Dissertations / Theses on the topic 'Micro refrigerator'

Cooling of electronic equipments has become an important issue as the advances in technology enabled the fabrication of very small devices. The main challenge in cooling is the space limitation. The use of miniature refrigerators seems to be a solution alternative for the cooling problem. The objective of this study is to design and simulate a vapor compression refrigeration cycle for a micro-scale refrigerator. A MATLAB code is developed for the simulations. The four components of the refrigerator, namely, the condenser, evaporator, compressor and the capillary tube are designed separately. The cycle is successfully completed nearly at the same point where it begins. The cold space temperature, ambient air temperature, condensation and evaporation temperatures, and the evaporator heat load are the predetermined parameters. A fan is used to cool the condenser, and the compressor is selected as isentropic. R-134A is selected as the refrigerant and a simple interpolation code is developed to obtain the thermophysical properties of R-134A. The original design is carried out with an isentropic compressor. For the purpose of comparison, a cycle with a polytropic compressor is also considered. Similarly, two alternative designs for the evaporator are developed and simulated. A second law analysis is performed at the end of the study.

A existência de ondas gravitacionais foi confirmada indiretamente pela observação astronômica de pulsares binários. Detectores de ondas gravitacionais tem sido desenvolvidos desde o trabalho pioneiro de Weber nos anos 60. Esforços estão sendo realizados no sentido de aumentar a sensibilidade dos detectores e realizar uma detecção direta, que ainda não foi confirmada. O Grupo GRAVITON está aperfeiçoando e melhorando a sensibilidade de um detector de ondas gravitacionais que se encontra no Laboratório de Estado Sólido e Baixas Temperaturas do Instituto de Física da Universidade de São Paulo (LESBT/IFUSP), na cidade de São Paulo com apoio da FAPESP (processo 2006/56041-3). Esse detector, denominado MARIO SCHENBERG, é composto por uma massa ressonante esférica de CuAl(6%) com 65 cm de diâmetro, com aproximadamente 1150 kg, que deverá atingir a sensibilidade h ~ 10-22 em uma banda passante de 50 Hz, em torno de 3200 Hz, quando estiver operando a temperaturas da ordem de 0,05 K. Atualmente o detector já tem toda a sua infraestrutura criogênica montada e testada para resfriamentos a 4 K e toda a suspensão da esfera bem como todo o sistema de filtragem mecânica construídos e montados. Já foram realizadas as primeiras corridas comissionadas em 2006, 2007 e 2008, quando foram realizados vários diagnósticos sobre o sistema e desde então vem sendo desenvolvidos os transdutores para colocar o detector novamente em operação com melhor sensibilidade. Paralelamente, foram realizadas melhorias no próprio detector em razão dos diagnósticos realizados. O trabalho aqui apresentado está associado ao projeto acima. O autor desenvolveu atividades associadas à construção e desenvolvimentos do detector, que podem ser divididas em três partes principais: na parte mecânica, foi desenvolvido, instalado e testado um novo sistema de isolamento vibracional da suspensão da esfera; na parte criogênica foram feitas novas conexões térmicas, cálculos de gastos de hélio líquido e feitos desenvolvimentos para o funcionamento do refrigerador por diluição; e na parte eletrônica foi feita a instalação da eletrônica responsável pela transdução do sinal, além do desenvolvimento de um novo par de antenas de microfita.
