Les filtres plissés à très haute efficacité (THE) jouent un rôle essentiel dans le maintien du confinement des installations industrielles à risques, les réacteurs nucléaires par exemple. Ces filtres permettent le piégeage des aérosols, qui sont la principale source de contamination externe. Dans ce cadre, l’IRSN (Institut de Radioprotection et de Sûreté Nucléaire) conduit des études pour caractériser les performances des filtres THE. L’objectif de la présente thèse a été de développer un modèle prédictif décrivant l’évolution de la résistance aéraulique du filtre au cours de son colmatage. Le colmatage d’un filtre plissé THE est piloté, d’une part par les conditions aérauliques imposées dans les réseaux de ventilation, d’autre part par la nature du média et les propriétés des particules collectées par le filtre. Ainsi, les travaux réalisés ont été répartis sur deux volets. La première partie a été dédiée à l’étude de l’aéraulique du filtre suivant une approche double expérimentale et numérique. Un banc expérimental permettant de mesurer les champs de vitesse dans un pli de filtre plissé THE a été développé. Les mesures réalisées ont servi à la validation des codes de calcul utilisés (ANSYS CFX® et GeoDict®) et au développement d’un modèle de perte de charge initiale des filtres plissés vierges. Une validation du modèle à l’échelle de filtres complets avec des géométries différentes a ensuite été effectuée. La deuxième partie a porté sur l’étude du colmatage des filtres plissés THE suivant une approche expérimentale et numérique. Un banc expérimental a été développé pour la détermination des paramètres entrant en jeu dans les modèles de colmatage (porosité, taille de particules …), puis des simulations numériques à l’aide du logiciel GeoDict ont été réalisées pour caractériser la croissance du dépôt sur la média. La combinaison des résultats expérimentaux et numériques a conduit au développement d’un modèle décrivant l’évolution de la résistance aéraulique du filtre THE au cours de son colmatage avec des particules microniques / High Efficiency Particulate Air (HEPA) pleated filters are used to ensure the containment of airborne contamination within nuclear facilities. These filters are often the last barrier before a potential release of radioactive substances into the environment. Predicting their performances (pressure drop and efficiency) is a main thrust of research for IRSN which aims at avoiding any unexpected failure of those devices on French nuclear facilities. The clogging phenomenon is mainly influenced, on one side, by the flow conditions imposed in the ventilation network and, on the other side, by the media characteristics and the particles properties which are collected by the filter. Consequently, the work carried out has been divided into two parts. In the first part, we focused on the study of the airflow behavior within a HEPA pleated filter with an experimental and numerical approach. An experimental bench has been developed to measure the airflow field at the entrance channel of a HEPA filter pleat. Then, the experimental measurements have been used to validate GeoDict® and ANSYS CFX® simulations. The combination of the experimental and the numerical results have been used to develop a model to predict the evolution of the initial HEPA filter pressure drop. The work performed in the second part deals with the clogging of the filter. An experimental bench, dedicated to the characterization of the parameters that intervene in the clogging models such as the porosity, the pressure drop and the particles size, has been developed. Then, numerical simulations conducted with GeoDict® have been realized to describe the cake formation on the surface of a HEPA pleated filter. Finally, we developed a model to predict the evolution of HEPA pressure drop during clogging with micronic particles

Submitted by Kilvya Braga (kilvyabraga@hotmail.com) on 2018-04-26T15:19:43Z No. of bitstreams: 1 BRUNO FERREIRA SILVA -DISSERTAÇÃO (PPGEM) 2016.pdf: 3315035 bytes, checksum: 915092a23718ba159ff32b574a8fb04e (MD5) / Made available in DSpace on 2018-04-26T15:19:43Z (GMT). No. of bitstreams: 1 BRUNO FERREIRA SILVA -DISSERTAÇÃO (PPGEM) 2016.pdf: 3315035 bytes, checksum: 915092a23718ba159ff32b574a8fb04e (MD5) Previous issue date: 2016-02-29 / Capes / Na indústria de petróleo, o transporte de óleos pesados em oleodutos requer altas potências de bombeamento devido a sua alta viscosidade, sendo este um dos inconvenientes na produção de óleos dessa natureza; surge assim a necessidade de desenvolvimento de novas tecnologias que otimize tais processos. Uma técnica economicamente viável e que pode ser usada, é o core anular flow, onde uma camada de água é responsável pela lubrificação do óleo que escoa no centro do duto. Desse modo, o presente trabalho apresenta um estudo energético e hidrodinâmico envolvendo o transporte de óleos pesados e água em dutos de conexões curvadas, através