This thesis investigates the refrigerant (R-134a) flow in a minitube experimentally. The small scale heat transfer is a relatively new research area and has been in favor since the end of 1970&rsquo / s. Refrigerant flow in mini- and microscale media is a potential enhancement factor for refrigeration technology in the future. For the forthcoming developments and progresses, experimental studies are invaluable in terms of having an insight and contributing to the establishment of infrastructure in the field in addition to leading the numerical and theoretical approaches. The studies in the literature show that low mass flow rate and constant wall temperature approach in minitubes and minichannels were not among the main areas of interest. Therefore, an experimental set-up was prepared in order to perform experiments of two-phase refrigerant flow in a 1.65 mm diameter copper minitube with the constant wall temperature approach. The design, preparation, and modifications of the experimental set-up are explained in this thesis. Two-phase flow and quality arrangements were done by pre-heating the refrigerant at saturation pressure and the constant wall temperature was achieved by a secondary cycle with water and ethylene glycol mixture as the working fluid. The heat transfer coefficient and the pressure drop for the two-phase flow with varying quality values and saturation temperatures of the refrigerant were calculated and compared with the results available in literature.

This experimental research aims at investigating the single-phase flow heat transfer and friction factor, flow boiling heat transfer and pressure drop, and flow visualisation in microchannels using de-ionized water. In the literature, many studies failed to explain the effect of aspect ratio on the single-phase and two-phase flow heat transfer rate and pressure drop. Because the channel aspect ratios and hydraulic diameters were varied together in those studies. Also, there is a discrepancy between past studies and the conventional theory for the flow boiling heat transfer characteristics. Accordingly, the objectives of this research can be listed as follows: (i) modifying the existing experimental facility to perform single-phase and two-phase flow heat transfer and pressure drop and two-phase flow pattern visualization experiments in microchannels, (ii) clarifying the fundamental aspects of flow boiling in micro passages, (iii) investigating the aspect ratio, heat flux, mass flux and vapour quality effects on flow patterns, heat transfer rate and pressure drop in single-phase and two-phase flow, (iv) comparing the obtained results with heat transfer and pressure drop correlations and flow pattern maps available in the literature. Consequently, the pre-existing experimental facility was modified in the current research by changing the pre-heaters, flowmeter and piping in order to achieve the goals of this study. Four copper rectangular microchannels were designed and manufactured. Three microchannel test sections having the same hydraulic diameter and length but different aspect ratios were investigated to reveal the effect of aspect ratio on the single-phase and two-phase flow heat transfer rate and pressure drop. The surface roughness of each microchannel was also examined. It was found that the surface roughnesses of all microchannels are similar. Moreover, an additional microchannel test section was used to examine the effect of heated length on the flow boiling heat transfer coefficient and pressure drop. The single-phase flow results demonstrated that the channel aspect ratio has no influence on the friction factor and heat transfer rate for the tested microchannels and experimental range. In the flow boiling experiments, bubbly, bubbly/slug, slug, churn and annular flow regimes were observed in the tested microchannels. The channel aspect ratio effect was found to be small on the observed flow patterns. The experimental flow patterns were predicted well by the flow pattern map proposed by Galvis and Culham (2012) except for the slug flow regime. The flow pattern maps of Sobierska et al. (2006) and Harirchian and Garimella (2009) reasonably predicted the experimental flow pattern data. The flow boiling heat transfer results showed that the prevailing heat transfer mechanism is nucleate boiling for the low and medium heat flux inputs. On the other hand, the dominant heat transfer mechanism is unclear at the high heat flux inputs while smaller aspect ratio microchannel has better heat transfer performance for low and medium heat flux inputs. However, at high heat flux inputs the channel aspect ratio effect was found to be insignificant on the flow boiling heat transfer coefficient. The experimental flow boiling heat transfer coefficient data were reasonably predicted by the correlations of Sun and Mishima (2009), Li and Wu (2010) and Mahmoud and Karayiannis (2011) from the literature. The flow boiling pressure drop characteristics were also examined in the tested microchannels. Outcome of the experiments consistently indicated a highly linear trend between the increasing flow boiling pressure drop and the heat and mass flux. Also, the flow boiling pressure drop increased with the increase in vapour quality. The effect of channel aspect ratio on the flow boiling pressure drop was also assessed. It was found that when the channel aspect ratio decreased, the flow boiling pressure drop increased. The experimental flow boiling pressure drop data were compared to correlations from the literature. Mishima and Hibiki (1996), Yu et al. (2002) and Zhang et al. (2010) correlations reasonably predicted the experimental flow boiling pressure drop results.

