Yes / A numerical study of natural convection heat transfer in water filled cavity has been examined in 3-D for single phase liquid cooling system by using an array of parallel plate fins mounted to one wall of a cavity. The heat generated by a heat source represents a computer CPU with dimensions of 37.5∗37.5mm mounted on substrate. A cold plate is used as a heat sink installed on the opposite vertical end of the enclosure. The air flow inside the computer case is created by an exhaust fan. A turbulent air flow is assumed and k-ε model is applied. The fins are installed on the substrate to enhance the heat transfer. The applied power energy range used is between 15 - 40W. In order to determine the thermal behaviour of the cooling system, the effect of the heat input and the number of the parallel plate fins are investigated. The results illustrate that as the fin number increases the maximum heat source temperature decreases. However, when the fin number increases to critical value the temperature start to increase due to the fins are too closely spaced and that cause the obstruction of water flow. The introduction of parallel plate fins reduces the maximum heat source temperature by 10% compared to the case without fins. The cooling system maintains the maximum chip temperature at 64.68°C when the heat input was at 40W that is much lower than the recommended computer chips limit temperature of no more than 85°C and hence the performance of the CPU is enhanced.

DC power distribution has gained popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) design of a high-efficiency interface converter to connect dc microgrids with the existing low-voltage ac distributions, with a special focus on common-mode (CM) voltage attenuation. Droop control has been used in dc microgrids to share loads among multiple sources. However, line resistance and sensor discrepancy deteriorate the performance. The quantitative relation between the droop voltage range and the load sharing accuracy is derived to help create droop design guidelines. DC system designers can use the guidelines to choose the minimum droop voltage range and guarantee that the sharing error is within a defined range even under the worst cases. A nonlinear droop method is proposed to improve the performance of droop control. The droop resistance is a function of the output current and increases when the output current increases. Experiments demonstrate that the nonlinear droop achieves better load sharing under heavy load and tighter bus voltage regulation. The control needs only local information, so the advantages of droop control are preserved. The output impedances of the droop-controlled power converters are also modeled and measured for the system stability analysis. Communication-based control is developed to further improve the performance of dc microgrids. A generic dc microgrid is modeled and the static power flow is solved. A secondary control system is presented to achieve the benefits of restored bus voltage, enhanced load sharing and high system efficiency. The considered method only needs the information from its adjacent node; hence system expendability is guaranteed. A high-efficiency two-stage single-phase ac-dc converter is designed to connect a 380 V bipolar dc microgrid with a 240 V split-phase single-phase ac system. The converter efficiencies using different two-level and three-level topologies with state-of-the-art semiconductor devices are compared, based on which a two-level interleaved topology using silicon carbide (SiC) MOSFETs is chosen. The volt-second applied on each inductive component is analyzed and the interleaving angles are optimized. A 10 kW converter prototype is built and achieves an efficiency higher than 97% for the first time. An active CM duty cycle injection method is proposed to control the dc and low-frequency CM voltage for grounded systems interconnected with power converters. Experiments with resistive and constant power loads in rectification and regeneration modes validate the performance and stability of the control method. The dc bus voltages are rendered symmetric with respect to ground, and the leakage current is reduced. The control method is generalized to three-phase ac-dc converters for larger power systems. / Ph. D. / DC power distribution gains popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) a high-efficiency converter design to connect dc microgrids with the existing low-voltage ac utility, with a special focus on controlling the dc bus to ground voltage. An analytical model is established to solve the power flow and voltage distribution in a generic dc system. The impact from cable resistance and measurement error on droop control is quantitatively analyzed, based on which droop design guidelines are proposed. DC system designers can use the conclusion to choose a minimum droop voltage range and guarantee a predefined load sharing accuracy. A nonlinear droop control method and a communication-based control method are proposed to further improve the dc system performance. The benefits include better load sharing, tighter voltage regulation and higher system efficiency. To connect dc grids with the low-voltage ac distribution, a high-efficiency bidirectional ac-dc interface converter is designed and built. Different converter topologies with stateof-the-art power semiconductor devices are evaluated. Based on the comparison, an interleaved converter is selected and achieves an efficiency higher than 97% with an optimized passive component design. This converter is also capable of generating symmetric dc bus to ground voltages using a dedicated common-mode voltage control system, and is thus suitable for bipolar dc distribution systems.

