All heat pump cycles have one common feature that connects them to one another; this feature is the presence of a heat exchanger. There are even some heat–driven cycles that are completely composed of heat exchangers, every heat exchanger fulfilling a different, though critical role. The need therefore exists to optimize heat exchangers, more specifically Compact Heat Exchangers (CHE). This study deals with the optimization of such a CHE by determining an optimal hydraulic diameter of the micro–channels in a CHE, for minimal hydraulic losses. Two Computational Fluid Dynamics (CFD) models were developed for a single micro–channel that is present in a CHE. The first model had a semi–circular cross–section, the second a triangular cross–section. The results were verified by comparing it with existing experimental data. Following the verification of the results, the micro–channel was optimized by implementing an optimum diameter for the lowest pressure drop over the micro–channel. This was done for both the semi–circular and triangular micro–channel cross–sections. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.

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.

The Reciprocating Mechanism Driven Heat Loop (RMDHL) is a novel heat transfer device that utilizes reciprocating flow, either single-phase or two-phase flow, to enhance the thermal management in high tech inventions. The device attains a high heat transfer rate through a reciprocating flow of the working fluid inside the heat transfer device. Although the concept of the device has been tested and validated experimentally, analytical or numerical studies have not been undertaken to understand its working mechanism and provide guidance for the device design. The objectives of this study are to understand the underlying physical mechanisms of heat transfer in internal reciprocating flow, formulate corresponding heat transfer correlations, conduct an experimental study for the heat transfer coefficient, and numerically model the single-phase and two-phase operations of the RMDHL to predict its performance under different working conditions. The two-phase flow boiling model was developed from the Rensselaer Polytechnic Institute (RPI) model, and a virtual loop written in C programming language was used to eliminate the need for fluid structure interaction (FSI) modelling. The accuracy of several turbulence formulations, including the Standard, RNG, and Realizable k-ɛ Models, Standard and SST k-ω Models, Transition k - - ω Model, and Transition SST Model, have been tested in conjunction with a CFD solver to select the most suitable turbulence modelling techniques. The numerical results obtained from the single-phase and two-phase models are compared with relevant experimental data with good agreement. Three-dimensional numerical results indicate that the RMDHL can meaningfully reduce the peak temperature of an electronic device and result in significantly more uniform temperature across the device. In addition to the numerical study, experimental studies in conjunction with analytical studies are undertaken. Experimental data and related heat transfer coefficient as well as practically useful semi-empirical correlations have been produced, all of which provide archival information for the design of heat transfer devices involving a reciprocating flow. In particular, this research will lead to the development of more powerful RMDHLs, achieve a heat flux goal of 600 W/cm2, and significantly advance the thermal management at various levels. Considering the other advantages of coolant leakage free and the absence of cavitation problems, the RMDHL could also be employed for aerospace and battery cooling applications.

Solid-fluid thermal interactions during unsteady flow in porous media play an important role in the regenerators of pulse tube cryocoolers. Pore-level thermal processes in porous media under unsteady flow conditions are poorly understood. The objective of this investigation is to study the pore-level thermal phenomena during pulsating flow through a generic, two-dimensional porous medium by numerical analysis. Furthermore, an examination of the effects of flow pulsations on the thermal dispersion and heat transfer coefficient that are encountered in the standard, volume-average energy equations for porous media are carried out. The investigated porous media are periodic arrays of square cylinders. Detailed numerical data for the porosity range of 0.64 to 0.84, with flow Reynold's numbers from 0-1000 are obtained. Based on these numerical data, the instantaneous as well as cycle-average thermal dispersion and heat transfer coefficients, to be used in the standard unsteady volume-average energy conservation equations for flow in porous media, are derived. Also, the adequacy of current applied cycle-average correlations for heat transfer coefficients and the inclusion of the thermal dispersion in the definition of an effective fluid thermal conductivity are investigated.

