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Sistema termoelétrico aplicado ao estudo dos efeitos de congelamento e de propriedades térmicas; Thermoelectric system applied to study effects of freezing and thermal properties

Karla Silva
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 14/12/2012 PT
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Módulos termoelétricos ou Peltier podem funcionar como bombas de calor promovendo o fluxo térmico entre suas faces opostas à custa da inserção de potência elétrica. O efeito Peltier profere a linearidade entre transferência de calor e corrente elétrica inserida, tornando o módulo um aparato de características potencialmente ideais aos estudos dos efeitos das trocas térmicas. Nesse contexto, o presente trabalho traz o projeto, montagem e avaliação de um protótipo termoelétrico idealizado para congelamento unidirecional por contato direto de amostras consistentes em placa Peltier. A instrumentação do sistema possibilitou o monitoramento da potência elétrica inserida na placa e das temperaturas em diferentes pontos da superfície da placa bem como do espaço interno do sistema. Os testes de avaliação do protótipo comprovaram a adequada montagem com garantia das leituras dos sensores, bem como a boa repetibilidade dos resultados em nível de significância de 5%. Os resultados obtidos para congelamentos de gel de gelatina 2% confirmaram a viabilidade de uso do aparato para estudos dos efeitos de diferentes taxas de congelamento sobre a estruturação do gelo, com alto nível de monitoramento das condições de avanço da frente de congelamento. A precisão do protótipo em detectar pontos de alterações energéticas na amostra...

Nanoscale heat conduction with applications in nanoelectronics and thermoelectrics

Yang, Ronggui, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 217 p.
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When the device or structure characteristic length scales are comparable to the mean free path and wavelength of energy carriers (electrons, photons, phonons, and molecules) or the time of interest is on the same order as the carrier relaxation time, conventional heat transfer theory is no longer valid. Tremendous progress has been made in the past two decades to understand and characterize heat transfer in nanostructures. However most work in the last decade has focused on heat transfer in simple nanostructures, such as thin films, superlattices and nanowires. In reality, there is a demand to study transport process in complex nanostructures for engineering applications, such as heat transfer in nanoelectronic devices and the thermal conductivity in nanocomposites which consists of nanowires or nanoparticles embedded in a matrix material. Another class of problems which are rich in physics and might be explored for better design of both nanoelectronic devices and energy conversion materials and devices are coupled electron and phonon transport. Experimentally, most past work has been focused on thermal conductivity characterization of various nanostructures and very little has been done on the fundamental transport properties of energy carriers.; (cont.) This thesis work contributes to the following aspects of heat transfer...

Solar Hot Water Heater Augmented with PV-TEM Heat Pump

Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
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Solar assisted heat pumps (SAHPs) can provide higher collector efficiencies and solar fractions when compared against standard solar hot water heaters. Vapour compression (VC) heat pumps require environmentally hazardous refrigerants and moving parts which increase maintenance costs and place limits on operating conditions. Thermoelectric modules (TEMs) use solid state electronics to pump heat without moving parts or refrigerants. This study investigated the performance of a SAHP system utilizing an array of thermoelectric modules as the heat pump. A component model was developed to simulate the TEM heat pump performance using TRNSYS software and results were compared against those determined experimentally. A simplified model was unable to accurately predict TEM performance. The model was therefore refined to incorporate empirically determined parameters. An apparatus was constructed to measure the performance of a small photovoltaic array driven TEM heat pump and simulation results using the component model were found to be in agreement. Several SAHP configurations were simulated using TRNSYS and compared with vapour compression SAHPs previously studied at the QSCL. The TEM ISAHP provided a Free Energy Ratio (FER) of 46% and a collector efficiency of 36% whereas the VC ISAHP provided 54% and 76% respectively. The photovoltaic (PV) driven ISAHP system was able to achieve a higher FER of 50% with the use of a 2 module PV array and a reduced thermal collector area. Further optimization is required to improve TEM heat pump performance in this configuration and reduce the photovoltaic array size. The use of a maximum power point tracker into the PV/TEM system should be investigated as panel efficiency losses reached up to 30% in the configuration studied. The possibility of incorporating combined PV/T modules could decrease costs and also warrants further study. Critical factors in the success of a TEM driven solar assisted heat pump will include the improvement of thermoelectric materials performance...