Este trabalho apresenta um método utilizando redes neurais artificiais para a determinação das amplitudes e fases dos componentes harmônicos presentes na corrente de carga monofásica. O número de harmônicos identificados é previamente selecionado. Os hamônicos identificados estão presentes na corrente de cargas não-lineares de um sistema de iluminação onde é considerada a variação no tempo das características da forma de onda desta corrente. Os harmônicos presentes no sistema degradam a qualidade de energia, sendo assim é apresentado um breve estudo sobre este tema e métodos para atenuar a distorção harmônica no sistema. Dentre estes métodos é dado ênfase na aplicação de filtros ativos de potência em paralelo com carga não-linear. O trabalho também apresenta um estudo sobre os mais comumente métodos utilizados na identificação harmônica. Dentre eles está o método baseado em redes neurais artificiais. Este método é validado com base nos dados levantados por meio de simulação e de forma experimental. / In this thesis artificial neural networks are employed in a novel approach to identifying harmonic components of the single-phase nonlinear load current, whose amplitudes and phase angles are subject to unpredictable changes in steady-state. An identified harmonics number is previously selected. These harmonics are present in the non-linear loads current of electrical illumination system. The harmonics in the power system degrade the power quality, then is exhibited a concise study dealing with power quality problems and methods to mitigate the harmonic distortion in the power system. Among these methods emphasis is given in the application of pure active power filters in parallel with the non-linear load. The thesis also shows a study about the more commonly methods used in the harmonic detection. Among them is the method based on artificial neural networks. Simulation and experimental results are presented to validate the proposed approach.

Pulse width modulation (PWM) rectifiers are extensively used in battery charger, regulated dc voltage source, UPS systems, ac line conditioner and motor drives. The conventional control schemes for these rectifiers require PLL, transformations, and input voltage sensing, which increase the cost and complexity of the controller. Simple control schemes based on resistance emulation control are developed in this thesis work for different PWM boost rectifiers. Modeling, analysis and design methods for these rectifier systems are presented. The effect of computational delay involved in digital implementation on the performance of the above rectifier systems is studied. A single-switch boost rectifier system is presented, which operates in DCM and in CCM for an output power less than and greater than 50% rated load, respectively, exploiting the best features of both the operating modes. A generalized feedforward control is presented to improve the dynamic response of output voltage of single-phase boost rectifiers against input voltage, load current and reference voltage disturbances. Feedforward control requires additional voltage and/or current measurements. A state observer is presented for estimating the inductor current of a buck rectifier, and two disturbance observers are presented to estimate the input voltage and the load current of a boost rectifier. These observers eliminate the need of additional sensors for implementing the feedforward control. The resistance emulation control is extended to four-wire PWM rectifier. Two control methods are presented. The first method makes the input currents of the rectifier proportional to their respective input voltages, while the second one balances its input currents even under unbalanced input voltage condition. A detailed analysis of line and neutral current distortions of four-wire converter is presented. A three-carrier based PWM scheme is presented, which significantly reduces the neutral current of the rectifier compared to conventional PWM scheme, when three single-phase inductors are used, and considerably reduces both line and neutral current distortions, when a three-limb inductor is used. A regenerative test setup containing two back-to-back connected three-phase PWM converters is presented for testing high-power converters in the active and reactive power circulation mode. The proposed scheme considerably reduces the cost of testing, and hence, the overall production cost of the converters compared to load-bank testing. A mathematical model is presented for the above system. A suitable control method is presented to control the two converters of the back-to-back system. A new PWM scheme is presented, which considerably reduces the requirement of the dc bus voltage of the back-to-back system compared to conventional PWM schemes. All theoretical predictions are experimentally validated. The experimental results are presented.