The continuous miniaturization of electronic devices and the corresponding increase in computing powers have led to a significant growth in the density of heat dissipation within these devices. This increase in heat generation has challenged conventional air fan cooling and alternative solutions for heat removal are required to avoid overheating and part damage. Micro/Mini channel heat sinks (M/MCHS) that use liquids for heat removal appear as an attractive solution to this problem as they provide large heat transfer area per volume. Mini/microchannels traditionally have suffered from geometrical and material restrictions due to fabrication constraints. An emerging new additive manufacturing technique called binder jetting has the potential to overcome some of those restrictions. In this study, a straight minichannel heat sink is manufactured from stainless steel using binder jetting, and it is experimentally evaluated. The hydraulic performance of the heat sink is tested over a range of Reynolds numbers (150-1200). The comparison between the hydraulic results and standard correlations confirms that the targeted geometry was produced, although the high surface roughness created an early transition from laminar-to-turbulent flow. The heat transfer performance was also experimentally characterized at different heat flux conditions ($3000W/m^2$, $5000W/m^2$, $6500W/m^2$), and a range of Reynolds numbers (150-800). These results indicated that convection heat transfer coefficients on the order of $1000 W/m^2-K$ can be obtained with a simple heat sink design. Finally, the effects of the contact resistance on the results are studied, and contact resistance is shown to have critical importance on the thermal measurements. / Master of Science / The continuous miniaturization of electronic devices and the corresponding increase in computing powers have led to a significant growth in the density of heat dissipation within these devices. This increase in heat generation has challenged conventional air fan cooling and alternative solutions for heat removal are required to avoid overheating and part damage. Micro/Mini channel heat sinks (M/MCHS) that use water instead of air for heat removal appear as an attractive solution to this problem as they provide large heat transfer area per volume due to the small channels. Mini/microchannels are distinguished from conventional channels by the hydraulic diameter, where they range from $10mu m$ to $2mm$. M/MCHS are typically manufactured from a highly conductive metals with the channels fabricated on the surface. However, mini/microchannels traditionally have suffered from geometrical and material restrictions due to fabrication constraints. Complex features like curves or internall channels are difficult or even impossible to manufacture. An emerging new additive manufacturing technique called binder jetting has the potential to overcome some of those restrictions. Binder jetting possess unique advantageous as it uses precise control of a liquid binder applied to a bed of fine powder to create complex geometries Furthermore, it does not require extreme heating during the fabrication process. The advantages of binder jetting include that it is low cost, high speed, can be applied to a variety of materials, and the ability to scale easily in size. In this study, a straight minichannel heat sink is manufactured from stainless steel using binder jetting, and this heat sink is experimentally evaluated. The hydraulic performance of the heat sink is tested over different water flow rates (Reynolds numbers between 150-1200). The comparison between the hydraulic results and standard correlations confirms that the targeted geometry was produced, although the high surface roughness created an early transition from laminar-to-turbulent flow. The surface roughness effect should be considered in future designs of additively manufactured minichannels. The heat transfer performance was also experimentally characterized at different heat flux conditions ($3000W/m^2$, $5000W/m^2$, $6500W/m^2$), and different water flow conditions (Reynolds numbers 150-800). These results indicated that convection heat transfer coefficients on the order of $1000 W/m^2-K$ can be obtained with a simple heat sink design. However, a mismatch between the experimental data and the correlation requires further investigation. Finally, the effects of the contact resistance on the results are studied, and contact resistance is shown to have critical importance on the thermal measurements.

