This study investigated the corrosion processes at the top and bottom of carbon steel pipelines transporting wet gases, and studied possible chemical mitigation strategies. First, immersion tests were carried out using carbon steel to study the effects of de-aeration with high purity nitrogen gas on the corrosion rate. Secondly, the corrosion rate was assessed for varying chloride ion concentrations in an aerated environment. In general, increasing de-aeration time changes the corrosion mechanism from mass transfer oxygen reduction to water reduction reaction. However, oxygen solubility controlled the corrosion process in aerated solution containing different chloride ion concentrations. A special two-electrode cell was designed for the top of the line corrosion (TLC) electrochemical measurements but a conventional three electrode cell was used for the bottom of the line corrosion (BLC) measurements. The TLC rate increases with temperature, and X-Ray Diffraction (XRD) confirmed the presence of chukanovite {Fe2(CO3)(OH)2}and possibly ferrous carbonate corrosion products at 40oC and 60oC respectively. However, for the BLC, the cementite phase remained on the metal surface after preferential dissolution of the ferrite phase in the carbon steel. Addition of acetic acid (HAc) locally dissolved the initially FeCO3 film formed on the metal surface, causing local corrosion damage. Addition of methyl di ethanol amine (MDEA) as a pH stabiliser reduced TLC and BLC rates due to enhanced stability of FeCO3 at pH 5.7-6.3. When Zn2+ ions were added as ZnCl2, both Fe2O3 and ZnCO3 were formed at reduced corrosion rate. Whenever FeCO3 film was damaged/dissolved by HAc addition of neither pH stabiliser; MDEA nor hydrate preventer; mono ethylene glycol (MEG) could not re-establish a protective film on the metal surface. The following organic inhibitors were investigated as potential mitigators of TLC: 2-mercaptobenzimidazole (2MBI), 2-amino-5-ethyl-1,3,4-thiodiazole (AETDA), 2-phenyl-2-imidazoline (2PI), dicyclohexylamine (DHA), and a commercial inhibitor formulation (CI-A). The inhibition efficiency (IE%) was found to increase in the order CI-A > 2MBI > AETDA > DHA. Their efficiency increases (except DHA) with inhibitor concentrations both at top and bottom of the line. 2MBI and CI-A behaved as mixed inhibitors but AETDA behaved as cathodic inhibitor, all were best fitted to a simple Langmuir adsorption isotherm. However, IE% of DHA decreased at higher inhibitor concentrations. Surprisingly, 2PI inhibitor increased the corrosion rate, and the corrosion rate further increased with increase inhibitor concentrations. Weight loss measurements results of TLC are also presented which showed lower inhibition efficiency for all the inhibitors investigated compared with electrochemical measurements in similar environments. The free energy of adsorption (∆Goads values for 2MBI and AETDA are around -36kJ.mol-1 while for CI-A the value was -15kJ.mol-1 (-7kJ.mol-1 in the presence of HAc). This is evidence for adsorption of 2MBI and AETDA on the metal surface by chemisorption with CI-A by physisorption. XPS analysis confirmed the presence of FeCO3 and FeOOH as corrosion products in the brine solution in the absence and presence of HAc containing different corrosion inhibitors.