The existence of gravitational waves has been confirmed indirectly by astronomical observation of binary pulsars. Gravitational wave detectors have been developed since the pioneering work of Weber in the 60s. Efforts are being made to increase the sensitivity of the detectors and perform a direct detection, wich has not been confirmed yet. The GRAVITON Group is enhancing and improving the sensitivity of a gravitational wave detector which is at the Laboratório de Estado Sólido e Baixas Temperaturas of the Instituto de Física of the Universidade de São Paulo (LESBT / IFUSP), in São Paulo city and is supported by FAPESP (processo 2006/56041-3). This detector, called MARIO SCHENBERG, consists of a spherical resonant mass of CuAl (6%) with 65 cm in diameter, and approximately 1150 kg, which should reach the sensitivity of h ~ 10-22 in a bandwidth of 50 Hz around 3200 Hz, when operating at temperatures of 0.05 K. Currently the detector already has all its infrastructure assembled and tested for cryogenic cooling down to 4 K and the whole suspension of the sphere as well as all mechanical isolation system constructed and assembled. Commissioning runs have already been done in 2006, 2007 and 2008, when several diagnoses on the system were performed and since then there have been many developments on the transducers to put back the detector into operation with improved sensitivity. At the same time, improvements have been made within the detector itself due to the diagnoses. The work presented here is associated with the above project. The author has developed activities and developments associated with the detector construction, which can be divided into three main parts: the mechanical part, in which a new system of vibration isolation was designed for the sphere suspension, installed and tested; the cryogenic part, in which new connections and thermal calculations of liquid helium boil-off rate were made as well as other developments for the operation of a dilution refrigerator; and the electronic part, in which the installation of the electronic signal responsable for the transduction was made, besides the development of a new pair of micro-strip antenna.

Chip scale refrigeration system is critical for the development of electronics with the rapid increase of power consumption and substantial reduction of device size, resulting in an emergent demand on novel cooling technologies with a high efficiency for the thermal management. In this thesis, active refrigeration devices based on Stirling cycle and an electrocaloric material, are designed and investigated to achieve a high cooling performance. Firstly, a new Stirling micro-refrigeration system composed of arrays of silicon MEMS cooling elements is designed and evaluated. The cooling elements are fabricated in a stacked array on a silicon wafer. A regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms, which are driven electrostatically. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally and out of phase such that heat is extracted to the expansion space and released from the compression space. A first-order of thermodynamic analysis is performed to study the effect of geometric parameters. Losses due to regenerator non-idealities and chamber heat transfer limitation are estimated. A multiphysics computational approach for analyzing the system performance that considers compressible flow and heat transfer with a large deformable mesh is demonstrated. The optimal regenerator porosity for the best system COP (coefficient of performance) is identified. To overcome the computational complexity brought about by the fine pillar structure in the regenerator, a porous medium model is used to allow for modeling of a full element. The analysis indicates the work recovery of the system and the diaphragm actuation are main challenges for this cooler design.The pressure drop and friction factor of gas flow across circular silicon micro pillar arrays fabricated by deep reactive ion etch (DRIE) process are investigated. A new correlation that considers the coupled effect of pillar spacing and aspect ratio, is proposed to predict the friction factor in a Reynolds v number range of 1-100. Silicon pillars with large artificial roughness amplitudes is also fabricated, and the effect of the roughness is studied in the laminar flow region. The significant reduction of pressure drop and friction factor indicates that a large artificial roughness could be built for pillar arrays in the regenerator to enhance the micro-cooler efficiency. The second option is to develop a fluid-based refrigeration system using an electrocaloric material poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer. Each cooling element includes two diaphragm actuators fabricated in the plane of a silicon wafer, which drive a heat transfer fluid back and forth across terpolymer layers that are placed between them. Finite element simulations with an assumption of sinusoidal diaphrahm motions are conducted to explore the system performance detailedly, including the effects of the applied electric field, geometric dimensions, operating frequency and externally-applied temperature span. Multiphysics modeling coupled with solid-fluid interaction, heat transfer, electrostatics, porous medium and moving mesh technique is successfully performed to verify the thermal modeling feasibility. The electrocaloric effect in thin films of P(VDF-TrFE-CFE) terpolymer is directly measured by infrared imaging at ambient conditions. At an electric field of 90 V/μm, an adiabatic temperature change of 5.2 °C is obtained and the material performance is stable over a long testing period. These results suggest that application of this terpolymer is promising for micro-scale refrigeration.

