EFFECTS OF OFF-CENTER ANGLE ON THE HEAT TRANSFER COEFFICIENT ON VERTiCAL TIER OF MULTIPLE SPHERICAL SURFACES Kaya, Ebubekir M.S., Department of Mechanical Engineering Supervisor: Assoc. Prof. Dr. Cemil Yamali December 2004, 112 pages The purpose of this study is to investigate the laminar film condensation phenomenon of steam on a vertical tier of multiple spherical surfaces by using both analytical and experimental methods. The analytical heat transfer results were obtained by following the Nusselt type of analysis and represented graphically. In addition, in order to observe the real behavior of the film condensation, an experimental setup was manufactured and experiments were done. In analytical section / mass flow rate, (mean) velocity, film thickness, local heat flux and local heat transfer coefficient values were obtained and plotted as depending on angular position. Moreover, mean heat flux and mean heat transfer coefficient variations were presented with respect to diameter of the sphere and sub-cooling. On the other hand, for the experimental section, heat flux and mean heat transfer coefficient values were obtained and expressed as depending on sub-cooling. To see the effects of off-center angle, setup was inclined for different angles and experiments were repeated for each inclination angle. At the end of the study, mean heat transfer coefficients belong to analytical and experimental studies were compared to each other as well as to the literature.

Utilizing an experimental facility which was prepared to conduct performance tests on heat exchangers; experiments were completed in an attempt to see verifiable improvements in overall heat transfer coefficient in engine coolant with nanoparticles suspended at different weight percentages. The different fluids tested were: base ELC (Extended Life Coolant), ELC with 0.002 wt% CNT (Carbon Nanotubes), ELC with 0.02 wt% CNT, ELC with 0.02 wt% MWNT's (Multiwalled Nanotubes) and water. The volume percents range from 0.00164 volume% to 0.0164 volume% which seemed quite small, but according to Caterpillar representatives, were the best concentration. These fluids were tested at standard flowrates which this type of heat exchanger would be used in as well as a higher air flowrate and lower coolant flowrates in an attempt to gather more verifiable data. Results were obtained regarding the change in heat transfer ability of engine coolant with suspended nanoparticles. For this system under these specific conditions, there was verifiably no increase in UA as nanoparticles were added to the coolant. The benefits of adding nanoparticles to engine coolant have potential to be great, but the cost of nanoparticles and difficulty keeping them suspended may outweigh any benefits obtainable in this type of set up.

High combustion temperatures are required in gas-turbine engines to achieve high cycle efficiencies. With increasing temperature, however, the life span of the turbine components are reduced. The ability to accurately predict engine component temperature as a function of combustion temperature is required to strike this balance correctly. An experimental heat transfer measurement technique is developed in this thesis, which builds on a large body of existing literature. The technique enables a detailed quantification of turbine heat transfer on test rigs which closely represent gas-turbine engine configurations. Fundamental improvements are made to existing methods, in the definition of the ‘semi- infinite limit’ for transient measurement techniques, in Infra-red camera calibration, and in thermal effusivity measurement. The improvements were developed from first principles, verified experimentally, and have been used on a world leading heat transfer rig (the FACTOR combustor-turbine interaction rig, run on the NG-Turb facility at DLR Göttingen). It was found that optimisation of a number of measurement parameters was required to minimise the measurement uncertainty. It is shown that the optimum measurement parameters are dependant, and sensitive to the specific configuration of the test rig. An experimental procedure was developed and tested, which has been ‘tuned’ for measurements on the FACTOR test rig. Despite the challenging measurement environment on the FACTOR rig, it was found that state-of-the-art heat transfer measurement uncertainties of approximately 5%, could nevertheless still be achieved, by using the new methods. General principles and rules are established which can be used to guide the design of future heat transfer measurements, with the aim of minimising measurement uncertainty.