de simulação numérica via software ANSYS CFX® Release 15.0. Foi utilizado um modelo matemático que considera o modelo de mistura para tratar o escoamento bifásico águaóleo pesado, tridimensional, transiente e isotérmico, assumindo regime laminar para fase óleo e turbulento para a fase água adotando o modelo de turbulência k . Uma análise energética envolvendo as bombas também foi realizada. Resultados de campos de pressão, velocidade e fração volumétrica para as duas fases são apresentados e analisados. Verifica-se que o uso da técnica core-flow proporciona uma redução de 95,4% na queda de pressão por fricção comparado ao escoamento monofásico de óleo pesado, e que o desligamento e repartida das bombas de óleo e água afetam a queda de pressão por fricção. / In the oil industry, the transport of heavy oils in pipelines requires high power pumping due to its high viscosity, which is one of the drawbacks in oils production of this nature, thus emerge a need to develop new technologies that optimize such processes. An economically viable technique that can be used is the core annular flow, where a water layer is responsible for oil lubrication flowing in the center of the duct. In this way, this work presents an energy and hydrodinamic study of the heavy oils transportation in curved connections by numerical simulation via software ANSYS CFX® Release 15.0. It used a mathematical model that considers the mixture model to treat heavy oil-water two-phase flow, three-dimensional, transient and isothermal, assuming laminar flow to oil phase and turbulent flow to water phase, using k-ε turbulence model. An energy analysis involving pumps was performed. Results of pressure, velocity and volumetric fraction fields for the two phases are show and analysed. It was verified that the use of core-flow technique has decreased in 95.4% friction pressure drop as compared to monophase heavy oil flow and that stop-and-go from the oil and water pumps affect pressure drop by friction

Submitted by RENATO ALVES CAVALCANTI (renatoa.cavalcanti@hotmail.com) on 2018-04-20T19:35:13Z No. of bitstreams: 1 DISSERTACAO final CD.doc: 1705472 bytes, checksum: b127c150094c6b7ffb1f73ed0519b39f (MD5) / Rejected by Alexandra Maria Donadon Lusser Segali null (alexmar@fcav.unesp.br), reason: Solicitamos que realize correções na submissão seguindo as orientações abaixo: O arquivo submetido ao repositório deve estar no formato pdf e não em formato word. Favor colocar em formato pdf desbloqueado segundo ite 13 do tutorial de auto arquivamento: Upload do arquivo 13. Para enviar o arquivo contendo sua dissertação ou tese é necessário que:  o arquivo esteja no formato Portable Document Format (PDF);  o arquivo não esteja protegido;  o trabalho (dissertação ou tese) esteja reunido em um único arquivo, inclusive os apêndices e anexos. https://portal.biblioteca.unesp.br/portal/arquivos/tutorial-para-o-autoarquivamento-de-dissertacoes-e-teses-2.0.pdf Agradecemos a compreensão on 2018-04-25T12:08:51Z (GMT) / Submitted by RENATO ALVES CAVALCANTI (renatoa.cavalcanti@hotmail.com) on 2018-04-25T14:47:43Z No. of bitstreams: 1 DISSERTACAO_RENATO_ALVES_CAVALCANTI.pdf: 1329114 bytes, checksum: 20d539d56c4e488c0120474cccea5252 (MD5) / Approved for entry into archive by Alexandra Maria Donadon Lusser Segali null (alexmar@fcav.unesp.br) on 2018-04-25T15:59:46Z (GMT) No. of bitstreams: 1 cavalcanti_ra_me_jabo.pdf: 1329114 bytes, checksum: 20d539d56c4e488c0120474cccea5252 (MD5) / Made available in DSpace on 2018-04-25T15:59:46Z (GMT). No. of bitstreams: 1 cavalcanti_ra_me_jabo.pdf: 1329114 bytes, checksum: 20d539d56c4e488c0120474cccea5252 (MD5) Previous issue date: 2018-02-23 / O objetivo deste trabalho foi desenvolver um protótipo de filtro para irrigação com crepina artesanal, submetido a regime de água residuária de suinocultura avaliando o desempenho hidráulico juntamente com a eficiência de filtragem, utilizando resíduos agrícolas (Maravalha, Casca de arroz, Crepina (sem elemento filtrante), Silagem de milho e Feno de Tifton (Cynodon spp.)) como elemento filtrante. A água residuária utilizada na experimentação foi coletada após a utilização do biodigestor. A hidrometria utilizando placa de orifício foi através do diferencial de pressão acusado na coluna de mercúrio tipo “U”. O processo de construção do protótipo de filtro foi a partir de meio metro tubo PVC azul, com diâmetro interno de 94,4 mm. Em uma das extremidades, foi inserido na parte interna, um cap PVC roscável de 1 polegada, com 115 furos de 1 milímetro. A esta peça, neste trabalho, foi atribuída o nome de Crepina. Pôde-se concluir nesta pesquisa, que o protótipo de filtro operou satisfatoriamente em termos estruturais e operacionais. O filtro operando sem elemento filtrante teve baixa perda de carga e eficiência de filtragem de sólidos totais de 34%. / The objective of this work was to develop a filter prototype for irrigation with artisanal crepine, to