Made available in DSpace on 2014-10-09T12:30:57Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:17Z (GMT). No. of bitstreams: 1 01386.pdf: 5820506 bytes, checksum: 6308c9f7dae1ed503a75ddfa5a2542db (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:30:57Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:17Z (GMT). No. of bitstreams: 1 01386.pdf: 5820506 bytes, checksum: 6308c9f7dae1ed503a75ddfa5a2542db (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

D.Ing.

M.Ing. / Heat pump water heaters (HPWHs) have been identified as a viable replacement for electrical resistance heaters due to their high efficiency and reliability. Heat exchangers are a crucial part of HPWHs, and play a vital role in improving the system's coefficient of performance (COP). Experimentally analysing a heat exchanger is usually a slow and highly labour intensive practice since vast amounts of data have to be logged and mathematically manipulated to obtain results. A lot of time and money could be saved, if this process were to be automated. The first objective of this study was to develop a software program capable of calculating the heat transfer correlation constants of a tube-in-tube heat exchanger using the modified Wilson plot technique from data obtained through water-to-water experimentation at different flow rates and temperatures. The data was to be captured by using data acquisition equipment capable of measuring temperature from several Ptl 00-type temperature sensors, pressure transducers as well as Coriolis flow meters, all within a few seconds, thus giving virtually steady-state measurements. The second objective of this study was to develop a software package, capable of capturing and manipulating data from a HPWH system using the same tube-in-tube heat exchanger and using R-22 as refrigerant. The software package had to be capable of capturing all the required experimental data from the system and calculate the local and average heat transfer coefficients on-line and display it to the user. It also had to capture it in the form of a spreadsheet data file for further manipulation. The success of the software package would depend on the results achieved, as well as the time saved with its implementation. To verify the results, the output of the program was compared with the findings of various other researchers. It was found that the output of the program compared well with the results obtained by other researchers, both for the average heat transfer coefficient as well as the local heat transfer coefficients. The time taken for a full set of data was as little as 30 minutes, compared to many hours previously needed to achieve stable results. The software package has thus succeeded in fulfilling its objective to reduce the time taken to achieve accurate results during heat transfer experimentation.

D.Ing. / According to literature 6% of South Africa’s primary energy consumption could be saved if heat pumps were used to their full technical potential. Although there is world-wide interest in the use of heat pumps and considerable effort has been expended on heat-pump research, heat pumps are not commonly used in South Africa. The objective of this thesis is to determine the possibility of a combined evaporator or condenser with a normal pump. This will reduce cost and space of a normal heat pump and make heat pumps economically more competitive against resistance element geysers. In order to investigate this combination research is done on semicircular heat exchangers, since this is the primary geometry of the heating channels in the condenser pump. Analyses is done experimentally on a standard 28.58 mm hard drawn copper tube, cut trough the middle, with a 1.6 mm copper plate in between to obtain a semicircular heat exchanger. Turbulent flow is investigated with the flat side of the semicircular heat exchanger being horizontal or vertical, a spiralled and a s-shape semicircular heat exchanger. In each case the heat transfer coefficient is determined with the use of the Wilson plot technique. It is found that there is a significant increase in Nusselt number for semicircular heat exchangers above a normal tube-in-tube heat exchanger but the pressure loss coefficient increase with an equal amount.

