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.     

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 %.     

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.     

Simulation des transferts de masse et de chaleur par modélisation à bas nombres de Reynolds dans un écoulement avec effusion locale en canalisation

The turbulence model developed by Lam and Bremhorst with low values of Reynolds numbers is used to simulate a flow between two impermeable plate walls. This model, corrected by Yap, is able to simulate flow inside the boundary layer on the part of porous walls submitted to the blowing. This model of flow inside a channel with local blowing is coupled with a model of transfers through the impermeable walls and the porous wall, and with the environment. The global model is validated by comparison between experimental results from the literature and those obtained by the authors from experiences carried out with the test facilities of a subsonic thermal wind tunnel.A simulation of non-isothermal flow inside a channel with local blowing shows the attractive way of wall cooling by blowing. The optimal injection ratio is found equal to 0.01.     

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.     

Constante de force du vibrateur CH en relation avec la structure électronique de la molécule

Une expression analytique est recherchée pour le calcul des constantes de force k _CH dans le cadre de la méthode CNDO/2. Les dérivées des intégrales de résonance qui apparaissent dans cette expression sont évaluées empiriquement. Un ajustement statistique est effectué sur les valeurs expérimentales, la paramétrisation originale de la méthode CNDO/2 étant conservée. Dans cette optique, deux évaluations des dérivées β_{μ, ν}′ et β_{μ, ν}″ des intégrales de résonance par rapport à la distance CH sont proposées:

1. β_{μ, ν}′ et β_{μ, ν}″ sont supposées constantes.

2. β_{μ, ν}′ et β_{μ, ν}″ sont posées proportionnelles aux dérivées S _{μ, ν}′ et S _{μ, ν}″ des intégrales de recouvrement.

L'une ou l'autre de ces évaluations traduit l'évolution des constantes de force grâce à l'effet conjugué d'une contribution ‘statique’ (fonction des éléments de la matrice densité) et d'une contribution ‘dynamique’ (dépendant de la variation pendant la vibration de ces mêmes éléments).

An analytical expression is sought for k _CH bond force constants, within the frame of the CNDO/2 method. Some integrals appearing in this expression are empirically valued. A statistical adjustment is effected on the experimental data, the original parameters of the CNDO/2 method having been preserved. Two appraisals of β_{μ, ν}′ and β_{μ, ν}″ derivatives of the resonance integrals are compared with each other.

1. β_{μ, ν}′ and β_{μ, ν}″ are constants.

2. β_{μ, ν}′ and β_{μ, ν}″ are in direct ratio to S _{μ, ν}′ and S _{μ, ν}″ derivatives of the overlap integrals.

Either of these valuations explains the evolution of the bond force constants, thanks to the aggregate effect of a ‘static’ contribution (function of elements of the density matrix) and thanks to a ‘dynamic’ contribution (depending on the variation during the vibration of these same elements).     

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.     

Identification du coefficient de transfert thermique sur la surface externe d'un échangeur de chaleur à tubes et ailettes planes en fonction de l'humidité de l'air,en l'absence de condensation

The following study, which is rather oriented towards experimentation, shows the influence of the humidity content of air on heat transfer. This first article concerns heat transfer between the external fluid (moist air) and the internal fluid (water containing glycol, whose thermal behavior inside circular tubes is well-known) in a heat exchanger of the same type as those used in automotive air conditioning (horizontal copper tubes and plane aluminium fins), in the absence of condensation. The most difficult part of this experimental work is the measurement and control of the air humidity, since one has to make sure that the measurement incertainties are not significant compared to the precision of the calculation of the heat transfer coefficient. The conclusion is that, for this type of exchanger, the heat transfer coefficient decreases with air humidity in the absence of condensation (dry wall). Some correlations have been developed with respect to the relative air humidity. An analog experimental investigation, but this time carried out in the presence of condensation (partially or completely wetted wall), is about to be completed; the obtained results will be communicated later on.     

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.     

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.     

Etude par résonance paramagnétique électronique de la variation en fonction de la température du couplage hyperfin du radical tert-butyle

Le radical tert-butyle obtenu par irradiation γ à 77°k du chlorure de tertiobutyle peut se présenter sous deux structures différentes. Une forme pyramidale, ayant pour origine les contraintes exercées par le réseau cristallin, et pour laquelle le couplage a ¹³C avec le carbone central est une fonction décroissante de la température. Une forme plane pour laquelle a ¹³C croît avec la température. L'étude du radical plan a permis de séparer les contributions relatives des vibrations, a ¹³C _v , et de la polarisation des électrons des couches internes et des liaisons, a ¹³C_pol, à la valeur du couplage a ¹³C. Le résultat obtenu confirme la relative insensibilité de a ¹³C_Pol, par rapport aux détails des substituants du carbone central, prévue par les calculs de Melchior.     

Mise en œuvre d'une méthode d'approximation diffuse adaptative pour la résolution des équations de diffusion et de transport

An adaptative diffuse approximation method for convection diffusion problems has been developed. The grid refinement method is first described and two examples are reported to illustrate the validity of the proposed method. The first one is a two-dimensionnal diffusion problem and the second one is a one-dimensionnal convection-diffusion problem.     

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.     

Les échangeurs à plaques: dix ans de recherche au GRETh: Partie 2. Dimensionnement et mauvaise distribution

In a first paper [RGT 397, January 1995], the thermal and hydraulic performances in single and two phase flows were presented. This paper describes design methods used for sizing plate heat exchangers. In a first part, an overall design method based on the temperature enthalpy diagram is presented. Afterwards, a model taking into account flow maldistribution effects is described.     

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.     

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.