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The storage of food at low temperatures is the most widely used form of preservation in the production and distribution of fresh food. The practice of storing raw milk refrigerated at the source of production was instituted by the Ministry of Agriculture, Livestock and Food Supply (MAPA) in 2002, aiming to reduce the losses due to the acidifying action of mesophiles. The research of contaminating microorganisms allows the identification of the flaws that lead to the proliferation of these bacterial groups with consequent loss of product quality, and allows the adoption of control measures in order to meet the standards established by the legislation. The objective was to determine the microbiological quality of raw milk from 12 farms located in Pernambuco, Brazil, through quantification of mesophiles and coliforms, which are the micro-organisms of choice for quality control of refrigerated raw milk, whose maximum amounts are established in the current legislation. After quantification and establishment of the cooling effect, 80 isolates of these microorganisms were stocked and passed through molecular tests for genotypic identification of the Escherichia coli strain O157: H7 species. Refrigeration for 24 hours did not interfere with microbial proliferation (P≥0.05) and E. coli O157: H7 was not identified. It is concluded that the milk produced in the properties of three municipalities that compose the milk basin of the state of Pernambuco presents low microbiological quality, which can result in harm to the producers.
O armazenamento de alimentos a baixas temperaturas é a forma de conservação mais utilizada na produção e distribuição de alimentos frescos. A prática de estocagem do leite cru refrigerado na fonte de produção foi instituída pelo Ministério da Agricultura, Pecuária e Abastecimento - MAPA em 2002, visando reduzir as perdas por ação acidificante de mesófilos. A pesquisa de micro-organismos contaminantes permite a identificação das falhas que levam a proliferação desses grupos bacterianos com consequente perda da qualidade do produto, e possibilita a adoção de medidas de controle de forma a atender os padrões estabelecidos pela legislação. Objetivou-se determinar a qualidade microbiológica do leite cru de 12 propriedades localizadas no agreste pernambucano, por meio de quantificação de mesófilos e coliformes, que são os micro-organismos indicadores de eleição para o controle da qualidade do leite cru refrigerado, cujo as quantidades máximas permitidas são estabelecidas na legislação vigente. Após a quantificação e o estabelecimento do efeito da refrigeração, 80 isolados desses micro-organismos foram estocados e passaram por testes moleculares para identificação genotípica da espécie Escherichia coli cepa O157:H7. A refrigeração por 24 horas não interferiu na proliferação microbiana (P≥0,05) e a E. coli O157:H7 não foi identificada. Conclui-se que o leite produzido nas propriedades de três municípios que compõem a bacia leiteira do estado de Pernambuco apresenta baixa qualidade microbiológica, o que pode resultar em prejuízo aos produtores.

Orientador: Ana Maria Centola Vidal Martins
Coorientador: Karina Paes Bürger
Banca: Arlindo Saran Netto
Banca: Angela Cleusa de Fátima Banzatto de Carvalho
Resumo: O objetivo do presente trabalho foi de avaliar 10 propriedades, produtoras de leite cru refrigerado, localizadas no Sudeste do Estado de São Paulo, quanto ás condições higiênico-sanitárias do rebanho bem como o manejo; e correlacionar á qualidade do leite produzido por cada uma delas com o estabelecido pela IN62 de 2011. Foi aplicado um checklist, foi avaliado o índice de mastite clínica e subclínica e identificados os micro-organismos envolvidos e seu perfil de sensibilidade á antimicrobianos, assim como analisados os parâmetros físicoquímicos, de Contagem de Células Somáticas (CCS) e microbiológicos estabelecidos pela legislação vigente. Após a aplicação do checklist era realizado o teste da caneca de fundo escuro e o California Mastits Tests (CMT), e colhidas amostras dos quartos positivos em tubos estéreis para isolamento microbiológico. As amostras de leite colhidas dos tanques de expansão eram submetidas à CCS, Contagem Bacteriana Total (CBT) e a análise de composição. Dos 1280 quartos mamários avaliados, 43,67% foram positivos ao CMT, caracterizando mastite subclínica e 2,89% dos quartos positivos ao teste da caneca de fundo escuro caracterizando mastite clínica. O Staphylococcus aureus foi o micro-organismo mais isolado, sendo que nos casos de mastite clínica ele representou 64,86%, seguido de S. epidermidis com 18,91% e 2,70% de S. intermedius. Nos casos de mastite subclínica o S. aureus foi isolado de 73,60% dos quartos analisados, seguido de S. epidermidis com 16,57%, Corynebacterium sp. com 3,66%, S. dysgalactiae com 3,08%, S. intermedius com 1,93%, Bacillus sp. com 0,58% e Enterococcus sp. com 0,59%. No teste de sensibilidade antimicrobiana, in vitro, o Florfenicol foi o antimicrobiano, do qual os micro-organismos isolados nos casos de mastite clínica apresentaram maior sensibilidade. Das 10 propriedades produtoras de leite sete apresentaram valores superiores ao de 500 mil céls./mL para ...