Most heat transfer tubes are designed for either fully uniform wall temperature or fully uniform wall heat flux boundary conditions under forced convection. Several applications, including but not limited to the solar collectors of renewable energy systems, do however operate with non-uniform boundary conditions. Limited research has been conducted on non-uniform wall heat flux heat transfer coefficients in circular tubes, especially for mixed convection conditions. Such works are normally numerical in nature and little experimental work is available. In this experimental investigation the effects of the circumferential heat flux distribution and heat flux intensity on the single phase (liquid) internal heat transfer coefficient were considered for a horizontal circular tube. Focus was placed on the laminar flow regime of water within a stainless steel tube with an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions, including fully uniform and partially uniform heat fluxes were studied for Reynolds numbers ranging from 650 to 2 600 and a Prandtl number range of 4 to 7. The heat flux conditions included 360˚ (uniform) heating, lower 180˚ heating, upper 180˚ heating, 180˚ left and right hemispherical heating, lower 90˚ heating, upper 90˚ heating and slanted 180˚ heating. Depending on the angle span of the heating, local heat fluxes of 6 631 W/m2 , 4 421 W/m2 , 3 316 W/m2 , 2 210 W/m2 and 1 658 W/m2 were applied. Results indicate that the local and average steady state Nusselt numbers are greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for case where the applied heat flux was positioned on the lower half (in terms of gravity) of the tubes circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Variations in the heat transfer coefficient were found to be due to the secondary buoyancy induced flow effect. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for fully uniform heating, lower 180° heating, and upper 180° heating. / Dissertation (MEng)--University of Pretoria, 2018. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Submitted by Maicon Juliano Schmidt (maicons) on 2015-03-20T19:50:03Z No. of bitstreams: 1 000002F2.pdf: 1101205 bytes, checksum: 35ea0ac880e5841836ff1b5e64d2f9ff (MD5) / Made available in DSpace on 2015-03-20T19:50:04Z (GMT). No. of bitstreams: 1 000002F2.pdf: 1101205 bytes, checksum: 35ea0ac880e5841836ff1b5e64d2f9ff (MD5) Previous issue date: 2012-11-27 / CNPQ – Conselho Nacional de Desenvolvimento Científico e Tecnológico / Componentes eletrônicos estão cada vez mais potentes, necessitando de dissipações térmicas maiores. Os ventiladores atuais, conhecidos comercialmente como "coolers", estão se tornando ineficientes para esta evolução por dependerem de uma maior vazão para atender a demanda de calor dissipado, o que também causa aumento no seu ruído. Como uma alternativa para aprimorar a troca de calor, estudam-se (micro) jatos sintéticos. Estes são produzidos através de uma cavidade selada por uma membrana oscilatória e uma placa com um orifício. A movimentação periódica da membrana produz um jato com valor positivo de quantidade de movimento, que pode ser direcionado para o resfriamento de um dispositivo eletrônico.Para análise térmica, um modelo numérico do dispositivo de refrigeração foi construído em ANSYS CFX 12.0. Variações nos números de Reynolds e Strouhal dos jatos sintéticos e posição da região aquecida na superfície de interesse foram realizadas e seu efeito no desempenho térmico analisado. Os resultados foram comparados a um escoamento convencional de mesma geometria em regime permanente e submetido à mesma vazão mássica média induzida por cada jato sintético. Para a configuração testada, observou-se que os (micro) jatos sintéticos podem fornecer um fluxo de ar mais direcionado para os "hotspots" com maior necessidade de resfriamento. Os resultados encontrados indicam um aumento de número de Nusselt até 122% em jatos sintéticos comparados aos escoamentos contínuos. Logo, confirmam o maior desempenho térmico do jato sintético em relação ao método convencional equivalente e justificam a necessidade de investigações adicionais nesta área. Isto indica que os jatos sintéticos podem ser personalizados ou direcionados especificamente para atender a demanda de resfriamento do problema de interesse. / The rising power consumption of electronic components requires higher and higher thermal dissipation. Current fan systems, commercially known as "coolers", are becoming ineffective to cope with this demand since their performance is dependent on the volumetric flow rate of the driving fan, which becomes more wasteful and noisy. An alternative to improve the heat exchange of current systems is the application of (micro) synthetic jets. These are produced by the oscillations in a cavity bounded by a membrane and a plate with an orifice. Membrane actuation produces a net forward momentum jet through the orifice, which can be applied to cool an electronic device. For this analysis, a numerical model of the cooling device was built on ANSYS CFX 12.0. Variations in jet Reynolds and Strouhal numbers and positioning of the heated region of interest were made and their effect on thermal performance analyzed. Results were compared to a conventional flow with the same geometry but subjected to a single-fan providing steady flow with the same average mass flow rate induced by each synthetic jet. For the configurations tested, it was found that (micro) synthetic jets may provide more directed air flow for "hotspots" with the greatest need of cooling. The results indicate a thermal performance up to 122% higher compared to their equivalent conventional cooling case. This confirmation of the higher thermal performance of synthetic jets relative to a convencional method and justifies the need for the current and additional investigations in this area. Results also indicate that synthetic jets can be customized and specifically directed to meet the cooling demand of the problem in question.