submit to the swine wastewater regime and to evaluate the hydraulic performance along with filtering efficiency, using agricultural residues (Shavings, Rice husk, Crepina, Corn silage and Tifton hay (Cynodon spp.)) as filter element. The wastewater used in the experiment was collected after using the biodigester. The hydrometry was through the pressure differential on the orifice plate accused in the mercury column type "U". The process of constructing the prototype filter was from half a meter blue PVC tube, with internal diameter of 94.4 mm. At one end, a 1-inch threadable PVC cap with 115 1-mm holes was inserted inside. This piece, in this work, was given the name Crepina. It could be concluded in this research that the prototype filter operated satisfactorily in structural and operational terms. And that artisanal crepine had a low head loss and a total solids filtration efficiency of 34%.

O presente trabalho envolve um estudo teórico e experimental do escoamento bifásico externo a banco de tubos. Inicialmente, apresenta-se uma ampla revisão da literatura sobre padrões de escoamento, fração de vazio e perda de pressão, durante escoamentos monofásicos e bifásicos externos a banco de tubos. Nesta análise são também descritos os métodos de previsão destes parâmetros. Verificam-se diferenças significativas entre as estimativas proporcionadas por eles, fato que indica a inexistência de métodos generalizados. Posteriormente é apresentada uma descrição detalhada da bancada experimental projetada e construída durante o doutoramento. O aparato completo compõe-se da seção de testes, circuito de água, sistema de compressão e condicionamento de ar, e seções de injeção dos fluxos e condicionamento do escoamento. A seção de testes consiste em um banco de tubos distribuídos segundo configuração triangular normal, com os tubos apresentando diâmetro externo de 19,1 mm, comprimento de 381 mm, e espaçamento transversal de 24 mm. Os experimentos foram realizados para escoamento vertical ascendente de misturas água-ar e velocidades superficiais da fase líquida e gás de 0,020 a 1,500 m/s e de 0,10 a 10,00 m/s, respectivamente. Neste estudo foram desenvolvidas técnicas inéditas para determinação experimental da fração de vazio superficial no interior do banco de tubos baseadas em sistemas óptico e de sensoriamento capacitivo. Os padrões de escoamento foram identificados subjetivamente através de visualização de imagens e vídeos do escoamento, e objetivamente com o auxílio do método de agrupamento de dados k-means utilizando parâmetros baseados no sinal de perda de pressão e do sensoriamento capacitivo. Identificou-se subjetivamente os padrões de escoamento bolhas, bolhas dispersas, bolhas grandes, agitante, intermitente e anular. Constatou-se equivalência entre os padrões de escoamento identificados através dos métodos objetivo e subjetivo. Resultados experimentais para fração de vazio foram obtidos através de técnicas óptica e capacitiva. Constatou-se que o traçador rodamina B utilizado no método óptico altera as condições do escoamento, ainda que em concentrações reduzidas. A partir dos resultados obtidos com o sensoriamento capacitivo estimou-se a fração de vazio para o padrão bolhas. Resultados para a parcela friccional da perda de pressão também foram levantados. Constata-se o incremento da fração de vazio e da parcela friccional da perda de pressão com as velocidades superficiais das fases líquida e gás. Os resultados para fração de vazio foram comparados com métodos de previsão da literatura, e de maneira geral os métodos preveem as tendências dos resultados experimentais apenas para vazões de líquido reduzidas. Analogamente, os resultados para perda de pressão foram comparados com estimativas segundo métodos da literatura, concluindo que os métodos não preveem satisfatoriamente os resultados obtidos. Desta forma, foram propostos novos métodos de previsão para padrões de escoamento, fração de vazio e parcela friccional da perda de pressão, desenvolvidos a partir de análises dos mecanismos dominantes do escoamento, e adotando parâmetros adimensionais para correlacionar os dados. Os métodos propostos preveem satisfatoriamente os resultados experimentais deste estudo e da literatura para escoamentos bifásicos água-ar. / The present thesis concerns a theoretical and experimental study of external two-phase flows across tube bundles. Initially, a comprehensive literature review covering flow patterns, void fraction and pressure drop for single and two-phase flows across tubes bundle is presented. The review also describes predictive methods for these parameters. A comparison of these methods reveals reasonable disagreement among their predictions, indicating the absence of generalized methods. Subsequently, the apparatus and instrumentation designed and built to obtain the experimental data are described. The experimental apparatus comprises the test section, a water loop, air