Abstract: The objective this study was to assay 10 dairy farms that produce refrigerated raw milk, located in the southeastern of São Paulo State, on the hygenicsanitary conditions and management of the herd and, if possible, to correlate the quality of the milk produced with the standards recommended by IN62. For this, a checklist was applied together with the assessment of the rate of clinical and subclinical mastitis, identification of the microorganisms involved and their sensitivity to antimicrobials. The microbiological, physical-chemical and SCC parameters were also analyzed according to the established by law. Following application of the checklist,test of the black deep mugand CMT were performed and samples from the positive mammary quarters were collected in sterile tubes for microbiological isolation. Bulk tank milk samples were submitted to SCC, TBC and composition analysis. From 1280 mammary quarters evaluated, 43.67% were positive for the CMT, characterizing subclinical mastitis; and 2.89%, in the test of the black deep mug, characterizing clinical mastitis. Staphylococcus aureus was the most frequently isolated microorganism, and in cases of clinical mastitis, represented 64.86% of all positive samples, followed by S. epidermidis 18.91% and S. intermedius 2.70%. In cases of subclinical mastitis, S. aureus was isolated from 73.60% of the quarters examined, followed by S. epidermidis 16.57%, Corynebacterium sp. 3.66%, S. dysgalactiae 3.08%, S. intermedius 1.93%, Enterococcus sp. 0.59% and Bacillus sp. 0.58%. Regarding the in vitro antimicrobial susceptibility test, florfenicol was the antibiotic to which the microorganisms isolated in clinical mastitis cases showed higher sensitivity. Of the 10 farms milk producers seven had values higher than the 500,000 cels. / mL for SCC in relation to that established by IN62, but for TBC and protein, all attended the set of at least 300,000 CFU / mL and 2.9 g / 100g, respectively, but for ...
Mestre

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Os refrigeradores de uso dom?stico s?o equipamentos que apresentam uma parcela significativa do consumo de energia el?trica nas resid?ncias brasileiras. O uso destes equipamentos com baixa efici?ncia energ?tica contribui para o aumento do consumo de energia. A efici?ncia energ?tica de um refrigerador ? fun??o da intera??o o fluido refrigerante e os componentes do ciclo termodin?mico. Altera??es na carga e/ou natureza do refrigerante podem modificar as press?es de condensa??o e/ou evapora??o. A capacidade volum?trica do compressor, a vaz?o m?ssica de refrigerante e a pot?ncia de compress?o s?o par?metros dependentes das press?es de evapora??o e condensa??o. Assim, os dispositivos de expans?o exercem um papel importante no equil?brio destas press?es, sendo fundamentais para o melhor desempenho do ciclo de refrigera??o. A presente pesquisa experimental tem como prop?sito investigar a sensibilidade dos par?metros de desempenho de um refrigerador dom?stico operando com R134a e em diferentes press?es de evapora??o. Para tanto, um refrigerador de pequeno porte foi instrumentado com sensores de temperatura, de press?o e de outras vari?veis de interesse, instalados ao longo do circuito frigor?fico, de modo a permitir o mapeamento t?rmico e a avalia??o dos par?metros de desempenho do equipamento. A varia??o da perda de carga no fluido refrigerante resultante da atua??o da v?lvula de expans?o de ajuste microm?trico modifica a temperatura de evapora??o, influenciando sensivelmente os par?metros de desempenho do ciclo termodin?mico de refrigera??o.
