Commande floue et thermique du bâtiment. Proposition d'une loi de chauffe adaptée à la régulation des bâtiments de type tertiaire à occupation discontinue

Fuzzy control and thermal properties of buildings: proposition of a heating law fitting the regulation of tertiary buildings with intermittence. In this article, we start with a general presentation of thermal regulation in buildings and of fuzzy logic applied to the control. Then, thanks to the numerical simulation of a building, we show why regulators based on the use of a straight line heating law are not adapted to the practice of intermittence. The command law we propose is based on fuzzy control. It takes into account both the exterior temperature and the thermal state of the building we estimate by means of the same function whatever the building. Simulating the opening of a window, we show that the closing degree of the regulation loop is very weak. The inference system incorporates constraints upon the conclusions which allow to do an off-line learning from the second week (respect of the open-loop) without any risk of obtaining an aberrant power because of a non-learned rule. The regulation we obtain limits considerably the disfunctioning caused by the open-loop control with intermittent regime. We conclude upon the interest of fuzzy control as regards to the temperature regulation of complex dynamic systems.     

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.     

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.     

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 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.     

Etude expérimentale du fonctionnement au sol d'une boucle diphasique à ammoniac en vue d'une régulation thermique en milieu spatial

Experimental terrestrial study of an ammonia two-phase loop for thermal control in spatial environment. This paper presents the terrestrial test results of a mechanically pumped two-phase fluid loop, using ammonia as the working fluid. The 600 W experimental set-up (main components, instrumentation) is described. The two-phase flow patterns observed are compared with several flow regime maps. The experimental results are in good agreement with the predictions of Taitel and Dukler. The condensation lengths and the mean heat transfer coefficients of the condenser are given. The friction pressure gradients of adiabatic two-phase flow increases with mass flow rate and quality. The wall temperatures and heat transfer coefficients of the evaporator show the influence of the liquid distribution on the heat transfer : it is better at the top than at the bottom of the tube and at the ends than in the center part of the evaporator. The best evaporation coefficients are obtained with high mass flow rates and low qualities. The results obtained show the good operation of the system.     

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.     

Théorème du viriel et dynamique des supernovae

The use of the transformation groups leads to dynamical solutions of the nonlinear gravitational hydrodynamical equations. These solutions, combined with the virial theorem, generalise the Chandrasekhar's static relation between the total and the internal energies. The new relations describe a collapse, an expansion or the sequence collapse/expansion of the external layers of a star (case of a supernova, for example).     

Influence des phénomènes d'hystérésis sur les propriétés hydriques de matériaux poreux: Comparaison de deux modèles de simulation du comportement thermohydrique de parois de bâtiment

Porous materials hysteretic moisture characteristics : influence on water content distributions in buildings walls. Comparison of two simulation models. Two models for determining the temperature and moisture distributions within a wall are presented. The first one is a direct application of the classical Philip and de Vries model. The second one, using the suction potential, is derived from Milly's researches and is more efficient because it allows the hysteresis phenomena to be taken into account. First, the different equilibrium and transfer parameters used in the models are reviewed. Indications are given on how they can be experimentally determined and results for two cement pastes and a cellular concrete are illustrated. Then, calculation results of water content distributions for the two models are presented for an homogeneous cellular concrete wall in steady environmental conditions. In that particular case, the two models estimations are in good agreement despite different numerical treatments and experimental procedures. Using Milly's model with cyclic boundaries conditions, a significant influence of hysteresis phenomena is clearly shown.     

Confinement thermique d'une enceinte par jet d'air en convection forcée

A global model is proposed for the heat transfer through a plane air jet which is used as a thermal seal of a case at temperatures higher than the ambient one. The model considers that forced convection predominates over the natural convection and uses an analogy between heat and mass transfers. The heat balance calculation is based on an estimation of the velocity and temperature profiles at returning slot of the jet. Hydrodynamically, the jet is assimilated to a free jet. Thermally, a new formulation for the temperature profile in the jet is proposed for the most general case whatever the temperatures at the blowing slot and on each side of the jet — room and case —. The predictions by the model are in good agreement with experimental results from the point of view of:

• •- the changes in longitudinal and transversal temperature in the jet;
• •- the flux exchanged between all compartments;
• •- the energy consumption required to assure a given difference between the case temperature and the ambient one.

