107 research outputs found
An oriented-design simplified model for the efficiency of a flat plate solar air collector
Get PDFIn systems design, suitably adapted physical models are required. Different modelling approaches for a solar air collector were studied in this paper. First, a classical model was produced, based on a linearization of the conservation of energy equations. Its resolution used traditional matrix methods. In order to improve the possibilities for use in design, the behaviour of the collector was next expressed in terms of efficiency. Lastly, simplified models constructed from the results obtained with the classical linearized model, and explicitly including the design variables of the collector, were proposed. These reduced models were then evaluated in terms of Parsimony, Exactness, Precision and Specialisation (PEPS). It was concluded that one of them (D2), using a low number of variables and of equations, is well suited for the design of solar air collector coupled with other sub-systems in more complex devices such as solar kiln with energy storag
Model and simulation of a solar kiln with energy storage
Get PDFA solar kiln with energy storage can be used for continuous drying. This kiln consisted of several units which were modeled to simulate it in operation. A model was proposed for each unit, and another based on laboratory tests for drying a wooden board by passing air across. These models were combined to produce a global model. Simulation results were then analyzed and showed that the use of storage was justified to reduce drying time. Moreover, with the judicious use of storage and air renewal, drying schedules could be produced for a better quality of dried wood
Heat transfer in mini-channels : unsteady behaviour and convolutive approach
No full textUn modèle semi-analytique permettant de simuler le transfert thermique conjugué dans un mini/macro canal plan soumis à des sources de chaleur surfaciques localisées sur les faces externes et variantes en fonction du temps, a été présenté et vérifié. Plus le diamètre hydraulique du canal est petit, plus la caractérisation expérimentale interne (mesure des températures et des flux) en régime thermique permanent ou transitoire à l'aide des capteurs internes est délicate. Une méthode non-intrusive permettant d'estimer les conditions internes à partir des mesures de température par thermographie infrarouge sur les faces externes et d'un modèle semi-analytique, a été effectuée. Comme le coefficient de transfert convectif forcé classique perd son sens en régime instationnaire, une approche alternative basée sur une fonction de transfert, valable pour un système linaire et invariant dans le temps a été mise en œuvre. Cette fonction peut être calculée analytiquement (uniquement pour une géométrie simple) ou estimée expérimentalement (géométrie complexe). Grâce au caractère intrinsèque de cette fonction de transfert, deux capteurs virtuels ont été conçus : capteur virtuel de température et détecteur d'encrassement permettent respectivement d'estimer les températures internes et de détecter l'encrassement qui peut avoir lieu dans l'échangeur à partir des mesures de températures sur les faces externesA semi-analytical model allowing to simulate the transient conjugate heat transfer in mini/macro plane channel subject to a heat source(s) localized on the external face(s), was presented and verified. The developed model takes into account advection-diffusion in the fluid and conduction in the solid. As the hydraulic diameter of the channel becomes small, the internal experimental characterization (measurement of temperature and heat flux) using internal sensors become tricky because internal sensors located may compromise the structural integrity of the whole system. A non-intrusive method for estimating the internal conditions from infrared temperature measurements on the external faces using the semi-analytical model was performed. Since the classic convective heat transfer coefficient loses its meaning in transient state, an alternative approach based on a transfer function, valid for Linear Time-Invariant (LTI) systems, was highlighted. This function can be calculated analytically only for a simple geometry. For complex geometries it can be estimated experimentally. Thanks to intrinsic character of this function, two characterization methods were designed. The first to estimate the temperature at a point from a measurement at another point in the system (virtual temperature sensor). The second method concerns the detection of fouling layers that may appear in the heat exchanger from temperature measurements on the external face
Thermal characterization of low density insulating materials.Application to low molecular weight aerogels
No full textLa problématique de la sauvegarde de l’énergie pose un certain nombre de défis à la science, en particulier celui de son efficacité. La conception et la caractérisation de nouveaux matériaux isolants thermiques plus performants se révèlent donc fondamentales dans cette perspective. Les aérogels se présentent comme de sérieux candidats dans ce domaine, leur procédé de fabrication confère à certains d’entre eux des caractéristiques extrêmes telles qu’une grande porosité et une faible masse volumique. La caractérisation thermique de tels matériaux est délicate, leur faible sensibilité aux flux thermiques qui les traversent rend les méthodes connues difficiles à mettre en œuvre. A travers l’étude d’échantillons d’aérogels de faible poids moléculaire conçus au LCPM, une méthode de caractérisation adaptée a été développée. Cette méthode de type « tri-couche » offre les avantages d’être robuste et de s’affranchir de la connaissance de paramètres difficiles à atteindre dans de tels cas. La description et la validation de cette méthode sont l’objet principal de ce travail. Par ailleurs, les mesures de conductivité thermique sous vide ont été exploitées et ont permis une compréhension plus poussée de la structure de ces aérogels. Les résultats obtenus dans cette étude ouvrent donc des perspectives en vue de l’optimisation de nouvelles solutions pour l’isolation thermiqueThe issue of preserving energy raises a number of challenges to science, particularly its efficiency. The conception and characterization of new more efficient thermal insulating materials prove fundamental in this regard. Aerogels appear as serious candidates in this area, their manufacturing process provides extreme characteristics such as high porosity and low density for some of them. Thermal characterization of such materials is tricky, their low sensitivity to heat flux makes well-known methods difficult to implement. Through the study of low molecular weight aerogel samples designed by the LCPM a characterization method suitable to these samples has been developed by the LEMTA. This “three-layers” method offers the advantages of being robust and to overcome the knowledge of parameters that are difficult to reach in such cases. Describing and validating this method is the main object of this work. In addition, thermal conductivity measurements under vacuum have been processed which allowed a deeper understanding of the structure of aerogels. The results obtained this study open perspectives for the optimization of new solutions for thermal insulatio