Sistemas de hidrantes na prevenção contra incêndios em área industrial; Fire hydrants systems in fire prevention in industrial area

Quadros, Clóvis Waldy Belaunzaran de
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Trabalho de Conclusão de Curso Formato: application/pdf
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Este trabalho apresenta os procedimentos necessários para a elaboração de um Projeto de Instalações de Hidrantes, conforme a NBR-13714:2000, que tem por finalidade proporcionar a proteção contra incêndios em instalações do tipo industrial, bem como, a liberação de Alvará de Proteção Contra Incêndios, expedido pelo Corpo de Bombeiros. Normalmente os incêndios inicialmente são de pequenas proporções, aumentando de intensidade à medida que passa o tempo sem o devido combate, ocorrendo uma propagação do calor, e a respectiva progressão do incêndio. Por isso, é de fundamental importância conhecer as características do fogo, sua forma de propagação e os respectivos métodos de extinção. Considerando que o presente trabalho direciona-se à área industrial, o mesmo terá foco na utilização de hidrantes como sistema de proteção, utilizando a água como substância responsável pelo combate ao fogo, devido a sua grande capacidade de absorção de calor. Para que o combate ao fogo seja eficiente e eficaz, faz-se necessário o perfeito dimensionamento do sistema, conhecendo-se os preceitos recomendados pelas normas, bem como, conhecer o princípio de funcionamento dos equipamentos que compõe o sistema e sua perfeita utilização. Desta forma...

Subcuticular microstructure of the hornet's gaster: Its possible function in thermoregulation

Ishay, Jacob S; Pertsis, Vitaly; Neufeld, Arnon; Bergman, David J
Fonte: BioMed Central Publicador: BioMed Central
Tipo: Artigo de Revista Científica
Publicado em 11/01/2004 EN
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The present study set out to elucidate the structure and function of the large subcuticular air sacs encountered in the gaster of the Oriental hornet Vespa orientalis (Hymenoptera, Vespinae). Gastral segments I, II, III, together with the anterior portion of segment IV, comprise the greater volume of the gaster, and inside them, beneath the cuticle, are contained not only structures that extend throughout their entire length, like the alimentary canal, and the nerve cord with its paired abdominal ganglia, situated near the cuticle in the ventral side, but also the heart, which is actually a muscular and dorsally located blood vessel that pumps blood anteriorly, toward the head of the hornet. The mentioned structures take up only a small volume of the gaster, while the rest is occupied by air sacs and tracheal ducts that also extend longitudinally. Interposed between the two air sacs, there is a hard partition and above it, at the center – a paired tracheal duct that extends the entire length of the air sacs. The endothelium of the air sacs is very anfractuous, thereby enlarging and strengthening the surface area. In each gastral segment there is an aperture for the entry of air, namely, a spiracle. Additionally, in each segment, in the antero-lateral aspect of its tergum and situated between two successive segments...

Thermoelectric performance of a driven double quantum dot

Juergens, Stefan; Haupt, Federica; Moskalets, Michael; Splettstoesser, Janine
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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In this paper we investigate the thermoelectric performance of a double-dot device driven by time-dependently modulated gate voltages. We show that if the modulation frequency {\Omega} is sufficiently small, not only quantized charge pumping can be realized, but also the heat current flowing in the leads is quantized and exhibits plateaux in units of kB T ln2 {\Omega}/2{\pi}. The factor ln2 stems from the degeneracy of the double-dot states involved into transport. This opens the possibility of using the pumping cycle to transfer heat against a temperature gradient or to extract work from a hot reservoir with Carnot efficiency. However, the performance of a realistic device is limited by dissipative effects due to leakage currents and finite-frequency operation, which we take into account rigorously by means of a generalized master equation approach in the regime where the double dot is weakly coupled to the leads. We show that despite these effects, the efficiency of a double-dot charge pump performing work against a dc-source can reach of up to 70% of the ideal value.; Comment: 16 pages, 9 figures

