Influence des différents scénarios techniques, énergétiques et économiques sur la décision d'installer un nouvel échangeur de chaleur

Influence of different technical, energetic and economic scenarios on the decision of installing a new heat exchanger. This article proposes to examine the decision of installing a new heat exchanger in a working industrial unit in order to improve its energy performances. This examination is based on a new graphical method permitting us to approach the decision while taking into consideration the operating, technological and conjunctural variables.The analysis of the results shows that improvement in energy performances of an industrial unit does not always contribute to improving its economic performances. Factors that govern this contribution are in increasing importance: the choice of heat exchanger's configuration, the energy scenarios, the energy to be saved and the choice of the construction material of the heat exchanger.     

Bilan énergétique pour une onde électromagnétique incidente sur l'interface séparant deux milieux mobiles

We are exhibiting the formula expressing the unitary quadrivector normal to a moving interface; moreover we are able to write the jump conditions for the electrodynamic stress-energy tensor M_ij for the case of an incident electromagnetic field. It appears that generally the energy balance is rather involved due to the surface forces of radiation pressure; the work done by these forces is not remaining zero and is a function of the M_ij tensor discontinuity.     

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.     

Interaction d'un écoulement de thermosiphon avec un panache thermique à symétrie axiale: étude expérimentale

The present work reports an experimental study of a thermosiphon effect on an axisymmetric thermal plume. An experimental apparatus composed of a circular disc heated at constant temperature was set up. The disc is placed at the entrance to an open-ended vertical cylinder of larger diameter. Thermal radiation emitted by the hot disc heats the cylinder wall. The heating of fluid to the cylinder-inlet is the cause of the thermosiphon effect around the thermal plume. First, we studied the flow generated by the thermal plume. The analysis of the average fields of velocity and temperature shows that the structure of a thermal plume generated by a hot obstacle is affected by the characteristics of the main flow around this obstacle. Furthermore, these results allowed us to rediscover the two classical zones which constitute a thermal plume. Secondly, we studied the thermosiphon effect on the thermal plume development. The average fields evolution of velocity and temperature as well as the flow visualization show the existence of three different zones. The first zone of the plume air feeding is characterized by the dynamic and thermal profiles in three extrema structures. These extrema disappear in the second zone where the profiles present only one maximum. In the last zone, the profiles are flattened and self-similar. Thus, the turbulence is fully developed. However, one observes an improvement in the amount of energy absorbed by the fluid and an increase in the flow rate inside the cylinder. A flow visualization with laser plan allowed us to show that the position of the vertical cylinder around the hot disc affects the flow structure plume and causes the appearance of a new zone at the entrance to the system. However, the analysis of the fluctuating fields related to two studied cases shows that the thermosiphon effect has an important influence on the turbulent intensity structure of the flow evolution.     

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.     

Les études Séismologiques en Tchécoslovaquie de 1951 à 1953

Ohne ZusammenfassungRapport national présenté devant l'Association Internationale de Séismologie et de Physique de l'Intérieur de la Terre pendant la 10ème Assemblée Générale de l'Union Géodésique et Géophysique Internationale (UGGI) à Rome en Septembre 1954.     

Les pseudopotentiels à norme conservée relativistes étendus

The extended norm-conserving pseudopotentials developed by Shirley et al., in the nonrelativistic case, has been extended to the relativistic case. We have discussed the importance of the relativistic terms added in the case of the hard pseudopotentials. This correction allows us to revise the precision linked to the nonlinearity of the electronic structure methods on a solid in the particular case of heavy atoms.     

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.     

Charge électrique résiduelle sur des poudres à l'extinction d'un plasma poussiéreux

La charge électrique résiduelle des poudres à l'extinction d'un plasma poussiéreux a été mesurée dans une décharge radio-fréquence. La force de gravité a été compensée par l'application d'une force de thermophorèse de même intensité et de direction opposée. Des poudres chargées positivement ou négativement ainsi que des poudres non chargées ont été observées. Ces charges résiduelles peuvent se maintenir pendant plus de 1 minute après l'arrêt de la décharge. La charge électrique résiduelle moyenne pour des poudres d'environ 200 nm de rayon a été mesurée à environ ?5e pour une pression de 1,2 mbar et ?3e pour une pression de 0,4 mbar. La charge électrique résiduelle sur les poudres étant reliée aux phénomènes de diffusion (ambipolaire et libre) des espèces chargées du plasma (ions et électrons), des mesures de densité électronique dans des plasmas post-décharges d'argon pur et d'argon/méthane (plasma poussiéreux) ont été effectuées afin de comprendre l'influence des poudres sur les mécanismes de perte des électrons. Cesmesures ont montré que la présence de poudres provoque d'abord une augmentation rapide de la densité électronique à l'arrêt du plasma suivie d'une décroissance accélérée.     

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.     

