Procédure rapide de choix de réseaux d'échangeurs de chaleur

A fast calculation of a reasonable solution for the choice of a heat exchanger network able to make some imposed cooling and warming up tasks, with the use of auxiliary utilities, is developed in this article.This problem boils down to the internal heat exchanger network choice with respect to a constraint limiting the exchange area, maximizing the heat recovery and minimizing the number of stream matching.These internal matches are made one by one with respect to the tasks to be effected at each step. Amongst the possible matches, we have retained the privilegied ones, ie matches saturating a maximum number of constraints. Some procedures are implemented so as to choose among this set of privilegied matches.The illustration with some literature problems shows that an algorithm made up with these procedures carries out a good compromise between the quality of the obtained solution and the computer time.     

Quelques nouvelles propriétés de régularité de l'opérateur de Gribov

In this work, we establish new regularity properties for Gribov's operator:H=A ^* A ₊ iA ^*(A+A ^*)A;(,)², whereA ^* andA are the creation and annihilation operators. Particularly, we prove that for all >0,H ^–1 is in the class of Carleman's operatorl ₁₊.     

Instabilités de calibration de phase

In very long baseline interferometry, the image reconstruction procedures are very sensitive to the potential instabilities of the phase calibration operation. The analysis presented in this Note reveals that these instabilities are due to the existence of secondary minima (more or less critical) of the phase calibration objective functional. By resolving the corresponding integer ambiguity problems in an appropriate manner, these minima can be explicitly identified and compared. The stability of the self-calibration procedures can thus be easily controlled. To cite this article: A. Lannes, C. R. Physique 4 (2003).     

Etude expérimentale de la dévolatilisation rapide de charbons pulvérisés

Experimental study of the rapid devolatilization of pulverized coals. Rapid devolatilization of various pulverized coals have been studied in a laboratory bench constituted by a flat flame burner of propane which reproduces thermal conditions of an industrial flame. The particles, which undergo a heating rate of 6.10⁶ K⁻¹.s⁻¹ with a peak temperature of 1 100 °C, are completely devolatilized within 24 ms. Fifteen coals, included in a wide range (anthracite to subbituminous coal) have been tested. The coal weight loss is globally proportional to the normalized volatile matter content with some exceptions which confirm the advantage of this laboratory bench. The formation of tars or hydrocarbons has been related to the coal weight loss. Carbon, hydrogen and nitrogen devolatilized fraction have been followed with total mass weight loss. In spite of the fact that hydrogen and carbon devolatilized fractions present a good correlation with the total mass weight loss, the nitrogen devolatilized fraction have an anarchic evolution. For high heating rates, the devolatilization of tars observed for the bituminous coals seems to explain this unpredictable phenomenon. These results will be valorized in comparison with those obtained in industrial flame conditions.     

Etude théorique de la durée de vie radiative du triplet métastable de quelques composés carbonylés

Les paramètres de phosphorescence (force d'oscillateur et durée de vie), sont déterminés théoriquement pour quelques composés carbonylés (formaldéhyde, benzaldéhyde, acétophénone, propiophénone, 2,4,5-triméthylbenzaldéhyde, et 3,4-diméthylacétophénone). La fonction d'onde de l'état singulet fondamental est obtenue à l'aide d'un calcul SCF utilisant la méthode CNDO/S de Del Bene et Jaffé. Les fonctions d'onde, les énergies et les moments de transition dipolaire électrique S_I ←S ₀ et T_J ←T ₁ sont déterminés à l'aide d'une interaction portant sur des configurations mono et diexcitées par rapport à l'état singulet fondamental.

Les paramètres de phosphorescence sont calculés en utilisant une approximation monocentrique et monoélectronique de l'opérateur hamiltonien de couplage spin-orbite. Cette approximation est justifiée par la comparaison des résultats obtenus avec les valeurs expérimentales, compte tenu de la concordance entre les conditions d'expérience et les hypothèses de calcul.     

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.     

Développement de lasers visibles pompés par diode à base de cristaux de Nd:GdCOB autodoubleurs de fréquence

We present the performances of a diode-pumped solid-state laser operating in the visible by self-frequency-doubling. A cw output power of 115 mW at 545 nm has been obtained in a stable concave–concave cavity by using a crystal of Nd:GdCa₄O(BO₃)₃ pumped by a 2 W high-brightness laser diode. In a plano–plano cavity, similar to a microchip laser, we have obtained, for the first time to our knowledge, an output power of 22 mW in the green.     

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.     

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.     

Étude phénoménologique de l'ébullition en espace confinéà partir d'un site de nucléation isolé

The aim of the present work is to identify the mechanisms of heat transfer enhancement during natural convective boiling in confined spaces. The influences of gap-size (0.3 to 2 mm), of pressure (1 to 3 bar), of heating surface orientation (vertical or horizontal) and that of the number of nucleation sites (no site, single site or several sites) for boiling R-113 in a narrow channel are investigated. Results for unconfined boiling are also presented as a reference. It is shown that latent heat transfer is improved when the bubbles are squeezed against the heated wall. The heat transfer enhancement is greater for a vertical than for a horizontal orientation because the two-phase fluid velocity is increased when the gap-size decreases, which enhances the sensible heat transfer. Finally, the confinement reduces the effect of an increase in pressure, which limits the influence of confinement. Indeed, both parameters lead to contrary effects on the bubble diameter.     

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.