Vers l'estimation des propriétés thermiques intrinsèques de matériaux semi-transparents à haute température à partir d'un modèle couplé résolu par la méthode de Monte-Carlo
No full textNational audienceLorsque l’on souhaite caractériser les propriétés thermiques de matériaux isolants hétérogènes et semi-transparents utilisés dans des procédés à haute température (ex: fours verriers), il est nécessaire de considérer le couplage entre la conduction et le rayonnement au sein de ces matériaux. Des travaux précédents [1] ont montré la possibilité d’estimer de façon fiable les conductivités thermiques effectives et les capacités thermiques de matériaux denses et épais optiquement jusqu’à 1200°C par l’utilisation d’un modèle conductif pur. De plus, en première approche, une séparation des contributions conductive et radiative a été effectué suivant l’approximation de Rosseland et l’évolution en température de la conductivité thermique effective estimée.Cependant, cette étude a révélé également les limites d’une telle approximation et la nécessité du développement d’un modèle direct robuste s’affranchissant d’hypothèses sur les propriétés radiatives et n’étant pas limitée aux modèles asymptotiques (milieu optiquement épais ou mince par exemple).Des études précédentes ont démontré la faisabilité et la pertinence de méthodes statistiques telles que les méthodes de Monte-Carlo lors, d’une part, de la résolution de problèmes thermiques couplés et transitoire dans le cadre de la caractérisation de milieu complexes [2] et d’autre part, pour la prise en compte du rayonnement semi-transparent dans des problèmes couplés conduction-rayonnement [3]. La présente étude expose le développement étape par étape d’un modèle couplé conduction-rayonnement recourant à de telles méthodes dans le but de séparer les contributions du rayonnement et de la conduction dans la conductivité thermique effective.[1] Penazzi L., Jannot Y., Meulemans J., Farges O., Schick V., Influence of radiation heat transfer on parallel hot-wire thermal conductivity measurements of semi-transparent materials at high temperature, International Journal of Thermal Sciences, vol. 179, art. 107690 (2022).[2] Sans M., Schick V., Parent G., Farges O., Experimental characterization of the coupled conductive and radiative heat transfer in ceramic foams with a flash method at high temperature, International Journal of Heat and Mass Transfer, vol. 148, art. 119077 (2020).[3] Penazzi L., Blanco S., Caliot C., Coustet C., El-Hafi M., Fournier R., Galtier M., Ibarrart L., Roger M., Toward the use of Symbolic Monte Carlo for conduction-radiation coupling in complex geometries, RAD-19 - 9th International Symposium on Radiative Transfer, pp. 311-318. (2019)
1D conduction radiation coupled heat transfer in an absorbing medium : Monte Carlo functionnal modeling
No full textThe data in this dataset pertains to three distinct Viskanta et al. problems.
Two problems involve a pure absorbing/emitting medium with black walls, and subsequently with gray walls.
The last problem entails an absorbing/emitting and scattering medium with black walls.
In each of these scenarios, conduction is in a steady state, and radiation occurs within the volume.
Temperature profiles and relative erros profiles are computed for various parameter values (wall temperature, Stark number, wall emissivity, scattering coefficient)
Simplified Estimation Method for the Determination of the Thermal Effusivity and Thermal Conductivity Using a Low Cost Hot Strip,” Meas.
Get PDFAbstract This paper presents the study of a hot strip made of thin rectangular electrical resistance with a thermocouple placed on its centre. The purpose was to simultaneously estimate thermal effusivity and conductivity in a limited time (t 2 < 180 s) using a low cost probe. Heat transfer has been modelled with the quadrupole formalism to simulate the evolution of the temperature at the centre of a hot strip set between two samples of material to be characterized when a heat flux step is applied. Simulation is used to fix the optimal dimensions of a hot strip that behaves as a hot plate (1D transfer) during a minimal time t 1 (>20 s) and that has higher sensitivity to the thermal conductivity between t 1 and t 2 (2D transfer). The thermal effusivity is estimated between 0 and t 1 by minimization of the quadratic errors between the experimental curve and the curve calculated by the classical hot plate model. The thermal conductivity is estimated between t 1 and t 2 but using the complete 2D model. To validate the model and the estimation process, experimental tests were realized on three materials with low diffusivities (a < 2 × 10 −7 m 2 s −1 ) and having typical area of 6 cm × 4 cm and typical thickness of 1.5 cm. Keywords: hot strip, transient method, thermal quadrupoles, effusivity, conductivity, parameter estimation method half-width of the hot strip (m) c s thermal capacity of the hot stri
A new quasi-steady method to measure gas permeability of weakly permeable porous media
No full textInternational audienceA new quasi-steady method for the determination of the apparent gas permeability of porous materials is presented in this paper along with the corresponding interpretative physical model derived from the unsteady flow equations. This method is mainly dedicated to the measurement of very low permeability of thin porous media, although thicker but more permeable samples may also be analyzed. The method relies on quasi-steady flow resulting from a (quasi) constant pressure maintained at the inlet face of the sample. Gas flow-rate, as low as 3 × 10 −10 m 3 /s, is determined from the record of pressure increase in a reservoir connected to the outlet face of the sample. An estimate of the characteristic time, t c , to reach quasi-steady flow after imposing a constant pressure at the inlet is derived. It is validated by direct numerical simulations of the complete unsteady flow, clearly defining the required experimental duration for the method to apply. Experimental results obtained on rather permeable and thick rock samples are reported showing an excellent agreement of the measured permeability with that determined independently on the same sample whereas the experimental value of t c is also in very good agreement with the predicted one. The method is further employed on a composite material sheet allowing the identification of an apparent gas permeability of about 10 −23 m 2
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