A presente dissertação trata de um estudo teórico-experimental sobre escoamento monofásico e bifásico no interior de tubos com fitas retorcidas. Esta técnica tem sido utilizada há várias décadas para a intensificação de troca de calor para escoamento monofásico e evaporação convectiva no interior de tubos. No entanto, com sua utilização, o aumento do coeficiente de troca de calor é acompanhado pelo incremento da perda de pressão. Portanto a compreensão dos fenômenos relacionados aos incrementos da perda de pressão e troca de calor são fundamentais para engenheiros projetistas. Neste estudo, inicialmente, é apresentada uma extensa revisão bibliográfica sobre padrões de escoamento, modelos para estimativa de fração de vazio e metodologias para previsão da perda de pressão em tubos com e sem fitas retorcidas para escoamentos bifásicos e monofásicos. Foram realizados experimentos em bancada experimental utilizando seção de testes com diâmetro interno de 15,9 mm e dois metros de comprimento com fitas apresentando razões de retorcimento de 3, 4, 9, 14 e , a última correspondente ao tubo sem inserto. Os experimentos foram executados para o refrigerante R134a, velocidades mássicas entre 75 e 250 kg/m²s, e títulos de vapor entre 5 e 95% no caso de escoamentos bifásicos. Resultados foram levantados para velocidades mássicas entre 100 e 450 kg/m²s para escoamentos monofásicos. Durante os ensaios foram levantados resultados de perda de pressão e identificados subjetivamente os respectivos padrões de escoamento. Como resultado final deste estudo é proposta uma correlação para previsão da perda de pressão durante o escoamento no interior de tubos com fitas retorcidas. / A theoretical and experimental study on single and two-phase flow inside tubes with twisted tapes inserts is presented. Twisted tape inserts have been used for decades as a technique of heat transfer enhancement. However, their heat transfer enhancement is accompanied by an increase of the pressure drop. Therefore, understanding the pressure drop and heat transfer mechanisms is fundamental for heat exchanger designers in order to optimize these devices when using twisted-tape inserts. Based on these aspects the present study concerns single and two-phase flows inside tubes with twisted tapes. Experiments were performed in an experimental apparatus for a 15.9 mm ID and twisted tape with twist ratios of 3, 4, 9, 14 and , the last one corresponding to a plain tube without tape. For two-phase flow, experiments were conducted for R134a, mass velocities from 75 to 250 kg/m²s and vapor qualities from 5 to 95%. In case of single-phase flow, experiments were performed for mass velocities from 100 to 450 kg/m²s. The experimental campaign was focused on pressure drop and flow pattern subjective identification. Additionally an extensive literature review on flow patterns, superficial void fraction, and methodologies for prediction of the pressure drop in plain tubes with and without twisted-tape inserts during single and two-phase flow is presented. As final result, a correlation for prediction of pressure drop during flow inside tubes with twisted tape inserts is proposed.

Apresenta um estudo teórico-experimental da ebulição convectiva do fluido refrigerante R-134a no interior de tubos lisos. Os parâmetros físicos disponíveis para medida foram: pressão, temperatura, vazão de refrigerante e potência de aquecimento, os quais, juntamente com o registro fotográfico, foram utilizados para caracterizar os padrões de escoamento e as transições, investigando-se os efeitos do diâmetro do tubo, da velocidade mássica e do fluxo de calor sobre a perda de pressão e a transferência de calor. Os principais padrões de escoamento visualizados foram: o intermitente, o anular e o estratificado, nos quais constatou-se que, as transições são governadas, principalmente, pelos efeitos da velocidade mássica e do diâmetro do tubo. Dentre estes padrões de escoamento, o anular e o estratificado foram modelados analiticamente. O modelo para o escoamento anular foi utilizado na obtenção de correlações para o fator de atrito interfacial e para espessura do filme de líquido. O modelo para o escoamento estratificado foi dividido em duas partes, uma destinada a obter a configuração da interface, a qual se mostrou côncava e a outra destinada à determinação dos fatores de atrito líquido-parede e interfacial os quais foram correlacionados / The research reports a theoretical and experimental study of convective boiling of refrigerant R-134a in smooth tubes. Tests have been carried out to measure the following physical parameters at the test section: mass