O biodiesel é um biocombustível proveniente de fontes renováveis, que pode ser obtido a partir da transesterificação de óleos vegetais e/ou gordura animal, produzindo alquil-ésteres de ácidos graxos e um importante resíduo, o glicerol. Este coproduto deve ser removido para a comercialização do biocombustível, pois concentrações altas podem causar danos aos motores de ciclo diesel, além da produção de gases altamente tóxicos durante a combustão. O objetivo deste trabalho foi o desenvolvimento de um procedimento analítico empregando um sistema de fase única para a determinação in situ de glicerol livre em biodiesel. Procedimentos analíticos para análises in situ trazem vantagens, tais como rapidez na resposta ao monitoramento de um processo industrial, evitando trabalhos de reprocessamento, ou no screening de amostras durante fiscalizações em campo, permitindo não só tomadas de decisões imediatas, como também no encaminhamento de apenas amostras duvidosas para análises laboratoriais mais detalhadas. As análises in situ devem ter como principais características a praticidade, rapidez, robustez, uso de reagentes estáveis e pouco tóxicos e possibilidade de emprego de instrumentos portáteis. O procedimento desenvolvido é baseado na oxidação do glicerol por periodato gerando formaldeído, ácido fórmico e iodato. O formaldeído reage com acetilacetona, em meio amoniacal, formando o composto 3,5-diacetil-1,4-dihidrolutidina (reação de Hantzsch). Este composto apresenta máximo de absorção em 410 nm e pode ser quantificado por espectrofotometria. A extração do analito, geralmente empregada nos procedimentos propostos na literatura, foi evitada. As amostras de biodiesel foram diluídas 10 vezes em etanol anidro. Aquecimento a 50ºC por 10 minutos foi empregado para acelerar a reação. A solução final deve conter, no mínimo, 75% de etanol para garantir a formação de uma única fase para que possa ser realizada a medida espectrofotométrica. Resposta linear foi observada entre 40 e 400 mg kg-1 de glicerol, sendo a curva analítica representada pela equação: A = 0,0018C + 0,0204, r2 = 0,999, em que C é a concentração de glicerol em mg kg-1. O limite de detecção foi estimado em 2,0 mg kg-1 (99,7% de nível de confiança; n = 10) e o coeficiente de variação em 2,1% (n=10). O procedimento proposto foi aplicado a amostras de biodiesel de diferentes fontes oleaginosas, com resultados concordantes com o procedimento de referência em nível de confiança de 95%. / Biodisel can be obtained from renewable sources by trans-esterification of vegetable oils or animal fats, producing alkyl esters of fatty acids and glycerol as side product. Glycerol need to be removed for commercialization of the biofuel, because high concentrations can damage motors, and produce highly toxic gases during combustion. The objective of this study was to develop a procedure for in situ analysis of free glycerol in biodiesel samples employing a single-phase system. Procedures for in situ analysis show advantages such as fast response in monitoring of industrial processes, avoiding the need for reprocessing of a production batch, or for screening of samples during field inspections, thus allowing fast decisions as well as reduction of the number of samples sent to a more detailed laboratory analysis. The in situ analysis requires practical, fast, and robust procedures, which uses stable and low toxicity reagents, and presents potential for use of portable instruments. The developed procedure is based on glycerol oxidation by periodate, which yields formaldehyde, formic acid and iodate. Formaldehyde reacts with acetylacetone in an ammoniacal medium, forming the 3,5-diacetyl-1,4-dihydrolutidine (Hantzsch\'s reaction). This compound can be quantified by spectrophotometry with maximum absorption at 410 nm. The analyte extraction, usually employed in previously proposed procedures, was avoided. The biodiesel samples were 10-fold diluted in anhydrous ethanol. In order to accelerate the reaction, the flasks were heated at 50 °C for 10 minutes. The final solution should contain at least 75% ethanol to ensure the formation of a single phase before the spectrophotometric measurements. A linear response was observed from 40 to 400 mg kg-1 of glycerol (A = 0.0018 C + 0.0204, r2 = 0.999, in which C is the concentration of glycerol in mg kg-1). The detection limit was estimated at 2.0 mg kg-1 (99.7% confidence level, n = 10)and the coefficient of variation was 2.1% (n = 10). The proposed procedure was applied to biodiesel samples from different oil sources and the results agreed with the reference procedure at the 95% confidence level.

