Two-phase flow, boiling, and condensation in microchannels have received considerable attention in the recent past due to the growing interest in the high heat fluxes made possible by these channels. This dissertation presents a study on the condensation of refrigerant R134a in small hydraulic diameter (100 < Dh < 160 mm) channels. A novel technique is used for the measurement of local condensation heat transfer coefficients in small quality increments, which has typically been found to be difficult due to the low heat transfer rates at the small flow rates in these microchannels. This method is used to accurately determine pressure drop and heat transfer coefficients for mass fluxes between 300 and 800 kg/m2-s and quality 0 < x < 1 at four different saturation temperatures between 30 and 60oC. The results obtained from this study capture the effect of variations in mass flux, quality, saturation temperature, hydraulic diameter, and channel aspect ratio on the observed pressure drop and heat transfer coefficients. Based on the available flow regime maps, it was assumed that either the intermittent or annular flow regimes prevail in these channels for the flow conditions under consideration. Internally consistent pressure drop and heat transfer models are proposed taking into account the effect of mass flux, quality, saturation temperature, hydraulic diameter, and channel aspect ratio. The proposed models predict 95% and 94% of the pressure drop and heat transfer data within ±25%, respectively. Both pressure drop and heat transfer coefficient increase with a decrease in hydraulic diameter, increase in channel aspect ratio and decrease in saturation temperature. A new non-dimensional parameter termed Annular Flow Factor is also introduced to quantify the predominance of intermittent or annular flow in the channels as the geometric parameters and operating conditions change. This study leads to a comprehensive understanding of condensation in microchannels for use in high-flux heat transfer applications.

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

<p>Härtill 5 uppsatser</p> / digitalisering@umu

Heat transfer and pressure drop during condensation of propane flowing through minichannels is investigated in this study. Studies of condensation of hydrocarbons are important for applications in the petrochemical industry. Insights into the mechanisms of propane condensation are required for accurate design of heat transfer equipment for use in hydrocarbon processing. At present, there is very little research on vertical condensation, especially of hydrocarbons, for the tube sizes and flow conditions of interest to the present study. An experimental facility was designed and fabricated to measure the frictional pressure drop and heat transfer coefficients during condensation of propane in plain tubes with an inner diameter of 1.93 mm. Measurements were taken across the vapor-liquid dome in nominal quality increments of 0.25 for two saturation temperatures (47°C and 74°C) and four mass flux conditions (75 – 150 kg m‾² s‾¹). The data were compared to the predictions of relevant correlations in the literature. The data from this study were also used to develop models for the frictional pressure drop and heat transfer coefficient based on the measurements and the underlying condensation mechanisms. These results and the corresponding correlations contribute to the understanding of condensation of hydrocarbons in vertical minichannels.

All modern High Temperature Reactors (HTR) thermal cycles have one thing in common: the use of some form of heat exchanger. This heat exchanger is used to pre-heat or cool the primary loop gas, from where the secondary power generation cycle is driven. The Compact Heat Exchanger (CHE) type offers high heat loads in smaller volumes. Various studies have been done to improve the heat transfer in the flow channels of these CHEs but little focus has been placed on the thermal design of surrounding material in such a heat exchanger. The focus of this study is on the effective material usage in a CHE. Three test cases were investigated (trapezoidal, serpentine and zigzag layouts with semi-circular cross-sections) all under the same boundary conditions. Computational Fluid Dynamics (CFD) was used to simulate these test cases and the results were evaluated according to four factors, the volume ratio, heat spots, temperature difference and the combined enhancement factor. From the results it was concluded that the zigzag layout performs best when evaluated according to the volume ratio and the temperature difference and gave the best overall enhancement factor. The serpentine layout performed the worst when evaluated according to the enhancement factor. / Thesis (MIng (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

All modern High Temperature Reactors (HTR) thermal cycles have one thing in common: the use of some form of heat exchanger. This heat exchanger is used to pre-heat or cool the primary loop gas, from where the secondary power generation cycle is driven. The Compact Heat Exchanger (CHE) type offers high heat loads in smaller volumes. Various studies have been done to improve the heat transfer in the flow channels of these CHEs but little focus has been placed on the thermal design of surrounding material in such a heat exchanger. The focus of this study is on the effective material usage in a CHE. Three test cases were investigated (trapezoidal, serpentine and zigzag layouts with semi-circular cross-sections) all under the same boundary conditions. Computational Fluid Dynamics (CFD) was used to simulate these test cases and the results were evaluated according to four factors, the volume ratio, heat spots, temperature difference and the combined enhancement factor. From the results it was concluded that the zigzag layout performs best when evaluated according to the volume ratio and the temperature difference and gave the best overall enhancement factor. The serpentine layout performed the worst when evaluated according to the enhancement factor. / Thesis (MIng (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