Household refrigerators are equipments that represent a significant portion on the eletricity consumption of Brazilian homes. The use of these devices with low energy efficiency contributes to increase the energy consumption. The energy efficiency of a refrigerator is a function of the interaction between the coolant fluid and the components of the thermodynamic cycle. Changes in load and/or nature of the coolant may modify the condensing and/or evaporation pressures. The volumetric capacity of the compressor, the mass flow of coolant and the compression power are dependent parameters of the condensation and evaporation pressures. Thus, the expansion devices exert an importante role in the balance of these pressures, being fundamental for the better performance of the refrigeration cycle. This experimental research aims to investigate the sensitivity of the performance parameters of a household refrigerator operating with R134a and at different evaporation pressures. Therefore, a small refrigerator was instrumented with temperature, pressure sensors and other variables of interest, installed along the cooling circuit, in order to allow the thermal mapping and the evaluation of the equipment performance parameters. The variation of pressure loss in the coolant fluid resulting from the operation of the expansion valve with micrometric adjustment that modifies the evaporation temperature, influencing significantly the performance parameters of the thermodynamic refrigeration cycle.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2008.
Jeremy P. Harvey, Committee Member ; Carl S. Kirkconnell, Committee Member ; Kurt D. Pennell, Committee Member ; S. Mostafa Ghiaasiaan, Committee Chair ; Prateen V. Desai, Committee Member ; Sheldon M. Jeter, Committee Member.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008.
Kirkconnell, Carl S., Committee Member ; Ghiaasiaan, S. Mostafa, Committee Chair ; Desai, Prateen V., Committee Member ; Jeter, Sheldon M., Committee Member.

The Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEGIS) at the European Organization for Nuclear Research (CERN) is an experiment investigating the influence of earth’s gravitational force upon antimatter. To perform precise measurements the antimatter needs to be cooled to a temperature of 100 mK. This will be done in a Penning trap, formed by several electrodes, which are charged with several kV and have to be individually electrically insulated. The trap is thermally linked to a mixing chamber of a 3He-4He dilution refrigerator. Two link designs are examined, the Rod design and the Sandwich design. The Rod design electrically connects a single electrode with a heat exchanger, immersed in the helium of the mixing chamber, by a copper pin. An alumina ring and the helium electrically insulate the Rod design. The Sandwich uses an electrically insulating sapphire plate sandwiched between the electrode and the mixing chamber. Indium layers on the sapphire plate are applied to improve the thermal contact. Four differently prepared test Sandwiches are investigated. They differ in the sapphire surface roughness and in the application method of the indium layers. Measurements with static and sinusoidal heat loads are performed to uncover the behavior of the thermal boundary resistances. The thermal total resistance of the best Sandwich shows a temperature dependency of T-2,64 and is significantly lower, with roughly 30 cm2K4/W at 50 mK, than experimental data found in the literature. The estimated thermal boundary resistance between indium and sapphire agrees very well with the value of the acoustic mismatch theory at low temperatures. In both designs, homemade heat exchangers are integrated to transfer the heat to the cold helium. These heat exchangers are based on sintered structures to increase the heat transferring surface and to overcome the significant influence of the thermal resistance (Kapitza resistance). The heat exchangers are optimized concerning the adherence of the sinter to the substrate and its sinter height, e.g. its thermal penetration length. Ruthenium oxide metallic resistors (RuO2) are used as temperature sensors for the investigations. They consist of various materials, which affect the reproducibility. The sensor conditioning and the resulting good reproducibility is discussed as well.