flow rate, pressure and pressure drop, refrigerant and surface temperatures and the electrical power. In addition to these parameters, a photographic study has been carried out from pictures taken at the test section exit in order to determine the flow regimes that intervene under the imposed operating conditions. Effects over the pressure drop and heat transfer of the mass flow rate, heat flux, quality, and tube diameter have been investigated. Three flow regimes have been found: the intermitent, the stratified and the annular. Flow regime transitions are apparently governed by the mass velocity and tube diameter. The annular and the stratified flow regimes have been semi-empirically modeled using a mechanistic approach. The annular flow model has been applied to develop correlations for two important physical parameters: the interfacial friction factor and the film thickness. Through the stratified model, the shape of the interface has been evaluated along with correlations for the liquid to wall and interface friction factors

A presente dissertação trata de um estudo teórico-experimental sobre escoamento monofásico e bifásico em um dissipador de calor baseado em microcanais. Este tipo de dissipador de calor tem sido usado para a intensificação da troca de calor em sistemas compactos e de alto desempenho. A intensificação da troca de calor promovida pelo escoamento em microcanais é acompanhada de um incremento na perda de pressão, portanto o estudo destes dois parâmetros é essencial para o entendimento dos fenômenos relacionados e fundamental para o desenvolvimento de ferramentas de projeto para dissipadores de calor baseados em microcanais. Inicialmente, um levantamento bibliográfico extenso sobre a ebulição convectiva em microcanais de reduzido diâmetro foi realizado. Este estudo da literatura trata de critérios de transição entre micro- e macro-escala, padrões de escoamento, métodos de previsão do coeficiente de transferência de calor e perda de pressão. Atenção específica foi dada a estudos de dissipadores de calor baseados em microcanais. Com base nesta análise da literatura, uma bancada experimental foi confeccionada para que dados experimentais de transferência de calor e perda de pressão pudessem ser levantados a partir de um dissipador de calor de microcanais. O dissipador de calor fabricado para este estudo é constituído de 50 microcanais retangulares dispostos paralelamente com 15 mm de comprimento, 100 µm de largura, 500 µm de profundidade e espaçados entre si de 200 µm. Experimentos foram executados para o R134a, velocidades mássicas de 400 a 1500 kg/m²s, título de vapor máximo de 0,35 e fluxos de calor de até 310 kW/m². Como conclusão deste trabalho observa-se perda de pressão elevada em relação aos valores fornecidos pelos métodos de previsão da literatura e um coeficiente de transferência de calor próximo ao estimado pelo modelo de três zonas proposto por Thome et al. (2004). / This study presents a theoretical and experimental investigation on single and two-phase flows in a microchannel based heat sink. Multi-microchannel heat sinks are able of dissipating extremely high heat fluxes under confined conditions. Such characteristics have attracted the attention of academia and industry and actually several studies are being carried out in order to evaluate and optimize such devices. Initially, an extensive investigation of the literature concerning convective boiling in micro-scale channels was performed. This literature review covers transitional criteria between micro- and macro-scale flow boiling, two phase flow patterns, heat transfer coefficient and pressure drop during convective boiling. Special attention was given to studies concerning microchannels based heat sinks. Based on this investigation, an experimental facility was built for performing heat transfer and pressure drop measurements during single-phase flow and flow boiling in microchannel based heat sinks. For this study, a microchannel based heat sink was also manufactured. The heat sink contains 50 rectangular parallel microchannels, 15 mm long, 100 µm wide by 500 µm deep and separated by 200 µm walls. Experiments were performed for R134a, mass velocity of 400-1500 kg/m²s, maximum vapor quality of 0,35 and heat fluxes up to 310 kW/m². The database obtained in the present study was compared against pressure drop and heat transfer coefficient prediction methods from the literature. It was found that no one method is accurate in predicting heat sink pressure drop while heat transfer coefficient results were accurately predicted by the 3-zone model proposed by Thome et al. (2004).