Submitted by Silvana Teresinha Dornelles Studzinski (sstudzinski) on 2015-07-08T14:34:31Z No. of bitstreams: 1 Matheus Vicenzo Lehnen.pdf: 7507080 bytes, checksum: 1036a30adcb3840ea0e5fcb545f29987 (MD5) / Made available in DSpace on 2015-07-08T14:34:31Z (GMT). No. of bitstreams: 1 Matheus Vicenzo Lehnen.pdf: 7507080 bytes, checksum: 1036a30adcb3840ea0e5fcb545f29987 (MD5) Previous issue date: 2012-05 / Milton Valente / Nos dias atuais, os componentes eletrônicos estão cada vez mais potentes e com mais dispositivos integrados e há a necessidade de uma dissipação térmica mais eficiente. Os atuais ventiladores e dissipadores de calor usando ar como fluido de trabalho estão ficando obsoletos. Por este motivo, torna-se necessário o desenvolvimento de um sistema mais eficiente. Existem três técnicas principais em estudo nesta área: resfriamento líquido, trocadores de calor compostos por microcanais e jatos sintéticos como transmissores de quantidade de movimento ao fluido. Entretanto, a análise em pequena escala encontra limitações experimentais de modo que uma abordagem por Dinamica de Fluidos Computacional (Computational Fluid Dynamics – CFD) é mais recomendável para caracterizar e validar o desempenho dos jatos sintéticos. O objetivo principal deste trabalho é realizar uma análise fluidodinâmica de jatos sintéticos e caracterizar a troca térmica de jatos sintéticos colidindo sobre uma superfície aquecida, através de simulação numérica. A flexibilidade da aproximação numérica também possibilita o estudo da sensibilidade do design a vários parâmetros físicos e geométricos, tais como o número de Reynolds, a frequência do atuador, o número de Prandtl, a distância da placa aquecida ao orifício da cavidade, o formato do orifício do atuador, a profundidade da cavidade e a espessura da placa do orifício. Os resultados caracterizam o efeito dos parâmetros físicos e geométricos de interesse na formação do jato e na dissipação térmica. O conhecimento agregado neste estudo permitiu determinar uma correlação para o número de Nusselt em função da frequência adimensional – o número de Strouhal – do número de Reynolds, do número de Prandtl e da distância adimensional da superfície aquecida ao orifício. Assim, é possível prever o comportamento de tais jatos sobre a superfície aquecida, e assim contribuir para os atuais estudos nesta linha de pesquisa. Os resultados apresentados tem então aplicação em estudos posteriores, de maior complexidade de design com atuadores combinados com trocadores de calor de aletas, coolers e micro canais, resultando em avanços na área de resfriamento de microchips. / Current electronic components are becoming ever more potent and densly integrated, which requires further increases in the efficiency of heat dissipation. With current fan-based heat dissipation techniques with air as the working fluid becoming outdated, there is a pressing need to develop more eficient methods to cope with demand. So far, three techniques have been the primary focus of studies in this area: liquid cooling, microchannel heat exchangers and synthetic jets used to promote increased momentum transfer. Analysis of such devices at the small physical scale of electronic components is somewhat problematic in experimental form so that a computational fluid dynamics (CFD) approach is recommended. The main objective of this study is thus to utilize a CFD approach to establish the performance characteristics of a synthetic jet impacting against a heated surface. The flexibility of a numerical approach also allows the examination of the sensibility of the design with respect to several physycal and geometric parameters such as Reynolds number, pulsing frequency, jet orifice shape and size, cavity size and distance between the heated surface and the device. Such results, provide insight in the effect of physical and geometric parameters in the jet formation and heat dissipation. The combined knowledge of this study allowed the development of a practical correlation for the Nusselt number based on the Strouhal number (normalized pulsing frequency), Reynolds number, Prandtl number and the distance between the heated surface and the synthetic jet. This result allows improved predictions of a jet impacting against a heated surface and, consequently, adds an important contribution to other studies in this area. It is expected that the results presented here will be the starting point for further work, in which increasingly complex geometries such as actuators combined with heat exchangers equipped with fins, coolers or microchannels are examined to further improve the knowledge in the field of electronic cooling.

