly, coupled heat transfers have been numerically simulated and results have been satisfactorily compared with experimental thermograms. Finally, optimization technics based on effective inverse methods have been used.These points have been illustrated with two examples : glass fiber/epoxy resin and glass fiber/polyester. Sufficient mechanical characteristics of the first one, which is cured in oven, and good surface aspect of the second, that is made by injection in heated and closed molds, had to be obtained. The results let foresee real improvement of the corresponding molding processes.     

Comparaison des fonctions de corrélation mono- et multimoléculaires des liquides et des solutions

Le théorème de corrélation microscopique-macroscopique correspondante est redérivé à l'aide de la théorie des groupes. La démonstration est fondée sur l'invariance de l'hamiltonien d'un échantillon de liquide sous l'effet des permutations des molécules identiques. Les conséquences et les limitations du théorème sont examinées en détail.     

Modélisation de séchoirs à tapis. Utilisation des réseaux de neurones

Modelling of convective layer dryers. Using neural networks. Dryer modelling is considered in this paper. A dryer scale approach is implemented in order to write the classical differential equations through parameters such as the heat transfer coefficient or drying kinetics. The behaviour of the dryers is described by a non-linear system which integrates these equations in a transfer network using the finite difference method. The finite difference method is easy to implement, but appears to be too slow for dryer designing. So, in the second part of the study, neural networks are used to model drying process in steady state. When applying neural networks method to the design of dryers, one of the main problems is to find necessary and sufficient inputs so that the neural networks can learn transfers laws. To reduce the problem, each output is defined by a single neural network and non-dimensional numbers are used. The following step deals with the determination of the number of neurones and the minimization of output error for each efficiency (change of training points). Then, neural networks are used to simulate different configurations of dryers. Results are compared with the finite difference method and an industrial application is studied in the last chapter.     

Confinement des radioéléments de haute activité et des combustibles nucléaires usés

This article gives a compilation of quantified data on high activity radiological products generated in Nuclear Power Plants since their beginning, comparing them to the capabilities of the main types of spent fuel storage, which are under development: i.e., glass canisters of fission products, after reprocessing, and Fuel assemblies themselves, for storage in ponds, and, more recently, for smaller quantities, under dry atmospheric conditions.The main conclusion underlines that today, quantities are large and increasing, and that industrial solutions other than ponds, do not exist at present and will not be implemented in the next twenty years, with the exception of dry centralized storage, based on the feed back experience which is now thirty years late compared to pond storage.Technical conclusions are, finally, drawn on the feedback knowledge and the continuous improvements implemented in the wet type of confinement system. To cite this article: J.P. Martin, W.J. Fournier, C. R. Physique 3 (2002) 867–877.     

Termes de signe positif et négatif en analyses exergétiques des flux de chaleur

Positive and negative terms in the exergetic analysis of heat fluxes. In various technical plants heat transfer rates at temperatures above that of the environment and heat transfer rates at temperatures below the temperature of the environment can be simultaneously observed. This is the case in cryogenic industrial processes where a cooling effect is produced from a heat source. This is also the case for all absorption refrigeration plants or absorption plants that provide refrigeration and heat pumping, in particular for air-conditioning of buildings. In these cases exergetic analysis leads to equations in which some terms are positive and others are negative. This can generate difficulties in the definition of exergetic, or rational, efficiencies. In this paper ways of resolving these difficulties are discussed.     

Etude des mouvements moléculaires en milieu liquide à partir du profil des bandes de vibration dans l'infrarouge et des fonctions de corrélation

We have studied the fundamental bands of chloroform and bromoform, both pure and in solution in various solvents; also the first harmonics of v ₁, and a few other harmonics and combination bands. The correlation functions of v ₁ and v ₂ and band moments of v ₁ have been calculated. The comparison of our results with those obtained in microwave and far IR and Raman spectra offers an opportunity to discern, in widths and correlation functions, what can be attributed to vibration and what originates in rotational diffusion. Our results are interpreted with the assumption that rotational diffusion is produced by small angles jumps and that the vibrational effect is very important. The v ₁ band shows an additional widening not accounted for in existing theories.     

