Modèle correctif de la mesure de la température d'un gaz par couple thermoélectrique. Application à une zone à fort gradient thermique

Corrective model of the gas temperature thermocouple measure. Application to an important thermal gradient zone. The gas temperature measurement with a thermocouple in important thermal gradient zones requires a corrective model. For example, such zones exist for thermal boundary layers near active walls. We have calculated these thermocouple thermal exchanges where connection wires are considered as fins with a variable ambient gas temperature. The heat exchanges by convection and radiation on the thermocouple head are analytically calculated, then a numerical method is used for fins where the space increment is the same as for the experimental measure. The corrective model in steady state is semi-analytical. Its validation is made with experimental results from studies of flows along a non-isothermal vertical wall in a cavity filled with wet air. Several applications are offered for many thermal curves, for more important gradient zones corrections are larger than 1.5 K for a K type thermocouple of 0.08 mm wire diameter.     

Sur une Resolution Stochastique de l'Equation de Schrödinger à Coefficients Analytiques

Under some hypotheses of analyticity and integrability we show the existence and uniqueness of a strong regular solution of the Schrödinger equation using a natural generalisation to the complex case of the Feynman-Kac formula. This explicit representation allows us to study in certain cases the asymptotic behavior of the solution when the Planck constanth tends to zero. The same method can be used for the solution of more general Schrödinger equations.

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Membre du Laboratoire Associé au C.N.R.S., n° 224 Processus Stochastiques et Applications     

Theorie statistique des alliages metalliques binaires liquides a l'aide d'une methode de groupes de sites—Analyse de l'ordre local: II. Application a des alliages présentant une courbe d'enthalpie libre de mélange dissymétrique par rapport à la concentration

The model proposed in the preceding article is modified as to explain the non-symmetry of the enthalpy of mixing. Studied alloys are supposed to be compound-forming alloys so that the local order is modified. The structure of the compound is assumed to be A₃B.     

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.     

Analyse du contraste d'un sous-joint de torsion (0 0 1) dans le silicium en MET à deux ondes

A quantitative analysis of the image of a low angle (0 0 1) twist boundary in silicon is performed using the two-beam dynamical theory of electron diffraction. The contrast features are discussed as functions of the thickness of the foil and possible elastic relaxation effects of the low angle twist boundary in the thin foil. To cite this article: R. Bonnet et al., C. R. Physique 3 (2002) 657–663.     

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.     

Auto-inflammation de substances explosives. Comparaison entre calcul analytique et numérique en vue d'une optimisation dans le domaine de la pyrotechnie

Self-ignition of energetic material was investigated in order to optimize safety in the field of pyrotechnic applications. Two approaches were used; the first one is relative to Frank-Kamenetskii stationary thermal explosion theory. The second approach consists of a choice of some numerical solutions of heat conduction equations in a non-stationary state. Comparison between these results was carried out in order to find the numerical scheme which is the most compatible with Frank-Kamenetskii stationary thermal explosion theory. Numerical data were used for three explosive substances. One of them was studied by the author. In all cases, the numerical stationary state is in agreement with the Frank-Kamenetskii stationary thermal explosion theory, more or less accurately. From this comparison, it may be concluded that it is preferable, for this kind of problem, to use an implicit scheme with linearization of the heat source term. Explicit numerical methods, with or without the addition of the heat term with the Zinn and Mader scheme are revealed to be less accurate and to need a greater optimization of spatial and temporal meshing.     

Sondes laser et méthodologies pour l'analyse thermique à l'échelle micrométrique. Application à la microélectronique

We have developed optical techniques for thermal characterisation at micrometric scale based upon the detection and the analysis of the modifications of a reflected laser beam. The modifications are induced by a well controlled thermal excitation originating from electric heating. According to this principle we have developed a very compact, high sensitivity and high resolution optical bench. It includes a homodyne stabilised Michelson interferometer, a reflectometer and a differential interferometer. It is capable of detecting a temperature variation as small as 10⁻³ K and a thermal dilatation as small as 10⁻¹⁵ m. It has a large bandwidth ranging from DC to 125 MHz. The optical bench also includes a microscope and a visualisation system allowing analysis at micrometric scale.In the field of thermal studies in microelectronics, we have been able to determine the temperature of running microelectronic components in order to estimate their quality and reliability, to study the temperature distribution and to detect hot spots in integrated circuits and to study thermooptical and thermophysical properties of materials used in microelectronics.     