compression and conditioning systems, and sets for fluid flow injections and conditioning. The test section is a normal triangular tube bundle, with 19.1 mm OD tubes, 381 mm long and transversal pitch of 24 mm. The experiments were performed for air-water upward vertical flow, for superficial liquid and gas velocities ranging from 0.020 to 1.500 m/s and 0.10 to 10.00 m/s, respectively. Innovative techniques to evaluate the void fraction within the bundle were developed based on capacitive and optical methods. The flow patterns were identified subjectively and objectively by k-means clustering method, using as clustering parameters the pressure drop and the capacitive signals. Bubbles, dispersed bubbles, large bubbles, churn, intermittent and annular flow patterns were identified subjectively. The data groups identified by the objective method are representative of the flow patterns. Void fraction measurements were obtained for bubbly flow using both techniques (optical and capacitive). The void fraction data based on the optical method had its experimental range limited due to changes in the flow characteristics caused by the addition of the fluorescent dye Rhodamine B. The experimental results indicate that the void fraction increases with increasing the superficial velocities of both phases. In general, the void fraction predictive methods available in the literature capture the trends of the experimental results only for reduced liquid flow rates. According to the experimental results, the frictional pressure drop increases asymptotically with increasing the flow rates of both phases. None of the predictive methods from literature evaluated in the present study predicted satisfactorily the experimental results. Methods for prediction of flow patterns, void fraction and frictional pressure drop parcel were also developed in the present study. These methods provided reasonable predictions of the experimental results obtained in the present study, and also from the literature for air and water flows across tube bundles.

The current study is a part of a long term experimental project devoted to investigate flow boiling heat transfer, pressure drop and flow visualization of R134a in small to mini/micro-diameter tubes. The experimental facility was first designed and constructed by X. Huo (2005) with the contribution of L. Chen (2006). In the present study, the experimental facility was upgraded by changing the heating system from AC to DC heating and also upgrading the logging system through using a faster data logger and developing a new Labview program. The objectives of the current study include (i) contribute in identifying the reasons behind the wide scatter in the published flow boiling heat transfer results, (ii) contribute in understanding the fundamentals of flow boiling heat transfer in mini/micro-diameter tubes and (iii) evaluation of the existing heat transfer and pressure drop prediction methods. Two sizes of stainless steel tubes were investigated in the current study; 0.52 mm and 1.1 mm diameter. In the current study, the 0.52 mm tube was roughly called a “micro-tube” whilst the 1.1 mm tubes were called “mini-tubes”. The present study proposes two possible reasons for the scatter in the published heat transfer results. The first reason is the variations in the heated length from one study to another–there is no criterion for choosing the heated length. The second reason is the variations in the inner surface characteristics of the channels from one study to another. These two important parameters were not taken into consideration by researchers in the past studies. Accordingly, the effect of the heated length was investigated in the current study using a seamless cold drawn tube with diameter of 1.1 mm and heated length ranging from 150 to 450 mm. The effect of the tube inner surface was also tested here by conducting the test in two stainless steel tubes with diameter of 1.1 mm and manufactured by two different processes. The first tube was manufactured by welding technique whilst the second tube was a seamless cold drawn tube. Both tubes were identical in design and dimensions. The inner surface of each tube was examined first using SEM analysis and demonstrated that, the surface morphology is completely different. The local heat transfer coefficient was determined through measuring the local wall temperature using 14 K-type thermocouples attached to the wall using thermally conducting but electrically insulating epoxy supplied by Omega. Pressure drop was measured directly across the heated section and a high speed camera was used for the flow visualization at 1000 frames/s. All measurements were recorded after the system attained steady state. The experimental conditions include mass flux range of 100 – 500 kg/m2 s, system pressure range of 6 – 10 bar, inlet sub-cooling of about 5K and exit quality up to about 0.9. The most frequently observed flow