Comportement thermique d'un générateur à sorption solide

The thermal behaviour of a solid sorption generator of active carbon/alcohol machine, is studied during heating and cooling phases with a preheated air flow. A bicylindrical walls generator that contains 0.9 kg of a granular adsorbent in the presence of residual gas is tested during a cycle of an average duration of 3 hours; the grains of active carbon are rod-shaped of 0.003 m diameter and 0.008 m average length. The thermal contact conductance of adsorbent to the wall has an important influence on the rate of heat transfer between the generator and the external source of heat. In the absence of alcohol, heat transfer occuring without mass transfer in active carbon is essentially due to the conduction. A numerical bidimensional model allows one to justify experimentally the observed evolution and proposes thermal contact conductance between active carbon pellets and the generator wall. A parametric study of the thermal contact conductance gives 6.5 W·m⁻²·K⁻¹ as the best value. A simulation of heating and cooling phases with average conductance values between 5 and 30 W·m⁻²·K⁻¹ gives model estimated heating and cooling phases duration.     

Gestion de la thermique dans les procédés d'adsorption de gaz

Thermal aspects of gas adsorption processes. The adsorption of gases is an exothermal phenomenon, and fixed bed adsorption operations produce high thermal effects. Similarly, applying temperature changes to a process can modify adsorption.The purpose of this paper is to illustrate the basic features of non-isothermal adsorption, the most salient of which is the effect of relative velocities of heat and concentration waves. It is shown that there may exist a temperature at which a reversal occurs in the order of those velocities. A simple purification process is suggested, based on the temperature modulation of the gas mixture fed to the column. Examples are calculated with data corresponding to carbon dioxide adsorption on 13X molecular sieves.     

Brouillage thermique d'un gaz cohérent de fermions

It is generally assumed that a condensate of paired fermions at equilibrium is characterized by a macroscopic wavefunction with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature; the condensate has then a finite coherence time, even when the system is isolated in its evolution and the particle number N is fixed. The loss of phase memory is due to interactions of the condensate with the excited modes that constitute a dephasing environment. This fundamental effect, crucial for applications using the condensate of pairs' macroscopic coherence, was scarcely studied. We link the coherence time to the condensate phase dynamics, and we show with a microscopic theory that the time derivative of the condensate phase operator ${\hat{\theta }}_0$ is proportional to a chemical potential operator that we construct including both the pair-breaking and pair-motion excitation branches. In a single realization of energy E, ${\hat{\theta }}_0$ evolves at long times as $-2{\mu }_{\mathrm{mc}}(E)t/ħ$, where ${\mu }_{\mathrm{mc}}(E)$ is the microcanonical chemical potential; energy fluctuations from one realization to the other then lead to a ballistic spreading of the phase and to a Gaussian decay of the temporal coherence function with a characteristic time $\propto N^{1/2}$. In the absence of energy fluctuations, the coherence time scales as N due to the diffusive motion of ${\hat{\theta }}_0$. We propose a method to measure the coherence time with ultracold atoms, which we predict to be tens of milliseconds for the canonical ensemble unitary Fermi gas.     

Implosion de cibles laser magnétisées: influence de la limitation du flux thermique

The influence of thermal conduction on laser inertial confinement fusion is studied with a sophisticated CEA (Commissariat à l'Énergie Atomique) computer code. The beneficial effect of magnetic field on gain, shown by I.R. Lindemuth and R.C. Kirkpatrick, is qualitatively confirmed by our results.     

Étude expérimentale de la résistance thermique de contact entre l'étain en cours de fusion et un creuset refroidi

This paper deals with the analysis of the thermal contact conditions during the melting of tin on different cooled walls, for different heat situations and for different cooling flows. Two experimental set ups are studied. A polished nickel substrate covers the melting-pot of the first one. Semi intrinsic thermocouples are implemented to measure the temperature of the substrate. This will allow a better study of the thermal contact resistance distribution on the wall. The second melting-pot is made of copper. Its surface roughness is variable. The heat system is stronger. The results obtained with the first experimental set-up show that the thermal contact resistance is time-dependent and non-uniformly spread. However, the heat evacuation is relatively uniformly spread while the heat power increases. Tests carried out with the second experimental set-up point out huge temperature oscillations which are attributed to unstable thermal contact conditions. This thermal behaviour can be explained by the buckling of the tin crust. The apparition of buckling seems to be favoured by a higher dissipated power and by a higher thermal conductivity of the melting pot.     

