In the past two decades, a growing interest in alternative energy resources as a replacement to the non-renewable resources used now days. These alternatives include geothermal energy which can be used to generate power and reduce the demands on energy used to heat and cool buildings. Ground-coupled ventilation system is one of the many applications of the geothermal energy that have a lot of attention in the early 80's and 90's but all designs of the system where based on single case situations. On the other hand, computational fluid dynamics tools are used to simulate heat and fluid flow in any real life situation. They start to develop rapidly with the fast development of computers and processors. These tools provide a great opportunity to simulate and predict the outcome of most problems with minimum loss and better way to develop new designs. By using these CFD tools in GCV systems designing procedure, energy can be conserved and designs going to be improved. The main objective of this study is to find and develop a CFD modeling strategy for GCV systems. To accomplish this objective, a case study must be selected, a proper CFD tool chosen, modeling and meshing method determined, and finally running simulations and analyzing results. All factors that affect the performance of GCV should be taken under consideration in that process such as soil, backfill, and pipes thermal properties. Multiple methods of simulation were proposed and compared to determine the best modeling approach. / Master of Science

Master of Science / Department of Mechanical and Nuclear Engineering / Steven Eckels and Mohammad Hosni / This research presents results from experimental and numerical investigations of two-phase flow pattern analysis in a staggered tube bundle. Shell-side boiling tube bundles are used in a variety of industries from nuclear power plants to industrial evaporators. Fluid flow patterns in tube bundles affect pressure drop, boiling characteristics, and tube vibration. R-134a was the working fluid in both the experimental and computational fluid dynamics (CFD) analysis for this research. Smooth and enhanced staggered tube bundles were studied experimentally using a 1.167 pitch to diameter ratio. The experimental tube bundles and CFD geometry consist of 20 tubes with five tubes per pass. High speed video was recorded during the experimental bundle boiling. Bundle conditions ranged in mass fluxes from 10-35 kg/m[superscript]2.s and inlet qualities from 0-70% with a fixed heat flux. Classification of the flow patterns from these videos was performed using flow pattern definitions from literature. Examples of smooth and enhanced bundle boiling high speed videos are given through still images. The flow patterns are plotted and compared with an existing flow pattern map. Good agreement was found for the enhanced tube bundle while large discrepancies exist for the smooth tube bundle. The CFD simulations were performed without heat transfer with non-symmetrical boundary conditions at the side walls, simulating rectangular bundles used in this and other research. The two-phase volume of fluid method was used to construct vapor interfaces and measure vapor volume fraction. A probability density function technique was applied to the results to determine flow patterns from the simulations using statistical parameters. Flow patterns were plotted on an adiabatic flow pattern map from literature and excellent agreement is found between the two. The agreement between simulation results and experimental data from literature emphasizes the use of numerical techniques for tube bundle design.

Computational fluid dynamics and heat transfer (CFD/HT) models have been employed as the dominant technique for the design and optimization of both new and existing data centers. Inviscid modeling has shown great speed advantages over the full Navier-Stokes CFD/HT models (over 20 times faster), but is incapable of capturing the physics in the viscous regions of the domain. A coupled inviscid-viscous solution method (CIVSM) for bounded domains has been developed in order to increase both the solution speed and accuracy of CFD/HT models. The methodology consists of an iterative solution technique that divides the full domain into multiple regions consisting of at least one set of viscous, inviscid, and interface regions. The full steady, Reynolds-Averaged Navier-Stokes (RANS) equations with turbulence modeling are used to solve the viscous domain, while the inviscid domain is solved using the Euler equations. By combining the increased speed of the inviscid solver in the inviscid regions, along with the viscous solver’s ability to capture the turbulent flow physics in the viscous regions, a faster and potentially more accurate solution can be obtained for bounded domains that contain inviscid regions which encompass more than half of the domain, such as data centers.