A presente dissertação de mestrado envolve o estudo da perda da pressão e dos padrões para escoamentos água/ar em um canal retangular segundo inclinações a partir do plano horizontal de -90° a 90°. Também foi avaliado o efeito nestes parâmetros de rotacionar a seção de testes em torno de seu eixo axial segundo ângulos de 45° e 60°, determinados a partir da condição de suas faces superior e inferior posicionadas horizontalmente. O texto se inicia com a apresentação de um amplo estudo da literatura sobre métodos de previsão de padrões de escoamento e perda de pressão durante escoamentos bifásicos no interior de dutos. Em seguida é descrito o aparato experimental projetado e construído para este estudo. Tal descrição inclui o detalhamento da seção de testes, que consiste em um canal retangular de seção transversal com dimensões de 6,0 x 6,5 mm². Resultados foram levantados para vazões mássicas entre 90 e 760 kg/m²s, correspondendo a velocidades superficiais entre 0,03 a 19,42 m/s e 0,1 a 0,76 m/s, para, respectivamente as fases gás e líquido. Mapas de escoamento foram desenvolvidos com base em imagens capturadas por câmera de alta velocidade, e também com base na técnica de agrupamento de dados k-means. Os escoamentos foram classificados segundo os padrões bolhas, intermitente e anular. As características hidrodinâmicas e as transições entre estes padrões foram significativamente alteradas pela inclinação do canal. Constatou-se também significativa influência da rotação em torno do eixo do canal, favorecendo efeitos de estratificação no escoamento. As transições obtidas experimentalmente foram comparadas com os métodos de previsão disponíveis na literatura. Os métodos de Taitel e Dukler (1976), para escoamento horizontal, e Taitel et al. (1980) para escoamento vertical ascendente, apresentaram as melhores previsões dos dados experimentais. Com o objetivo de estimar a parcela gravitacional da perda de pressão, levantou-se resultados para a fração de vazio superficial avaliada com base na velocidade média de bolhas alongadas e no tratamento de imagens no caso do escoamento em bolhas. Resultados de perda de pressão por atrito foram comparados com vinte métodos de previsão da literatura. As correlações de Mishima e Hibiki (1996) e Zhang et al. (2010) proporcionaram as melhores previsões para escoamento horizontal. Para escoamentos inclinados, comparou-se 25 métodos da literatura para a determinação da perda de pressão por atrito com os resultados de perda de pressão experimentais, com a perda de pressão gravitacional estimada considerando 20 métodos para a fração de vazio superficial. Desta análise constatou-se que a combinação dos métodos de Mishima e Hibiki (1996) e Zhang et al. (2010) para a previsão da parcela de perda de pressão devido ao atrito e o método de Spedding e Chen (1984) para previsão da fração de vazio superficial, utilizado para a determinação da parcela gravitacional, proporcionam previsões satisfatórias dos dados experimentais. / The present dissertation concerns a study on pressure drop and flow patterns for air/water flows inside a rectangular channel positioned according to inclination angles, relative to the horizontal plane, from -90° to 90°. The effects on flow patterns and pressure drop of rotating the test section relative to its longitudinal axis according to angles of 45° and 60° are also evaluated. Initially, a broad review of the literature concerning experimental studies and predictive methods for flow pattern and pressure drop inside channels is presented. Then, the experimental facility developed for this study is described. In this description, the test section, consisting of a rectangular channel with cross-sectional area of 6.0 x 6.5 mm² is detailed. Experimental data were obtained for mass velocities from 90 to 760 kg/m²s, corresponding to gas and liquid superficial velocities from 0.03 to 19.4 m/s and from 0.1 to 0.76 m/s, respectively. Flow patterns maps were developed based on the following approaches: analyses of two-phase flow images from a high speed video camera; and using the k-means clustering algorithm based on pressure drop and optical signals. The bubbly, intermittent and annular flow patterns were characterized. From the analyses of the data, it was found that the flow pattern transitions are significantly affected by the flow inclination and channel rotation. Two-phase flow stratification effects are enhanced by rotating the channels. Among the flow pattern predictive method evaluated in the present study, Taitel and Dukler (1976), for horizontal channels, and Taitel et al. (1980), for upward flow, provided the best predictions of the data obtained in the present study. In order of estimating the gravitational parcel of the pressure drop, superficial void fraction results were obtained based on the mean velocity of elongated bubbles, for intermittent flow, and on the image processing of bubbles, for bubbly flow. Experimental results for frictional pressure drop were compared against 20 predictive methods available in the literature. The methods of Mishima e Hibiki (1996) and Zhang et al. (2010) performed the best for horizontal flows. The frictional pressure drop predictions were also evaluated for inclined flows by comparing the measured total pressure drop against the corresponding calculated values based on the combination of 25 frictional pressure drop predictive methods and the gravitational parcel of pressure drop estimated according to 20 predictive methods for superficial void fraction. From this analysis, it was found that the combination of Mishima e Hibiki (1996) and Zhang et al. (2010) methods for frictional pressure drop with the method of Spedding e Chen (1984), for void fraction, used to determine the gravitational parcel, provide satisfactory predictions of the experimental data.