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.     

Comportement thermique d'un matériau de référence poreux soumis à des atmosphères de chauffe différentes

This study presents an analysis of the temperature evolution in a porous sample. Various situations have been examined: the porous solid is initially dry or water saturated, and the heating fluid is dry air or steam. This analysis showed experimentally the optimal phases of using steam to heat a material. Moreover, as humidity in the solid considerably modifies the heating kinetics, the study has also been carried out on a non-porous solid. The treatment of the temperature curves under an adimensionnal form shows a singular behaviour of those temperatures following the heating fluid nature and its temperature. Equations giving temperature versus time have been proposed. They are based on the definition of a time lag and a time constant, both being defined as a function of the single fluid temperature parameter. Finally, in the particular case of the non-porous solid heated by dry air, with additional approximations, it has been possible to check that the experimental time constant is very close to a fundamental time constant.     

Contrôle thermique de parois de chambre de combustion. Banc d'essai du laboratoire aquitain de recherche en aérothermique

Thermal control of combustion chamber walls. LARA's test bench. Convective heat transfer along the wall of a combustion chamber is a major source of concern for a gas turbine engine manufacturer like Turboméca. This topic led to a joint research project with the University of Pau : to this end, a specific test bench with up-to-date means of measurement (IR thermography, laser doppler velocimetry) was created. An initial literature study had shown that multi-hole cooling had not been previously studied with a geometry design close to the real case. The present test facility provides and controls the dynamic and thermal parameters of a combustion chamber and the multi-hole cooling was set up on a flat plate simulating the wall chamber. This plate is submitted on one side to the hot gases resulting from combustion and on the other side to the cooling air flow. Dynamic and thermal entry conditions of the hot and cold flows were carefully controlled and are now presented. The wide possibilities of the testing facility have been explored offering scopes for other experimental studies.     

Modélisation du comportement thermique d'un espace urbain: Calcul de la réponse en température des structures et de l'air ambiant

This paper presents theoretical modelling works on the thermal behaviour of an urban space (street). A calculation code, named Codyflow, has been developed in order to simulate the thermal response of an urban system to climatic solicitations. The model allows us to take into consideration, on the one hand, the radiative flux soliciting the urban canyon, on the other hand, heat transfers inside the system. The airflow solicitation, which plays a part in the convective exchanges and in the ambient air temperature, is calculated by the CFD code Fluent. Some simulation results, obtained by the code Codyflow, are presented. They bring to the fore the influence of many factors on the thermal response of the urban canyon: the geometrical configuration, the used materials (through their thermophysical characteristics) and the airflow solicitation. These results allow us to predict the thermal behaviour of urban spaces for the benefit of architects and urban designers in the conception phase of an urban plan.     

Efficacité des techniques de régénération d'eau pour une usine de fabrication de papier intégrée sans effluent. Analyse exergétique

Water regeneration efficiency for pulp and paper plant. Exergy analysis. The water network of a pulp and paper mill is contaminated by a large number of components which are brought into the system as wood constituants or process additives. Keeping the contaminant concentrations below certain limits, to safeguard paper quality and process runnability, must be part of any systems closure strategy. This can be achieved by purging contaminants using well known separation techniques such as evaporation, freeze crystallization and membrane filtration. The purpose of this study was to compare the thermodynamic efficiency of these techniques by means of an advanced exergy analysis. The contaminated stream was modelled using key components for both organic and inorganic contaminants. The activity coefficients were calculated using the Chen algorithm. The approach developed by Brodyanski, Sorin and Le Goff was used to determine the exergy efficiency. This analysis shows that freeze crystallisation is the most efficient technique when the exergy of the output stream is considered as a useful effect. When only the useful work obtained by separation is considered, membrane separation becomes the most efficient process. The analysis of the process incorporating a separation operation produces the actual efficiency. This example illustrates the advantage of using exergy methods to determine the efficiency of processes based on different physico-chemical phenomena.