Submitted by IGOR SEICHO KIYOMURA null (igorseicho@gmail.com) on 2016-09-12T14:07:49Z No. of bitstreams: 1 Dissertação_Igor Seicho Kiyomura.pdf: 3224196 bytes, checksum: d71f4ad45145d3dc7f441a0090d3b373 (MD5) / Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-09-14T19:13:44Z (GMT) No. of bitstreams: 1 kiyomura_is_me_ilha.pdf: 3224196 bytes, checksum: d71f4ad45145d3dc7f441a0090d3b373 (MD5) / Made available in DSpace on 2016-09-14T19:13:44Z (GMT). No. of bitstreams: 1 kiyomura_is_me_ilha.pdf: 3224196 bytes, checksum: d71f4ad45145d3dc7f441a0090d3b373 (MD5) Previous issue date: 2016-07-29 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Nas últimas décadas surgiu a necessidade de dissipar maiores quantidades de energia térmica, fato que acarretou no aumento do número de estudos em ebulição nucleada e convectiva com o objetivo de produzir trocadores de calor cada vez mais eficientes e compactos. A busca de produtos cada vez mais eficientes e compactos e a procura de novas técnicas para melhorar a transferência de calor, garantindo a integridade física do equipamento, continuam crescendo e a tendência é que continuará assim nos próximos anos. Uma das técnicas que está sendo amplamente pesquisada na comunidade cientifica é o uso de nanofluidos. Os nanofluidos foram desenvolvidos com o intuito de melhorar a condutividade e a difusividade térmica em relação aos fluidos tradicionais. Muitos experimentos com nanofluidos têm sido desenvolvidos nos últimos anos, mas ainda existem muitas divergências a respeito do efeito desses fluidos sobre o fenômeno de ebulição. Dentro deste contexto, o presente trabalho tem como objetivo a análise teórico-experimental do efeito de superfícies nanoestruturadas e da concentração do nanofluido, a ser depositado sobre a superfície aquecedora, sobre o coeficiente de transferência de calor em regime de ebulição nucleada. Para tanto, testes foram realizados para fluxos de calor que correspondem ao regime de ebulição nucleada da água deionizada, à temperatura de saturação (Tsat = 99 °C) e à pressão atmosférica (patm = 98 kPa), sobre superfícies aquecedoras de cobre com diferentes rugosidades. As superfícies nanoestruturadas foram produzidas por deposição de nanopartículas de maguemita, por meio do processo de ebulição da solução Fe2O3-água deionizada para diferentes concentrações mássicas previamente estabelecidas. As superfícies foram submetidas a ensaios metalográficos, de molhabilidade e de rugosidade permitindo a avaliação das modificações estruturais, topográficas e químicas das superfícies, antes e após os testes no regime de ebulição nucleada. Os resultados para o coeficiente de transferência de calor foram relacionados com as características geométricas e morfológicas das superfícies de teste, levando em consideração os aspectos relacionados à interação fluido/superfície, como, o ângulo de contato e a molhabilidade. / In the last decade, the necessity to dissipate large quantities of heat energy increased, thus leading to an increase on the number of studies in nucleate pool boiling and flow boiling with the aim of producing more compact and efficient heat exchangers. The search for increasingly efficient and compact products and for new techniques to improve the heat transfer, ensuring the physical integrity of the equipment, keep growing and it will remain so in the next years. One of the techniques being widely researched in the scientific community is the use of nanofluids. The nanofluids have been developed in order to improve the thermal conductivity and diffusivity compared to traditional fluids. Although many experiments with nanofluids have been developed in recent years, there are still many differences related to the effects of these fluids on the pool boiling phenomenon. In this context, this work aims to analyze the effects of nanostructured surfaces and different nanofluid concentrations, which are deposited on the heating surface, on the heat transfer coefficient during the nucleate boiling regime. Therefore, tests were performed to heat fluxes values corresponding to the nucleate boiling regime for deionized water, at saturation temperature (Tsat = 99 °C) and atmospheric pressure (patm = 98 kPa), on copper heating surfaces with different roughness values. The nanostructured surfaces were produced by maghemite nanoparticle deposition, which is achieved by boiling selected mass concentrations of a Fe2O3-deionized water nanofluid. Prior and after each boiling test, the characteristics of the test surfaces were evaluated by applying the metallographic, wettability and surface roughness tests. The results for the heat transfer coefficient were related to the geometrical and morphological characteristics of the test surfaces, taking into account the aspects of the flu-id/surface interaction such as, the contact angle and wettability. / FAPESP: 2014/07949-9