Submitted by Silvana Teresinha Dornelles Studzinski (sstudzinski) on 2016-02-04T15:21:28Z No. of bitstreams: 1 Marino Trisch_.pdf: 3535397 bytes, checksum: 4cc7a6dc219d9c91a6de57725e4515d1 (MD5) / Made available in DSpace on 2016-02-04T15:21:28Z (GMT). No. of bitstreams: 1 Marino Trisch_.pdf: 3535397 bytes, checksum: 4cc7a6dc219d9c91a6de57725e4515d1 (MD5) Previous issue date: 2015-06-22 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho apresenta um estudo experimental relacionado ao resfriamento de dispositivos eletrônicos utilizando jatos sintéticos direcionados de modo que o jato flua tangencialmente à superfície aquecida, utilizando para isso uma bancada experimental especialmente desenvolvida. Para o desenvolvimento deste trabalho foram analisados outros estudos relacionados ao assunto, simulados e experimentais, utilizando neste caso um alto-falante como membrana montada em conjunto com a estrutura da bancada para formar a câmara e consequentemente o gerador de jatos sintéticos. O jato sintético gerado irá resfriar um elemento de aquecimento que simula o funcionamento de um dispositivo eletrônico, posicionado tangencialmente em diversas posições de distância em relação à saída do jato. Os procedimentos de teste de resfriamento foram realizados na bancada experimental em diversos modos de funcionamento do elemento de aquecimento, utilizando temperatura média de 80 °C semelhante à temperatura máxima de trabalho de dispositivos eletrônicos. Para a geração do jato sintético foram aplicados sinais senoidais em frequências de pulsação entre 20 e 120 Hz e com amplitude de aproximadamente 7,52 V_p, que resulta em 20 Wrms de potência no gerador de jatos sintéticos. Nos testes utilizando potência fixa do elemento de aquecimento, a temperatura no elemento de aquecimento é monitorada. Em outro modo de teste, foi mantida uma temperatura constante e monitorada a potência máxima correspondente dissipada no elemento de aquecimento. Por fim, também foi realizado comparativo entre resfriamento eletrônico utilizando jatos sintéticos e método tradicional com a utilização de ventiladores, onde são utilizados três diferentes tamanhos de coolers acoplados à bancada experimental e arrefecendo o mesmo elemento de aquecimento, verificando e comparando velocidades e rendimento entre os métodos de resfriamento. / This paper presents an experimental study related to the cooling of electronic devices using synthetic jets directed so that the jet flows tangentially to the heated surface. A custom-built experimental test bench especially developed based on other studies related to the subject. In this case, a speaker was used as a membrane and installed in a cavity in the test bench to form the synthetic jet generator. The synthetic jet cools a heating element that simulates the operation of an electronic device, positioned tangentially at various distance in relation to the exit plane of the synthetic jet. Cooling test procedures were performed in the custom-built experimental test bench in various operation modes of the heating element, using an average temperature of 80 ° C which is similar to the operating temperature of electronic devices. To generate the synthetic jet, sinusoidal input signals were applied with frequencies between 20 and 120 Hz and with amplitude of approximately 7.52 Vp which resulted in 20 Wrms power consumed by generator. In tests using a fixed power dissipated by the heating element, the temperature drop is monitored in the heating element. In the other test mode, the temperature on the heating element was set at a constant value the maximum power dissipated in the heating element was measured. Finally, comparisons were also performed between the cooling performance of synthetic jets and the conventional method with the use of three different coolers sizes. The same tests were performed on the same heating element and the corresponding velocities and cooling performance between the two methods were compared.

In this thesis, we consider two models from physics, which are characterized by the interplay of thermodynamical and fluid mechanical phenomena: demixing (spinodal decomposition) and Rayleigh--Bénard convection. In both models, we investigate the dependencies of certain intrinsic quantities on the system parameters. The first model describes a thermodynamically driven demixing process of a binary viscous fluid. During the evolution, the two components of the mixture separate into two domains of the different equilibrium volume fractions. One observes a clear tendency: Larger domains grow at the expense of smaller ones, and thus, the average domain sizes increases --- a phenomenon called coarsening. It turns out that two mechanisms are relevant for the coarsening process. At an early stage of the evolution, material transport is essentially mediated by diffusion; at a later stage, when the typical domain size exceeds a certain value, due to the viscosity of the mixture, a fluid flow sets in and becomes the relevant transport mechanism. In both regimes, the growth rates of the typical domain size obey certain power laws. In this thesis, we rigorously establish one-sided bounds on these growth rates via a priori estimates. The second model, Rayleigh--Bénard convection, describes the behavior of a fluid between two rigid horizontal plates that is heated from below and cooled from above. There are two competing heat transfer mechanisms in the system: On the one hand, thermodynamics favors a state in which temperature variations are locally minimized. Thus, in our model, the thermodynamical equilibrium state is realized by a temperature with a linearly decreasing profile, corresponding to pure conduction. On the other hand, due to differences in the densities of hot and cold fluid parcels, buoyancy forces act on the fluid. This results in an upward motion of hot parcels and a downward motion of cold parcels. We study the dependence of the average upward heat flux, measured in the so-called Nusselt number, on the temperature forcing encoded by the container height. It turns out that the efficiency of the heat transport is independent of the height of the container, and thus, the Nusselt number is a constant function of height. Using a priori estimates, we prove an upper bound on the Nusselt number that displays this dependency --- up to logarithmic errors. Further investigations on the flow pattern in Rayleigh--Bénard convection show a clear separation of length scales: Along the horizontal top and bottom plates one observes thin boundary layers in which heat is essentially conducted, whereas the large bulk is characterized by a convective heat flow. We give first rigorous results in favor of linear temperature profiles in the boundary layers, which indicate that heat is indeed essentially conducted close to the boundaries.