regimes in the 0.52 mm tube were found to be slug (elongated bubble), transition to annular and annular flow regimes. In the 1.1 mm tube, the observed regimes were found to be slug, churn and annular. The transition from slug flow to annular flow in the 0.52 mm tube occurred smoothly with little disturbances at the liquid vapour interface compared to the 1.1 mm tube. Additionally, increasing the heated length of the 1.1 mm tube was found to shift the transition to annular flow to occur at lower vapour quality. The heat transfer results demonstrated that the behaviour of the local heat transfer coefficient in the 0.52 mm diameter tube is different compared to that in the 1.1 mm tubes. Also, the tube inner surface characteristics and the heated length were found to strongly influence the local behaviour of the heat transfer coefficient. Flow boiling hysteresis was investigated and the results indicated that hysteresis exists only at very low heat fluxes near the boiling incipience. Existing heat transfer and pressure drop correlations were examined using the results of the 0.52 and 1.1 mm seamless cold drawn tubes. The pressure drop data were predicted very well using the Muller-Stienhagen and Heck (1986) correlation, the homogeneous flow model and the correlation of Mishima and Hibiki (1996). On the contrary, all macro and microscale heat correlations failed to predict the current experimental data. The mechanistic models failed to predict the data of all tubes with the same accuracy. Accordingly, two heat transfer correlations were proposed in the current study. The first correlation is based on dimensionless groups whilst the second is based on the superposition model of Chen (1966). Both correlations predicted the current experimental data and the data of Huo (2005) and Shiferaw (2008) very well.

Dans le séchage des fruits et légumes, beaucoup de questions non résolues subsistent. Le «Swell-drying» est un procédé de séchage couplant une étape de déshydratation par air chaud à la texturation par Détente Instantanée Contrôlée (DIC). Dans le cadre de ce travail de recherche, la carotte et la pomme sont utilisées comme aliment modèle pour le traitement de la plupart des végétaux. La littérature scientifique sur le séchage des agromatériaux est unanime sur l’effet positif de la température de l'air, l’effet négatif de l'humidité relative de l'air et de l'épaisseur vis-à-vis du temps de séchage. Cependant, malgré un nombre important d’études, des conclusions contradictoires sont notées en termes de corrélations entre la cinétique de séchage et la vitesse de l'air qui, selon certains auteurs, n’entraîne aucune modification du séchage; alors que certains travaux ont tenté d'établir des modèles empiriques entre la diffusivité effective et la vitesse de l'air ; ce qui est fondamentalement erroné. L’objectif de cette thèse est d’analyser les phénomènes de transfert pendant le processus du séchage et d'identifier le phénomène limitant entre les transferts internes et externes. Ce travail aboutir à l’identification de la vitesse critique de l'air CAV capable de rendre la diffusion interne de l’eau comme phénomène limitant. La CAV doit donc dépendre de la diffusivité effective et de la taille du produit. À partir de résultats expérimentaux qui couvrent de larges plages de température d’air, de teneur en eau, et de taux d'expansion absolu, un modèle empirique a été établi. / Great research works support the sector of drying of fruits and vegetables in a number of ways. Each year, experimental results and mathematical modelling try through conventional and innovative technological processes to protect agricultural products from seasonality and variability in the market prices. Nonetheless, there still exist considerable issues to improve and control this strategic agricultural sector. Swell-drying is very relevant drying process coupling hot air dehydration with Instant Controlled Pressure Drop (DIC). In the present work, carrot and apple were used as a fruit and vegetable model. Scientific literature on drying is unanimous that the drying time decreases with increasing air temperature, decreasing air relative humidity, and reducing thickness. However, the effect of airflow velocity leads to contradictory conclusions. Some authors have claimed that it does not trigger any modification of drying; while some articles tried to establish empirical models of the effective diffusivity versus airflow velocity; what is fundamentally erroneous. This thesis aims at analyzing external and internal transfer phenomena during drying. So, it was possible to recognize the Critical Airflow Velocity CAV depending on the effective diffusivity and the product size, to have the internal water transfer as the limiting phenomenon. From experimental results, an empirical model was established between the effective moisture diffusivity and airflow temperature, water content, and absolute expansion ratio.