O biodiesel é um biocombustível proveniente de fontes renováveis, que pode ser obtido a partir da transesterificação de óleos vegetais e/ou gordura animal, produzindo alquil-ésteres de ácidos graxos e um importante resíduo, o glicerol. Este coproduto deve ser removido para a comercialização do biocombustível, pois concentrações altas podem causar danos aos motores de ciclo diesel, além da produção de gases altamente tóxicos durante a combustão. O objetivo deste trabalho foi o desenvolvimento de um procedimento analítico empregando um sistema de fase única para a determinação in situ de glicerol livre em biodiesel. Procedimentos analíticos para análises in situ trazem vantagens, tais como rapidez na resposta ao monitoramento de um processo industrial, evitando trabalhos de reprocessamento, ou no screening de amostras durante fiscalizações em campo, permitindo não só tomadas de decisões imediatas, como também no encaminhamento de apenas amostras duvidosas para análises laboratoriais mais detalhadas. As análises in situ devem ter como principais características a praticidade, rapidez, robustez, uso de reagentes estáveis e pouco tóxicos e possibilidade de emprego de instrumentos portáteis. O procedimento desenvolvido é baseado na oxidação do glicerol por periodato gerando formaldeído, ácido fórmico e iodato. O formaldeído reage com acetilacetona, em meio amoniacal, formando o composto 3,5-diacetil-1,4-dihidrolutidina (reação de Hantzsch). Este composto apresenta máximo de absorção em 410 nm e pode ser quantificado por espectrofotometria. A extração do analito, geralmente empregada nos procedimentos propostos na literatura, foi evitada. As amostras de biodiesel foram diluídas 10 vezes em etanol anidro. Aquecimento a 50ºC por 10 minutos foi empregado para acelerar a reação. A solução final deve conter, no mínimo, 75% de etanol para garantir a formação de uma única fase para que possa ser realizada a medida espectrofotométrica. Resposta linear foi observada entre 40 e 400 mg kg-1 de glicerol, sendo a curva analítica representada pela equação: A = 0,0018C + 0,0204, r2 = 0,999, em que C é a concentração de glicerol em mg kg-1. O limite de detecção foi estimado em 2,0 mg kg-1 (99,7% de nível de confiança; n = 10) e o coeficiente de variação em 2,1% (n=10). O procedimento proposto foi aplicado a amostras de biodiesel de diferentes fontes oleaginosas, com resultados concordantes com o procedimento de referência em nível de confiança de 95%. / Biodisel can be obtained from renewable sources by trans-esterification of vegetable oils or animal fats, producing alkyl esters of fatty acids and glycerol as side product. Glycerol need to be removed for commercialization of the biofuel, because high concentrations can damage motors, and produce highly toxic gases during combustion. The objective of this study was to develop a procedure for in situ analysis of free glycerol in biodiesel samples employing a single-phase system. Procedures for in situ analysis show advantages such as fast response in monitoring of industrial processes, avoiding the need for reprocessing of a production batch, or for screening of samples during field inspections, thus allowing fast decisions as well as reduction of the number of samples sent to a more detailed laboratory analysis. The in situ analysis requires practical, fast, and robust procedures, which uses stable and low toxicity reagents, and presents potential for use of portable instruments. The developed procedure is based on glycerol oxidation by periodate, which yields formaldehyde, formic acid and iodate. Formaldehyde reacts with acetylacetone in an ammoniacal medium, forming the 3,5-diacetyl-1,4-dihydrolutidine (Hantzsch\'s reaction). This compound can be quantified by spectrophotometry with maximum absorption at 410 nm. The analyte extraction, usually employed in previously proposed procedures, was avoided. The biodiesel samples were 10-fold diluted in anhydrous ethanol. In order to accelerate the reaction, the flasks were heated at 50 °C for 10 minutes. The final solution should contain at least 75% ethanol to ensure the formation of a single phase before the spectrophotometric measurements. A linear response was observed from 40 to 400 mg kg-1 of glycerol (A = 0.0018 C + 0.0204, r2 = 0.999, in which C is the concentration of glycerol in mg kg-1). The detection limit was estimated at 2.0 mg kg-1 (99.7% confidence level, n = 10)and the coefficient of variation was 2.1% (n = 10). The proposed procedure was applied to biodiesel samples from different oil sources and the results agreed with the reference procedure at the 95% confidence level.

