Heat transfers in a double-skin roof ventilated by natural convection in summer time

International audienceThe double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in the channel underneath drives off the residual heat. The bi-dimensional numerical simulation of the heat transfers through the double skin reveals the most important parameters for the system's efficiency. They are, by order of importance, the sheet metal surface emissivity, the screen internal and external surface emissivity, the insulation thickness and the inclination angle for a channel width over 6 cm. The influence of those parameters on Rayleigh and Nusselt numbers is also investigated. Temperature and air velocity profiles on several channel cross-sections are plotted and discussed

Phase-change materials to improve solar panel's performance

International audienceHigh operating temperatures induce a loss of efficiency in solar photovoltaic and thermal panels. This paper investigates the use of phase-change materials (PCM) to maintain the temperature of the panels close to ambient. The main focus of the study is the computational fluid dynamics (CFD) modeling of heat and mass transfers in a system composed of an impure phase change material situated in the back of a solar panel (SP). A variation of the enthalpy method allows simulating the thermo-physical change of the material properties. The buoyancy term in Navier-Stokes' momentum conservation equation is modified through an additional term which forces the velocity field to be non-existent when the PCM is solid. For validation purposes, isotherms and velocity fields are calculated and compared to those from an experimental set-up. Results show that adding a PCM on the back of a solar panel can maintain the panel's operating temperature under 40 °C for 80 minutes under a constant solar radiation of 1000 W/m2

Volumetric Monitoring and Modeling of Indoor Air Pollutant Dispersion by the Use of 3D Particle Tracking Velocimetry

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Application of two-color LIF thermometry to nucleate boiling

International audienceThe laser-induced fluorescence (LIF) thermometry is applied to measure the temperature field surrounding a single vapor bubble growing at an artificial nucleation site. In order to correct measurement errors due to the non-uniformity of the incident laser intensity, the two-color LIF thermometry technique is used in this nucleate boiling experiment. This technique is based on the use of two fluorescent dyes: the temperature sensitive dye Rhodamine B and the temperature insensitive dye Sulforhodamine-101. The concentration of the dyes is optimized by analyzing the behavior of fluorescence intensities. The mapping between the two images is determined through a geometrical calibration procedure. This technique presents a success in correcting the non uniformities due to the reflection of the light at the bubble surface and to the temperature gradient. The obtained temperature fields show that the two-color LIF is a promising technique in the investigation of nucleate boiling

Solar PV Passive Temperature Control Using Phase Change Materials

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Thermal behavior of a passive solar wall with silica aerogel and phase change materials

International audienceThe present contribution discusses the thermal behavior of a passive solar wall composed of a glazing facing the outside, a channel filled with highly insulating and translucent silica aerogel, and glass bricks filled with an eutectic phase change material (PCM), facing the inside. First, the experimental building with the tested wall, south-oriented, is described. Second, the experimental results on four different weather conditions are detailed. Thanks to a numerical model, the wall behavior is simulated for the French climatic zones of Nice (Mediterranean climate), La Rochelle (oceanic climate), Embrun (inland mountain climate) and Nancy (cold continental climate). Results show that the wall is translucent and may save 27% to 83 % on the building energy bill for heating, depending on the building location. The wall performs best on cold sunny climates, where the phase change can occur all year. The air temperature inside the building is always 1 to 9°C above the external temperature. Overheating may occur in summer for Mediterranean and oceanic climates when the PCM stays in liquid state. This may be prevented by the use of shading

Phase Change Materials (PCM) for cooling applications in buildings: A review

International audienceCooling demand in the building sector is growing rapidly; thermal energy storage systems using phase change materials (PCM) can be a very useful way to improve the building thermal performance. The right use of PCM in the envelope can minimize peak cooling loads, allow the use of smaller HVAC technical equipment for cooling, and has the capability to keep the indoor temperature within the comfort range due to smaller indoor temperature fluctuations. This article presents an overview of different PCM applications in buildings for reducing cooling loads under different climate conditions, and the factors affecting the successful and the effective use of the PCM. Many drawbacks have been found in PCM applications, mainly the intense impact of summer weather conditions over the PCM performance, which prohibits its complete solidification during night, and thus, limiting its effectiveness during the day. Proposed solutions are reviewed in this article. Finally, a topology diagram is presented to summarize the steps leading to an effective use of PCM in building applications