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.

This study consists of an experimental investigation into the fluid flow and heat transfer aspects of microchannels. Rectangular copper microchannels of hydraulic diameters 1.05 mm, 0.85 mm and 0.57 mm were considered. Using water as the working fluid, heat transfer and pressure drop characteristics were determined under a constant surface heat flux for different inlet configurations in the laminar and transitional regimes. Three inlet geometries were experimentally investigated: a sudden contraction inlet, a bellmouth inlet and a swirl-generating inlet. The influence of the inlet conditions on the pressure drop, Nusselt number and critical Reynolds number was determined experimentally. Pressure drop results showed good agreement with existing correlations for adiabatic conditions. Diabatic friction factor results for the sudden contraction and bellmouth inlets were overpredicted when using the friction factor results from literature. It is noted that a relationship between the pressure drop and heat flux existed in the laminar regime, where an increase in the heat input resulted in a decrease in the friction factor. The bellmouth inlet condition showed an enhancement of the heat transfer in the transition regime compared with the sudden contraction inlet. The critical Reynolds number for the onset of transition for the sudden contraction inlet was found to be approximately 1 950, with a sharp rise to the turbulent regime thereafter. The bellmouth inlet influenced the originating point of the transition regime, which commenced at a Reynolds number of approximately 1 600. A smoother and more gradual increase to the turbulent regime was observed as an effect of the bellmouth inlet over the sudden contraction inlet. The swirl-generating inlet condition produced higher friction factor results in all three flow regimes. Transition occurred at a Reynolds number of approximately 1 500 and the turbulent regime was quickly ii reached thereafter. The turbulent regime friction factor was found to be significantly higher with the swirl inlet compared with both the sudden contraction and bellmouth inlets. Nusselt numbers continued to increase until the onset of the transition regime, and did not converge to a constant value as stated in theory. Similar enhancement of the transition regime with the bellmouth inlet was observed for the Nusselt numbers as with the friction factors. The initial turbulent regime results followed the trend of the theory for both the sudden contraction and bellmouth inlet conditions for most of the data sets, with deviation occurring in some of the 0.57 mm test cases. The swirl inlet Nusselt number results were significantly underpredicted by the theory in the early turbulent regime. / Dissertation (MEng)--University of Pretoria, 2014. / gm2014 / Mechanical and Aeronautical Engineering / unrestricted

The prewetting of a trickle-bed reactor has important implications in the design and operation of these reactors. This is because the prewetting changes the flow morphology (shape and texture) of the liquid flowing through the bed and leads to the existence of multiple hydrodynamic states. The extent of this change in flow morphology can be seen in the effect the prewetting of the reactor has on the pressure drop, liquid holdup and gas-liquid mass transfer. The following prewetting procedures were used: -- Levec-wetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flow is reintroduced -- Kan-wetted: the bed is operated in the pulse flow regime and liquid and gas flow rates are reduced to the desired set point -- Super-wetted: the bed is flooded and gas and liquid flow are introduced once draining commences For the pressure drop: -- The different prewetting procedures resulted in two distinct regions (Upper region Kan and Super-wetted, Lower region Dry and Levec-wetted) -- There was no significant difference between the Dry and Levec-wetted beds -- The pressure drop in the Kan and Super-wetted beds can be as much as seven times greater than the pressure drop in the Dry and Levec-wetted beds For the liquid holdup: -- The different prewetting procedures resulted in four distinct regions (Kan-wetted, Super-wetted, Levec-wetted, Dry bed) -- The liquid holdup in the Kan-wetted bed can be as much as four times greater than the liquid holdup in the Dry bed -- The liquid holdup in the Levec-wetted can be as much as thirty percent lower than the liquid holdup in the Kan-wetted bed For the gas-liquid mass transfer: -- The different prewetting procedures resulted in three distinct regions (Kan and Super-wetted, Levec-wetted, Dry bed) -- The volumetric gas-liquid mass transfer coefficient in the Kan and Super-wetted beds can be as much as six times greater than the mass transfer coefficient in the Dry bed -- The volumetric gas-liquid mass transfer coefficient in the Kan and Super-wetted beds can be as much as two and a half times greater