Submitted by Maicon Juliano Schmidt (maicons) on 2015-03-30T19:21:01Z No. of bitstreams: 1 Jéferson Diehl de Oliveira.pdf: 1682128 bytes, checksum: 754d96de0ab9ab919137513dac6fe705 (MD5) / Made available in DSpace on 2015-03-30T19:21:01Z (GMT). No. of bitstreams: 1 Jéferson Diehl de Oliveira.pdf: 1682128 bytes, checksum: 754d96de0ab9ab919137513dac6fe705 (MD5) Previous issue date: 2013-01-31 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho apresenta um estudo experimental da transferência de calor e queda de pressão na ebulição convectiva do hidrocarboneto isobutano (R-600a) em um mini-tubo horizontal e circular de 2,6 mm de diâmetro interno. Também é analisada a influência do diâmetro sobre a queda de pressão em escoamento adiabático. No caso dos experimentos envolvendo a ebulição convectiva, os testes foram realizados com uma temperatura de saturação de 22 ºC, com uma pressão média de saturação de 323,5 kPa, com velocidade mássica variando entre 188 e 370 kg/(m²s) e fluxo de calor constante na seção de teste entre 28 e 134 kW/m². Os resultados obtidos experimentalmente foram comparados com resultados de correlações para a queda de pressão por atrito e o coeficiente de transferência de calor propostas por diferentes autores para ebulição em canais de pequeno diâmetro. Também foram observados os padrões de escoamento ao longo dos testes com o auxílio de uma câmera com alta velocidade de captura e relacionados com o fluxo de calor, título de vapor e o coeficiente de transferência de calor. / This work presents an experimental study of heat transfer and pressure drop in flow boiling of hydrocarbon isobutane (R-600a) in a horizontal and circular mini-tube with 2.6 mm of inner diameter. It is also analyzed the influence of diameter on the pressure drop in adiabatic flow. In the case of experiments involving flow boiling, the tests were performed at a saturation temperature of 22°C with a mean saturation pressure of 323.5 kPa, with mass velocity ranging between 188 and 370 kg/(m²s) and constant heat flux in the test section between 28 and 134 kW/m². The results obtained experimentally were compared with results of correlations for pressure drop due to friction and heat transfer coefficients proposed by different authors for boiling in channels of small diameter. It was also observed flow patterns along the tests with the aid of a camera with high speed capture and related with heat flux, vapor quality and heat transfer coefficient.

Submitted by Nara Lays Domingues Viana Oliveira (naradv) on 2015-06-23T18:20:38Z No. of bitstreams: 1 DANIELDEITOS.pdf: 2301580 bytes, checksum: 0e88198dccb50c04a43cba524f61d255 (MD5) / Made available in DSpace on 2015-06-23T18:20:38Z (GMT). No. of bitstreams: 1 DANIELDEITOS.pdf: 2301580 bytes, checksum: 0e88198dccb50c04a43cba524f61d255 (MD5) Previous issue date: 2012-06 / Banco Santander / Banespa / Este trabalho apresenta uma análise do desempenho hidráulico do lado do escoamento do ar de uma nova geometria de tubos para evaporadores de túneis de congelamento. Estes evaporadores são trocadores de calor de tubo-aletados e fluxo cruzado. A nova geometria dos tubos consiste em um perfil tipo gota, que é testada e comparada com tubos circulares, já estudados e com resultados disponíveis na literatura. O objetivo do trabalho é determinar experimentalmente valores para perda de carga para as duas geometrias, comparar os resultados e apontar a geometria que apresenta as melhores características de perda de carga. É apresentada uma ampla revisão feita na literatura disponível sobre estudos realizados com relação ao efeito de diversos parâmetros no desempenho destes trocadores de calor, entre eles a geometria dos tubos, e os resultados indicam que tubos elípticos e achatados apresentam melhor desempenho térmico e menor perda de carga quando comparados com os tradicionais tubos circulares. O procedimento de cálculo para perda de carga para trocadores de calor tubo-aletados, também é apresentado. Para os evaporadores testados, foram mantidos constante o espaçamento entre tubos e o espaçamento entre aletas, variando apenas a geometria do tubo. As aletas usadas foram do tipo plana. Os testes foram realizados em túnel de vento construído em conformidade com a norma ANSI/ASHRAE 51-1999. Um ventilador gera um fluxo de ar que passa pela serpentina aletada e uma placa de bocais. Para diferentes velocidades de ar são obtidas a perda de carga no trocador de calor, as temperaturas médias de entrada e saída para o ar, a velocidade e a vazão de ar. Os resultados da perda de carga em função da velocidade do ar mostram que os tubos tipo gota tem desempenho hidráulico superior aos tubos circulares. Foi obtida uma redução média da perda de carga de 26% e do fator de atrito de 16%, comparando o tubo tipo gota com o tubo circular, para os ensaios realizados com regimes de escoamento que vão do laminar ao de transição. Uma análise da potência de ventilação mostrou novamente uma vantagem dos tubos tipo gota e na análise de uma situação real para um modelo de túnel de congelamento, os tubos tipo gota apresentaram uma redução da potência de ventilação total de 12,87%, quando comparado com os tubos circulares. Os modelos são