Modélisation dynamique en thermique du bâtiment

This work describes the advantages provided by applying the method of spectral analysis to solve problems in the thermal analysis of buildings. A reduced spectrum of the model is obtained with the analysis of the dynamic responses of the system. The model obtained is simple and efficient, since it describes with accuracy the main dynamic behaviour of the structure. The aim is to use the model for the control of ambiant temperatures in buildings. These temperatures do not require accurate precision but a good restitution of involved dynamics.     

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.     

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 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.     

Modèle correctif de la mesure de la température d'un gaz par couple thermoélectrique. Application à une zone à fort gradient thermique

Corrective model of the gas temperature thermocouple measure. Application to an important thermal gradient zone. The gas temperature measurement with a thermocouple in important thermal gradient zones requires a corrective model. For example, such zones exist for thermal boundary layers near active walls. We have calculated these thermocouple thermal exchanges where connection wires are considered as fins with a variable ambient gas temperature. The heat exchanges by convection and radiation on the thermocouple head are analytically calculated, then a numerical method is used for fins where the space increment is the same as for the experimental measure. The corrective model in steady state is semi-analytical. Its validation is made with experimental results from studies of flows along a non-isothermal vertical wall in a cavity filled with wet air. Several applications are offered for many thermal curves, for more important gradient zones corrections are larger than 1.5 K for a K type thermocouple of 0.08 mm wire diameter.     

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.     

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.     

Modélisation des jets turbulents subsoniques fortement chauffés

Among different approaches of the statistic treatment of the transport equations for variable-density fluid flows, a formulation is adopted which isolates the turbulent mass-fluxes contribution. It deals with centred statistical moments only. This choice enables the discussion of the transposal of constant-density closure-schemes to mass-weighted variables of heated free shear flows. The discussion focuses on first- and second-order closures for diffusion terms. The content of standard closure schemes in Favre variables is detailed. Concluding points are given concerning the relevancy of the different initial statitistical treatments to set the proper formal frame for deriving closure-schemes. Parabolic numerical simulations of heated jets including first- and second-order closures are then examined. The prediction of experimentally-asserted features of density effects on the mean field of variable density jets is verified. Some effects on turbulent quantities are underlined too. Comparaisons between mass-fluxes predicted at first- and second-order reveal that a first-order closure is likely to be sufficient.     

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₊.     

Identification des propriétés thermiques de composites fibres de verre/résines thermodurcissables: Application à l'optimisation des procédés de moulage

Identification of glass fiber/thermosetting resins composites thermal properties. Application to the optimization of molding processes. A procedure has been developed to optimize molding processes of thermosetting composite materials. Three stages have been distinguished. In the first one, some thermal and kinetic properties have been measured by differential scanning calorimetry. Thermal conductivities have been identified afterwards in thick instrumented pieces, placed in a thermally regulated press. High dependences of thermal conductivities on temperature and transformation degree have been shown. Secondly, coupled heat transfers have been numerically simulated and results have been satisfactorily compared with experimental thermograms. Finally, optimization technics based on effective inverse methods have been used.These points have been illustrated with two examples : glass fiber/epoxy resin and glass fiber/polyester. Sufficient mechanical characteristics of the first one, which is cured in oven, and good surface aspect of the second, that is made by injection in heated and closed molds, had to be obtained. The results let foresee real improvement of the corresponding molding processes.     

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.     

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.     

Distribution des températures à l'intérieur d'un jet de gaz chaud injecté dans un liquide

The distribution of temperatures in a high generating pressure impingement nitrogen jet submerged in an oil bath is determined by means of thermocouples. An analogy with homogeneous underexpanded jets is used to describe the nature of disturbances in the first portion of this flow. Further away, the radial distribution of the temperature excess can be assimilated to a Gaussian curve, but it remains still very narrow compared to the jet envelope. The axial evolution of the half-property radius is small and the corresponding temperature distributions are never fully self-preserving. The temperature excess at the jet centre decreases less rapidly than that of a homogeneous jet injected into a medium with a different density. Mixing is less effective and it occurs more by the contribution of the heavier phase than by the radial diffusion of the lighter phase. The results are satisfactorily correlated with the ratio of the mass flow densities of the liquid and of the gas.     

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.     

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.