than the mass transfer coefficient in the Levec-wetted bed While an increase in the liquid flow rate results in an increase in the pressure drop, liquid holdup and gas-liquid mass transfer for all of the experiments, the effect of increasing gas flow on the measured variables were more pronounced for the prewetted beds. In a prewetted bed (Kan, Super and Levec-wetted) an increase in the gas flow rate causes an increase in the volumetric gas-liquid mass transfer coefficient and a decrease in the liquid holdup. The decrease in the liquid holdup is due to the fact that the increased gas flow rate causes the films around the particles to thin and spread out. In the dry bed the flow is predominantly in the form of rivulets and the increase in gas flow rate does not affect the liquid holdup. In the case of the volumetric gas-liquid mass transfer coefficient the increased gas flow rate causes an increase in the mass transfer coefficient regardless of the prewetting procedure. This increase is due to the effect that the gas flow rate has on the liquid holdup as well as the increase in the gas-liquid interfacial area due to the increased gas-liquid interaction. If the pulsing in the Kan-wetted bed is induced by increasing the gas flow rate and keeping the liquid flow rate constant the results are significantly different. The pressure drop in the gas-pulsing experiments was lower than the pressure drop in the recorded in the Kan and Super-wetted beds, but higher than the pressure drop in the dry and Levec-wetted beds. However, the liquid holdup in the gas-pulsing experiments was higher than the liquid holdup in any of the other beds. The volumetric gas-liquid mass transfer coefficient in the gas-pulsing experiments was lower than the mass transfer coefficients of the Kan and Super-wetted beds, but higher than the mass transfer coefficients in the dry and Levec-wetted beds. The multiple operating points obtained from the different prewetting procedures are by no means the only possible operating points. By simply decreasing the draining time in the Levec-wetted bed steady state operating points can be found between those of the Super and Levec-wetted beds. This alludes to the fact that the operating conditions determined from the different prewetting modes are only boundaries and that the actual operating point can lie anywhere between these boundaries. The existence of these multiple hydrodynamic states complicates things further when a correlation is developed to determine the pressure drop, liquid holdup or the volumetric gas-liquid mass transfer coefficient. No correlation tested was able to accurately predict the pressure drop, liquid holdup or volumetric gas-liquid mass transfer coefficient in the dry or prewetted beds. / Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2007. / Chemical Engineering / unrestricted

Cette étude concerne l'optimisation, la conception et la caractérisation de microréacteurs, de type multicanaux, appliqués à l'électrosynthèse organique de composés fluorés à intérêt médical tels que le 2-Fluoro- 2-Deoxy-D-Glucose (18FDG). Les microsystèmes ont connu un développement important ces dernières années dans le domaine de la chimie fine où la volonté est de développer des outils toujours plus compétitifs. Les microréacteurs appliqués à la synthèse offrent l’avantage d’un rapport surface sur volume de la zone réactionnelle élevé (> 100 cm-1), ce qui améliore nettement les transferts de masse et d’énergie et permet de traiter de très faibles quantités dans des conditions plus sûres et plus respectueuses de l’environnement. L’élément de base du microréacteur est souvent constitué d’un simple microcanal qu’il est nécessaire de dupliquer pour fournir le débit de production adapté à une application donnée. Ainsi, un microréacteur sera souvent composé d’une série de microcanaux disposés en parallèle et connectant un canal distributeur et un canal collecteur. Cette configuration peut entraîner une faible uniformité de la distribution de l’écoulement dans les différents microcanaux de réaction et il est particulièrement important d’optimiser la géométrie du microréacteur complet pour tendre vers une distribution uniforme des temps de séjour (DTS). Dans le cas de la synthèse électrochimique, les microcanaux sont directement gravés dans deux électrodes placées en vis-à-vis et séparées par une membrane échangeuse d’ions. Une optimisation préliminaire de la DTS au sein d’une électrode composée de microcanaux parallèles de section rectangulaire est réalisée. L’arrivée et la sortie du fluide s’effectue par l’intermédiaire de deux canaux distributeur et collecteur de section également rectangulaire, mais non constante. L’optimisation vise à déterminer une évolution linéaire optimale de la largeur de ces canaux distributeur et collecteur. Un modèle analytique basé sur des hypothèses simplificatrices permet de calculer les différentes pertes de charge ainsi que les débits dans chaque microcanal, dans