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.     

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 %.     

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.     

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₊.     

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.     

Modélisation numérique des écoulements réactifs dans les foyers de turboréacteurs

Some assumptions have to be made to deal with combustion and aerodynamical phenomena simultaneously. We propose a turbulent combustion model where we consider a one step reaction for chemical modelling. Consequently, only two variables are sufficient to describe the problem. In fact, the combustion can be characterized by the consumption of one of the two reactive species. In a first step, to obtain the instantaneous consumption rate, we model the Lagrangian equation of the fuel mass fraction by considering only the equilibrium state. This state is calculated in order to obtain the same temperature as with a detailed kinetic scheme. In a second step, the mean consumption rate is calculated with the instantaneous consumption rate and a presumed probability density function. This model has been tested on many configurations, particularly, on a non-premixed flame and an experimental industrial combustor. Results from these validations show that this model can be used to predict temperature level in an industrial combustor.     

Ecriture de l'énergie cinétique à l'aide d'opérateurs quasi-moments

Cet article concerne l'utilisation des quasi-moments ?π _m , définis par

,

pour exprimer l'opérateur correspondant à l'énergie cinétique de N particules en Mécanique Quantique. La condition de Wilson-Howard portant sur les coefficients s^ml est interprétée comme la condition pour que les opérateurs ?π _m soient hermitiques quand on utilise l'élément de volume s dq ₁ … dq ₃ N (s=[dét {s^ml }]^-1). La condition générale pour qu'il soit possible de trouver un élément de volume avec lequel les opérateurs ?π _m sont hermitiques est donnée et différentes expressions de l'opérateur énergie cinétique sont établies quand cette condition est remplie et quand elle ne l'est pas.     

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.     

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.     

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.     

Stabilité des Moments Magnétiques Localisés dans les Métaux

On étudie le cas d'un état lié virtuel dégénéré de spin et d'orbite et on discute l'influence de la dégénérescence orbitale d'une part sur l'apparition de magnétisme de spin et d'orbite, d'autre part sur l'ordre des transitions magnétiques. Toute la discussion est faite dans l'approximation de Hartree-Fock en utilisant le formalisme d'Anderson. On trouve soit des solutions non magnétiques, soit des solutions magnétiques de spin avec le moment orbital bloqué, soit des solutions magnétiques de spin et d'orbite; dans ce dernier cas, la transition du cas non magnétique au cas magnétique de spin et d'orbite est généralement une transition du premier ordre analogue aux transitions rencontrées dans le diagramme de Clapeyron pour l'équilibre liquide-vapeur. La discussion générale dépend des valeurs respectives des intégrales de Coulomb et d'échange par rapport `a la largeur de l'état lié virtuel. Ce traitement peut ?etre étendu `a température finie: la température n'a en fait une grande influence que dans le cas o`u la largeur de l'état lié virtuel est de l'ordre de kT pour les températures usuelles, c'est-`a-dire dans le cas des métaux de terres rares.

Ce mod`ele s'applique aux trois cas physiques suivants: 1. (1) Il permet de clarifier le concept de niveau lié virtuel p et d : les états liés virtuels p ne sont pas magnétiques 7`a cause de leur grande largeur et les états liés virtuels d peuvent ?etre magnétiques de spin avec le moment orbital bloqué `a cause de l'importance des intégrales d'échange.

2. (2) Il permet de retrouver que le mod`ele ionique est valable pour les métaux de terres rares, `a l'exception du cérium et de l'ytterbium.

3. (3) Il permet d'expliquer le comportement anormal du cérium et de l'ytterbium. On peut obtenir théoriquement un diagramme de phase avec un point critique qui rend correctement compte du diagramme de phase anormal du cérium : la transition est du premier ordre en dessous de la température critique et du second ordre au dessus. On peut aussi expliquer les principales propriétés des deux phases cubiques faces centrées α et γ du cérium. Enfin, on donne une interprétation qualitative du diagramme de phase et des expériences de résistivité sous pression de l'ytterbium.

     

Pattern analysis of Békésy audiograms

A data-dependent method for the objective classification of Békésy audiograms is introduced. The technique uses orthogonal transformation from the pattern space to a reduced dimensionality feature space that allows a classification process to be implemented digitally. It is argued that this technique, with accretion of audiograms, could form the basis of a data-based system for determining and classifying the coherent structures of the various Békésy patterns with as few as possible a priori hypotheses made about the Békésy "types."     

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.     

Séparation physique de molécules chirales

One examines the order of magnitude of the speed of separation of optically active molecules bearing a permanent dipole under the effect of a rotating electric field. To cite this article: Y. Pomeau, C. R. Physique 3 (2002) 1269–1271.     

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.     

Etude numérique des jets turbulents à température élevée

Numerical prediction of the structure of high temperature axisymmetric turbulent jets. Turbulent axisymmetric jets at high temperature are studied numerically by using first and second order turbulence models. Regarding the temperature fields, on which we concentrate in this work, predictions with both types of models do not show large differences. In general, predictions agree well with the measurements; the existing differences are usually favorable for the second order model. The effect of solving a transport equation for the scalar dissipation rate on the prediction of the mechanical to scalar time scale ratio and on the prediction of the scalar fluctuations is studied. The influence of varying the density ratio on parameters such as the axial decay rates of the temperature and velocity and the turbulence intensity are studied. Two definitions of the mixing efficiency are introduced. According to both definitions, the mixing efficiency decreases with increasing effects of buoyancy.     

Contrôle de matériaux d'origine biologique par méthode photothermique: intérêt de la détection pyroélectrique

Photothermal methods are well adapted to thin multilayer material analysis. In the particular case of biological materials, which are thermally fragile, low excitation power is required. We have studied whole human blood sedimentation by using photothermal radiometry and a photopyroelectric technique. Results obtained with random- and sine-modulated excitation are discussed. Evolution laws of plasma thicknesses and optical absorption coefficients during the process have been determined by identifying the parameters using different estimation methods. It appears that results are consistent with theoretical predictions of our 2D-thermophysical model and also with the values measured by the well-known Westergreen reference method, usually used in clinical analysis. Finally, in order to extend the investigation field of the pyroelectric method, particularly in the domain of biological materials, we have analysed the possibility of using the sensitive pyroelectric sensors without direct thermal contact with the sample and detecting the temperature changes at the sample surface through a thin air monolayer.