Gestion thermique des batteries Li-ion par l'utilisation de matériaux à changement de phase

International audienceLes batteries lithium-ion nécessitent une gestion thermique pour que soit assuré leur bon fonctionnement. Des systèmes actifs comme un refroidissement par air en convection forcée ou par un liquide de refroidissement sont employés actuellement. L’utilisation de matériaux à changement de phase (MCP), en tant que système passif, représente une alternative intéressante. Dans ce papier une comparaison est proposée entre un système comportant un MCP et un système à air en convection forcée

Transfert de chaleur dans les blankets aérogels de silice

International audienceLes blankets aerogels de silice sont des matériaux composites composés de nanoparticules de silice interconnectées et de renforts fibreux. Leur structure meso-poreuse en fait d’excellents isolants thermiques et suscitent beaucoup d’intérêt dans le secteur du bâtiment. La présente étude propose des modèles de conductivité thermique des différents modes de tranfert thermique au sein de ce matériau ainsi qu’un modèle de conductivité thermique effective. On s’intéresse tout particulièrement à l’évolution de la conductivité thermique effective en fonction de la densité du matériau, de sa température, de la pression et de l’humidité relative. Les résultats issus des modèles sont comparés à quelques valeurs expérimentales

Suivi Lagrangien tridimensionnel de particules en grand champ pour l'étude des écoulements d'air dans les bâtiments

Alors que la majorité des méthodes actuelles de suivi Lagrangien de particules ne permettent que la mesure d'écoulements 2D ou 3D à petite échelle, le présent document décrit un algorithme complet de suivi Lagrangien 3D en grand champ. Des bulles de savon gonflées à l'hélium servent de traceurs. L'éclairement est fourni par des sources continues de type halogènes. Des cameras synchrones sont préalablement calibrées suivant des algorithmes connus afin d'avoir un repère 3D commun. Apres suppression du fond, une procédure spéciale supprime les larges spots créées par les images des bulles se rapprochant des caméras. Les centres de masse des bulles restantes sont ensuite calculés. Le suivi temporel se fait soit par corrélation croisée, soit par régression linéaire tempéré par l'emploi d'un critère de qualité. La triangulation 3D se fait par une méthode de moindres carrés. L'algorithme complet est testé dans plusieurs configurations : dans une cellule expérimentale de taille 3.1mx3.1mx2.5m aux parois gris clair ; (ii) dans une salle de taille 5.5mx3.7mx2.4m aux parois noires ; (iii) au dessus d'une source de chaleur; (iv) à l'intérieur d'une maquette d'avion de taille réelle; (v) dans un réservoir 100cmx100cm 100cmx rempli d'une solution aqueuse en utilisant une lumière continue et des particules creuses de 10um de large. Dans chaque cas, des lignes directrices sont données pour un positionnement optimal des cameras et des sources de lumière. Les résultats montrent que l'algorithme est capable de suivre plus de 1400 traceurs dans des volumes allant jusqu'a 3mx3mx1.2m.While most research on particle tracking velocimetry (PTV) is devoted either to 2D flows or to small scale 3D flows, this paper describes a complete 3D PTV algorithm and some applications to indoor airflow velocity measurement. A particle detection procedure specially adapted to the physical characteristics of neutrally buoyant helium filled soap bubbles is described. In particular, overlarge particle images are removed. Particle center are calculated by a weigh averaging method. Two temporal tracking schemes are presented and compared. The first one is based on fast normalized cross-correlation with Lagrangian extrapolation in image space to solve ambiguities. The second uses polynomial regression to find an estimated position and applies a quality criterion based on a minimization of changes in particle acceleration; to increase the measurement area and the number of trajectories, the correspondence problem is addressed by a new procedure involving fundamental matrices using both three and two 3D calibrated cameras. 3D triangulation is done by a least squares method. Some guidelines are given in terms of camera and light positioning for 3D PTV in large volumes with various wall colors. Applications of the algorithm include Lagrangian tracking: (i) in a light-gray walled 3.1mx3.1mx2.5m high test-room; (ii) inside a black walled 5.5mx3.7mx2.4m high test-room; (iii) over a heat source and inside; (iv) in experimental aircraft cabin; (v) in a 100cmx 100cmx100cm tank filled with an aqueous solution using continuous light and 10 m-large hollow particles. Results show that the algorithm is capable of tracking more than 1400 tracers in volumes up to 3mx3mx1.2m high.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF