Conductivité thermique de mousses cellulaires de carbone: Mesures par méthode flash et interprétation

This paper deals with the thermal diffusivity measurement of cellular carbon foams by the flash method. The thermal conductivity is obtained from the thermal diffusivity by using specific heat and density of carbon foam. These materials constitute non-homogeneous porous semitransparent media, hence the classic experimental set-up of the flash method must be adapted. A study is carried out in order to show the influence of the sample thickness, of the temperature, of the enclosed gas pressure and of the gas composition on the effective thermal conductivity. Identification functions are established in the monodimensional and bidimensional cases. Results indicate the dependence of the equivalent thermal conductivity upon the sample thickness. This result, attributed to radiative transfer, is confirmed by modelling. Radiative thermal conductivities are obtained with a Monte-Carlo code. Experimental and theoretical results are compared. It is demonstrated that the concept of effective conductivity is not relevant to these materials.     

Dynamique non-linéaire d'instabilités de surface : gouttes et colonnes liquides formées par ruissellement de films

The destabilization of a thin film by gravity is of central importance in many industrial devices, especially those involved in Thermics (film boiling, heat exchangers). In the simplest case of a liquid film hanging below a solid ceiling, spatial structures are forced by the destabilizing action of gravity (Rayleigh-Taylor instability): one observes regular lattices of pendant drops, that in turn become instable by coalescences and falling of the drops. When the film is supplied with liquid at a constant rate, other flow regimes are observed: periodic emission of drops, formation of regular arrays of liquid columns, liquid sheet. Liquid columns exhibit a collective dynamics that is typical of non-linear systems: modulations of their spatial periodicity, diffusion of the perturbations, self-sustained oscillations, propagation of solitary dilation waves, coalescences and nucleations of the columns, etc.     

Apports de la colorimétrie et de la spectroradiométrie à la caractérisation in situ des peintures paléolithiques de la grotte Chauvet (Ardèche,France)

The colour of the Palaeolithic paintings on cave walls has been studied primarily in a qualitative way and, more rarely, from the physical angle. Our approach is based on the production of colorimetric measurements in caves, by means of the acquisition of energy radiance spectra with a portable spectroradiometer that can function without contact, several metres away from the decorated walls being studied, thus respecting the constraints of conservation. The colour of parietal works is an element that needs to be taken into account for their archaeological study and conservation. But observation is insufficient and only the measurement of colour makes it possible to compare distant works on rocky supports with different colourings and surface conditions.This article shows the methodology used, and it takes stock of the experience acquired in the research programme on the Chauvet Cave. It also attempts to show the benefits of taking spectroradiometric measurements on prehistoric artworks. However, the in-situ study of their colour cannot be a substitute for chemical and mineralogical analyses and the characterization of the materials, either sampled or in place. Its interest lies in its capacity for detecting even minimal changes in condition and composition. So our objective has been to establish comparisons between the graphic entities to identify the origins of the variations detected in their optical signatures. These variations can be linked to the nature of the colouring materials, but also to their position on the wall, a consequence of the application methods and sometimes also of later removals through taphonomic processes such as water run-off. Alterations (patinas) and coverings (notably veils of calcite or clay) have also been taken into account in our analysis of the data. Hence, we carried out experiments in calcification of surfaces to evaluate the case – quite frequent in caves – of calcite covering a coloured surface, outside of any other phenomenon of alteration. The first results lead us assume that it is possible to identify the signature of a veil of calcite on given colouring materials and also to present hypotheses about the nature of colouring materials covered by a veil of calcite.     

Etude du désenfumage par extraction mécanique dans un local siège d'un feu: expériences en grandeur et simulations numériques par modèles de zones et de champs

This study regards conditions of control of fire-induced smoke in a 7-m × 7-m rectangular cell, for an ‘intermediate configuration’ of a volume larger than that of an apartment room but smaller than that of an entrance hall, of a fire source a little less powerful than ‘design fires’ considered in standard approaches, of a fan-powered exhaust rate a little higher than rates typically required in regulations.From case to case were experimentally modified the following parameters: ceiling height (3 m, or 6.4 m), source type and power (between 100 and 500 kW), exhaust volume flow rate (from a few tenths of m³.s⁻¹ to a few m³.s⁻¹). Each experiment was supposed to reach a steady-state configuration, which has proved more or less true in practice. Smoke stratification within the cell has proved less sharp for a fire source non located in the centre of the cell, and the mean temperature of gases in the exhaust inlet has proved lower than under the cell ceiling, which suggested a phenomenon of ‘plugholing’ (air mixing upstream). In spite of the weaknesses of the computer models, and of a certain unaccuracy of measurements, plugholing phenomenon could be recognized in simulation results yielded by the zone model (which were in a better accordance with test results for higher exhaust rates), since the field model gave rise to a better agreement between computed and measured results for lower heat release rates and/or higher rates of smoke exhaust.     

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.     

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.     

Étude théorique de la conductivité, photoconductivité et de l'absorption photoinduite du composé KTa0.52Nb0.48O3 dopé au fer

The conductivity, the photoconductivity and the light induced absorption in a potassium tantalate niobate crystals doped with iron: KTa_0.52Nb_0.48O₃ (KTN:Fe) are studied using a charge-transport model called `three-valence states'. The results of our calculations involve the nonlinear dependence of both the photoconductivité and the photoinduite absorption according to light intensities.     

Etude théorique du spectre infra-rouge des complexes liés par liaison hydrogène à l'état gazeux: complexe ClH … O(CH3)2, son homologue deutérié et complexes voisins

Un modèle théorique est proposé en vue de reconstituer des spectres d'absorption IR des complexes du type ClH … O(CH₃)₂ en phase gazeuse. Ces complexes sont assimilés à des supermolécules triatomiques linéaires AH … B pour lesquelles la vibration v _AH et la vibration v _AH … B sont couplées par un potentiel anharmonique. Le terme couplant ces deux vibrations peut être atteint de manière précise par la connaissance de l'évolution du spectre en fonction de la température. Le modèle permet de reproduire de façon satisfaisante les spectres I.R. de ClH … O(CH₃)₂ et de ClD … O(CH₃)₂. On reproduit également mais de façon moins précise les spectres de quelques complexes voisins pour lesquels on ne possède pas de résultats en fonction de la température.