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The capability of computational fluid dynamics (CFD), in particular the FLUENT ™ commercial software suite, to predict wind loadings on heliostats has been investigated. If CFD proves useful in this area then the overall development costs of heliostats and concentrating solar thermal power plants could be reduced. Due to the largest loading on the heliostat originating from wind loads, by using CFD to determine these loads it could be possible to ensure heliostats are not overdesigned. This thesis contains a first study within the Solar Thermal Energy Research Group (STERG) at Stellenbosch University into the use of CFD for determining heliostat wind loads. The relevant theoretical background concerning the turbulence models used in this study, namely, the RNG k-ε, Realisable k-ε and SST k-ω turbulence models is reiterated. The „standard‟ k-ε model and the large eddy simulation (LES) approach, due to their relevance to bluff body flows, are also revisited. Some analysis is also provided around each model to gain insight as to the role of respective modelling sensitivities and their advantages. Previous work done in the area of heliostat wind studies is reviewed. The geometric considerations when dealing with heliostats leads onto the discussion concerning the requirement of modelling boundary layer profiles. Hence some background is provided on boundary layer modelling techniques. Further insight is drawn from more general previous bluff body CFD reported in the literature, from which observations and recommendations regarding the use of variations of the k-ε turbulence model can be inferred. The simulation procedure from geometry creation to results obtained for the flow over a vertical flat plate is reported. This investigation led to the conclusion that the Realisable k-ε should be used for the heliostat simulations on account of its accurate drag prediction under steady state flow conditions. It was also found that for transient simulations for heliostat like geometries, the SST k-ω model appears most suitable. The Realisable k-ε model is then used to model the flow about a heliostat using the same procedures as for the flat plate; both with flat and boundary layer inlet profiles. The overall conclusions drawn from this work are that the Realisable k-ε would not be suitable for predicting wind loads used in the final design of heliostats although it may be used with flat velocity and turbulence profiles to compare differences between early heliostat designs. The conclusion that the Realisable k-ε model should not be used to predict the flow field in the vicinity of a heliostat is also reached. It is recommended that further work should be carried out by using more advanced modelling techniques, such as the LES, to determine wind loads on heliostats. Furthermore, additional studies focused on accurately reproducing the velocity and turbulence profiles should be done. Lastly a larger set of data containing the orientations mentioned in literature should be generated using the methods contained within this study. / AFRIKAANSE OPSOMMING: Die vermoë van Numeriese Vloei Meganika (NVM), spesifiek die van die FLUENT ™ kommersiële sagtewarepakket, om die windlaste op heliostate te voorspel was ondersoek. As daar gevind word dat NVM wel betekinsvolle resultate kan lewer, kan dit die totale ontwikkelingskoste van heliostate en gekonsentreerdesonkragstasies verlaag. Wind plaas die grootste las op heliostate, dus deur gebruik te maak van NVM om die windlaste op heliostate te voorspel, kan dit gebruik word om te verseker dat heliostate nie oorontwerp word nie. Hierdie tesis bevat „n eerste studie binne die Sontermiese Energie Navorsings Groep aan die Universiteit van Stellenbosch, wat die gebruik van NVM om windlaste op heliostate te voorspel ondersoek. Alle relevante teoretiese agtergrond wat turbulensiemodelle aanbetref, naamlik die RNG k-ε, Realiseerbare k-ε en SST k-ω turbulensiemodelle, word bespreek. Hulle relevansie tot stompligaamvloei toegestaan, word die „standaard‟ k-ε model en die groot werwel simulasie (GWS) benaderings ook bespreek. Elke model word bespreek om die leser insig te gee in dié model se sensitiwiteite en voordele. Vorige studies wat betrekking het tot die studie van heliostate en wind word bespreek. Die geometrie van heliostate lei tot „n bespreking oor die noodsaklikheid vir „n model vir die grenslaagprofiel, dus word grenslaagmodelleringstegnieke bespreek. Verdere insig word verkry van vorige NVM studies uit die literatuur met meer algemene stomp liggame, wat waarnemings en voorstelle vir die gebruik van die k-ε turbulensiemodel en variante verskaf. Die simulasieproses, vanaf geometrieskepping tot die resultate vir die vloei oor 'n vertikale vlak, word bespreek. Hierdie ondersoek het tot die gevolgtrekking gelei dat die realiseerbare k-ε model gebruik moet word vir die heliostaat simulasies, as gevolg van die akkurate sleurvoorspellings onder bestendigetoestande. Daar was ook gevind dat vir heliostaatagtige liggame onder oorgangskondisies, die SST k-ω model mees geskik sal wees. Die Realiseerbare k-ε model word dan gebruik om die vloei om 'n heliostaat te modelleer deur gebruik te maak van dieselfde proses wat gebruik word om vloei oor 'n plat plaat te analiseer: albei met plat en grenslaaginlaatprofiele. Die gevolgtrekkings van hierdie studie is dat die Realiseerbare k-ε model nie gebruik kan word tydens die finale ontwerpfase om die windlaste op 'n heliostaat te voorspel nie. Dit kan wel gebruik word met plat snelheids- en turbulensieprofile om die versikille tussen vroeë heliostaatkonsepte te vergelyk. Daar was ook bepaal dat die Realiseerbare k-ε model nie gebruik moet word om die vloeiveld om 'n heliostaat te voorspel nie. Daar word voorgestel dat verdere studies in hierdie vakgebied met meer gevorderde modelleringstegnieke aangepak word. Dit word aanbeveel dat verdere werk uitgevoer moet word deur die gebruik van meer gevorderde modellering tegnieke, soos GWS, om die wind kragte op heliostats te bepaal. Verder, studies wat akkurate snelheid en turbulensieprofiele produseer sal nog bygelas moet word. Laastens 'n groter stel data met oriëntasies soos wat in die literatuur beskryf word, moet deur middel van die metodes van dié studie gegenereer word.