La première partie de la thèse comporte l’étude des machines à froid, en tenant compte de la vitesse finie des processus. L’approche est basée sur une nouvelle méthode d’optimisation des processus et cycles à vitesse finie, la Méthode Directe d’étude et évaluation des irréversibilités. Les performances de ces cycles sont évaluées en prenant en compte les irréversibilités internes générées par la vitesse finie, notamment (1) les pertes de pression dues au laminage, (2) les pertes de pression dues à la vitesse finie du piston, (3) les pertes de pression dues aux frottements interne et mécanique et (4) l’irréversibilité due aux pertes de chaleur. On obtient ainsi directement l'expression du rendement ou du coefficient de performance et de la génération d’entropie en fonction de la vitesse des processus et d'autres paramètres géométriques et fonctionnels. Le travail proposé pour cette partie de thèse analyse la génération des irréversibilités dans une machine thermique fonctionnant selon le cycle inverse quasi-Carnot (Machine Frigorifique à compression mécanique des vapeurs), en proposant un schéma de calcul complètement analytique. A l’aide de ce schéma de calcul on peut développer des études de sensibilité et d’optimisation de ces machines, sans avoir besoin d’utiliser de tableaux des vapeurs saturés.La deuxième partie du mémoire présente l’application des modèles thermodynamiques (la Méthode Directe, la Méthode de la Thermodynamique en Dimensions Physiques Finies (TDPF), la méthode isotherme de Schmidt, la méthode adiabatique de Finkelstein) dans l’étude des machines Stirling – moteurs et récepteurs et confrontation avec l’expérience.La Méthode de la TDPF est une méthode qui regroupe les techniques de la thermodynamique en temps, vitesse et dimensions géométriques finies. Cette méthode introduit les exo-irréversibilités dues aux transferts de chaleur finis entre les réservoirs (source chaude, puits froid, régénérateur) et le fluide de travail et, de plus, considère les contraintes qui se présentent à l’ingénieur (la pression maximale, le volume maximum, les températures des réservoirs chaud et froid, la vitesse de rotation). La méthode isotherme de Schmidt est une méthode zéro-dimensionnelle qui permet l’étude de la machine divisée en trois volumes isothermes. Elle permet de décrire l’évolution de paramètres, comme le volume instantané (chaud, froid ou de régénération) ou la pression en fonction du temps. L’analyse des processus de transfert de la chaleur et d’écoulement du gaz de travail, ayant lieu dans le moteur Stirling d’un micro-cogénérateur, est effectuée en utilisant un model adiabatique monodimensionnel. Cette analyse repose sur la division du moteur Stirling en 5 volumes de control auxquels on applique les équations des gaz parfaits et les équations de conservation de masse et d’énergie.Les résultats expérimentaux seront confrontés à ceux obtenus par les quatre méthodes de calcul, ce qui permettra de définir les paramètres d’ajustage afin de valider les modèles thermodynamiques. Cette confrontation permettra le développement d’une autre méthode, une combinaison des trois approches utilisées afin de modéliser au mieux le fonctionnement du système, préservant les avantages de chacune sur des intervalles de vitesse de rotation donnés.Des études de sensibilité et d’optimisation de paramètres géométriques et fonctionnels seront effectuées afin de proposer des améliorations de mise au point système pour fournir puissance et de rendement plus élevés
This paper presents the author's overall results obtained in his doctoral thesis, on: The analysis of entropy generation and the evaluation of the performances of the inversed cvasi-Carnot cycle; The application of the Direct Method, Finite Physical Dimensions Thermodynamics method (TDFF), Schmidt’s isotherm method and Finkelstein’s adiabatic model in the study of Stirling engines – engines and machines that function on reversed cycles (receivers) and the confrontation of analytical results with the experimental ones. The first part of the thesis covers the study of the refrigeration machines, considering the finite speed of the processes. The study is based on a new method to optimize the processes and the cycles with finite speed, the Direct Method of study and the evaluation of the irreversibilities. The performance of these cycles are evaluated using analytical relations, considering internal irreversibilities generated by finite speed, especially the pressure losses due to (1) throttling (2) finite speed of the piston (3) internal and mechanical friction (4) irreversibilities due to heat losses. These irreversibilities are introduced in the expression of the First Principle of Thermodynamics for processes with finite speed, and