Orientador: Elaine Maria Cardoso / Resumo: Nas últimas décadas surgiu a necessidade de dissipar maiores quantidades de energiatérmica, fato que acarretou no aumento do número de estudos em ebulição nucleada e convectivacom o objetivo de produzir trocadores de calor cada vez mais eficientes e compactos. Abusca de produtos cada vez mais eficientes e compactos e a procura de novas técnicas paramelhorar a transferência de calor, garantindo a integridade física do equipamento, continuamcrescendo e a tendência é que continuará assim nos próximos anos. Uma das técnicas que estásendo amplamente pesquisada na comunidade cientifica é o uso de nanofluidos. Os nanofluidosforam desenvolvidos com o intuito de melhorar a condutividade e a difusividade térmicaem relação aos fluidos tradicionais. Muitos experimentos com nanofluidos têm sido desenvolvidosnos últimos anos, mas ainda existem muitas divergências a respeito do efeito dessesfluidos sobre o fenômeno de ebulição. Dentro deste contexto, o presente trabalho tem comoobjetivo a análise teórico-experimental do efeito de superfícies nanoestruturadas e da concentraçãodo nanofluido, a ser depositado sobre a superfície aquecedora, sobre o coeficiente detransferência de calor em regime de ebulição nucleada. Para tanto, testes foram realizadospara fluxos de calor que correspondem ao regime de ebulição nucleada da água deionizada, àtemperatura de saturação (Tsat = 99 °C) e à pressão atmosférica (patm = 98 kPa), sobre superfíciesaquecedoras de cobre com dif... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In the last decade, the necessity to dissipate large quantities of heat energy increased,thus leading to an increase on the number of studies in nucleate pool boiling and flow boilingwith the aim of producing more compact and efficient heat exchangers. The search for increasinglyefficient and compact products and for new techniques to improve the heat transfer,ensuring the physical integrity of the equipment, keep growing and it will remain so in thenext years. One of the techniques being widely researched in the scientific community is theuse of nanofluids. The nanofluids have been developed in order to improve the thermal conductivityand diffusivity compared to traditional fluids. Although many experiments withnanofluids have been developed in recent years, there are still many differences related to theeffects of these fluids on the pool boiling phenomenon. In this context, this work aims to analyzethe effects of nanostructured surfaces and different nanofluid concentrations, which aredeposited on the heating surface, on the heat transfer coefficient during the nucleate boilingregime. Therefore, tests were performed to heat fluxes values corresponding to the nucleateboiling regime for deionized water, at saturation temperature (Tsat = 99 °C) and atmosphericpressure (patm = 98 kPa), on copper heating surfaces with different roughness values. Thenanostructured surfaces were produced by maghemite nanoparticle deposition, which isachieved by boi... (Complete abstract click electronic access below) / Mestre

Em sistemas frigoríficos de compressão a vapor, o evaporador é o equipamento responsável direto pela retirada de calor de ambientes refrigerados. Por esta razão, este componente é a fonte de investigação do presente estudo, o qual teve por objetivo principal a busca por alternativas para a melhoria de desempenho térmico em sistema frigoríficos. Isto posto, o trabalho foi desenvolvido em três frentes de estudo, as quais encontram-se interligadas entre si: desenvolvimento de programa de simulação de serpentinas resfriadoras; realização de ensaios para determinação das capacidades de trocadores de calor; e revisão bibliográfica circunstanciada do coeficiente de transferência de calor do lado do ar. O programa de simulação desenvolvido, o qual tomou como base o modelo proposto por Rich, mostrou-se ser uma ótima ferramenta de trabalho, tanto no meio acadêmico como no industrial. Como conseqüência da elaboração do programa, fez-se uso de correlações já existentes para determinação dos coeficientes de transferência de calor do lado do refrigerante, Bo Pierre, e do lado do ar, McQuiston, sendo esta última escolhida a partir de um sumário de correlações levantadas quando no estudo dos coeficientes de transferência de calor descritos acima. Para que o programa fosse validado como ferramenta de trabalho, foram realizadas simulações dos ensaios das serpentinas testadas e seus resultados confrontados. Tais resultados apresentam uma adequada concordância,possibilitando validar o programa. Os ensaios foram realizados segundo recomendações da norma ASHRAE 25/1992. Dos dados obtidos nos testes, também foi proposto uma correlação para o fator j-Colburn para superfícies secas; no entanto, tratou-se apenas de especular acerca de seu comportamento com as demais correlações, as quais obtiveram boa concordância. / In refrigerating systems of compression to vapour, the evaporator is the direct responsible equipment for remove of heat of cooling environments. In this way, the component is the source of investigation of the present study, which had for main objective the search for alternatives to improvement the performance thermal in refrigeration systems. The research was developed in three studies fronts, that are connected each other: development of program of simulation of cooling coils; tests to find the performance of heat exchange; and revision bibliographical of the coefficient of heat transfer of air. The developed simulation program, based in the Rich\'s model, it showed to be a good work tool, as in the academic, as in the industrial behaviour. As consequence of the elaboration the program, it was used existents correlations to calculation the coefficients of refrigerant heat transfer, Bo Pierre, and to air, McQuiston, the last one was choose from a correlation\'s summary. Same simulations were made to validated the program of the tested serpentines and the results were confronted. Such results presented an appropriate agreement that contributed to validate the program. The tests were accomplished according to recommendations of the norm ASHRAE 25/1992. One correlation was proposed for the factor j-Colburn for dry surfaces in base of the testes results; however, the purpose was to speculated its behaviour with the other correlations, which obtained good agreement.

