No description available.

[pt] Escoamentos bifásicos no regime de golfadas são caracterizados pela alternância de pacotes de líquido e grandes bolhas de gás na tubulação, sendo associados a altas perdas de carga, além de trazer uma indesejada intermitência aos escoamentos. O desenvolvimento do regime de golfadas em tubulações horizontais se dá a partir do escoamento estratificado em decorrência de dois fatores: do crescimento natural de pequenas perturbações (por um mecanismo de instabilidade do tipo Kelvin-Helmholtz) ou devido à acumulação de líquido causada por mudanças de inclinação no perfil do duto. O presente trabalho consiste da simulação numérica do surgimento das golfadas em ambas as situações descritas acima, assim como do subseqüente desenvolvimento do escoamento neste padrão para um regime estatisticamente permanente. A previsão do escoamento é obtida utilizando-se uma formulação unidimensional baseada no Modelo de Dois Fluidos. Parâmetros médios das golfadas (comprimento, velocidade e freqüência) são comparados com estudos numéricos e experimentais da literatura, obtendo-se uma concordância bastante satisfatória, especialmente dada a simplicidade de uma formulação unidimensional. / [en] Slug flow is a two-phase flow pattern which is characterized by the periodic presence of packs of liquid and long bubbles in the tube, associated with high pressure-drops and an often undesired intermittency in the system. The development of the slug pattern in horizontal pipes is caused by two reasons: the natural growth of small disturbancies at the interface (by a Kelvin-Helmholtz instability mechanism) or the liquid accumulation at valleys of hilly terrain pipelines with sections of different inclinations. The present work consists of the numerical simulation of the onset of slugging in both situations, as well as the subsequent development of statistically steady slug flow in the pipe. The prediction of the flow is obtained through a one- dimensional formulation based on the Two-Fluid Model. Averaged slug parameters (length, velocity and frequency) are compared with previous numerical studies and experimental correlations avaiable in the literature, and a very satisfactrory agreement is obtained, specially given the simplicity of a one dimensional formulation.

Thermally developing, electro-osmotically generated flow has been analyzed for a circular microtube under imposed constant wall temperature (CWT) and constant wall heat flux (CHF) boundary conditions. Established by a voltage potential gradient along the length of the microtube, the hydrodynamics of such a flow dictate either a slug flow velocity profile (under conditions of large tube radius-to-Debye length ratio, a/lambda_d) or a family of electro-osmotic flow (EOF) velocity profiles that depend on a/lambda_d. The imposed voltage gradient results in Joule heating in the fluid with an associated volumetric source of energy. For this scenario coupled with a slug flow velocity profile, the analytical solution for the fluid temperature development has been determined for both thermal boundary conditions. The local Nusselt number for the CHF boundary condition is shown to reduce to the classical slug flow thermal development for imposed constant wall heat flux, and is independent of Joule heating source magnitude. For the CWT boundary condition, a local minimum in the streamwise variation in local Nusselt number for moderate positive dimensionless inlet temperature is predicted. For negative dimensionless inlet temperature, which arises if the fluid entrance temperature is below the tube wall temperature, the fluid is initially heated, then cooled, resulting in a singularity in the local Nusselt number at the axial location of the heating/cooling transition. The thermal development length is considerably larger than for traditional pressure-driven flow heat transfer, and is a function of the magnitudes of Peclet number and dimensionless inlet temperature. For the EOF velocity profile scenario, numerical techniques were used to predict the fluid temperature development for both wall boundary conditions by utilizing a finite control volume approach. In addition to Joule heating as an energy source, viscous dissipation is also considered. The results predict that for decreasing a/lambda_d, the local Nusselt number decreases for all axial positions and the thermal development shortens for both wall boundary conditions. Viscous dissipation has significant effect only at intermediate values of a/lambda_d. Results predict local Nusselt numbers to increase for a CWT boundary condition and to decrease for an imposed constant wall heat flux with increasing viscous dissipation.

No description available.

Für die Untersuchung von Luft/Wasser-Strömungen wurde ein horizontaler Acrylglas-Kanal mit rechteckigem Querschnitt gebaut. Der Kanal ermöglicht Gleich- und Gegenstrom-Versuche bei Atmosphärendruck, insbesondere die Untersuchung der Schwallströmung. Es wurden optische Messungen mit einer Hochgeschwindigkeits-Kamera durchgeführt, die durch synchronisierte dynamische Druckmessungen ergänzt wurden. Für die Analyse der Bilder wurde eine Methode zur Erfassung der Phasengrenze entwickelt und diese anhand möglicher Anwendungen getestet. Die Druckmessungen zeigten, dass der Druck bei Schwallströmungen um einige Kilopascal ansteigt und wieder abfällt, sobald der Schwall aus dem Kanal austritt. Zudem wurden Geschwindigkeiten in der flüssigen Phase mittels nicht invasiver Verfahren gemessen. Das durchschnittliche Geschwindigkeits-Profil am Kanaleintritt wurde mit Ultraschall-Köpfen bestimmt. Die Ermittlung des Geschwindigkeitsfeldes in einem Schwall erfolgte mit PIV (Particle Image Velocimetry).