Etude numérique des effets de densité dans un jet de mélange turbulent en microgravité

Numerical study of density effects on a turbulent mixing jet in microgravity. The aim of this paper is to present density effects on a turbulent heterogeneous mixing free jet in microgravity. The influence of the inlet density ratio S between the jet and the surrounding gas is pointed out from numerical simulations based on second order single point closure schemes. The mean quantities, entrainment, expansion and second order moments are presented. The density ratio varies from 0,14 to 5,11. Results are discussed in comparison with both experimental data and asymptotical considerations. It is shown that the density ratio effects on the jet development vanish far from the exit leading to self-similar profiles (full self-preservation according to George [1]).     

Dimérisation des sels de l'ion nitrosodisulfonate à l'état solide

La dimérisation de plusieurs sels du radical libre nitrosodisulfonate est étudiée par R.S.E. Les spectres d'échantillons polycristallins de deux formes cristallographiques des sels de sodium, potassium et rubidium, sont caractéristiques d'un état triplet accessible thermiquement. Ces spectres sont attribués à des paires de groupements NO radicalaires, couplés par échange et sont décrits par l'Hamiltonien de spin:

Les paramètres de déplacement en champ nul, les composantes du tenseur g le long des directions principales X, Y, Z du tenseur fin et la composante du tenseur A le long de l'axe Z (dans le cas où la structure hyperfine est résolue), ont été mesurés et discutés.

A partir de ces mesures, on montre que l'axe Z est dirigé suivant les orbitales 2p des atomes d'azote et d'oxygène, et que l'axe X est dirigé selon la liaison NO. Par suite, l'arrangement des deux groupements NO d'une paire est toujours sensiblement rectangulaire.

Dans le cas des deux formes cristallines du potassium et du rubidium, l'écart singulet triplet dépend de la température, ainsi que les paramètres de déplacement en champ nul, ces variations avec la température sont attribuées à l'expansion thermique du réseau.

La présence d'une structure hyperfine bien résolue dans certains de ces sels, et la forte anisotropie des largeurs de raies dans les autres sels, suggèrent que les excitations sont fortement localisées, et que si elle sont mobiles leur fréquence de saut est inférieure à 10⁷ Hz.     

Méthode d’épuration des gaz rares au moyen de décharges électriques de haute fréquence

Raw krypton and xenon gases obtained from the distillation of air contain impurities such as CF₄ and CH₄, which preclude their use in many applications. These impurities can be abated by having the gas circulating through a microwave-sustained electric discharge. The use of this technique for production proves to be beneficiai in terms of energy consumption, reduction of gas losses, easiness and safety of operation. Plasma purification is therefore an useful extension of the range of available technologies for the design of high performance pure krypton/xenon production plants. It further demonstrates the feasibility and interest of achieving selective chemistry in a plasma that is not in thermodynamic equilibrium.     

Sur la remanence de la magnétisation par déplacement des parois de Bloch et la cinétique de sa desaimantation

Sans résumé

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Essai d'interprétation des déplacements et élargissements par la pression des raies du spectre de vibration-rotation des molécules diatomiques

An interpretation is given of the dependence on the rotational quantum number J of the pressure shift and broadening of the lines of the vibration-rotation band of a hydracide molecule perturbed by a noble gas. It is shown that, with the fundamental hypothesis of adiabatic collisions between the absorbing molecule and the noble gas atoms, the impact theory can describe the J dependence. This results from the fact that one introduces the average over the sine of the phase shift η, which is J dependent, and not the average of the phase shift itself which is vanishing. With a hard-sphere model good agreement is obtained between the calculated and the observed shifts. Further, a rough estimation of the breadth is made.     