Submitted by JOSIANE SANTOS DE OLIVEIRA (josianeso) on 2018-11-12T13:35:25Z No. of bitstreams: 1 Melissa Grahl Figueredo_.pdf: 8476500 bytes, checksum: 0e323d417d0ad90bf62e4ef266aa2f7d (MD5) / Made available in DSpace on 2018-11-12T13:35:25Z (GMT). No. of bitstreams: 1 Melissa Grahl Figueredo_.pdf: 8476500 bytes, checksum: 0e323d417d0ad90bf62e4ef266aa2f7d (MD5) Previous issue date: 2018-07-03 / Programa de Bolsas de Estudo Talentos Tecnosinos / Este trabalho apresenta o estudo para a caracterização do escoamento bifásico ar-água em um minicanal de 2,6 mm de diâmetro interno para os regimes de escoamento pistonado e bolha isolada, por meio do emprego de quatro pares de sensores ópticos: dois deles formados por emissores IR e fotodiodos como receptores, diferenciados pelo comprimento de onda dos emissores e pela área ativa dos receptores, o outro é o sensor de tubo para líquidos, formado por um emissor IR e um fototransistor como receptor e, por fim, o sensor composto por um LED branco como emissor e um LDR como receptor. Os testes foram realizados para vazões de água de 50, 75 e 100 ml/min e volumes de ar de 0,1, 0,06 e 0,02 ml, captando-se simultaneamente os sinais dos sensores e suas respectivas imagens com uma câmera de alta velocidade. Os resultados obtidos foram a fração de vazio, as curvas de calibração para os sensores, a comparação das respostas dos quatro sensores empregados e a medida da velocidade das bolhas. A calibração dos sensores é dada por meio de uma curva que relaciona a fração de vazio e a tensão do sensor. Relacionando-se as imagens e os sinais dos sensores, foi possível identificar os padrões de escoamento pistonado para os volumes de ar de 0,10 e 0,06 ml e o de bolhas isoladas para o volume de 0,02 ml. A partir da análise dos sinais dos sensores observou-se que para volume de ar de 0,02 ml o sensor de tubo para líquidos, diferentemente dos outros sensores, identificou o padrão levemente alongado da bolha. A verificação dos sinais dos sensores foi feita por meio das áreas medidas pelos sensores e aquelas obtidas através das imagens, ficando com EMR entre -9,36% e 4,49%, sendo os piores resultados os encontrados para o volume de ar de 0,02 ml. O resultado para os valores normalizados das áreas medidas pelos sensores mostrou que o sensor LDR possui resposta mais lenta durante as mudanças entre as fases líquida e gasosa, visto que a área medida por ele ficou maior em relação aos outros sensores. Nota-se ainda que o sensor de tubo para líquido foi aquele que obteve os menores tempos durante as mudanças de fases. As curvas de calibração obtidas foram melhor aproximadas por exponenciais de segunda ordem com R2 entre 0,928 e 0,897 (LDR). Por fim, a aplicação dos sensores em pares possibilitou a medida da velocidade das bolhas para as diferentes vazões de água, resultando em EMR entre -4,92 e 2,17%. / This paper presents an air-water two-phase flow characterization in a small diameter tube of 2.6 mm internal diameter for plug and isolated flow, using four pairs of optical sensors: two of them consist in IR emitter whit different wavelength and photodiodes receivers, whit distinct reception active areas, the other one is a tube liquid sensor based in an IR emitter and a phototransistor receiver and the last one is a LDR as receiver and a white LED emitter. The tests were performed for 50, 75 and 100 ml/min for water flow and 0.1, 0.06 and 0.02 ml for air volume, capturing sensors signals and its respective images in a high-speed camera. The results obtained where void fraction, sensors calibrations curves, a comparison between the four sensors response and bubbly velocity. The sensor calibration process relates void fraction and sensor signal. Images and sensor signal showed that is possible to recognize plug pattern for air volumes of 0.10 and 0.06 ml and isolated bubbly pattern for 0.02 ml. Signal sensors comparison allowed to identify that the tube liquid sensor is better in recognizing a bubbly-plug pattern. The sensors verification has compared the areas measured by the sensor and the image, the EMR results are between -9.6% and 4.49%. The worst results are for air volume of 0.02 ml. LDR sensor response is slower than the others sensors are during phase changing, since the area measured by it has been higher than the other sensors. On the other hand, tube liquid sensor showed to be faster sensor in phase changing. Despite sensors differences, four calibrations curves were obtaining and defined by second order exponentials whit R2 between 0.928 and 0.897 (LDR). Finally, the sensors in pairs allowed to measure mean velocity of bubbles for different water flow, resulting in EMR between -4.92 and 2.17%.

Enormous quantities of heat are available in air, soil, water, exhaust air from buildings, and in waste water of any kind. However these heat sources are use-less for heating purposes since their temperatures are lower than the tempera-ture required for heating. Heat pumps can be used to extract heat from these sources with a small expenditure of additional energy and up-grade and deliver the energy as useful heat for room heating. The heat pump cycle employs the well-known vapour compression cycle. The amount of heat delivered by a heat pump is equal to the amount of energy extracted from the heat source plus the heat equivalent to the compression work of the heat pump. Heat pumps, of course, are being generally accepted as outstanding energy saving units due their coefficient of performance (COP). Heat pumps for house heating have been used extensively in many countries and are especially common in Sweden. The annual growth rate of heat pump usage in Sweden is the same as in rest of Europe. According to the Swedish heat pump association, between 1986 to August 2003, the number of installed heat pump units in Sweden was 332,309. The demand for heat pumps started to increase from the year 1995 and in the year 2002, approximately 40,000 heat pump units were installed. Among the many types available, single-family heat pumps providing heating capacity of about 5 kW are widely popular. The main drawbacks of heat pumps are the complexity of the systems, high cost, need of technical knowledge, safety hazards and environmental effects of certain refrigerants, etc. An efficient heat pump with small refrigerant charge would have less of some of these drawbacks and could be a competitive alterna-tive to other heating processes. In this study, methods of refrigerant charge minimisation without reducing the performance of a small capacity (5 kW) heat pump have been investigated. Work has been focused on finding refrigerant charge distribution in different components of the heat pump, on finding out the solubility of refrigerant (pro-pane) with different compressor lubrications oils, on testing different types of compact heat exchangers, on constructing new minichannel heat exchangers and on finding correlations for calculating the heat transfer of minichannel heat exchangers. The results included in this thesis have been presented in four con-ference papers and five journal papers of which two were published and three were submitted for publication. / QC 20100707