Superhydrophobic surfaces have gained attention as a potential mechanism for increasing condensation heat transfer rates. Various aspects related to condensation heat transfer are explored. Adiabatic, air-water mixtures are used to explore the influence of hydrophobicity on two-phase flows and the hydrodynamics which might be present in flow condensation environments. Pressure drop measurements in a rectangular channel with one superhydrophobic wall (cross-section approximately 0.37 X 10 mm) are obtained, revealing a reduction in the pressure drop for two-phase flow compared to a control scenario. The observed reduction is approximately 10% greater than the reduction that is observed for single-phase flow (relative to a classical channel). Carbon nanotubes have been used to create superhydrophobic coatings due to their ability to offer a relatively uniform nanostructure. However, as-grown carbon nanotubes often require the addition of a thin-film hydrophobic coating to render them superhydrophobic, and fine control of the overall nanostructure is difficult. This work demonstrates the utility of using carbon infiltration to layer amorphous carbon on multi-walled nanotubes to achieve superhydrophobic behavior with tunable geometry. The native surface can be rendered superhydrophobic with a vacuum pyrolysis treatment, with contact angles as high as 160 degrees and contact angle hysteresis less than 2-3 degrees. Drop-size distribution is an important aspect of heat transfer modeling that is difficult to measure for small drop sizes. The present work uses a numerical simulation of condensation to explore the influence of nucleation site distribution approach, nucleation site density, contact angle, maximum drop size, heat transfer modeling to individual drops, and minimum jumping size on the distribution function and overall heat transfer rate. The simulation incorporates the possibility of coalescence-induced jumping over a range of sizes. Results of the simulation are compared with previous theoretical models and the impact of the assumptions used in those models is explored. Results from the simulation suggest that when the contact angle is large, as on superhydrophobic surfaces, the heat transfer may not be as sensitive to the maximum drop-size as previously supposed. Furthermore, previous drop-size distribution models may under-predict the heat transfer rate at high contact angles. Condensate drop behavior (jumping, non-jumping, and flooding) and size distribution are shown to be dependent on the degree of subcooling and nanostructure size. Drop-size distributions for surfaces experiencing coalescence-induced jumping are obtained experimentally. Understanding the drop-size distribution in the departure region is important since drops in this size are expected to contribute significantly to the overall heat transfer rate.

With the increasing pressure of providing reliable potable water in a sustainable way, it is important to understand water phase change phenomena (condensation and evaporation) as the water phase change is involved in many processes such as membrane distillation and solar still which can be a feasible choice of supplementing the present potable water access. In the present thesis, we first elucidate the role of wettability of water condensation substrate by combining the droplet growth dynamics and droplet population evolution. The results show that wettability has a negligible effect on water condensation rate in an atmospheric environment. After confirming the role of substrate wettability, we provide a quantitative analysis of the effect of substrate geometry on water condensation in the atmospheric environment. The analysis can help to predict the efficiency of water condensation rate with a given substrate of a certain geometry with the aid of computational simulation tools. The results show that water condensation can be increased by 40% by rationally designing the geometry of the condensation surface. However, the condensation rate in the atmospheric environment is relatively slow due to the presence of non-condensable gas. In order to increase the condensation rate, a relatively pure vapor environment is desired, in which condensed water will be the major heat transfer barrier. Coalescence induced jumping of condensed droplets on superhydrophobic surfaces is an interesting phenomenon to help faster removal of condensed droplets. However, it is still not clear how to optimize the overall heat transfer efficiency by condensation on such surfaces. We observed an interesting phenomenon on a superhydrophobic nano-cones array, on which water preferentially condenses within larger cavities among the nanocones. Droplets growing form larger cavities have larger growth rate. This finding can possibly provide a solution to optimizing heat transfer efficiency. Finally, a nylon-carbon black composite is prepared by electrospinning to enhance water evaporation under solar radiation. The composite shows an interesting light absorption property. In a wet state, the composite can absorb around 94% of the incident sunlight. The composite also shows strong mechanical and chemical stability. Thus, the composite is considered to be a practical candidate to be applied in the solar distillation process.