Modélisation d'un échangeur de chaleur compact à courants croisés séparés par des couches de matériau à changement de phase

A mathematical model was developed to predict the thermal behaviour of a crossflow compact heat exchanger with layers of phase change material (PCM) operating under winter conditions. The model is validated and a parametric study is conducted to determine the design and operating conditions that promote condensation and may lead eventually to the frosting of the exchanger. The transient and steady-state thermal efficiency of the exchanger are also predicted as a function of the number of thermal units NTU_f, of the ratio of the products of the mass flow rate by the heat capacity on the cold and hot sides C and of the Biot number Bi. Results indicate that for Bi = 0.6 (which corresponds to a 3 mm thick PCM layer), condensation occurs when NTU_f ≥ 2.0 for C = 1.0 and when NTU_f ≥ 5.0 for C = 0.5. If the thickness of the PCM layer increases, condensation is avoided and the duration of the heat recovery period is prolonged. At the same time however, the steady state thermal efficiency diminishes. A heat exchanger for which Bi = 0.6, C = 0.5 and NTU_f = 4.0 appears to be a good compromise for acceptable heat recovery and efficiency. In this case, the heat recovery period lasts 1.6 h and the steady state thermal efficiency levels off at 64 %.     

Étude théorique des transferts couplés de chaleur et de masse lors de la désorption du couple zéolithe 13X-eau

A mathematical model based on scale changing has been established, in order to study heat and mass transfers in a zeolithe bed during water desorption. The model has been solved numerically by the finite-volume method, for the case of two-dimensional transfers. The numerical simulation is used to present the time-space evolution of the solid temperature, the pressure and the moisture content in the reactor and to determine the sensitivity to some parameters (heating temperature, outlet temperature, heat coefficients and reactor geometry). The effects of the particle diameter, the kinetic constant of desorption as well as the effective thermal conductivity are discussed.     

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.     

Efficacité des techniques de régénération d'eau pour une usine de fabrication de papier intégrée sans effluent. Analyse exergétique

Water regeneration efficiency for pulp and paper plant. Exergy analysis. The water network of a pulp and paper mill is contaminated by a large number of components which are brought into the system as wood constituants or process additives. Keeping the contaminant concentrations below certain limits, to safeguard paper quality and process runnability, must be part of any systems closure strategy. This can be achieved by purging contaminants using well known separation techniques such as evaporation, freeze crystallization and membrane filtration. The purpose of this study was to compare the thermodynamic efficiency of these techniques by means of an advanced exergy analysis. The contaminated stream was modelled using key components for both organic and inorganic contaminants. The activity coefficients were calculated using the Chen algorithm. The approach developed by Brodyanski, Sorin and Le Goff was used to determine the exergy efficiency. This analysis shows that freeze crystallisation is the most efficient technique when the exergy of the output stream is considered as a useful effect. When only the useful work obtained by separation is considered, membrane separation becomes the most efficient process. The analysis of the process incorporating a separation operation produces the actual efficiency. This example illustrates the advantage of using exergy methods to determine the efficiency of processes based on different physico-chemical phenomena.     

Une nouvelle méthode calorimétrique périodique de mesure de l'émissivité des matériaux opaques à température ambiante

This paper presents a new calorimetric periodic technique for measuring emissivity of opaque materials at room temperature, without surface temperature measurement. The sample is in a vacuum chamber, so the thermal losses are only radiative. The presented technique requires thermal modulation of one side of the sample (the front side). The measured signals are the sample's front side temperature and the infrared flux of the other side (back side). Experimental data and a heat conduction model are compared, yielding the Biot number which characterizes the thermal losses. Using the identified value of the Biot number, it is possible to access the total hemispherical emissivity. Measurements have been carried out on a PVC sample (5 mm thick) coated with black paint and on a PVC sample coated with aluminium paint. The results are concordant with the expected ones ; the repeatability error is about 3 %.     

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.