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 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%.

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.

Heavy oil production has drawn more and more attention in petroleum industry. The amount of heavy oil in the world is twice more than the conventional oil (low viscosity), which has been consumed rapidly from the past. The understanding of flow patterns and pressure losses in multiphase flow with high viscosity oil are vital to assist the design of transportation pipeline. This thesis involves experimental investigation of two phase and three phase flows under high oil viscosity conditions (up to 17000cP) in horizontal pipelines. The multiphase (oil/water/solid/gas) facility was designed and constructed at Cranfield University and consists of 6m long horizontal pipeline of 0.026m diameter along with instrumentations. The principal objectives of the work were to study the effect of viscosity, water cut, temperature variance, and flow conditions on flow patterns and pressure drops for (oil/gas and oil/water) two phase flows; to compare the measured flow parameters and phase distribution with those predicted from models found in the literature for two phase flows; and to conduct an experimental study of gas injection effect on pressure gradient in (oil/water/gas) three phase flow. Due to the nature of heavy oil reservoirs, sand is associated with oil/water mixture when extracted; therefore sand concentration effect on pressure drop in (oil/water/sand) three phase flow is also examined. For oil-air flow, a smooth oil coating was observed in the film region of slug flow, while a ripple structure of oil coating film was found at higher superficial air velocity for slug flow regime and annular flow regime. The ripple structure was believed to increase the effective roughness of the pipe wall, which resulted in higher pressure gradients. The pressure drop correlations from Beggs and Brill (1973) and Dukler et al. (1964) were used to compare with experimental pressure gradients for oil/air flow. It was found that these correlations failed to predict the pressure gradients for heavy oil/air flows in this work. Several new heavy oil/water flow patterns were named and categorized based on observations. Though the heavy oil viscosity is an essential parameter for oil continuous phase flow on pressure drop, it had no significant effect beyond Water Assist Flow (WAF) condition, as a threshold was found for water cut with fixed superficial oil velocity. The transition criterion by McKibben et al. (2000b) for WAF was found to be able to predict this threshold reasonably well. Core Annular Flow (CAF) models were found to greatly under predict the pressure gradients mainly due to the coating (oil fouling) effect associated with this study. A new coating coefficient was introduced to models presented by Bannwart (2001) and Rodriguez et al (2009). The addition of solid in the mixed flow led to minor increase in the pressure gradient when the particles were moving with the flow. However, higher sand concentration in the system led to higher pressure gradient values. The addition of gaseous phase to the oil/water flow was more complex. The gaseous injection was beneficial toward reducing the pressure gradient when introduced in oil continuous phase only at very low water cuts.

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.

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.

Reservoir releases for generating power need to be reconciled with efforts to maintain healthy ecosystems in regulated rivers having irregular channel topography. Fluctuating, complex flow patterns near river obstructions such as boulders and large woody debris provide unique habitat for many aquatic organisms. Numerical modeling of the flow structures surrounding these obstructions is challenging, yet it represents an important tool for aquatic habitat assessment. Moreover, efforts for modeling the morphologically and biologically important transient flows, as well as quantifying their impacts on physical fish habitat during the unsteady-flow period remain rare. In this dissertation, the ability of two- (2-D) and three-dimensional (3-D) hydraulic models to reproduce the localized complex flow features at steady base and peak flows is examined first. The performance of the two hydraulic models is evaluated by comparing the numerical results with measurements of flow around a laboratory hemisphere and boulders located at a reach of the Smith River in Virginia. Close agreement between measured values and the velocity profiles predicted by the two models is obtained outside the wakes behind these obstructions. However, results suggest that in the vicinity of theses obstructions the 3-D model is better suited for reproducing the circulation flow behavior favored by many aquatic species over a broad range of flows. Further, time-dependent flow features affecting channel morphology and aquatic physical habitat are investigated using the numerical models for the same reach in the Smith River. Temporal variation measurements of water surface elevation and velocity profile obtained in the field during a reservoir release are in good agreement with the numerical results. A hypothetical "staggering" flow release scenario simulated by the 3-D model leads to reduced erosional area and longer refugia availability for juvenile brown trout during hydropeaking. Finally, an unsteadiness parameter β is proposed for determining whether an unsteady flow regime can be either modeled using a truly dynamic flow approach or a quasi-steady flow method. / Ph. D.

Yes / In this paper we consider four restricted cases of the generalised communicating P systems and study their computational power, by providing improved results, with respect to the number of compartments involved. We illustrate the expressive power of these devices by modelling several problems, such as producer/consumer, work ow patterns, broadcasting problem and comparative operations. We also present some relationships between generalised communicating P systems and P colonies, tissue-like P systems with very simple components. / MG and FI were supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0688, CSVE by grant No. 120558 of the National Research, Development, and Innovation Office, Hungary.