This thesis investigates the refrigerant (R-134a) flow in three minichannels having different geometries experimentally. During the last 40 years heat transfer in small scales has been a very attractive research area. Improvements in heat transfer in the refrigeration applications by means of usage of micro/minichannels provide significant developments in this area. Also it is known that experimental studies are very important to constitute a database which is beneficial for new developments and research. During the two-phase flow experiments conducted in the minichannels, low mass flow rates and constant wall temperature approach, which are the conditions in the evaporators of the refrigerator applications were applied because one of the purposes of this study is to determine the most ideal minichannel among the tested minichannels for usage in the evaporator section of the refrigerators. Two-phase flow experiments were made with refrigerant R134a in the three minichannels having hydraulic diameters of 1.69, 3.85 and 1.69 mm respectively. As distinct from the others, the third minichannel has a rough inner surface. Comparison of the experimental results of the three minichannels was made in terms of forced convection heat transfer coefficients and pressure drop at constant quality and mass flux values. As a result of the experiments, the most ideal minichannel among the tested minichannels was determined for the evaporator applications in the refrigerators.

A study of heat transfer and pressure drop in zero ozone-depletion-potential (ODP) ‎refrigerant blends in small diameter tubes was conducted. The azeotropic refrigerant ‎blend R410A (equal parts of R32 and R125 by mass) has zero ODP and has properties ‎similar to R22, and is therefore of interest for vapor compression cycles in high-‎temperature-lift space-conditioning and water heating applications. Smaller tubes lead to ‎higher heat transfer coefficients and are better suited for high operating pressures.‎ Heat transfer coefficients and pressure drops for R410A were determined experimentally ‎during condensation across the entire vapor-liquid dome at 0.8, 0.9xPcritical and gas ‎cooling at 1.0, 1.1, 1.2xPcritical in three different round tubes (D = 3.05, 1.52, 0.76 mm) ‎over a mass flux range of 200 < G < 800 kg/m2-s. A thermal amplification technique was ‎used to accurately determine the heat duty for condensation in small quality increments ‎or supercritical cooling across small temperature changes while ensuring low ‎uncertainties in the refrigerant heat transfer coefficients. ‎ The data from this study were used in conjunction with data obtained under similar ‎operating conditions for refrigerants R404A and R410A in tubes of diameter 6.22 and ‎‎9.40 mm to develop models to predict heat transfer and pressure drop in tubes with ‎diameters ranging from 0.76 to 9.40 mm during condensation. Similarly, in the ‎supercritical states, heat transfer and pressure drop models were developed to account for ‎the sharp variations in the thermophysical properties near the critical point.‎ The physical understanding and models resulting from this investigation provide the ‎information necessary for designing and optimizing new components that utilize R410A ‎for air-conditioning and heat pumping applications.‎

Les travaux de cette thèse portent sur le refroidissement descomposants électroniques de puissance par métal liquide. Les efforts se sontconcentrés plus particulièrement autour de deux fonctions : la pompeélectromagnétique servant à mettre le fluide en mouvement et le refroidisseur àminicanaux situé sous la source de dissipation.Le mémoire de thèse se structure en quatre chapitres équivalents. Dans lepremier, l’apport des métaux liquides pour le refroidissement des composantsactifs de puissance est démontré. Dans un deuxième temps, l’étude théorique etexpérimentale d’une pompe électromagnétique à conduction est effectuée. Lesystème de refroidissement est plus particulièrement abordé dans le troisièmechapitre. Enfin, des réflexions sur la mise en oeuvre des refroidisseurs à métauxliquides en électronique de puissance sont discutées dans la dernière partie.Grâce à elles, nous voyons que le champ d’application de ces travaux favorisel'émergence de solutions innovantes pour la gestion thermique des composantsélectronique de puissance. / The work presented in this Phd manuscript deals with cooling powerelectronics devices using an electrical conductive fluid. Two important functionshave been considered: the study and the realization of the electromagnetic pumpwhich circulated the fluid in the cooling loop. The second function was study andrealization of the cooler which evacuated the heat from the electronics device.This document has four chapters: introduction to power electronics coolingsystem with liquid metal, electromagnetic pump study, cooler study, and at lastreflections on realizing liquid metal cooler for power electronics devices. Theresults of this work concern a wide range of applications, especially towards newthermal management solutions of power electronics devices.