A presente dissertação tem como objetivo principal estudar a propagação acústica em escoamentos monofásicos. Para tal, são analisados sinais transientes de pressão fornecidos por sensores instalados em posições conhecidas na linha de teste, através do estudo de técnicas de análise de sinais, a fim de investigar se as variações do conteúdo espectral dos sinais são influenciadas pela ocorrência de vazamentos no duto. A análise dos sinais foi realizada nos planos temporal, frequencial, tempo-frequência e estatístico. Os resultados experimentais foram obtidos no oleoduto piloto do NETeF - Núcleo de Engenharia Térmica e Fluidos da USP - Universidade de São Paulo, com uma seção de testes com 1500 metros e diâmetro de 51,2 mm, com escoamento monofásico de água. Os resultados obtidos através da análise tempo-frequência mostraram-se satisfatórios, sendo esta técnica capaz de identificar a composição espectral instantânea de um sinal, ou seja, foi eficiente na identificação de picos de amplitude da frequência ao longo do eixo temporal. Além disso, a análise probabilística, através do desvio-padrão do sinal também mostrou-se eficiente exibindo uma disparidade significativa entre os sinais com e sem vazamento. / The present dissertation reports on the study of the acoustic propagation in single-phase flow. It analyzes the transient signals provided by pressure sensors in known locations in the test line through the study of signal analysis techniques to investigate if the variations in spectral content of the signals are influenced by the occurrence of leaks in the pipe. The analysis of signals was performed in the time, frequency, time-frequency and statistical plans. The experimental results were obtained in a 1500 meter-long and 51.2 millimeter-diameter pilot pipeline at the Center of Thermal Engineering and Fluids, with single-phase flow of water. The results obtained by time-frequency analysis were satisfactory, allowing identifying the spectral composition of an instantaneous signal, i.e., the analysis was effective in identifying the frequency amplitude peaks along the time axis. Moreover, probabilistic analysis using the standard deviation of the signal was also efficient, displaying a significant disparity between the signals with and without leakage.

Os motores de indução monofásicos necessitam de dois enrolamentos estatóricos para criação de um campo magnético girante: um enrolamento principal e um enrolamento auxiliar. Este último tem o objetivo de criar um campo girante apenas na partida do motor, desligando-se logo em seguida, através de um interruptor centrífugo acoplado ao eixo do rotor da máquina. O objetivo deste trabalho é avaliar a influência das diferentes configurações de enrolamentos sobre as características externas do motor de indução monofásico. Sendo assim, através desta análise, pretende-se verificar se há algum benefício em seu desempenho. Neste trabalho, duas configurações de enrolamentos foram desenvolvidas e testadas em laboratório. Na primeira configuração, o enrolamento principal foi alocado na parte inferior das ranhuras. Na segunda configuração, as posições dos enrolamentos foram invertidas. As duas configurações de enrolamento foram aplicadas a um protótipo de 0,37 kW, quatro pólos. A simulação foi realizada através de um programa computacional de elementos finitos, possibilitando a extração e análise de resultados, com as densidades de fluxo nas duas configurações. Os resultados dos ensaios realizados no protótipo permitiram analisar o comportamento de grandezas como corrente elétrica, torque, potência e rendimento que são apresentados e comparados no final deste trabalho. / The single phase induction motor needs two stator windings to produce the rotating magnetic field: a main winding and an auxiliary winding. The aim of the auxiliary winding is to create the rotating electromagnetic field when the machine is starting and afterwards turned off, generally through a centrifugal switch coupled to the shaft of the machine. The main purpose of this work is to evaluate the influence that the position of the two windings have on the external characteristics of the single phase induction motor. For this purpose, two different kinds of winding configurations were carried out and simulated in a prototype. In the first configuration, the main winding was located in the bottom of the slot. In the second configuration, the positions of the windings were inverted. Performance analysis and computational simulation by Finite Element Method were carried out after the prototype 0.37 kW, four poles was prepared. In this way, through this analysis, improvements on its performance are expected. The electrical quantities such as flux density, electric current, torque, power and efficiency are compared in the end of this document.