Dissipadores de calor baseados em microcanais são apresentados como solução para a remoção de fluxos de calor elevados em espaços restritos, pois proporcionam elevados coeficientes de transferência de calor quando comparados a canais convencionais. Tais trocadores também proporcionam elevadas razões entre a área superficial em contato com o refrigerante por unidade de volume do dissipador. Além dos microcanais, a utilização de nanofluidos também se apresenta como tecnologia com potencial de incremento do coeficiente de transferência de calor. Os nanofluidos consistem na adição de nanopartículas a um fluido base visando alterar suas propriedades de transporte termodinâmicas. Neste contexto, o objetivo do presente estudo é avaliar o coeficiente de transferência de calor para escoamentos monofásicos e ebulição convectiva de nanofluidos aquosos no interior de microcanais. Para isto, foram realizados experimentos em canais com diâmetro de 1,1 mm e comprimento de 200 mm para água deionizada, nanofluidos de alumina com diâmetros de 20-30 e 40-80 nm, nanofluidos de dióxido de silício com diâmetros de 15 e 80 nm, e nanofluidos de cobre com diâmetro de 25 nm. Estas soluções foram ensaiadas para concentrações volumétricas de nanopartículas de 0,001, 0,01 e 0,1, velocidades mássicas de 200, 400 e 600 kg/m2s e fluxos de calor de 20 a 350 kW/m2. A análise dos resultados revelou que a adição de nanopartículas a água deionizada proporciona o incremento do número de Nusselt para escoamentos monofásicos, principalmente na região inicial do tubo. Concluiu-se que os efeitos da adição de nanopartículas a um fluido base no coeficiente de transferência de calor durante a ebulição convectiva estão relacionados ao recobrimento da superfície com uma camada porosa. A deposição de nanopartículas com diâmetro inferior a 30 nm resultou na redução do coeficiente de transferência de calor e das instabilidades térmicas do escoamento em relação a água deionizada. O coeficiente de transferência de calor e as instabilidades térmicas não apresentaram variações significativas da deposição de nanopartículas com diâmetro superior a 40 nm. Por meio da análise da textura das superfícies recobertas e do critério de nucleação proposto por Kandlikar et al. (1997) concluiu-se que tal comportamento encontra-se associado aos efeitos do acabamento superficial na densidade de cavidades de nucleação ativas. / Microchannels based heat exchangers were introduced as a solution to high heat flux removal in restrict spaces due to their high heat transfer coefficients compared to heat exchangers based on conventional channels. The high ratio of surface are per volume is an additional advantage to microchannels in relation to conventional channels. Beside the microchannels technology, the nanofluids also present itself as a technique with potential to increase the heat transfer coefficient. Nanofluids consist of a solution containing nanoparticles dispersed in a base fluid with the goal to improve its thermodynamic and transport properties. In this context, the objective of the present study is to evaluate the heat transfer coefficient for single-phase flow and convective boiling of aqueous nanofluids inside microchannels. Experiments were performed for channels with internal diameter of 1.1mm and 200 mm long for DI-water, nanofluids containing alumina- (nanoparticles diameters of 20-30 and 40-80 nm), silicon dioxide (nanoparticles diameters of 15 and 80 nm), and copper (nanoparticles diameter of 25 nm). These solutions were evaluated for volumetric concentrations of 0.001, 0.01 and 0.1%, mass velocities of 200, 400 and 600 kg/m2s and heat fluxes from 20 to 350 kW/m2. The analysis of the results revealed that the addition of nanoparticles to DI-water causes an increment in the Nusselt number for single phase flows, especially at the inlet of the tube. The results for flow boiling indicated that the effects of adding nanoparticles to the base fluid are related to the deposition on the heating surface of a nanoparticles porous layer due to the boiling process. The deposition of nanoparticles smaller than 30 nm promoted a reduction of the heat transfer coefficient compared to DI-water on a clean surface, and thermal instabilities were minimized. For the deposition of nanoparticles larger than 40 nm these parameters did not presented significant variations in comparison to DI-water. A combined analysis of the surfaces finishing and the criterion of Kandlikar et al. (1997) for bubble nucleation revealed that such behaviors are correlated to the effects of the surface texture associated to the boiling process on the density of active nucleation cavities.