ferramentas importantes para o projeto de trocadores de calor, assim foram analisadas diferentes correlações para predição do coeficiente de atrito do escoamento do ar através das aletas e comparadas com os dados experimentais. Verificou-se que a correlação de Gray e Webb (1986) foi a que melhor se ajustou, com um erro médio de 15% e a dispersão entre os valores medidos e calculados de 31% para tubos circulares e de 50% para o tipo gota. / This study presents an analysis of hydraulic performance of the air side for a new tube geometry of evaporators used in freezing tunnel. These evaporators are heat exchangers with finned tubes and air in cross flow. The new geometry consists in a wing profile tube that is tested and compared to circular tubes, which are already studied and with available results in literature. The objective of this work is determining experimentally the pressure drop for both geometries, comparing the results and pointing out the geometry that shows the best features for pressure drop. A comprehensive literature review is also presented and it focuses on studies concerning the effects of several parameters in these heat exchangers performance, including tube geometry. Several results have shown that elliptical tubes have better thermal performance and lower pressure drop when compared to the conventional circular tube. A review of the procedures calculation for fin-tube heat exchangers pressure drop is shown. For the evaporators tested, the spacing between tubes and fins, was kept constant by varying only the tube geometry and the fins were flat. The tests were carried out in a wind tunnel, built in accordance with ANSI/ASHRAE 51-1999. A fan generates an air flow that passes through the finned coil and a nozzle plate. For different air velocities and the pressure drop in the heat exchanger, input and output average air temperature, the velocity and flow of the air were registered. The results showed the dependence on the air velocity and that wing profile tubes have better hydraulic performance than circular tubes. An average reduction of 26% for pressure drop and 16% for friction factor for wing tubes were obtained in comparison to circular tubes, in tests in the regimes from laminar to transition flow. An analysis of ventilation power showed the advantage of wing tubes, and the evaluation of a real situation, on a freezing tunnel, the wing tubes show an average reduction of 12,87 % for total ventilation power when compared with circular tubes. The theoretical models are important tools for designing the evaporator, and in this study some correlations were evaluated to prediction of the air flow friction factor and were compared to experimental results. The Gray and Webb correlation (1986) presented the best fit, with 15% of average error and the dispersion between the measured and calculate values was 31% for circular tubes and 50% for wing tubes.

Afin de permettre la mise en place d’une filière de valorisation du CO2 issu de l’industrie sidérurgique, cette étude a pour but de proposer un nouveau procédé de dépoussiérage des fumées de hauts fourneaux qui serait placé à la suite des filières actuelles de traitement pour améliorer leur épuration. L’originalité de la technologie retenue, un lit granulaire arrosé, permet de maintenir la perte de charge constante en facilitant le réentraînement en continu des particules collectées à l’aide d’un écoulement de liquide (formant un film à la surface des collecteurs) pour ainsi pallier le problème de colmatage. Une étude hydrodynamique et des mesures d’efficacité vis-à-vis de particules de diamètre compris entre 0,5 et 10 µm ont montré que la perte de charge et l’efficacité initiale augmentaient avec la diminution du diamètre des collecteurs, ce qui est en accord avec les résultats de la littérature pour des lits granulaires secs. La présence d’un filet d’eau augmente l’efficacité initiale de collecte au détriment toutefois d’une perte de charge plus élevée. Ces observations sont liées à la diminution de la porosité du lit lorsque ce dernier est arrosé par un débit d’eau. Un bon compromis entre perte de charge et efficacité a été trouvé avec un lit composé de 50 cm de billes de 5 mm de diamètre et un débit d’eau de 12 L.min-1 pour un débit de gaz à traiter de 20 m3.h-1. Des essais de colmatage longue durée ont mis en évidence que l’efficacité du lit reste globalement constante au cours du temps, à une valeur proche de la valeur initiale. La perte de charge, bien que supérieure en début d’expérience à celle d’un lit granulaire sec, se stabilise rapidement à une valeur bien plus faible que celle mesurée pour un lit sec pour une même masse de particules collectées. Les expériences ayant été menées sur 30 h dans des conditions défavorables, nous pouvons nous attendre à ce que dans des conditions réelles de fonctionnement la perte de charge soit stable sur des temps beaucoup plus longs. Ainsi la présence de l’eau remplit bien le rôle prévu en limitant l’évolution du colmatage du lit granulaire par le réentrainement des particules collectées par le lit. Cette étude s’est achevée par le développement d’un modèle permettant de déterminer la perte de charge et l’efficacité