le cas d’un écoulement laminaire de liquide. Les résultats obtenus sont ensuite confirmés par des simulations numériques 3-D, plus précises. Un modèle hybride combinant les simulations numériques pour les canaux distributeur et collecteur et le modèle analytique pour les microcanaux parallèles est également développé. Il permet d’augmenter la finesse du maillage dans les zones sensibles de l’écoulement, sans nécessité d’accroître les ressources informatiques (mémoire et temps de simulation). Les résultats obtenus montrent un très bon accord entre les simulations numériques 3-D, le modèle hybride et le modèle analytique. Sur un exemple de 10 microcanaux parallèles, il est montré que dans le cas de la géométrie initiale, pour laquelle les canaux collecteur et distributeur sont de section constante, des écarts de l’ordre de 50 % existent entre les débits traversant les microcanaux latéraux et centraux. Après optimisation, cet écart est réduit à moins de 0,1 %. Le modèle analytique est ensuite étendu au cas d’écoulements gazeux en prenant en compte les effets non linéaires et antagonistes de la raréfaction et de la compressibilité de l’écoulement. La raréfaction est ici caractérisée par un nombre de Knudsen compris entre 0 et 0,1 et se traduit pas des sauts de vitesse à la paroi ; les écoulements dans ce régime modérément raréfié sont alors correctement modélisés par les équations compressibles de Navier Stokes associées à des conditions de glissement du second ordre en Knudsen, en prenant en compte la géométrie tridimensionnelle des microcanaux de réaction et des canaux collecteur et distributeur. / This study focuses on the optimisation, design and characterization of microreactors, of multichannel type, applied to the organic electrosyntheses of fluorinated compounds of medical interest such as the 2-Fluoro-2-Deoxy-D-Glucose (18FDG). Microsystems have known an important development these last years in the field of fine chemicals where the aim is to develop increasingly competitive tools. The microreactors applied to synthesis offer a reaction zone with high surface to volume ratio (> 100 cm-1), which significantly improves mass and energy transfers and allows treating small quantities in safer conditions and a better respect of environment. The basic element of the microreactor is often composed of a single microchannel, which is necessary to duplicate in order to provide the suitable production rate for a given application. Thus, a microreactor is often composed of a series of microchannels arranged in parallel and connecting a distributing channel to a collecting one. This configuration can result in poor uniformity of flow distribution among the reaction microchannels and it is particularly important to optimize the geometry of the microreactor in order to obtain a uniform residence time distribution (RTD). In the case of electrochemical synthesis, microchannels are directly etched into two electrodes facing each other and separated by an ion exchange membrane. A preliminary optimisation of the RTD in an electrode composed of parallel microchannels with rectangular cross-section is performed. The fluid inlet and outlet are connected to a distributing and a collecting channel with non constant rectangular cross-section. The aim of the optimisation is to determine an optimal linear evolution of the width of the distributing and collecting channels. An analytical model based on simplifying assumptions allows calculating the various pressure drops and the flowrate in each microchannel, in the case of a laminar liquid flow. The obtained results are then confirmed by more accurate 3-D numerical simulations. A hybrid model combining numerical simulations for the distributing and collecting channels and the analytical model for the parallel microchannels is also developed. This model allows a more refined mesh in the sensitive areas of the flow, without requiring additional numerical effort (memory and simulation time). The results show a good agreement between the 3-D numerical simulations, the hybrid model and the analytical model. On an example of 10 parallel microchannels, it is shown that in the case of the initial geometry (with a constant cross-section of collecting and distributing channels), the flowrate difference through the lateral and the central microchannels is in the order of 50%. After optimization, this difference is reduced to less than 0.1%. The analytical model is then extended to the case of gas flows, taking into account nonlinear and antagonist effects of rarefaction and compressibility. Rarefaction is characterized by the value of the Knudsen number which remains lower than 0.1; the flow in this moderately rarefied regime is accurately modelled by the compressible Navier-Stokes equations associated with second-order slip boundary conditions, taking into account the three-dimensional geometry of the reaction microchannels and of the collecting and distributing channels.