Orientadores: Carlos Teofilo Salinas Sedano, Kamal Abdel Radi Ismail / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-25T08:57:38Z (GMT). No. of bitstreams: 1 Silva_RaqueldaCunhaRibeiroda_D.pdf: 3855986 bytes, checksum: dc3f065365a2224c57128ddbedffab68 (MD5) Previous issue date: 2014 / Resumo: O fenômeno de formação de frost é encontrado em muitas aplicações comerciais e industriais de baixa temperatura. A camada de gelo que se forma em uma superfície fria cresce ao longo do tempo, tornando-se mais densa e por este motivo, a formação de frost em equipamentos é responsável pela queda no seu desempenho. O processo de formação de frost é um fenômeno transiente e complicado de avaliar, no qual os processos de transferência de calor e de massa ocorrem simultaneamente em um meio poroso. Neste trabalho apresentam-se medições experimentais da espessura de frost ao redor de cada um dos tubos de um arranjo triangular de cilindros verticais, realiza-se a estimação experimental de uma correlação empírica para a obtenção do número de Nusselt Global sobre o arranjo de tubos com formação de frost baseada nas medições experimentais feita numa seção de teste no interior de um túnel de vento do Laboratório de Armazenamento térmico e tubos de Calor da FEM. Para verificação é feita a estimação de uma correlação empírica para o Número de Nusselt Local em cada cilindro baseado em dados empíricos da literatura e um procedimento inverso de aproximação sucessiva utilizando a solução numérica unidimensional que simula o problema direto de formação de frost em geometria cilíndrica. O mesmo procedimento utilizando os dados experimentais deste trabalho. Encontra-se que os valores dos Números de Nusselt para as duas formas de estimação tem boa concordância. Comparações dos resultados numéricos e experimentais são realizadas e mostra-se boa concordância dos resultados das simulações numéricas para o crescimento da camada de frost utilizando ambas as correlações empíricas / Abstract: In practice, it is found the phenomenon of frost formation in a lot of commercial and industrial refrigeration applications. The layer of ice that it is formed in a cold surface grows over time, becoming more dense and bigger, and for that reason, this formation of frost in refrigeration equipment is responsible for performance decrease in this equipment. The formation of frost is a complicated and transient process, where different process of heat transfer and mass occurs simultaneously in a porous and fine layer. The convective and mass diffusive coefficients are difficult to determinate in fact of the complex physic phenomenon and the variation of the different properties that occurs over time like velocity, temperature, humidity and the geometry of this problem himself. In this work some experimental results of the frost thickness around every cylinder in a triangular arrangement of vertical cylinders are presented, an estimated experimental correlation in order to find Nusselt Global number in this triangular arrangement of cylinders is presented. This correlation is based in the experimental measurements in a wind tunnel situated in the Laboratory of Thermal storage and Fluids in FEM. In order to verify this correlation, another empiric correlation have been estimated in order to find the Nusselt number Global and Local in each cylinder. In this case, it is used data reported in the literature and an inverse method that approximates the numerical solution in one dimension that simulates this problem in cylindrical frost formation geometry. Then the same procedure has been made using the experimental results founded in this work. The different results of the two different correlations have an acceptable concordance. Different comparisons of the numerical and experimental results have been made for the frost growth always validating the results with the numerical simulations of the frost growth using both of the empirical correlations and comparing them with the experimental results that have been founded in this work and with other experimental results founded in the literature / Doutorado / Termica e Fluidos / Doutora em Engenharia Mecânica

This thesis deals with the calculation procedures for determining the heat transfer coefficient for steam condensers. The first section provides a basic overview for industrial steam condensers and possible procedure for selecting the appropriate type of tube condenser. The next section describes the types of condensation and equations for determining the heat transfer coefficient for various geometries of tubular condensers. In the final section there is performed practical calculation of condenser in Maple software using the above relations. The results obtained in this calculation are compared with the results obtained by professional computational software Chemcad and HTRI.

This thesis in introductory part aims to analyze the available literature on the heat transfer coefficient in labyrinth seals and rotating discs of steam turbines. The available experiment studies were processed to summarize heat transfer coefficients on the rotating parts of the turbine. Then, this thesis specifies a design calculation to determine the heat transfer coefficient in selected parts of the turbine, exercisable for specific geometric and operating parameters. The outcome of this work is simulation of rotor dilation of operating steam turbine in the program Ansys during cold start of turbine.