Étude expérimentale et théorique du microgrippage dans les contacts élastohydrodynamiques

This paper describes experimental research conducted on a high-speed twin-disk machine. The twin-disk machine, originally designed to simulate the operation of gears, is used at high speed and low load to simulate the operation of high-speed rolling bearings at the inner ring - rolling element contact. Operating conditions, ie, rolling and sliding speeds, contact pressure, lubricant, temperature, disk material combination and surface finish are typical of aerospace practice. The lubricant used in these tests is a tetra ester of five centistoke viscosity at 100 °C, qualified for use in gas turbine engine lubrication systems under the MIL-L-23699 specifications. In the experimental procedure, the scuffing limit is reached by increasing progressively the sliding speed, other operating conditions such as the normal load and the rolling speed being kept constant. The evidence of disk scuffing is a sudden increase of the friction force, which stops automatically the machine at an earlier stage of the damage process.Moreover, a model to estimate the surface and subsurface temperature distribution in the contacting bodies is presented and used to complement our experimental results.Scuffing damages at sliding speeds of up to 40m.s⁻¹ have been obtained. A total of 32 tests have been carried out. Damages observed on the surface of the disks after tests at very high sliding speeds, appear to be similar in nature to those of the inner raceway of rolling bearings subject to skidding. They consist of transfer of component surface material in microscopic patches from a location on one contacting surface to a location on the other contacting surface. The scuffing limits of two material combinations (M50-M50 and M50-16NCD13), for two mean rolling speeds (25 and 50 m/s), three maximum Hertz pressures (0.5, 0.8 and 1 Gpa) and three oil feed temperatures (40, 80 and 120 °C) are presented and discussed.Finally, from this experimental and theoretical investigation, it is proposed to call ‘micro-scuffing damage’ the surface distress observed as a consequence of skidding.     

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.     

Remarques sur la structure électronique des métaux et alliages

We study the validity of the perturbation expansion in the calculation of the effect of a small, periodic potentialV on an electron gas. We show that for two and three dimensions, the total energy of the system up to second order inV, as well as the total electronic density up to first order, is correctly given by such an expansion. On the other hand, this method can lead to uncorrect and diverging results as far as the electronic density per unit energy and all the abovementioned quantities in a one dimensional model are concerned. Some consequences and applications of these results are then briefly reviewed.     

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 %.     

Convection sous-vestimentaire. Analyse dimensionnelle et mesure des flux échangés

This article presents an experimental study that quantifies heat and mass transfers between the skin and a fabric layer. The experiment is carried out with a sponge plate heated to skin temperature, about 35°C, and saturated in moisture. Analysis by holographic interferometry provides the determination of the convective heat fluxes. Radiative flows are calculated by the radiosity method. We show the influence of the underclothing air layer, and that of the fabric using different textiles. The radiative properties also modify the underwear exchanges. The influence of mass transfer on convective phenomena was studied by comparison with a dry plate. The results that are obtained, under the form of heat fluxes, are analysed. They can be used to create a data base to check the validation of models treating heat exchanges between skin and clothes.     

Étude analytique des instabilités induites à la transition ébullition nucléée — ébullition en film

After a reminder of the static stability criterion of a system with non-linear characteristics, the thermal stability of a single-dimensional system was studied with the help of simple analytical methods. The influence of axial conduction, connected in particular with limit conditions, was highlighted and it was demonstrated that even if the static stability criterion is verified, the system can present second order instability over part of its intrinsic character. Multiple stationary states can also be superimposed on the instability. A more sophisticated analytical method allowed the existence of several temperature profiles to be verified on a heated conductor wire, dependent on the value of the amperage of the electric current. An experimental visualization confirms the existence of three temperature profiles, corresponding to three different heat exchange regimes.     

Présentation simple de la pression émise par un transducteur focalisant

The numeric computing of the pressure field emitted by a focusing transducer is relatively long. That is why we have developed a simple method to predict the pressure of such transducers. This model requires to compute only one integral and is called 1-dimensional model. It provides a rapid calculation of the radial and axial pressures. These pressures will be compared with the results of the 2-dimensional model and validated by experiment.     

Influence des conditions d'émission sur un écoulement de type jet plan laminaire isotherme ou chauffé

We propose numerical solutions for a laminar jet, accounting for emission conditions at the exit of the nozzle. Two emission cases are considered in this study: velocity and temperature profiles are uniform or parabolic, respectively. A finite difference scheme is developed for the resolution of the equations governing the isothermal and non-isothermal free jet and wall jet developing tangentially along an adiabatic flat plate. The analysed results are the centerline velocity and centerline temperature for the free jet, and wall temperature and shear stress for the wall jet. The results obtained are compared to another method that is based on two constraints of integration, i) conservation of momentum and ii) conservation of energy, to replace the emission conditions at the exit of the nozzle for the resolution of equations. Our results of the velocity and temperature profiles compare well with those obtained by the latter method solely in the plume region, where buoyancy forces are responsible for flow.