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

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

Made available in DSpace on 2014-12-17T14:08:45Z (GMT). No. of bitstreams: 1 Dissert_Glydianne.pdf: 2012971 bytes, checksum: 9ff5c7d76d98c33ed2140632388bede0 (MD5) Previous issue date: 2011-09-09 / The oil companies in the area in general are looking for new technologies that can increase the recovery factor of oil contained in reservoirs. These investments are mainly aimed at reducing the costs of projects which are high. Steam injection is one of these special methods of recovery in which steam is injected into the reservoir in order to reduce the viscosity of the oil and make it more mobile. The process assisted gravity drainage steam (SAGD) using steam injection in its mechanism, as well as two parallel horizontal wells. In this process steam is injected through the horizontal injection well, then a vapor chamber is formed by heating the oil in the reservoir and, by the action of gravitational forces, this oil is drained down to where the production well. This study aims to analyze the influence of pressure drop and heat along the injection well in the SAGD process. Numerical simulations were performed using the thermal simulator STARS of CMG (Computer Modeling Group). The parameters studied were the thermal conductivity of the formation, the flow of steam injection, the inner diameter of the column, the steam quality and temperature. A factorial design was used to verify the influence of the parameters studied in the recovery factor. We also analyzed different injection flow rates for the model with pressure drop and no pressure drop, as well as different maximum flow rates of oil production. Finally, we performed an economic analysis of the two models in order to check the profitability of the projects studied. The results showed that the pressure drop in injection well have a significant influence on the SAGD process. / As empresas da ?rea de petr?leo em geral est?o ? procura de novas tecnologias que possam elevar o fator de recupera??o do ?leo contido em seus reservat?rios. Esses investimentos t?m como principal objetivo reduzir os custos dos projetos de produ??o de petr?leo, que s?o elevados. A inje??o de vapor representa um desses m?todos especiais de recupera??o, em que vapor ? injetado no reservat?rio com o objetivo de reduzir a viscosidade do ?leo e torn?-lo mais m?vel. O processo de drenagem gravitacional assistida por vapor (SAGD) utiliza a inje??o de vapor em seu mecanismo, assim como dois po?os horizontais paralelos. Neste processo o vapor ? injetado atrav?s do po?o injetor horizontal, em seguida uma c?mara de vapor ? formada no reservat?rio aquecendo o ?leo e, pela a??o das for?as gravitacionais, este ?leo ? drenado para baixo onde se encontra o po?o produtor. O presente trabalho tem como objetivo analisar a influ?ncia da perda de carga e calor ao longo do po?o injetor no processo SAGD. Foram realizadas simula??es num?ricas atrav?s do simulador t?rmico STARS da CMG (Computer Modelling Group). Os par?metros estudados foram ? condutividade t?rmica da forma??o, a vaz?o de inje??o de vapor, o di?metro interno da coluna, o t?tulo do vapor e a temperatura. Um planejamento fatorial foi utilizado para verificar a influ?ncia dos par?metros estudados no fator de recupera??o. Foram tamb?m analisadas diferentes vaz?es de inje??o para o modelo com perda de carga e sem perda de carga, assim como diferentes vaz?es m?ximas de produ??o de ?leo. Finalmente, foi realizada uma an?lise econ?mica dos dois modelos com a finalidade de analisar a rentabilidade dos projetos estudados. Os resultados mostraram que as perdas de carga no po?o injetor t?m uma influ?ncia significativa no processo SAGD.

Les particules ultrafines (PUF) sont de nos jours susceptibles de se retrouver massivement dans l’air des lieux de travail et dans l’environnement, notamment de par leur génération non-intentionnelle par certains procédés industriels. Du fait de la toxicité de plus en plus avérée de ces particules, l’air contaminé doit être extrait des lieux de travail et filtré avant d’être rejeté dans l’atmosphère. Les filtres classiquement utilisés sont des filtres à fibres plissés présentant l’inconvénient vis-à-vis des PUF d’être rapidement et irréversiblement colmatés. Afin de trouver une alternative à ces filtres, il a été décidé d’étudier les lits granulaires. Pour ce faire, la cinétique de colmatage des lits granulaires par des PUF a dans un premier temps été étudiée à l’échelle macroscopique et microscopique par suivi des évolutions des performances ainsi qu’en procédant à des observations de structure de dépôt. Evaluer la capacité des lits granulaires à se positionner en tant qu’alternative aux médias fibreux peut nécessiter la connaissance des performances des lits granulaires dans un grand nombre de configurations. Pour s’affranchir des expériences correspondantes, un modèle théorique de prédictions des performances des lits granulaires en cours de colmatage a été développé. Ce modèle a par la suite permis par une optimisation multicritère de trouver la configuration optimale d’un lit granulaire amélioré. Enfin, des essais préliminaires très prometteurs d’une manche granulaire permettant d’augmenter la surface de filtration et l’efficacité de collecte ont posé les bases d’une potentielle utilisation des lits granulaires pour la filtration de PUF dans l’industrie / The air of workplaces and the environment can be contaminated by ultrafine particles (UFP) coming mainly from a non-intentional generation emitted by some industrial processes. The toxicity of these particles being more and more admitted nowadays, the polluted air of the workplaces has to be extracted and filtered in order to protect the workers and the public, respectively. The commonly used filters are pleated fiber filters which are rapidly and sometimes irreversibly clogged by the UFP. In order to find an alternative to these pleated filters, it was decided to study the granular beds. To do so, the clogging kinetic of granular bed by UFP was studied. This was achieved by conducting both macroscopic and microscopic studies of the granular bed clogging consisting in the monitoring of the evolution of the performances and in performing visualizations of UFP deposit structures. Evaluate the ability of granular beds to be an alternative to fiber filters can require the knowledge of the granular bed performances evolution for a large number of configurations. In order to avoid the realization of the corresponding experiments, a theoretical model was developed. Then, this model permitted by a multi-criteria optimization method to find the optimal configuration of an improved granular bed. Finally, some preliminary and very promising tests of a cylindrical granular bed filter permitting to increase the surface filtration and the collection efficiency laid the groundwork of a potential use of granular beds for the industrial UFP filtration