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Call number: LD2668 .T4 1985 S337 / Master of Science

O presente estudo envolve a análise teórico-experimental da transferência de calor através do mecanismo de ebulição em um único tubo e em banco com três tubos horizontais. A análise da literatura permitiu levantar os parâmetros que podem influenciar o coeficiente de transferência de calor na ebulição em banco de tubos e algumas correlações. O aparato experimental foi adaptado para realização de ensaios envolvendo refrigerantes halogenados, 3 tubos dispostos em fileiras paralelas e 3 distintos diâmetros. Nos experimentos foram utilizados os refrigerantes R-11, R-123 (baixa pressão) e R-134a (média pressão), tubos de latão aquecidos internamente com resistências elétricas, com fluxo de calor específico variando entre 1 e 40 kW/'M POT.2'. Através dos resultados foram observados alguns comportamentos inéditos na influência do acabamento superficial e em banco de tubos. Com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição em banco de tubos. Os resultados obtidos por esta correlação apresentaram reduzidos desvios em relação aos experimentais. / The research reported herein is a theoretical and experimental investigation of nucleate boiling heat transfer in an isolated tube and a row of three horizontal tubes. The literature review provided enough information to raise the intervening physical parameters and several correlations. The experimental bench has been developed and adapted to perform experiments with several refrigerants, three different tube diameters, and to accommodate a row of three parallel tubes. The experiments have been carried out with refrigerants R-11, R-123 and R-134a. Heating of the brass tubes has been provided by tubular electrical heaters inserted inside the tubes. The heat flux varied from 1 to 40 kW/'M POT.2'. Experiments have been carried out by successively heating two and three tubes. Effects of boiling in tubes underneath (lower level) have been investigated. Finally a correlation for the heat transfer coefficient in successive tubes of a tube bank has been developed. The correlation presents good accuracy with respect to data from the present investigation.

Direct numerical simulations, where both the large and small scales in the flow are fully resolved, provide an excellent instrument to validate multiphase flow processes and also further our understanding of it. Three multiphase systems are studied using a finite difference/front-tracking method developed for direct numerical simulations of time-dependent system¬¬s. The purpose of these studies is to demonstrate the benefit in developing accurate sub-grid models that can be coupled with the direct numerical simulations to reduce the computational time. The primary reason to use the models is that the systems under consideration are sufficiently large that resolving the smallest scales is impractical. The processes that are examined are: (1) droplet motion and impact (2) nucleate boiling and (3) convective mass transfer. For droplet impact on solid walls and thin liquid films, the splash characteristics are studied. The collision of a fluid drop with a wall is examined and a multiscale approach is developed to compute the flow in the film between the drop and the wall. By using a semi-analytical model for the flow in the film we capture the evolution of films thinner than the grid spacing reasonably well. In the nucleate boiling simulations, the growth of a single vapor from a nucleation site and its associated dynamics are studied. The challenge here is the accurate representation of the nucleation site and the small-scale motion near the wall. To capture the evaporation of the microlayer left behind as the base of the bubble expands we use a semi-analytical model that is solved concurrently with the rest of the simulations. The heat transfer from the heated wall, the evolution of the bubble size and the departure diameter are evaluated and compared with the existing numerical results. The mass transfer near the interface, without fully resolving the layer by refining the grid is accommodated by using a boundary layer approximation to capture it. The behavior of the concentration profile is taken to be self-similar. A collection of potential profiles is tested and the accuracy of each of these models is compared with the full simulations.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 333-336. / by Hsueh-Rong Chang. / Sc.D.

In observations of steady-state nucleate boiling, fluctuations in the temperature and heat flux might initially appear to be completely random. However, it was shown that, for a vertically mounted platinum wire in liquid nitrogen, the fluctuations about the steady-state exhibit an average counterclockwise circulation when the heat flux is plotted versus the superheat temperature. An area associated with the average circulation was proposed as a numerical measure of stability for steady-state nucleate boiling. The mechanisms for the generation of these fluctuations are thought to be the feedback of the bubbles rising past the wire and the differential heating and cooling that this engenders. Data similar to the data on the vertical wire have been obtained using the same wire mounted horizontally. Although the counterclockwise circulation mentioned above is still seen, the measure of stability as proposed earlier, is less useful for prediction of the transition to film boiling. This reduced sensitivity can be attributed to the fact that the possibility of feedback through the rising bubbles has been eliminated.

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

O presente estudo envolve a análise teórico-experimental da transferência de calor através do mecanismo de ebulição em um único tubo e em banco com três tubos horizontais. A análise da literatura permitiu levantar os parâmetros que podem influenciar o coeficiente de transferência de calor na ebulição em banco de tubos e algumas correlações. O aparato experimental foi adaptado para realização de ensaios envolvendo refrigerantes halogenados, 3 tubos dispostos em fileiras paralelas e 3 distintos diâmetros. Nos experimentos foram utilizados os refrigerantes R-11, R-123 (baixa pressão) e R-134a (média pressão), tubos de latão aquecidos internamente com resistências elétricas, com fluxo de calor específico variando entre 1 e 40 kW/'M POT.2'. Através dos resultados foram observados alguns comportamentos inéditos na influência do acabamento superficial e em banco de tubos. Com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição em banco de tubos. Os resultados obtidos por esta correlação apresentaram reduzidos desvios em relação aos experimentais. / The research reported herein is a theoretical and experimental investigation of nucleate boiling heat transfer in an isolated tube and a row of three horizontal tubes. The literature review provided enough information to raise the intervening physical parameters and several correlations. The experimental bench has been developed and adapted to perform experiments with several refrigerants, three different tube diameters, and to accommodate a row of three parallel tubes. The experiments have been carried out with refrigerants R-11, R-123 and R-134a. Heating of the brass tubes has been provided by tubular electrical heaters inserted inside the tubes. The heat flux varied from 1 to 40 kW/'M POT.2'. Experiments have been carried out by successively heating two and three tubes. Effects of boiling in tubes underneath (lower level) have been investigated. Finally a correlation for the heat transfer coefficient in successive tubes of a tube bank has been developed. The correlation presents good accuracy with respect to data from the present investigation.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22894号 / 工博第4791号 / 新制||工||1749(附属図書館) / 京都大学大学院工学研究科原子核工学専攻 / (主査)教授 横峯 健彦, 教授 佐々木 隆之, 講師 河原 全作 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM