Fluid flow accompanied by heat transfer, solidification and interrelated chemical reactions play a key role during Continuous Casting (CC) of steel. Generation of defects and production issues are a result of the interaction between mould flux, steel grade and casting conditions. These issues are detrimental to both productivity and quality. Thus, the development of reliable numerical models capable of simulating fluid flow coupled to heat transfer and solidification are in high demand to assure product quality and avoid defects. The present work investigates the influence of steel grade, mould powder and casting conditions on process stability by including heat and mass transfer through liquid steel, slag film layers and solidifying shell. The thesis addresses the application of a numerical model capable of coupling the fluid flow, heat transfer and solidification developed by Swerea MEFOS; based on the commercial CFD code FLUENT v12. The Volume of Fluid (VOF) method, which is an interface tracking technique, is coupled to the flow model for distinction of the interface between steel and slag. The current methodology not only allows the model to describe the behaviour of molten steel during solidification and casting but also makes the assessment of mould powders performance possible.  Direct prediction of lubrication efficiency, which is demonstrated by solid-liquid slag film thickness and powder consumption, is one of the most significant advantages of this model. This prediction is a direct result of the interaction between metal/slag flow, solidification and heat transfer under the influence of mould oscillation and transient conditions.  This study describes the implementation of the model to analyse several steel and mould powder combinations. This led to detection of a combination suffering from quality problems (High Carbon Steel + High Break Temperature Powder) and one, which provides the most stable casting conditions (Low Carbon Steel + Low Break Temperature Powder).  Results indicate the importance of steel pouring temperature, mould powder break temperature and also solidification range on the lubrication efficiency and shell formation. Simulations illustrate that Low Carbon Steel + Low Break Temperature Powder delivers the best lubrication efficiency and thickest formed shell. In contrast, High Carbon Steel + High Break Temperature Powder conveys the minimum lubrication efficiency. Therefore, it was concluded that due to absence of proper powder consumption and solidification rate the latter combination is susceptible to production defects such as stickers and breakouts during the casting sequence.

Additive manufacturing (AM) provides unique opportunities to push the boundaries of material properties and free form fabrication. However with this novel manufacturing technique a number of defects not commonly found in conventional processes such as machining or casting can arise. Both experimental and numerical studies can help better understand the printed material on a more fundamental level in order to optimize the process and mitigate these defects. Electron microscopy can provide essential information about the as-built microstructure and characteristic defects while numerical modelling can help determine a correlation between process parameters and the resulting properties. First, an initial investigation of directed energy deposition (DED) processed 4140 steel was conducted using various microscopy methods to better understand the defects and microstructure of the printed alloy. A martensite dominate microstructure within a bainitic matrix with increasing degrees of tempering further down the build was revealed. Additional sample preparation was conducted with a focused ion beam and analyzed with the transmission electron microscope to investigate features such as grain boundaries, mechanical twins and interplanar spacing. This interplanar spacing was measured for a number of different diffraction images and compared with the theoretical values. The deviation between the measured and theoretical values can be attributed to defects such as residual stress which causes lattice strain and consequently a smaller or larger spacing between atomic planes. Lastly, diffraction images were characterized and compared with the literature to determine the Miller indices and the specific zone axis orientations. A thermo-mechanical-metallurgical finite element model for 42CrMo4 steel was then developed in ABAQUS to identify the correlation between processing parameters and resulting properties by predicting the temperature history, and resulting residual stresses and metallurgical phase fractions for the DED process. A pre-processing framework was implemented in order to allow the modelling of complex geometries and laser trajectories while experiments were conducted to validate the fidelity of the model. Four separate cases were fabricated with varying processing parameters and geometries. In addition to in-situ temperature measurements, post-build residual stress and substrate distortion data was also collected. Furthermore, metallurgical analysis was performed for each case and compared with the simulated phase fractions. The accuracy of the distortion profile increased with increasing dwell time while the accuracy in predicting the metallurgical phase fractions and residual stresses demonstrated the opposite trend. / Graduate / 2022-07-05

Geothermal energy consumption is projected to increase along with other renewable energy in the future. Therefore, it is important to have a better understanding on the evolution of geothermal systems to optimize the exploitation of such resources. Generally, numerical models are used as a fundamental tool to study a potential geothermal field. However, current modeling practices tend to focus on the shallow area around the heat source, while ignoring the deeper part below the heat source. The purpose of this project is to observe the influence of lower boundary at the bottom of intrusion towards the evolution of geothermal system, while changing the permeability and topography of host rock systematically, using a software from USGS called HYDROTHERM. Simulations differed in three main aspects: 1) having a layer below, or having the bottom boundary directly below intrusion, 2) different topographies with volcanic significance, and 3) varying permeabilities of the host rock. The study is based on a fossil geothermal system, the Cerro Bayo laccolith in Chachahuén volcanic complex (Neuquén Basin), Argentina. The input parameters were obtained in several ways. ILMAT Geothermometry analysis provide the temperature value related to the intrusion. The whole rock data is used to determined density of the intrusion by calculating partial molar volume of the oxides. The other parameters, e.g. densities of the host rock and the impermeable layer, permeability, porosity, and thermal conductivity were obtained from literature. The result from numerical modeling shows that the bottom boundary below intrusion strongly affect the entire system evolution. The added layer (with constant permeability) has strong influence on the life-span of the system. Additionally, while taking into account on the variation of topography and permeabilities, the models show two temperature anomalies: 1) A caldera volcano’s geometry “traps” heat below the caldera, whereas shield and strato-volcano geometries “push” heat away from below the volcanic edifice, and 2) a low temperature anomaly develops beneath the intrusion in all high permeability models with an added layer. Finally, this assessment could prove to be useful as prior knowledge for optimizing the extraction of heat from a given geothermal field, as well as future investigations towards geological applicability of numerical models of geothermal systems, hydrothermal alteration, and ore formation processes.

The black colour of an asphalt pavement causes it to reach very high temperatures throughout the summer months. Asphalt binder is a temperature dependent material, so these high temperatures can result in damage to the road surface. This report explores the use of light coloured surface coatings to decrease the temperature of an asphalt pavement. A field testing method was developed to compare the effect of several surface materials on temperature. To support this field test, a method was developed to characterize the surface colour (albedo) through the use of a simple light meter. As well, the durability of the surface coatings applied to asphalt pavement surfaces was examined using the Wet Track Abrasion Test, and methods for further testing were suggested. A numerical model was developed in Abaqus to predict the temperature effects based on the surface colour and climate conditions. This model can be used to predict the temperature in an asphalt concrete pavement at the surface and throughout the depth of the pavement. Two versions of this model were created: A complete model, which is used when all climate data is available, and a simplified model, which uses estimated values to replace any data that is not available The temperature difference between white and black painted asphalt concrete surfaces was found to be as much as 17C. Using light coloured surfaces with albedo values in the range of 0.2 to 0.3 yielded a temperature decrease of approximately 7 to 10C as compared to a black painted surface. Microclimate effects were found to be significant; wind speed can drastically affect the temperature of a pavement. The use of hydrated lime in conjunction with a polymer modified asphalt surface course yielded good results for both temperature reduction and durability. It should be considered for future work. / Thesis / Master of Applied Science (MASc)

<p>  </p> <p>Architectured metamaterials are artificial systems with unique structural characteristics. They show distinct deformation behaviors and improved mechanical properties compared to regular materials. For example, mechanical metamaterials demonstrate negative Poisson's ratios, whereas regular materials have positive values. In theory, the auxetic behaviors arise from periodic cellular architectures regardless of the materials utilized. While this premise is mostly true for macroscopic metamaterials, it may not work well at a very small lengthscale since chemistry may play a critical role in nanostructures. However, this fundamental idea has not been addressed due to the lack of powerful manufacturing strategies at the nanoscale. The majority of architectured metamaterials are manufactured from top down with their unit size of microns or larger. On the other hand, there are also molecular auxetics which are natural crystals and thus are not designable. Therefore, there is a significant gap in lengthscale from 10 nm to 1 µm. DNA self-assembly is a bottom-up approach that can construct complex nanostructures based on sequence complementarity. Examples include DNA origami structures and DNA tile assemblies. This dissertation bridges the gap in the lengthscale by introducing nanoscale auxetic units from DNA and investigates relevant structural properties and mechanical behaviors. This study addresses the premise of metamaterials and elucidates the structure-property relation. The findings from this work formulate design principles for DNA based auxetic metastructures. </p> <p>In this work, we built several two-dimensional (2D) auxetic nanostructures from wireframe DNA origami. They serve as the model systems to demonstrate the feasibility of constructing nanoscale auxetics via DNA self-assembly. DNA origami structures are commonly constructed by a long ‘scaffold’ strand with many ‘staple’ oligonucleotides. Since the DNA metastructures are too small to directly apply external forces, we implemented chemical deformation by inserting ‘jack’ edges. Like a car jack, the length of the jack edges can be modulated via two-step DNA reactions: toehold-mediated strand displacement and annealing with a new set of jack staples. The DNA nanostructures reconfigure accordingly. To complement the experiment, we performed molecular dynamics (MD) simulations based on coarse-grained models using an open-source oxDNA platform. In the numerical computation, external loads were directly applied to deform the metastructures, providing details of structural deformation. We discovered that the auxetic behaviors of DNA metamaterials can be estimated by architectural designs, however the material properties are also crucial in the structures and deformations. Our mechanistic study provided general design guidelines for 2D auxetic DNA metamaterials. We also designed and constructed a Hoberman flight ring from DNA, a simplified planar version of Hoberman sphere. This structure consists of six equilateral triangles that are topologically organized into two layers, resembling a trefoil knot. The DNA flight ring deploys upon external forces, expanding (open state) or contracting (closed state) by sliding the two layers of triangles. This is the first synthetic deployable nanostructure and offers a versatile platform for topological research.</p> <p>This thesis also investigates 3D effects in DNA assemblies and related mechanics. We used a DNA origami tile designed with an intrinsic twist as a model system and explored its cyclization process using MD simulations. The numerical computation revealed the detailed process where the structure untwists and curves for cyclization simultaneously under external forces. The force and energy required to overcome the initial curvature and cause the 3D deformation were also calculated. The results agree well with the previous experiment and theory, further verifying the simulation method. Direct mechanical forces and DNA responses were realized experimentally with 3D DNA crystals built from triangular DNA tiles. Nanoindentation was performed on macroscopic ligated crystals using atomic force microscopy (AFM). MD simulations were performed in parallel, which revealed the full spectrum of several distinct deformation modes from linear elasticity to structural failure. The combined experiment, computation, and theoretical calculation showed that the complex behaviors can only be understood fully by considering the structure and its components. </p> <p>The scientific findings from this thesis should contribute to the construction of auxetic metastructures, the design methods for DNA based metamaterials as well as the prediction of their structural properties and mechanical behaviors. This thesis will pave the way for building architectured materials from DNA with tailored properties and functionalities, opening the door for new opportunities and unique applications.</p>

Rock wedge stability is a common concern in underground excavations. Since the wedge stability influences the support design, it is important to use a wedge stability analysis method that can capture factors like excavation geometry, joint parameters and properties, rock mass properties, rock cover, and stress field as accurately as possible. This thesis compared the Limit Equilibrium Method (LEM) and the Discrete Element Method (DEM) using the software UnWedge (LEM) and 3DEC (DEM). The objectives included studying how factors like excavation and joint geometry, stress field, rock cover, and rock mass properties could be considered by the methods and how that affected the wedge stability and support design. Nine different analysis cases were defined with the aim of capturing wedge stability analysis parameters representable for both the civil engineering and mining sector. The study found that 3DEC allows for more accurate modelling of excavation and joint geometry, considering joint density and full 3D geometries, while UnWedge has limitations in creating intersecting tunnels and does not consider joint density. Only 3DEC, with stress redistribution and a plastic material model, captures rock mass failure mechanisms other than wedge failure. Based on the study, it is recommended to use DEM for situations where the excavation geometry cannot be assumed as two-dimensional with constant cross-section, and, when stress-induced rock mass failure is expected, use DEM with stress redistribution. It is also recommended to use information about joint lengths if available and apply engineering judgement when studying results of wedge volume and support force.

No

Yes / Cooling towers are a relatively inexpensive and consistent method of ejecting heat from several industries such as thermal power plants, refineries, and food processing. In this research, an earlier model from literature was to be validated across three different case studies. Unlike previous models, this model considers the height of the fill as the discretised domain, which produces results that give it in a distribution form along the height of the tower. As there are limitations with the software used (gPROMS) where differential equations with respect to independent variables in the numerator and denominator cannot be solved, a derivative of the saturation vapour pressure with respect to the temperature of the air was presented. Results shown were in agreement with the literature and a parametric sensitivity analysis of the cooling tower design and operating parameters were undertaken. In this work the height of fill, mass flowrates of water and air were studied with respect to sensitivity analysis. Results had shown large variations in the outlet temperatures of the water and air if the mass flows of water and air were significantly reduced. However, upon high values of either variable had shown only small gains in the rejection of heat from the water stream. With respect to the height of the fill, at larger heights of the fill, the outlet water temperature had reduced significantly. From a cost perspective, it was found that a change in the water flowrate had incurred the largest cost penalty with a 1% increase in flowrate had increased the average operating cost by 1.2%. In comparison, a change in air flowrate where a 1% increase in flowrate had yielded an average of 0.4% increase in operating cost.

Yes / Natural draft cooling towers are subjected to dynamic air temperature and humidity ratio of air throughout the day. Their performances based on steady state model is restricted to single air temperature and humidity ratio of air values and thus are not accurate. In this work, a dynamic model for a natural draft cooling tower is developed. Air temperature and relative humidity data for Johannesburg were acquired to develop dynamic correlations for these two parameters and are embedded in the process model together with the dynamic model of the collection basin of the tower. A simple algorithm is proposed to solve the resulting boundary value problem. Finally, the common perception that seawater/river water should be used instead of freshwater as cooling medium is tested via optimisation where the operating costs of the process for two cooling mediums is minimised. The freshwater has been found to be cost effective cooling medium.

No