initiale d’un lit granulaire arrosé, ce qui a abouti sur un exemple de pré-dimensionnement à échelle industrielle / In order to enable the establishment of a CO2 recovery chain sector from the steel industry, this study aims to propose a new filtration device for the dust removal of blast furnace fumes that would be placed after the current treatment chain to improve their purification. The originality of the chosen technology, a trickle bed, permits to maintain a constant pressure drop by re-entraining continuously the particles trapped on collectors thanks to a water film flow to overcome the clogging problem. A hydrodynamic study and efficiency measurements lead with particles (in a range of diameters between 0.5 and 10 μm) showed that the pressure drop and the initial efficiency increase with the decrease of the collector diameter, which is in agreement with the results of the literature for dry granular beds. The presence of water increases the initial collection efficiency at the expense of a higher pressure drop. These observations are related to the decrease of the bed porosity in presence of a water flow. A good compromise between pressure drop and efficiency was found for an air flow rate of 20 m3.h-1, a liquid flow rate of 12 L.min-1 and using a 50 cm bed composed of glass beads of 5 mm diameter. Long-term clogging tests showed that the efficiency of the bed remains globally constant over time at a value close to the initial one. The pressure drop, although higher at the beginning of the experiment than that of a dry granular bed, stabilizes rapidly at a much lower value than that measured for a dry granular bed for the same mass of particles collected. The experiments have been carried out over 30 h under disadvantageous conditions, and we can expect that under real operating conditions the pressure drop will be stable over much longer times. Thus, the presence of water plays its part by limiting the clogging evolution of the granular bed by the re-entrainment of particles collected by the bed. This study was completed by the development of a model to determine the pressure drop and initial efficiency of a trickle bed, which resulted in an example of pre-sizing of a process on an industrial scale

The growing demand for the development of efficient miniature cooling systems has led to stimulating numerous investigations on two-phase flow boiling in small to microscale tubes. Because of the variation in properties of synthetic cooling fluids, this causes an inaccuracy of existing flow boiling prediction models or correlations in the literature to interpolate or extrapolate the two-phase flow boiling heat transfer and pressure drop. The purpose of this investigation was to study experimentally the parametric aspects of flow boiling characteristics inside vertical stainless-steel tubes with four different internal diameters (1.1, 2.01, 2.88 and 4.26 mm). The R245fa (1,1,1,3,3-pentafluoropropane, HFC-245fa) was used as working fluid. The experiments were carried out under a system pressure range of 185 - 310 kPa (which correspond to a saturation temperature range of 31 - 46 °C), mass flux range of 200 - 500 kg/m²s, heat flux range of 3 - 188.5 kW/m², vapour quality up to the onset of dryout and 5 K inlet subcooling. Flow pattern visualisations, two-phase pressure drops and saturated flow boiling heat transfer coefficients were presented. The experimental data of R134a employed for comparison is acquired from the previous studies of Huo et al. (2007), Shiferaw et al. (2011) and Mahmoud et al. (2014a). These studies were carried out in the same experimental facility and under the similar operating conditions. The Two-phase flow regimes inside four tubes were visualised in a borosilicate glass tube located at the heated section outlet to capture the dominant flow patterns which assist to elucidate the heat transfer results. The flow boiling visualisation was recorded by a high-speed camera with experiments of increasing and decreasing heat flux. The four observed flow regimes are identified as bubbly flow, slug flow, churn flow and annular flow. In increasing heat flux experiments, the churn and annular flows were only the dominant patterns in all four tubes. The slug flow was often discerned at lower mass flux except for the tube of 1.1 mm where it was not observed at all. This is contrary to decreasing heat flux experiments where all flow patterns including the bubble flow were observed in all the tubes. This shows a strong impact of hysteresis, which is a result of nucleation sites remained active as the heat flux is reduced. The flow patterns and transition boundaries for R245fa are affected by mass flux, system pressure, and tube diameter. The vapour quality corresponding to flow pattern transition boundary tends to decrease with increasing mass flux and tends to increase with increasing system pressure and decreasing tube diameter. Except for the bubbly-slug boundary, its vapour quality decreases with decreasing tube diameter. The experimental flow pattern maps of R245fa were fairly predicted with the predictive models developed for mini- and micro-channels by Tibiriçá et al. (2017). The two-phase pressure drop of