Rendimento de painéis solar térmicos poliméricos unglazed e glazed

Mestrado em Engenharia Físicaas energias renováveis representam uma solução limpa para a diminuição da dependência da nossa sociedade nos combustíveis fósseis. Recentemente verificou-se um aumento na procura de combustíveis fósseis em todo mundo, levando a um acentuado aumento no seu preço. Considerando que uma fracção considerável do nosso consumo energético é usado para aquecimento de água, a conversão solar térmica torna-se um tema de elevado interesse. Actualmente, existem vários sistemas solares térmicos para aquecimento de água com eficiências e tempos de recuperação de investimento razoáveis. Aqueles que usam absorsores poliméricos, apresentam ser uma solução economicamente eficiente para aplicações de aquecimento de água a baixa temperatura. Neste trabalho apresentam-se os resultados da análise do desempenho de um painel comercial solar térmico polimérico sem cobertura, unglazed, usado primariamente para o aquecimento de piscinas, que usa como absorsor tiras de epdm (ethylene propylene diene monomer). Este polímero tem boas propriedades mecânicas e boa resistência aos UV. O objectivo principal deste trabalho é comparar a eficiência e as alterações físicas no colector quando é introduzida uma cobertura transparente e estruturada de PC (policarbonato), transformando-o num painel solar térmico glazed. ABSTRACT: renewable energies represent a clean solution to decrease the dependence of our societies on fossil fuels. In recent times we have witnessed an increase in the demand for fossil fuels worldwide, leading to ever increasing prices. Considering that a considerable fraction of our energy consumption goes to water heating applications, solar thermal conversion becomes a relevant approach. Presently, several solar water heating systems with reasonable efficiencies and with acceptable payback times are available. Those using polymeric solar absorbers can be an economical and efficient solution for low temperature water heating applications. This work presents the results of the performance analysis of an unglazed commercial polymeric solar thermal collector, primarily used for swimming pool heating, made from epdm (ethylene propylene diene monomer). This polymer shows good mechanical properties and good UV resistance. The main goal of this work is to compare the efficiency and the changes on material physical properties when this unglazed collector is covered with a transparent PC (polycarbonate) structured sheet, transforming it into a glazed solar thermal collector

Dripping to jetting transition for cross-flowing liquids

International audienceWe experimentally study drops formed from a nozzle into an immiscible, cross-flowing phase. Depending on the operating conditions, drops are generated either in dripping or jetting mode. We investigate the impact of the continuous and dispersed phase velocities, dispersed phase viscosity, and interfacial tension on the drop generation mode and size. We find that a dripping to jetting transition (DJT) takes place at a critical inner Weber number, a function of the outer capillary and Ohnesorge numbers. Two jetting regimes occur depending on the phase velocity ratio. When the continuous phase velocity is significantly greater (respectively lower) than the dispersed phase velocity, jet narrowing (respectively widening) occurs. In jet widening, the critical inner Weber number depends little on the outer capillary number whereas in jet narrowing, it sharply decreases as the outer capillary number increases. We propose a comprehensive model to describe the DJT based on the attached drop equation of motion. The model satisfactorily predicts the DJT and the effect of the outer capillary number on the critical inner Weber number. It also well accounts for the drop diameter in jet narrowing

Drying intensification by vibration: fundamental study of liquid water inside a pore

[EN] Vibration is a promising way to intensify the drying process through heating-up due to viscous dissipation, activation of internal liquid transfer and increase of external transfer. To better assess the possible contribution of these effects, we choose a multiscale approach. This paper is focused on the pore scale, simulated by a capillary tube partially filled with water subjected to sinusoidal vibrations. We studied the displacement of water inside this tube through image analysis. This configuration mimics the moisture transfer inside the pores of a porous media during drying. The experimental device developed in this study is applicable to a wide range of configurations, such as symmetrical or asymmetrical vibrations.Chen, W.; Colin, J.; Casalinho, J.; Ben Amara, M.; Stambouli, M.; Perré, P. (2010). Drying intensification by vibration: fundamental study of liquid water inside a pore. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 1911-1918. https://doi.org/10.4995/IDS2018.2018.7961OCS1911191