its application leads directly and through analytical means to the expressions of efficiency or coefficient of performance and entropy generation, function of the finite speed of the processes and other geometrical and functional parameters of the machine. The proposed study for this first part of the thesis analyzes the generation of thermal irreversibilities in a thermal machine functioning on a cvasi-Carnot reversed cycle (refrigerating machine with mechanical compression of vapor-IFV) proposing a completely analytical calculation scheme. With this calculation scheme sensitivity studies and optimization of these types of machines were developed, without having to use saturated vapor tables.The second part of the thesis presents the application of thermodynamic models (Direct Method, Finite Physical Dimension thermodynamics method, Schmidt's isotherm model and Finkelstein’s adiabatic model) in the study of Stirling engines – engines and machines that function on reversed cycles (receivers) and the confrontation of analytical results with the experimental ones. The Direct Method consists in the study and assessment of the irreversibilities generated in thermal machines by analyzing the cycle step by step (progressive) and the direct integration of the equation the First Principle of Thermodynamics combined with the Second Principle of Thermodynamics with finite speed, for each process of the cycle. This provides analytical expressions for power and efficiency or coefficient of performance COP, function of the speed of the processes and other geometric and functional parameters

Les SQUID (Superconducting QUantum Interference Device), à haute température critique, permettent la réalisation de magnétomètres directionnels à haute sensibilité, pour des fréquences allant du continu à plusieurs centaines de kHz. Les seuils de détection atteints en chambre blindée autorisent la caractérisation fine du biomagnétisme, dont celui du muscle cardiaque. L'objectif de cette thèse était de réaliser un magnétomètre portable, opérant la détection des signaux cardiaques en milieu magnétiquement non-blindé. La modélisation du signal magnétique cardiaque par un moment magnétique variable permet de montrer qu'une mesure optimale de ce champ peut être réduite à celle de sa composante normale au plan d'étude. Nous montrons également qu'une résolution de $100\,\mathrm(fT)/\sqrt(\mathrm(Hz))$ dans une bande passante de 100 Hz est nécessaire à sa caractérisation. Cependant, de nombreuses sources magnétiques contrarient toutes mesures sans blindage. Ces sources parasites, que nous avons analysées, peuvent être discriminées par leur évolution spatiale. Un sytème gradiométrique permet alors d'extraire le signal magnétique cardiaque en réalisant un filtrage spatial. Un tel dispositif, mis en oeuvre avec deux flux-gates et associé à un processeur de signal numérique (DSP), permet la mise en évidence des pics magnétiques du signal cardiaque et montre l'utilité et la souplesse du traitement numérique en temps réel pour notre application, y compris en milieu ouvert. Un magnétomètre à SQUID dc refroidi par un système cryogénique portable, du type tube à gaz pulsé, s'est révélé inadapté à la magnéto-cardiographie, le bruit lié à ce cryogénérateur pertubant trop largement les mesures dans la bande passante utile. Plusieurs systèmes gradiométriques à SQUID, refroidis par azote liquide, ont été mis en oeuvre. Le signal magnétique cardiaque a été mesuré sans blindage. Des techniques de réductions du bruit à basse fréquence du capteur permettront une meilleure caractérisation du signal cardiaque.

碩士
國立臺灣大學
機械工程學研究所
96
This research apply Micro-channel Evaporator to Vapor Compression Refriger- ation System (VCRS) for electrical Cooling. Steady and transient States experimental investigations are conducted to analysis the thermal properties depends on the design experimental factor.The cooling capacity of the VCRS varied from 250W to 400W, with COP from 1.7 to 3, at pressure ratios of 1.6 to 2.8.The highest overall second-law efficiency was 70%. In steady state experiment. Micro-channel evaporator efficiently make the heat exchange between CPU base and the evaporator, improving the coefficient of perfor- mance .Because of the icreacing heat load ,too much gass R-134a decreased the heat transfer coefficient.This research discussed the mechanism of the two phase flow in high-heat-flux micro-channel ,using evaporator thermal resistance model in order to predict and control the CPU temperature. In transient experiment.Transient evaporator temperature can control R-134a quality and mass flow rate .A complete experimental and numerical medole was built and tested in order to find out optimimal control of the CPU temperature.