This experimental study explores the heat transfer from heated bare and finned tubular surfaces to particulates in packed bed cross flow. The results from this experiment will be used to help select the type of particulates that will be used. Additionally, these results will assist in estimating heat transfer in prototype and commercial particle to fluid heat exchangers (PFHX). This research is part of larger effort in the use of particulates in concentrating solar power technology. These solid particles are heated by concentrated sunlight to very high temperatures at which they are a suitable heat source for various thermal power and thermochemical cycles. Furthermore, one of the advantages of this concept is the ability to store thermal energy in the solid particles at relatively low cost. However, an important feature of any Particle Heat Receiver (PHR) system is the PFHX, which is the interface between the solar energy system and the thermal power or chemical system. In order to create this system material data is needed for the design and optimization of this PFHX. The paper focuses on the heat transfer properties of particulates to solid surfaces under plug flow conditions. The particulates will be evaluated for three grain sizes of sand and two grain sizes of proppants. These two materials will be tested at one, five and ten millimeters per second in order to see how the various flow rates, which will be required for different loads, will affect the heat transfer coefficient. Finally the heat transfer coefficient will also be evaluated for both finned and non-finned heat exchangers to see the effect that changes in the surface geometry and surface area have on the heat transfer coefficient. The heat transfer coefficient will help determine the appropriate material that will be used in the PHR system.

Made available in DSpace on 2014-06-11T19:23:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-12-27Bitstream added on 2014-06-13T18:50:59Z : No. of bitstreams: 1 mendes_faa_me_ilha.pdf: 2673818 bytes, checksum: 120558e9283b734ad04dda5b07ec06d0 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O princípio de funcionamento de um sistema gas-lift é bastante simples, baseando-se na injeção de um gás inerte próximo à extremidade submersa de um tubo utilizado para ascensão do líquido (riser). Esse sistema tem sido utilizado em diversos setores da indústria, dentre os quais se destaca o setor petroquímico. Apesar de ter sido concebido há mais de um século, e a despeito do considerável número de artigos publicados sobre o assunto, vários aspectos relativos às características do escoamento bifásico dentro do riser e ao funcionamento do sistema são, ainda, pouco conhecidos. No presente trabalho, um sistema air-lift de pequeno porte foi inteiramente desenvolvido e testado, permitindo observar o comportamento do sistema quando submetido a variações na vazão de ar, na razão de submersão e na geometria do injetor – contendo três (3F) ou quinze furos (15F). Ensaios de visualização foram conduzidos, possibilitando a caracterização do escoamento bifásico no interior do riser. Medições experimentais foram, também, realizadas para a obtenção da eficiência e das curvas características do sistema. Os resultados quantitativos foram interpretados em associação com imagens estáticas do escoamento, capturadas com uma câmera fotográfica digital, e com imagens dinâmicas, tomadas com o auxílio de uma filmadora de alta velocidade. A partir destas imagens, foi possível, ainda, a estimativa de alguns importantes parâmetros característicos do escoamento pistonado, associados ao deslocamento da chamada bolha de Taylor. Os dados assim obtidos foram comparados com valores oriundos de correlações empíricas propostas por outros autores, apresentando boa concordância. Com relação aos resultados quantitativos, observou-se que, sob determinadas condições de operação, o injetor 3F é capaz de atribuir ao sistema um rendimento significativamente maior... / The operation principle of a gas-lift system is very simple: it is based on the injection of an inert gas close to the immersed edge of a tube, which is used for the rising of the liquid (riser). This system has been used in several industrial areas and more often in the petrochemical industry. Despite the fact that it was conceived over a century ago and that there have been a considerable amount of articles published on the subject, several aspects concerning the two-phase flow characteristics inside the riser as well as its operational system are still not very well known. In the present work, a small size air-lift system was thoroughly developed and tested, which facilitated the visualization of the system behavior when submitted to variations in the air flow, the immersion ratio and the injector geometry – containing three (3H) or fifteen holes (15H). Observation experiments were carried out making it possible to characterize the two-phase flow inside the riser. Experimental measurements were also taken in order to obtain the system characteristic efficiency and slopes. The quantitative results were analyzed together with the statistic images of the flow - taken by a digital photo camera, and with the dynamic images – taken by a high-speed filming camera. Through these images, it was also possible to estimate some important characteristic parameters of the slug flow associated to the displacement of the so-called Taylor bubble. The data obtained this way were compared to the figures originated from experimental correlations proposed by other authors, and they showed a fine agreement. Regarding the quantitative results, it was observed that under certain operation conditions the 3F injector is able to give the system a significantly greater efficiency than when the 15F injector is used