R245fa is affected by mass flux, heat flux, system pressure, tube diameter and surface topography. The two-phase pressure drop increases with increasing mass flux and heat flux (vapour quality) and decreases with increasing system pressure and tube diameter. The two-phase pressure drop of the coated tube is higher than that of the uncoated tube. This is attributed to the coated tube having a higher surface roughness compared to the uncoated tube. The comparison between R245fa and R134a shows that the measured two-phase pressure drop of R245fa is dramatically higher than that of R134a. This arises from the difference in physical properties of the two fluids. The experimental data of two-phase pressure drop for 4.26 mm tube were reasonably predicted by Müller-Steinhagen and Heck (1986) correlation. Further, the experimental data of 2.88 mm and 2.01 mm tubes were well predicted by Chisholm (1973a), and Kim and Mudawar (2013), respectively. The experimental data of 1.1 mm tube were not well predicted by any of the selected predictive methods. The local heat transfer coefficient of all tubes increases with increasing heat flux for low and intermediate vapour qualities. After this vapour qualities, the heat flux effect diminishes. Then, the local heat transfer coefficient increased slightly with vapour quality, especially for higher heat flux near the outlet of the tube. However, the dryout inception in the 1.1 mm tube occurs after the intermediate vapour quality value and expands along the high vapour quality region. The behaviour of the local heat transfer coefficients of 1.1 and 2.88 mm tube is slightly dependent on the mass flux and vapour quality. Contrarily, there is insignificant effect of mass flux along 2.01 and 4.26 mm tube. This gives an indication of the contribution of nucleate boiling in the heat transfer process at lower and medium heat fluxes and nucleate boiling plus convective evaporation at higher heat fluxes near the tube outlet. Further, the local heat transfer coefficient increases as the system pressure increases. The tube diameter has a strong influence on the enhancement of local heat transfer coefficient. The enhancement in average heat transfer coefficient approaches 83% when the tube diameter is reduced from 4.26 to 1.1 mm. The trend of the local heat transfer coefficient of R134a was almost similar to that of R245fa with the exception of local dryout. The average heat transfer coefficient of R134a is about 106-151% larger than that of R245fa for the operational range studied. The dominant heat transfer mechanism is also represented by nucleate boiling for both fluids, particularly for 4.26 mm tube tested in this study. Also, the average heat transfer coefficient was enhanced by 33% when the inner tube surface coated with a copper coating. Finally, the correlation of Fang et al. (2017) predicted all experimental data for the four tubes with fair and similar accuracy.

The environmental concerns in recent times have grown especially after signing Montreal protocol. In the last ten years, researchers have focussed mainly on understanding the boiling and condensation phenomena of HFC refrigerants in minichannels. As global warming concerns are growing day by day, due to high global warming potential, HFCs are not the ultimate option. In the near future, HFCs will probably be replaced by environmentally friendly refrigerants. Therefore, to find the potential replacements of HFCs and also to get a deeper understanding of the boiling phenomena in minichannels, more and more fluids having low GWP (Global Warming Potential) and ODP (Ozone Depletion Potential) should be tested. Recent efforts to protect the environment have led to a growing interest for natural refrigerants. However in the literature, flow boiling data of natural refrigerants in minichannels are scarce. To meet the environmental concerns and to understand the behaviour of natural refrigerants in minichannels and the performance compared to HFCs, flow boiling experiments in single circular vertical minichannels of internal diameters of 1.70 and 1.224 mm were performed using ammonia and propane as working fluids. Flow boiling heat transfer results of ammonia and propane with 1.70 mm channel showed that the heat transfer coefficient was a function of heat flux and the effect of mass flux was insignificant. The heat transfer coefficient of ammonia in 1.224 mm was dependent on heat flux at low vapour qualities then a clear dependence of the heat transfer coefficient on the mass flux was observed at higher vapour qualities. The heat transfer results of ammonia and propane were compared with well known correlations and among them Cooper (1989) correlation in case of ammonia and Liu and Winterton (1991) and Cooper (1984) pool boiling correlations in case of propane best predicted the experimental heat transfer data. Results of the two phase pressure drop studies of ammonia and propane showed that the two phase pressure drop increased with the increase of mass flux, with the increase of heat flux and with the decrease of saturation temperature. The comparison of the two phase pressure drop experimental data with well known predicting models showed that