La convection turbulente dans des géométries régulières et complexes trouve son importance dans de nombreuses applications industrielles, notamment les échangeurs de chaleurs (dissipateurs à picots). L’objectif de ce travail est de faire une étude diagnostic sur la qualité d’échange thermique dans des géométries représentatives d’échangeurs de chaleur, en utilisant la simulation numérique comme outil d’investigation. L’approche qui sera utilisée est basée sur la résolution des équations de Navier-Stockes (moyennées) RANS avec les modèles statistiques.La finalité des résultats est de proposer des solutions pour promouvoir l’échange de chaleur dans ce type de configuration à petites échelles (micro systèmes de dissipateurs de chaleur à picots). Pouvoir déterminer une nouvelle approche pour le calcul du coefficient d’échange moyen par convection (coefficient d’échange convectif) dans des géométries complexes et compactes, et adopter une nouvelle approche pour calculer les températures de référence. Proposition et adaptation d’une nouvelle forme de géométrie pour une éventuelle optimisation du modèle existant (picots en forme losange). / The turbulent convection in regular and complex geometries is important in many industrial applications including mini/micro heat exchangers (pin-fins heat sinks). The objective of this work is to conduct a parametric study of the quality of heat exchange in representative forms of geometries of mini-channels, by using numerical simulation as an investigative tool. The approach taken is based on solving Averaged Navier-Stokes equations, RANS approach with the statistical models.The purpose of the results is to propose solutions to promote the exchange of heat in this type of configuration (micro systems of pin-fins heat sinks). Get a new approach for the calculation of averaged heat transfer coefficient in complex and compact geometries, and adopt a new approach to calculate reference temperatures. Suggestion and adaptation of a new form of geometry for an eventual optimization of the existing model (diamond shaped pin-fins).

Os motores de indução monofásicos necessitam de dois enrolamentos estatóricos para criação de um campo magnético girante: um enrolamento principal e um enrolamento auxiliar. Este último tem o objetivo de criar um campo girante apenas na partida do motor, desligando-se logo em seguida, através de um interruptor centrífugo acoplado ao eixo do rotor da máquina. O objetivo deste trabalho é avaliar a influência das diferentes configurações de enrolamentos sobre as características externas do motor de indução monofásico. Sendo assim, através desta análise, pretende-se verificar se há algum benefício em seu desempenho. Neste trabalho, duas configurações de enrolamentos foram desenvolvidas e testadas em laboratório. Na primeira configuração, o enrolamento principal foi alocado na parte inferior das ranhuras. Na segunda configuração, as posições dos enrolamentos foram invertidas. As duas configurações de enrolamento foram aplicadas a um protótipo de 0,37 kW, quatro pólos. A simulação foi realizada através de um programa computacional de elementos finitos, possibilitando a extração e análise de resultados, com as densidades de fluxo nas duas configurações. Os resultados dos ensaios realizados no protótipo permitiram analisar o comportamento de grandezas como corrente elétrica, torque, potência e rendimento que são apresentados e comparados no final deste trabalho. / The single phase induction motor needs two stator windings to produce the rotating magnetic field: a main winding and an auxiliary winding. The aim of the auxiliary winding is to create the rotating electromagnetic field when the machine is starting and afterwards turned off, generally through a centrifugal switch coupled to the shaft of the machine. The main purpose of this work is to evaluate the influence that the position of the two windings have on the external characteristics of the single phase induction motor. For this purpose, two different kinds of winding configurations were carried out and simulated in a prototype. In the first configuration, the main winding was located in the bottom of the slot. In the second configuration, the positions of the windings were inverted. Performance analysis and computational simulation by Finite Element Method were carried out after the prototype 0.37 kW, four poles was prepared. In this way, through this analysis, improvements on its performance are expected. The electrical quantities such as flux density, electric current, torque, power and efficiency are compared in the end of this document.