Ventilation naturelle d'un local par une grande ouverture extérieure : Calcul à l'aide d'un code de champ

This paper is dedicated to the numerical study of natural ventilation in a room through a large external opening using the CFD code Fluent. Bidimensional numerical simulations are performed for wind speeds up to 5.55 m·s⁻¹ (20 km·h⁻¹). We propose a two-step solution procedure with grid refinement. When combined with appropriate thermal boundary conditions, this technique appears very efficient at limiting local convergence problems. Through an analysis of the flow pattern in the cavity, we qualitatively explain those results and define a critical Archimedes number. Calculated air change rates are compared to values deduced from empirical correlations of the literature. Agreement is in general poor, which can be explained by the differences between the conditions of our simulations and those of the experiments that lead to those correlations. However, from our simulations, we derived very clear correlations between the air change coefficient and the Archimedes number. In the last part, our results are compared with published data from two experiments. Agreement, in terms of air change coefficient, is within 26 % on average with the Porto test cell data which is closest to our numerical conditions. One should be careful about the transposition of our results to different ventilation scenarios.     

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.     

Étude aérodynamique dans une machine à compression rapide

This study deals with aerodynamics phenomena in a rapid compression machine. The mixture is initially at rest in the cylinder, and the piston is at bottom dead center; therefore there is no flow generated during the intake stroke. Moreover, flat and parallel piston and cylinder-head faces avoid squish flow. A corner vortex is generated when the piston surface, moving toward the top dead center, scrapes the boundary layer on the cylinder wall. This vortex is visualized by shadowgraph and then modeled to evaluate its characteristics. During the expansion, the gas trapped in the crevices between the piston, piston ring and cylinder wall flows back into the combustion chamber. Both crevice jet and corner vortex exist in real engines and their interactions with combustion are important: the corner vortex impairs flame kernel development and the crevice gases constitute a major source of unburnt gas.     

Étude de deux cas d'écoulements thermoconvectifs le long d'une plaque verticale non uniformément chauffée dans une cavité

Study for two types of thermoconvective flows along a non-isothermal vertical wall in a cavity. For the study of the natural convection along a non-uniformly heated vertical wall in a cavity, we experimentally defined thermal curves. These curves were corrected by our semi-analytical corrective model. The two configurations, the first one instable, with a warm part lower to a cold part, and the other one, with warm and cold parts inverted are studied. We calculated convective flux densities and we showed a different comportment according to the configuration. We proposed some values for the convective exchange coefficient in the case of regular profiles with no extreme. We recommended some correlations for the stable areas of the flow close to the active wall.     

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.     

Application, à un cristal de béryl,d'une nouvelle méthode d'étude par réflexion

The vibrational frequencies of beryl are determined from a measurement of the optical constants with a method utilizing reflexion from samples with and without an evaporated layer of an optically known substance.     

Ionisation et fragmentation en spectrométrie de masse VIII. Énergies d'activation de deux fragmentations compétitives,dissociation de l'acétate de phényle et de l'acétanilide sous impact électronique

Activation energies are calculated for the two concurrent fragmentations of phenyl acetate and acetanilide. These results show that C₆H₆O^.+ and C₆H₇N^.+ ions have respectively a phenol- and aniline-like structure near the threshold. The competitive kinetic shift, associated with the formation of CH₃CO⁺ ion, is estimated.     

Commande floue et thermique du bâtiment. Proposition d'une loi de chauffe adaptée à la régulation des bâtiments de type tertiaire à occupation discontinue

Fuzzy control and thermal properties of buildings: proposition of a heating law fitting the regulation of tertiary buildings with intermittence. In this article, we start with a general presentation of thermal regulation in buildings and of fuzzy logic applied to the control. Then, thanks to the numerical simulation of a building, we show why regulators based on the use of a straight line heating law are not adapted to the practice of intermittence. The command law we propose is based on fuzzy control. It takes into account both the exterior temperature and the thermal state of the building we estimate by means of the same function whatever the building. Simulating the opening of a window, we show that the closing degree of the regulation loop is very weak. The inference system incorporates constraints upon the conclusions which allow to do an off-line learning from the second week (respect of the open-loop) without any risk of obtaining an aberrant power because of a non-learned rule. The regulation we obtain limits considerably the disfunctioning caused by the open-loop control with intermittent regime. We conclude upon the interest of fuzzy control as regards to the temperature regulation of complex dynamic systems.     

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 %.