Confined acoustic phonons in si nanomembranes: impact on thermal properties

Chávez Ángel, Emigdio
Fonte: [Barcelona] : Universitat Autònoma de Barcelona, Publicador: [Barcelona] : Universitat Autònoma de Barcelona,
Tipo: Tesis i dissertacions electròniques; info:eu-repo/semantics/doctoralThesis; info:eu-repo/semantics/publishedVersion Formato: application/pdf
Publicado em //2014 ENG
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La tendencia de la miniaturización tecnológica ha conducido a la generación de densidades de potencia que exceden los 100 watts/cm2, los cuales están en el orden del calor producido en los reactores termonucleares. La necesidad de nuevas técnicas y procesos de enfriamiento ha posicionado al manejo térmico en el escenario científico en estos últimos años. Por otro lado, la ingeniería de la conducción térmica puede abrir rutas nuevas rutas para recolección energética, por ejemplo, a través de la generación termoeléctrica. Como consecuencia, el control y la ingeniería de fonones a nivel nanométrico es una indispensable herramienta para el ajuste de propiedades físicas deseadas en dispositivos en la búsqueda de un adecuado compromiso entre performance y el consumo de potencia. En el siguiente trabajo se presenta un exhaustivo estudio teórico y experimental de la dependencia con el grosor de las propiedades térmicas en membranas de silicio con espesores que fluctúan entre los 9 a los 2000 nm. La relación de dispersión fononica y la correspondiente velocidad de fase han sido investigadas por medio de la espectroscopia inelástica de la luz Brillouin. La reducción de la velocidad de fase/grupo del modo fundamental de flexión fue observada y teóricamente explicada. Además...

Optimization of the heat pumping capacity of a thermoelectric heat pump

Heavner, David A.
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado
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A steady state model of a thermoelectric heat pump fitted with pin fin heat exchange surfaces is developed from the equations governing the operation of a thermoelectric device and pressure drop/ heat transfer correlations for staggered tube banks in cross flow. The model computes current draw, heat pumped at the cold side, heat rejected at the hot side, hot and cold side exhaust temperatures, and hot and cold side pin base temperatures when given the following parameters: ambient temperature, thermoelectric geometry factor, number of thermocouple junctions, input voltage, pin diameter, pin height, transverse pin stagger, longitudinal pin stagger, contact resistance between the thermoelectric modules and the heat ex change surfaces, and fan performance curves for the hot and cold sides. Results of a FIDAP numerical model of a 4 row heated pin system are presented and compared to the heat transfer and pressure drop correlations chosen for use in the model. Nine tests were run with a working thermoelectric heat pump unit under various air flow and voltage input to collect data for the purpose of comparison with the model. Using empirically determined contact resistance values, the model predicted Qc (heat pumped from the cold side) values that agreed with experiment within about 10%...

Effect of heat transfer on the performance of a thermoelectric heat pump driven by a thermoelectric generator

Chen,Lingen; Meng,Fankai; Sun,Fengrui
Fonte: Sociedad Mexicana de Física Publicador: Sociedad Mexicana de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/2009 EN
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A model of a thermoelectric heat pump driven by a thermoelectric generator with external heat transfer irreversibility is proposed. The performance of the combined thermoelectric heat pump device obeying Newton's heat transfer law is analyzed using the combination of finite time thermodynamics and non-equilibrium thermodynamics. Two analytical formulae for heating load versus working electrical current, and the coefficient of performance (COP) versus working electrical current, are derived. For a fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the heating load and the COP of the combined thermoelectric heat pump device. For a fixed total number of thermoelectric elements, the ratio of the number of thermoelectric elements of the generator to the total number of thermoelectric elements is also optimized for maximizing both the heating load and the COP of the combined thermoelectric heat pump device. The influences of thermoelectric element allocation and heat transfer area allocation are analyzed by detailed numerical examples. The optimum working electrical currents for maximum heating load and maximum COP at different total numbers of thermoelectric elements and different total heat transfer areas are provided...