Effet de la pression sur la cinétique de pyrolyse de biomasse lignocellulosique

International audienceFace au besoin de réduire des émissions de dioxyde de carbone, la biomasse est une alternative permettant de passer du déstockage de carbone fossile à un flux de carbone renouvelable. Cette biomasse, qui peut être produite par des cultures dédiées, être des coproduits d'autres cultures (agriculture ou sylviculture) ou provenir des résidus industriels/agriculture, nécessite plusieurs étapes pour être convertie en biocarburant 1. Dans la voie BtL, qui voit en ce moment un regain d'intérêt pour la production de jet fuel, la pyrolyse est un prétraitement intéressant 2,3. Plusieurs travaux montrent le potentiel de la pyrolyse de la biomasse sous pression, car ce paramètre peut influencer la taille de particule, les produits de réaction, la température de réaction et ainsi les coûts du procédé 4,5. Usuellement, les phénomènes thermochimiques liées à la pyrolyse sont déterminés par thermogravimétrie (TGA et/ou DSC) qui permet d'étudier la cinétique de la perte de masse et de flux de chaleur 6,7. Souvent, les expériences thermogravimétriques se composent d'un essai à blanc suivi d'un essai avec l'échantillon. Par correction (substruction) on obtient la variation de masse. Cependant, cette méthode ne permet pas de corriger la dérive temporelle de la balance (souvent de plusieurs µg/h). La communauté scientifique constate que les études avec des balances thermogravimétriques posent encore des problèmes de calibration et de reproductibilité 8,9. Dans ce travail, nous utilisons une balance à lévitation magnétique (HP-TGA 75, TA-Instruments) capable de travailler sur une large gamme de pression : 0,2 à 80 bar. Des cinétiques ont été réalisées à température linéairement croissante, jusqu'à 800°C avec de la poudre d'épicéa sur une faction de tamisage de 0.063 à 0.08 mm pour différents niveaux de pression sous atmosphère neutre (N2). Dans le cas de pyrolyse sous pression, le changement de volume de l'échantillon suite aux réactions (variation de la flottabilité) et le changement de viscosité des gaz avec la température et/ou pression doivent être pris en considération. Nous proposons un protocole rigoureux de traitement des données. Les cinétiques corrigées seront présentées. L'objectif du travail en cours sera d'intégrer l'effet de la pression dans une approche DAEM 10

Macroscopic formulation of non-Fickian diffusion in bio-based materials using memory functions: experimental identification on minute samples.

International audienc

Highly deformable food products: in-situ measurements during drying and physical modelling

International audienceConvective drying at low temperature is a process commonly used in the industry, especially for products that are prone to thermal degradation such as food products. Some advantages of convective drying, compared to other types of drying, are the low-cost infrastructures, easy process control and flexibility. Nevertheless, even if convective drying is performed at low temperature level, the water removal produces sample shrinkage, generally triggered by cell collapse. For a better understanding of the physical behavior of bioproducts during convective drying, our team built an experimental device capable to accurately and continuously measure the mass and shape of samples during convective drying (Figure 1). This device was used to study the drying behavior of several highly deformable food products [1, 2]. Clear differences in physical behavior between vegetable products were observed, for example, an isotropic shrinkage behavior was observed for potato and an anisotropic behavior for apple. These results were explained by the anatomical structure of the products, observed by environmental scanning electron microscopy, ESEM (Figure 2). The experimental data was also used to analyze the volume-moisture content relation during drying and to propose an extended van Meel model [3]. This model is able to easily predict the drying behavior of a highly hydrated and deformable product

Mass diffusivity of various building materials determined by inverse analysis of RH evolution at the back face of the sample

International audienceThis work used a new method for the determination of the mass diffusion coefficients of hygroscopic materials [1]. The experiment consists in submitting one face of the sample to a variation in time of the relative humidity (RH) and measuring the RH on its back face. The imposed RH and temperature are measured during the test and serve as boundary conditions in a comprehensive computational code to solve heat and mass transfer in porous media. This new method was used to characterize a various building materials such as autoclave aerated concrete, plaster, massive wood or wood based materials. This paper includes the determination of sorption isotherms of the selected materials which are needed for the determination of mass diffusivity. The study also confirms that accurate values of mass diffusivity can be obtained even if sample are not at the equilibrium, which allows the characterization time to be drastically reduced

Inverse analysis of oxygen diffusivity in oak wood using the back-face method: application to cooperage

International audienceDuring the aging of wines and spirits in oak barrels, oxygen diffusion through the wood occurs and leads to mild oxygenation of the beverage. In this study, the oxygen diffusivity in oak wood was determined by inverse analysis using the back-face method. In the measurement setup, a defined oxygen concentration was applied to the front of a sample and the variation of oxygen concentration due to gas diffusion through the sample was measured on the back. The experiment was carried out simultaneously on several samples. It was then possible to study 36 samples and to assess the effect of several parameters in a reasonable time. A finite volume model using the actual experimental conditions as boundary conditions was implemented for the identification of the diffusion coefficient. The obtained values range between 4.64 × 10−11 and 2.02 × 10−9 m2 s−1 and highlight the high heterogeneity of oak wood. Such low values, compared to the diffusion of oxygen in air (a factor 105 lower), reflect the huge tortuosity of oak wood in its tangential direction