Water is undoubtedly a key life element. Its importance is very clear from a religious perspective: “We made from water every living thing. Will they not then believe?” Surah Al-Anbiya verse 30 Also as highlighted in the United Nations resolution 64/292 which recognizes water as a basic necessity for human survival. As the world water demand grows, so does the need to use renewable water sources most available in the form of saline ocean water. Desalination of this water for potable use relies mainly on thermal and membrane-based technologies, mainly multi-stage flash (MSF), multi-effect distillation (MED), and seawater reverse osmosis (SWRO). However, these mature technologies are recognized for their high energy and chemicals use. To cope with these challenges, development of novel desalination processes is required to assure more sustainable water supply for the future. Membrane distillation (MD) has emerged as a process which combines advantages of both membrane and thermal technologies. It has a potential of being cost effective by utilizing renewable or waste heat energies as a driving force. Air gap membrane distillation (AGMD) is one of the four main MD configurations. AGMD’s main feature is the presence of an air gap which is enclosed between the membrane behind which flows the hot feed and condensation surface behind which flows a coolant. While improving the heat transfer across the membrane, the air gap negatively affects mass transfer resistance thereby reducing vapor flux and increasing process footprint. This dissertation investigates the effect of condensation plate surface modifications on AGMD process efficiency. The modifications are made by utilizing three different approaches including alterations of the surface shape and surface coating (to modify its contact angle) and by varying module inclination angle. A numerical simulation is carried out to determine the key factors which facilitate AGMD vapor flux increase. The second part of this thesis focuses on developing a promising novel approach utilizing Peltier’s process as a heat source to operate the MD process with less energy requirement. The morphological modifications of a plate surface positively affected vapor flux because of the air gap reduction. The highest vapor fluxes were observed when condensation plate had hydrophilic coatings. Based on the observed results, a thin film-wise condensation was suggested as a primary condensation mechanism. The formed film reduced the air gap thickness and this effect was more prominent at 45° when condensation plate was positioned over the membrane surface. A 2-dimensional mathematical model was developed and the model results agreed with the experimental data. Finally, the thermocouple-based MD concept was introduced and experimentally validated.

Multiphase flow provides a solution to the high heat flux and precision required by modern-day gadgets and heat transfer devices as phase change processes make high heat transfer rates achievable at moderate temperature differences. An application of multiphase flow commonly used in industry is the condensation of refrigerants in inclined tubes. The identification of two-phase flow patterns, or flow regimes, is fundamental to the successful design and subsequent optimisation given that the heat transfer efficiency and pressure gradient are dependent on the flow structure of the working fluid. This study showed that with visualisation data and artificial neural networks (ANN), a machine could learn, and subsequently classify the separate flow patterns of condensation of R-134a refrigerant in inclined smooth tubes with more than 98% accuracy. The study considered 10 classes of flow pattern images acquired from previous experimental works that cover a wide range of flow conditions and the full range of tube inclination angles. Two types of classifiers were considered, namely multilayer perceptron (MLP) and convolutional neural networks (CNN). Although not the focus of this study, the use of a principal component analysis (PCA) allowed feature dimensionality reduction, dataset visualisation, and decreased associated computational cost when used together with multilayer perceptron neural networks. The superior two-dimensional spatial learning capability of convolutional neural networks allowed improved image classification and generalisation performance across all 10 flow pattern classes. In both cases, the classification was done sufficiently fast to enable real-time implementation in two-phase flow systems. The analysis sequence led to the development of a predictive tool for the classification of multiphase flow patterns in inclined tubes, with the goal that the features learnt through visualisation would apply to a broad range of flow conditions, fluids, tube geometries and orientations, and would even generalise well to identify adiabatic and boiling two-phase flow patterns. The method was validated by the prediction of flow pattern images found in the existing literature. / Dissertation (MEng)--University of Pretoria, 2021. / NRF / Mechanical and Aeronautical Engineering / MEng / Restricted

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The aim of this thesis is to design a surface condenser with lateral exhaust. A research in the field of surface condensers with lateral exhaust takes place before the design. Core of the thesis is a thermodynamic design of the heat exchanger, calculation of pressure losses on the side of coolant water, check of the tube bundles from standpoint of oscillation and a design of air removal pipeline. Finally a design of the condenser is supported with a drawing attached in the supplement.

This work describes the creation of detailed FEM models of the selected area. Primarily is focused on the process of creating a detailed FEM model of the part of airplane using the static condensation. This work also contains a description of the process stress analysis of part of the fuselage of the airplane EV-55 Outback.

This thesis is focused on separation water drops from jet of steam. Liested are some of basic principles of separation of moisture and particles, as well as equipment, which are using these principles. Furthemore, this thesis include the part devoted to water drop formation during condesation of vapor. Last part is focused on the design of cyclone separator.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely interfacial heat transfer and combustionanalysis approach.After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities,a new solver, including the two developed models for phase change, was implemented under the name of interPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physicalbehavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties.It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advancedsolvers within the phase change class.