     

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.     

Structure moyenne et analyse intégrale du panache thermique des convecteurs électriques

In this study, we present an experimental and numerical analysis of thermal plumes issued from electric convectors. The study of the mean velocity and temperature profiles measured within the thermal plume shows that the self-similarity of the profiles is only partly achieved. We developed an integral model that takes into account the settlement flow region by the variations of the entrainment coefficient and the ratio between the temperature and velocity profile widths. A comparison between computed and experimental results proves that the model gives a satisfactory prediction of the physical phenomena. We propose finally a simplified thermal plume model for an integration into a thermal building code.     

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 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 %.     

Étude théorique du comportement thermique d'un capteur solaire plan à air à couverture combinée plastique-vitre

The air heater studied in this paper is made of an external transparent plastic cover, such as plexiglas, the role of which is to protect the collector from accidentally thrown stones and an internal transparent glass cover which produces the necessary greenhouse effect for heating the absorber. An analysis of the unsteady state heat exchanges in such a collector is presented. It is shown that at quasi-steady state the energy balance equations of the components of the heater cascade into a single first order differential equation, which is able to predict the thermal behaviour of the collector. The solution of this differential equation is written down as an explicit expression of the local temperature of the fluid flowing in the collector in terms of the time-dependent incident solar intensity. The effect of various parameters such as the inlet fluid temperature, the mass flow rate and the depth of the air channel on the thermal performances of the combined plastic-glass air collector is also studied.     

Amélioration du transfert thermique par un dépôt poreux sur la paroi d'un échangeur tubulaire

A study of flow regime and heat transfer in an annular heat exchanger partially filled with a porous medium is presented in this work. Constant heat flux and constant wall temperature boundary conditions on the inner cylinder are considered, while the outer cylinder is assumed adiabatic. The study is for both the thermal entry region and the thermally fully developed region. The flow in the porous region is modelled either by the Darcy-Brinkman equation for which an exact solution is developed or by the Darcy-Brinkman-Forchheimer equation in order to take into account inertial effects. For this case a numerical solution based on a control volume method is discussed. The results emphasize the effect of the porous layer attached to the inner cylinder on the thermal development length and heat transfer rate. It is shown that the porous substrate reduces the thermal entry length. When the effective thermal conductivity of the saturated porous medium is of the order of the fluid thermal conductivity, the local Nusselt does not vary monotonically with the thickness of the substrate. However, the use of a porous matrix always leads to an increase in the heat transfer rate provided its thermophysical properties and thickness are well chosen.     

Transfert thermique pour la condensation du R123, du R134a et de leurs mélanges,en écoulement forcé entre deux plaques planes horizontales. Etude numérique.

Heat transfer for forced convection condensation of R123, R134a and their mixtures flowing between two horizontal parallel plates. Numerical study. Film condensation of pure and binary mixtures flowing between parallel plates is treated numerically. The coupled equations of mass, momentum, species and energy conservations for the two phases are solved with an implicite scheme. In this study, we retained the pressure forces, the liquid and vapor interfacial shear stress, the Dufour effect, the inertia and enthalpy convection terms, the turbulence in the two phases and the variation of the physical properties with the temperature and concentration. The results obtained for the condensation of refrigerants R123 and R134a, show strong influence of the composition of mixture on the mean heat transfer coefficient and the total pressure loss. The calculated mean Nusselt number is in good agreement with the experimental correlations of Mochizuki and Inoué [6] and Akers and Rosson [24]. A new correlation for the mean heat transfer for forced convection condensation of pure refrigerants R123 and R134a and their mixtures between horizontal flate plates is proposed.     

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.