Le phénomène de Rayleigh-Bénard correspond à l'état instable dans lequel se trouve une couche horizontale d'un fluide dilatable, soumise à un gradient de température DT. Si ce dernier dépasse une valeur critique DTc, des mouvements convectifs naissent à l'intérieur du fluide. Concernant les fluides à seuil, le phénomène devient plus complexe. Le seuil s'ajoute aux forces stabilisatrices au sein du fluide et modifie de manière fondamentale le transfert de matière et le transfert thermique. Au départ, le fluide est au repos ; le gradient de vitesse est alors nul et la viscosité efficace infinie partout. L'approche de stabilité linéaire est incapable de fournir une solution aux équations d'écoulement car on doit perturber, par les forces d'Archimède, un fluide d'une viscosité infinie. Dans ce travail de thèse, des expériences de Rayleigh-Bénard ont été effectuées sur des solutions à base de Carbopol 940 présentant un seuil de contrainte. Le dispositif expérimental nous a permis d'avoir des résultats quantitatifs et qualitatifs intéressants. Les mouvements thermoconvectifs ont ensuite été filmés par la technique d'ombroscopie. L'effet non-linéaire au début de la convection a été observé. / Rayleigh-Bénard convection phenomena correspond to the unstable state of an horizontal and dilatable fluid layer under a temperature gradient DT. If it exceeds a given critical value DTc, convective movements appear. The phenomena becomes more complex for yield stress fluids. This threshold is added to stabilizing forces exerced within the fluid and alters the fundamental heat and mass transfer. The fluid is initially at rest and therefore the velocity gradient is zero, and the effective viscosity is infinite everywhere. The linear stability approach is unable to respond because we have to disturb Archimedes forces in a fluid with infinite viscosity. In this thesis, Rayleigh-Bénard experiments were performed with Carbopol 940 solutions which expressing a yield stress. The experimental apparatus allowed us to obtain interesting quantitative and qualitative results. The non-linear effect at the beginning of convection was observed and thermoconvective movements were observed using shadowgraphy technique.

Made available in DSpace on 2014-06-11T19:23:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-12-20Bitstream added on 2014-06-13T19:47:44Z : No. of bitstreams: 1 souza_eg_me_ilha.pdf: 959550 bytes, checksum: b4784dbcc883b1be2d0c6b7cce83f54b (MD5) / Fundação de Ensino Pesquisa e Extensão de Ilha Solteira (FEPISA) / O objetivo deste trabalho é a análise numérica e experimental de escoamento viscoso, incompressível, permanente, com transferência de calor, em um canal estreito contendo um arranjo de obstáculos retangulares. A análise experimental envolveu determinação de coeficiente de transferência de calor médio bem como o número de Nusselt médio e medidas de temperatura em esteira térmica para comparação com os resultados obtidos por simulação numérica. Para a análise numérica usamos o programa comercial de mecânica dos fluidos e transferência de calor computacional ICEPAK®. Verificamos que quanto mais adentro o obstáculo estiver no arranjo maior é a transferência de calor por convecção forçada. Determinamos coeficientes de transferência de calor médio e número de Nusselt médio (com incerteza entre 6 e 15%) e verificamos que o efeito da posição diminui à medida que a velocidade aumenta. Concluímos também que ambos os modelos de turbulência utilizados, k-ε padrão e k-ε RNG, foram incapazes de predizer o efeito da posição apropriadamente. Entretanto, o modelo k-ε RNG apresentou melhor comportamento, pois o seu uso resultou em soluções com valores de temperatura intermediários aos experimentais / The purpose of this work is the study of the numerical and experimental viscous incompressible steady flow with heat transfer into a narrow channel containing a rectangular array of obstacles. The experimental approach involves determining the coefficient of heat transfer and temperature measurements in thermal wake for comparison with the results obtained in numerical simulations. For the numerical analysis we use the commercial program of fluid mechanics and heat transfer computational ICEPAK™. We confirmed that in the last lines of the array the biggest is the heat transfer by forced convection. We determined the average heat transfer coefficients (with uncertainty between 6 and 15%) and found that the effect of the position decreases as flow speed increases. We use in the simulations the k-ε turbulence model and the k-ε RNG turbulence model. We conclude that both turbulence models used were unable to predict the effect of the position properly. However, the k-ε RNG model showed better behavior. The numerical temperatures with this model were consistent to the experimental temperature