Microchannel heat exchangers present many advantages, such as reduced size, high thermal efficiency and low fluid inventory; and are increasingly being used for heat transfer in a wide variety of applications including heat pumps, automotive air conditioners and for cooling of electronics.However, the fundamentals of fluid flow and heat transfer in microscalegeometries are not yet fully understood. The aim of this thesis is to contribute to a better understanding of the underlying physical phenomena in single-phase and specially flow boiling heat transfer of refrigerants in small channels. For this purpose, well-characterized heat transfer experiments have been performed in uniformly heated, single, circular, vertical channels ranging from 0.64 to 1.70 mm in diameter and using R-134a, R-22 and R-245fa as working fluids. Furthermore, flow visualization tests have been carried out to clarify the relation between the two-phase flow behavior and the boiling heat transfer characteristics. Single-phase flow experiments with subcooled liquid refrigerant have confirmed that conventional macroscale theory on single-phase flow and heat transfer is valid for circular channels as small as 640μm in diameter. Through high-speed flow boiling visualization of R-134a under non adiabatic conditions seven flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Annular-type flow patterns are dominant for vapor qualities above 0.2. Onset of nucleate boiling and subcooled flow boiling heat transfer of R-134a has been investigated. The wall superheat needed to initiate boiling was found as large as 18 ºC. The experimental heat transfer coefficients have been compared to predictions from subcooled flow boiling correlationsav ailable in the literature showing poor agreement. Saturated flow boiling heat transfer experiments have been performed with the 640 μm diameter test section. The heat transfer coefficient has been found to increase with heat flux and system pressure and not to change with vapor quality or mass flux when the quality is less than ∼0.5. For vapor qualities above this value, the heat transfer coefficient decreases with vapor quality. This deterioration of the heat transfer coefficient is believed to be caused by the occurrence of intermittent dryout in this vapor quality range. The experimental database, consisting of 1027 data points, has been compared against predictions from correlations available in the literature. The best results are obtained with the correlations by Liu and Winterton (1991) and by Bertsch et al. (2009). However, better design tools to correctly predict the flow boiling heat transfer coefficient in small geometries need to be developed. Dryout incipience and critical heat flux (CHF) have been investigated in detail. CHF data is compared to existing macro and microscale correlations. The comparison shows best agreement with the classical Katto and Ohno (1984) correlation, developed for conventional large tubes. / QC 20101101

Les travaux présentés dans cette thèse concernent l'étude de l'évolution des régimes d'écoulements diphasiques lors de l'évaporation progressive dans un canal horizontal. Le but est de mieux comprendre l'écoulement à l'intérieur d'un tube récepteur d'une centrale solaire à concentration à génération directe de vapeur. Cette technologie, présentée comme une amélioration des systèmes actuels pouvant permettre une réduction des coûts, consiste en la production de vapeur directement sous l'effet du rayonnement solaire concentré. La prévision de l'écoulement liquide-vapeur alors généré dans le tube est encore de nos jours difficile, c'est pourquoi le recours à la simulation numérique est intéressant. Pour cela un modèle a été développé permettant la simulation de ces écoulements, depuis le début de la création de la vapeur jusqu'à l'existence de larges poches. Basé sur le modèle diphasique VOF du code Fluent, par l'ajout de fonctions personnalisées et d'une phase dispersée supplémentaire, il permet de modéliser différents phénomènes liés au processus d'évaporation : création en paroi, transport, recondensation et création de larges structures. Ce développement a été mis en oeuvre pour simuler des écoulements en évaporation, permettant de reproduire l'évolution des régimes d'écoulement. La validation est faite grâce à une étude expérimentale de la littérature, en comparant les régimes d'écoulements obtenus pour différents débits de liquide et sous l'effet de différents flux de chaleur. Enfin, le modèle a été appliqué à la simulation de la génération de vapeur dans le tube récepteur d'une centrale solaire, mettant en évidence l'apparition et l'évolution des différents régimes d'écoulement. Au vu du peu d'installations expérimentales trouvées dans la littérature sur le sujet, et afin de valider au mieux les fonctions développées, une installation expérimentale a été conçue et dimensionnée. / This PhD thesis is about the study of two-phase flow patterns evolution during progressive evaporation in horizontal tubes. The goal is to better understand the flow regimes inside a receiver tube of a concentrated solar power plant with direct steam generation. This technological evolution allows vapor production directly inside the solar field, which can lead to coast reductions. A two-phase liquid-vapor flow occurs inside the tube, which is currently still difficult to predict. Numerical simulation is an interesting way to investigate these complex phenomena. A model has been developed in order to simulate the flow patterns, from first vapor generation to large vapor slugs. It is based on Fluent software's two-phase VOF model, to which are added user-defined functions and a new dispersed phase. Different phenomena linked to the evaporating process are taken into account: vapor creation at the wall, its transport, recondensation and large structures creation. The model is used to simulate evaporating flows, and retrieves well two-phase flow patterns evolution. Validation is made using experimental data from the literature, by comparing flow regimes obtained for different flow rates and heat fluxes. Finally numerical simulation of direct steam generation inside a concentrated solar plant receiver is conducted, clearly showing apparition and evolution of two-phase flow patterns. Because few experimental data where found in the literature concerning evaporating two-phase flows visualization, a new experimental apparatus has been conceived and sized in order to better validate our numerical results.