Eco-friendly non-CFC refrigerants were introduced in the Air Conditioning and Refrigeration industry during the last few years to reduce damage to the stratospheric ozone layer. The HFC refrigerant R-134a, which has zero Ozone Depletion Potential (ODP), is being used extensively as a replacement for R-12 and also in some centrifugal chillers as a replacement for R-11. However, the disadvantage of R-134a is its comparatively high global warming potential (GWP). Owing to energy crisis and also to reduce the indirect warming impact resulting from electrical energy usage, the new refrigeration systems should be operated at the lowest possible condensing temperatures. In view of this, several active and passive techniques for augmentation of condensation heat transfer and reduction of condensation temperature are gaining increasing attention. Passive augmentation methods are more popular than active ones. To this end, micro-finned tubes of various geometrical shapes are being explored for compact heat exchangers in the refrigeration industry as the best choice. Towards understanding the enhancement in condensation heat transfer coefficients in micro-finned tubes, a test facility has been fabricated to measure the condensing coefficients for R-134a refrigerant. Condensation experiments have been conducted on single plain and finned tubes of outer diameter 19 mm with a refrigerant saturation temperature of 400C and tube wall temperatures 350C, 320C, 300C and 280C respectively. Water is used as the cooling medium inside the tubes with the flow rate varying from 180 lph to 600 lph. The condensing coefficient typically ranged from 0.9 – 1.4 kW/(m2 K) for plain tubes and from 4.2 to 5.8 kW/(m2 K) for the finned tubes. The results of the plain v tube are found to compare favourably with the Nusselt’s theory, leading to a validation of the experimental procedure. Upon comparing the results of finned and plain tubes, it is found that provision of fins result in an enhancement factor of 3.6 to 4.6 in the condensation heat transfer coefficients. This level of enhancement is larger than that resulting from the enhanced surface area of the finned tube surface, suggesting that, apart from the extended area, the surface tension forces play an important role in the augmentation process by driving the condensate from the fin crests to the valleys in between the fins. The measured augmentation factors have also been cross-checked using the Wilson plot method. Detailed error analysis has been performed to quantify the uncertainty in the condensation heat transfer coefficient. The performance of a bank of tubes has been determined based on the measurements carried out on practical condensers of two large chillers with refrigerating capacities of 500 TR and 550 TR. On comparing the finned tube bank results and the single finned tube results, it is found that the average condensation heat transfer coefficient in a bank of tubes having N rows varies as N ¯1/6. The deterioration is in agreement with the relation proposed by Kern.

博士
國立彰化師範大學
工業教育與技術學系
99
This study was aimed to understand the current conditions, the effects of different background, and the relationship among entrepreneurial environment (EE), dynamic capabilities (DC) and entrepreneurial performance (EP) in micro- entrepreneurship management of the refrigeration and air conditioning repair and maintenance services industry. First of all, this study was implemented by reviewing related literatures and researches. The depth interview and the expert review investigate were constructed the connotation of questionnaires. Then, the research data was collected by questionnaires investigation, 2,448 samples were collected in Taiwan, 355 eate was 14.50%. The acquired data was analyzed with statistical methods of frequency distribution, percentage, ffective questionnaires were back, and the effective return rmean, standard deviation, independent sample t-test, one way analysis of variance, Pearson correlation analysis, and structural equation model (SEM), etc. According to the analytical result, I was concluded that as follow: 1. Entrepreneurship manager had agreed with the contents among EE, DC and EP. 2. The effects of different company capital, vocational training, location, entrepreneurial time, and educational background on EE were statistically significant. 3. The effects of different location, entrepreneurial time, age, and educational background on DC were statistically significant. 4. The effects of different company capital, location, entrepreneurial time, and educational background on EP were statistically significant. 5. The correlation among EE, DC and EP were between low and high positive. 6. Confirmatory factor analysis of measurement model among EE, DC and EP were good. 7. The relationships of SEM among EE, EC and EP were demonstrated to be positive structural relationships.