none of the correlations predicted the ammonia data well and that Müller Steinhagen and Heck (1986) was well in agreement with the propane data. Dryout of propane in 1.70 mm and 1.224 mm internal diameter channels was also investigated. Dryout heat flux was observed to increase with the increase of mass flux, with the decrease of vapour quality and with the increase of internal diameter. The effect of saturation temperature on the dryout heat flux was insignificant. The experimental dryout data were compared with macro and micro scale correlations and among them Bowring (1972) and Callizo et al. (2010a) gave best predictions. The heat transfer and pressure drop results of ammonia and propane and dryout results of propane were compared with R134a data taken on the same test rig by Owhaib (2007) and Ali (2010). The comparison of heat transfer showed that the heat transfer coefficient was a function of heat flux and the effect of mass flux was insignificant in all tested conditions except ammonia in 1.224 mm tube where the heat transfer coefficient was dependent on heat flux at lower vapour qualities and a clear dependence of mass flux was observed at higher vapour qualities. The heat transfer data of ammonia, propane and R134a were compared with correlations and among them Cooper (1989) correlation gave best predictions. The comparison of pressure drop results showed that the two phase pressure drop of all fluids was increased with the increase of mass flux, with the increase of heat flux and with the decrease of saturation temperature. At equal heat flux and mass flux, the two phase pressure drop of ammonia was increased with the decrease of internal diameter but the diametric effects of R134a were unclear. Müller Steinhagen and Heck (1986) and Zhang and Webb (2001) best predicted the experimental data of two phase pressure drop of ammonia, propane and R134a among the correlations considered for comparison. The dryout data of propane were also compared with dryout data of R134a and it was observed that the dryout heat flux of propane and R134a increased with the increase of mass flux, with the decrease of vapour quality and with the increase of internal diameter. The effect of saturation temperature on the dryout heat flux of propane and R134a was insignificant. The correlation of Bowring (1972) for conventional channels and the microscale correlation of Callizo et al. (2010a) were among the correlations which gave best predictions of experimental data of dryout. / QC 20120210

This thesis investigates the performance of compact brazed plate heat exchangers (CBE) operating as evaporator in heat pump applications. The thesis, and the performances investigated, has been divided into three main sections; One zone evaporator performance; Two zone evaporator performance; and finally Local performance. The 'One zone evaporator performance' section considers the evaporator as one "black box". It was found that "approaching terminal temperatures" were obtained as low overall heat flux is employed. It was also found that the total area averaged film heat transfer coefficient was affected by changes of the brine mass flow rate. This indicates that the widespread Wilson plot method may not be used to determine flow boiling heat transfer coefficients. Further, it seems that co- and counter-current flow configuration performs equally well if the superheat is kept low. A numerical simulation of the above investigations indicates that a nucleate boiling model better predicts the performance compared to a convective evaporation model. Finally, the significant impact of the refrigerant inlet distributor design was illustrated using several CBEs with different inlet geometries but with identical heat transfer surfaces. The 'Two zone evaporator performance section' considers the evaporator as two "black boxes", i.e. the boiling and superheating boxes. Thermochromic liquid crystals (TLC) was used to determine the boiling heat transfer area. The resulting flow boiling heat transfer coefficient was found to correlate with heat flux. The superheated heat transfer area was then estimated using single phase correlations. It was observed that the TLC measurements and the predicted superheating area did not agree. Possible causes for this deviation were discussed. The most likely explanation found was the presence of mist flow at the higher vapor quality range in the boiling section of the evaporator. The 'Local Performance' section considers local pressure drop and flow boiling heat transfer. The Chisholm parameter was found not to be a constant and was found to correlate well with the kinetic energy per volume. The resulting predictions of the pressure drop were better than ± 10%. The resulting local flow boiling heat transfer coefficient, at different vapor quality, mass flux and heat flux, was compared to flow boiling correlations available in the literature. It was found that the saturated nucleate pool boiling correlation by Cooper (1984) and narrow channel flow boiling correlations (Tran 1999, Lazarek and Black 1982) predicted the experimental data better than several traditional flow boiling correlations, developed for larger tubes. / QC 20100524