Resumo: O princípio de funcionamento de um sistema gas-lift é bastante simples, baseando-se na injeção de um gás inerte próximo à extremidade submersa de um tubo utilizado para ascensão do líquido (riser). Esse sistema tem sido utilizado em diversos setores da indústria, dentre os quais se destaca o setor petroquímico. Apesar de ter sido concebido há mais de um século, e a despeito do considerável número de artigos publicados sobre o assunto, vários aspectos relativos às características do escoamento bifásico dentro do riser e ao funcionamento do sistema são, ainda, pouco conhecidos. No presente trabalho, um sistema air-lift de pequeno porte foi inteiramente desenvolvido e testado, permitindo observar o comportamento do sistema quando submetido a variações na vazão de ar, na razão de submersão e na geometria do injetor - contendo três (3F) ou quinze furos (15F). Ensaios de visualização foram conduzidos, possibilitando a caracterização do escoamento bifásico no interior do riser. Medições experimentais foram, também, realizadas para a obtenção da eficiência e das curvas características do sistema. Os resultados quantitativos foram interpretados em associação com imagens estáticas do escoamento, capturadas com uma câmera fotográfica digital, e com imagens dinâmicas, tomadas com o auxílio de uma filmadora de alta velocidade. A partir destas imagens, foi possível, ainda, a estimativa de alguns importantes parâmetros característicos do escoamento pistonado, associados ao deslocamento da chamada bolha de Taylor. Os dados assim obtidos foram comparados com valores oriundos de correlações empíricas propostas por outros autores, apresentando boa concordância. Com relação aos resultados quantitativos, observou-se que, sob determinadas condições de operação, o injetor 3F é capaz de atribuir ao sistema um rendimento significativamente maior... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The operation principle of a gas-lift system is very simple: it is based on the injection of an inert gas close to the immersed edge of a tube, which is used for the rising of the liquid (riser). This system has been used in several industrial areas and more often in the petrochemical industry. Despite the fact that it was conceived over a century ago and that there have been a considerable amount of articles published on the subject, several aspects concerning the two-phase flow characteristics inside the riser as well as its operational system are still not very well known. In the present work, a small size air-lift system was thoroughly developed and tested, which facilitated the visualization of the system behavior when submitted to variations in the air flow, the immersion ratio and the injector geometry - containing three (3H) or fifteen holes (15H). Observation experiments were carried out making it possible to characterize the two-phase flow inside the riser. Experimental measurements were also taken in order to obtain the system characteristic efficiency and slopes. The quantitative results were analyzed together with the statistic images of the flow - taken by a digital photo camera, and with the dynamic images - taken by a high-speed filming camera. Through these images, it was also possible to estimate some important characteristic parameters of the slug flow associated to the displacement of the so-called Taylor bubble. The data obtained this way were compared to the figures originated from experimental correlations proposed by other authors, and they showed a fine agreement. Regarding the quantitative results, it was observed that under certain operation conditions the 3F injector is able to give the system a significantly greater efficiency than when the 15F injector is used / Orientador: Sérgio Said Mansur / Coorientador: Edson Del Rio Vieira / Banca: André Luiz Seixlack / Banca: Ricardo Augusto Mazza / Mestre

Für die Untersuchung von Luft/Wasser-Strömungen wurde ein horizontaler Acrylglas-Kanal mit rechteckigem Querschnitt gebaut. Der Kanal ermöglicht Gleich- und Gegenstrom-Versuche bei Atmosphärendruck, insbesondere die Untersuchung der Schwallströmung. Es wurden optische Messungen mit einer Hochgeschwindigkeits-Kamera durchgeführt, die durch synchronisierte dynamische Druckmessungen ergänzt wurden. Für die Analyse der Bilder wurde eine Methode zur Erfassung der Phasengrenze entwickelt und diese anhand möglicher Anwendungen getestet. Die Druckmessungen zeigten, dass der Druck bei Schwallströmungen um einige Kilopascal ansteigt und wieder abfällt, sobald der Schwall aus dem Kanal austritt. Zudem wurden Geschwindigkeiten in der flüssigen Phase mittels nicht invasiver Verfahren gemessen. Das durchschnittliche Geschwindigkeits-Profil am Kanaleintritt wurde mit Ultraschall-Köpfen bestimmt. Die Ermittlung des Geschwindigkeitsfeldes in einem Schwall erfolgte mit PIV (Particle Image Velocimetry).

No description available.