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.     

Étude statistique et dynamique de la propagation d'épidémies dans un réseau de petit monde

We study numerically in this work the statistical and dynamical properties of the clusters in a one dimensional small world model. The parameters chosen correspond to a realistic network of children of school age where a disease like measles can propagate. Extensive results on the statistical behavior of the clusters around the percolation threshold, as well as the evoltion with time, are discussed. To cite this article: N. Zekri, J.P. Clerc, C. R. Physique 3 (2002) 741–747.     

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.     

Simulation numérique de l'écoulement et du transfer de chaleur autour d'un cylindre

This paper deals with the numerical study of heat transfer and air flow around a heated cylinder in crossflow. A finite-volume method with a third-order convective scheme (Quick) has been used along with the Trio software to solve the heat and momentum balance equations. Local velocity and temperature fields have been determined. Besides, transient (Strouhal number) and time-averaged (wall shear stress, pressure coefficient and Nusselt number) results have been obtained and compared with the literature data. A good agreement has been obtained for both the local and averaged values.     

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.     

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.     

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₊.     

Thermo-frigopompe à absorption pour production simultanée de chaleur et de fraîcheur utiles

Absorption heat-cold-pump for simultaneous production of useful heat and coolness. A new absorption heat-cold-pump for the simultaneous production of useful cold (12 °C – 7 °C) and of useful heat (80 °C – 85 °C) is presented. The system operates with a couple of components with comparable volatilities, for example the following saturated hydrocarbons, butane + hexane, butane + octane, butane + decane, propane + octane. The working pair is separated in a rectification column. The components are remixed in a reverse-rectification column fitted with an incorporated heat exchanger. The COP and the exergy efficiency are presented for a number of modelized internal structures of the heat-cold-pump.     

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.     

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 %.     

Transfert de chaleur par rayonnement dans un four d'incinération industriel: application de la méthode des zones

In the context of resource conservation, incineration of domestic wastes is a technique allowing an interesting potential in energy valorization. However, because of the variability (annual, monthly) in quality and quantity of raw material, one of the characteristics of waste incineration cogeneration power plants is their frequent partial or excess loading. In order to describe their behaviour at these non-nominal loads, there is a need for simulation tools which predict with accuracy the behaviour of the principal components of the plant at these loads. This paper presents an original approach for predicting the heat transfer in a waste incinerator. In particular, the model developed is applied to and validated with measurements from a waste incinerator located at the Cottendart waste incineration cogeneration power plant in Switzerland. The part of the incinerator studied is characterized by high gas temperatures (1 000-500 °C) for which radiative heat transfer represents the most important part of the global heat exchange. A three-dimensional model using nodal analysis is proposed. The radiative heat exchange is treated using the zone method. With a fine enough mesh, the behaviour of the system in regions far from the nominal point can be predicted with very high accuracy. This paper describes the model and the simulation results.     

Fluctuations de temperature et flux de chaleur en aval d'une source linéaire placée dans une couche limite turbulente

The purpose of the present communication is to present and discuss some recent experimental results concerning the dispersion downstream of a heated line source located, in a turbulent boundary layer, successively at two distances from an adiabatic wall. Information on the mean and fluctuating temperature fields and associated heat fluxes are presented and analysed by testing closure assumptions for model dispersion at first and second orders. This model, which only requires the use of temporal velocity scales of the flow, leads to a rather good agreement with measurements.     

Caractérisation expérimentale et numérique des transferts de chaleur à travers un jet d'air plan horizontal

This study reports on heat transfer through a plan horizontal air jet. The jet maintained at a higher temperature than the ambience allows a thermal sealing at the upper side of the opening case.The thermal and hydrodynamic measurements taken on the experimental device and the numerical simulations carried out by using the Phoenics package distinguish two types of flow. For the small velocities of the jet or the high differences of temperature, the natural convection predominates: the hot jet diffuses upwards and can not seal the case. Inversely, for the high values of the jet velocities, the forced convection predominates: the case is correctly sealed but the power consumption increases like the flowrate. A dimensionless number called deflection modulus allows to characterize the transition from one flow pattern to another and it also allows to minimize the energy consumption.     

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.     

Evolution avec la température de la réflectivité, de l'indice complexe et de la sélectivité spectrale du nitrure de zirconium

The reflectance of zirconium nitride has been measured at 20, 500 and 700°C, within the range 0.22 to 5.50 μ m. The optical constants have been determined using Kramers-Kronig analysis. The spectral selectivity of this compound noticeably depends on temperature; the absorptance increases from 0.28 at 20°C to 0.36 at 700°C while the emittance increases from 0.06 to 0.14.     

Simulation numérique du comportement d'un béton cellulaire dans sa phase d'élaboration sous gradient de pression de vapeur à haute température

A numerical simulation of the coupled heat and mass transfers is carried out to analyse the thermal and moisture properties of cellular concrete during a manufacturing cycle. This cycle consists of two phases: a vacuuming phase is created first, then the material is baked under high energy and saturated vapour conditions. A 2D code was developped using a finite volume method. The numerical results were validated by means of a comparison study of the problem, in order to improve the process with a view to energy consumption and product quality.     

Modélisation de la décharge luminescente dans un écoulement de gaz en régime turbulent

In this paper we propose a model of a glow discharge in a turbulent flow. The electron density is calculated using a conservation equation. We assume that the gas glow acts on the electron density and the Shwartz model is used to model the change of diffusivity due to turbulence. In order to show the effects of the turbulence on the electron density, we use a 1D model of a stable electric discharge in to a turbulent flow. The model shows that the increase in turbulent diffusivity at high Reynolds numbers tends to flatten the electron density profiles. Theoretical results are in good agreement with the reported measures. Next, the model was applied to a 2D argon axisymmetric turbulent compressible steady flow. This study shows that when plasma oscillations and turbulence fluctuations of the neutral gas are correlated the temperature profile flattens. Finally, we study electronic distribution into a 3D plasma column in a dissymmetrical flow.     

Amélioration du coefficient de qualité de résonateurs à ondes transverses sur plaques fines de quartz

Resonators based on transverse waves propagating in thin quartz plates have been optimized using a specific fabrication step in order to improve their quality coefficient and to minimize their insertion losses. Systematic measurements have been performed, allowing to emphasize the efficiency of the proposed approach. The possibility to fabricate resonators operating in the frequency range 100–250 MHz and exhibiting quality coefficient times frequency product greater than 10 ¹³ is there demonstrated. This last figure corresponds to more than the state of the art in surface acoustic wave resonators.