[pt] O presente trabalho é um estudo experimental das características de um escoamento de ar em forma de jato espiralado, incidindo ortogonalmente sobre uma placa. Os objetivos do estudo são: avaliar a influência da presença de uma componente circunferencial de velocidade na distribuição dos coeficientes locais de troca de calor, obter campos de velocidade instantâneos no plano axissimétrico assim como informações sobre as características da turbulência no escoamento. Durante os experimentos se investigou a influência da distância jato/placa e da intensidade do escoamento espiralado (número de Swirl). Como etapa preliminar, foi conduzido um experimento de jato livre, para validação das técnicas de medição de velocidade utilizadas. Os resultados foram comparados com os da literartura e uma boa concordância foi obtida. A distribuição espacial dos coeficientes de troca de calor foi avaliada impondo-se um fluxo de calor constante na placa e medindo a distribuição radial de temperatura através de diversos termopares. Coeficientes locais puderam então ser estimados. Os campos de velocidades radial e axial instantâneos foram adquiridos experimentalmente através da utilização da técnica de Particle Image Velocimetry (PIV) e perfis de velocidade tangencial (média e flutuações) foram obtidos a partir da técnica Laser Doppler Velocimetry (LDV). Os resultados mostraram que os padrões de escoamento mudam significativamente quando a componente circunferencial de velocidade é introduzida. Para o valor mais alto do Número de Swirl foram verificadas fortes reversões do escoamento na região de estagnação. / [en] The present work is an experimental study of the characteristics of a swirling impinging air jet. The goals of the study are: to evaluate the influence of the presence of a circumferential velocity component in the distribution of the local heat transfer coefficients, to obtain instantaneous velocity fields in the axisymmetric plane, as well as information about the turbulence characteristics in the flow. During the experiments, the influence of the impingement distance and swirl intensity were investigated. As a preliminary validation of the velocity measurement tecniques, an experimental investigation of an axisymmetric free jet was conducted. The results were compared with literature showing good agreement. The spatial distribution of heat transfer coefficients was evaluated by imposing a constant heat flux condition to the plate and measuring temperature of several points along the radial distance of the plate with thermocouples. Local coefficients could then be estimated. Instantaneous axial and radial velocity fields were obtained with Particle Image Velocimetry (PIV) and tangential velocity profiles (mean and fluctuations) obtained by using Laser Doppler Velocimetry (LDV). The results showed that the flow patterns change significantly when the tangential component is added. For the highest value of Swirl number, strong recirculation zones were observed in the stagnation region.

In this research, an elastic cylinder that utilized vortex-induced vibration (VIV) was applied to improve convective heat transfer rates by disrupting the thermal boundary layer. Rigid and elastic cylinders were placed across a fluid channel. Vortex shedding around the cylinder led to the periodic vibration of the cylinder. As a result, the flow-structure interaction (FSI) increased the disruption of the thermal boundary layer, and therefore, improved the mixing process at the boundary. This study aims to improve convective heat transfer rate by increasing the perturbation in the fluid flow. A three-dimensional numerical model was constructed to simulate the effects of different flow channel geometries, including a channel with a stationary rigid cylinder, a channel with a elastic cylinder, a channel with two elastic cylinders of the same diameter, and a channel with two elastic cylinders of different diameters. Through the numerical simulations, the channel maximum wall temperature was found to be reduced by approximately 10% with a stationary cylinder and by around 17% when introducing an elastic cylinder in the channel compared with the channel without the cylinder. Channels with two-cylinder conditions were also studied in the current research. The additional cylinder with the same diameter in the fluid channel only reduced the surface wall temperature by 3% compared to the channel without any cylinders because the volume of the second cylinder could occupy some space, and therefore, reduce the effect of the convective heat transfer. By reducing the diameter of the second cylinder by 25% increased the effect of the convection heat transfer and reduced the maximum wall temperature by around 15%. Compared to the channel with no cylinder, the introduction of cylinders into the channel flow was found to increase the average Nusselt number by 55% with the insertion of a stationary rigid cylinder, by 85% with the insertion of an elastic cylinder, by 58% with the insertion of two cylinders of the same diameter, and by approximately 70% with the insertion of two cylinders of different diameters (the second cylinder having the smaller diameter). Furthermore, it was also found that the maximum local Nusselt number could be enhanced by around 200%-400% at the entrance of the fluid channel by using the elastic cylinders compared to the channel without cylinders.