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This thesis investigates the flow field and pressure distributions around cylinders at trans-critical Reynolds numbers using the k-ε Realizable turbulence model. A steady state 2-D and 3-D Fluent® model is successfully developed to evaluate the effects of changing various modelling parameters on the static pressure distribution around an infinite and finite cylinder. These parameters include surface roughness, cylinder rotation and air viscosity at the cylinder surface. The subsequent results obtained are compared to each other and to data trends from literature as well as measured experimental results and are found to be in good agreement. In addition a method for calibrating all developed methods based on their shear stress curves over a flat plate model is also successfully developed. The main objective is to find an appropriate single parameter which can be used for the rigorous adjustment of the pressure distribution around a cooling tower, which will allow for improved sensitivity analysis and modelling of cooling tower performance under wind conditions with and without meridional ribs located on the outer shell surface. / AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek die vloeiveld en druk verdelings rondom silinders by trans-kritiese Reynolds getalle deur gebruik te maak van die k-ε Realizable turbulensie model. ‘n Bestendige toestand 2-D en 3-D Fluent® model is suksesvol ontwikkel om die uitwerking van die verandering van verskeie model parameters op die statiese druk verdeling rondom ‘n oneindige en eindige silinder te evalueer. Die laasgenoemde parameters sluit in oppervlak grofheid, silinder rotasie en lug viskositeit by die silinder wand. Die daaropeenvolgende resultate wat verkry word, word met data tendense uit die literatuur asook gemete data vanuit eksperimente vergelyk en goeie ooreenkoms i.t.v die data tendense is gevind. Verder is ‘n metode vir die suksesvolle kalibrasie van die ontwikkelde numeriese tegnieke ontwikkel. Die laasgenoemde kalibrasie metode is gebaseer op die vergelyking van skuifspanning kurwes vir vloei oor ‘n plat plaat model. Die hoofdoel van die navorsing is om ‘n geskikte enkele parameter te vind wat gebruik kan word vir die effektiewe aanpassing van die druk verdeling rondom ‘n koeltoring wat sal lei tot verbeterde sensitiwiteits analise en modellering van koeltoring verrigting onder wind toestande met en sonder meridionale ribbes geleë op die buitenste dop oppervlak.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Modern Computational Fluid Dynamics modelling (CFD) are becoming common design and analysis tools in the engineering field. Nowadays, project designs involve the use of CFD techniques along with physical scale modelling to analyse the complex rapidly varied and turbulent flows which would not be easily analysed by physical modelling. In particular, the consideration and/or use of CFD modelling in the Hydraulic Engineering field remains on the increase. Apart from being used for comparison with other design techniques, CFD may in future become a standalone modelling technique in hydraulic structures design. This research aims to use CFD models to validate the simulation of the flow over two ogee dam spillways which are installed in the Hydraulic Laboratory of Stellenbosch University. To achieve this simulation of the flow which involves an interaction between water and air, the flow behaviour has been mapped by the Volume of Fluid (VOF) and the realisable "𝑘−𝜀" turbulence numerical models. The Volume of Fluid (VOF) and the realisable "𝑘−𝜀" models simulate the free surface of two-phase flow and the flow turbulence, respectively. Firstly, the study embarks with details on the actual design approaches of a typical ogee dam spillway. It subsequently presents the geometry and dimensions of the physical models, the testing procedure and the experimental test results achieved from this modelling exercise. For CFD modelling, a commercially available Computational Fluid Dynamics (CFD) package, Ansys-Fluent, was used. To model the physical model, the use of Reynolds-averaged Navier-Stokes equations in combination with the realisable k-ε eddy-viscosity closure model was adopted. The process of CFD model development and the underlying theory of it are discussed in this thesis. Different test scenarios including steady and fully hydrodynamic states simulation for two and three-dimensional geometries were considered in this simulation to achieve the most accurate results. In order to determine the required mesh size, the mesh sensitivity tests were conducted on the 2 dimensional and 3 dimensional models. Finally, the pressure readings and water levels produced by numerical models are discussed through a validation process by comparing the CFD model results with the results obtained from physical models. The outcome proved that CFD models are able to map the behaviour of both flow phases since they exhibited a close correlation to those achieved in the physical models. Even though some slight differences in values were revealed, the graphical trend remains reasonably similar for all test results. / AFRIKAANSE OPSOMMING: Moderne gerekenariseerde vloeidinamika numeriese modelle (CFD) word deesdae dikwels deur ingenieurs gebruik. Projekontwerpe sluit tans die gebruik van CFD tegnieke asook fisiese skaalmodellering in om komplekse, vinnig-veranderede en turbulente vloei te ontleed. Hierdie tipe vloeie is moeilik om met fisiese modellering te ontleed. Die gebruik van CFD numeriese modelle in hidrouliese ingenieurswese is besig om toe te neem, Die bevindinge van CFD modelering word tans vergelyk met die bevindinge van ander ontwerptegnieke, maar in die toekoms mag dit moontlik gebruik word as die enigste modelleringstegniek in hidrouliese struktuurontwerp. Die doel met hierdie navorsing is om CFD modelering te gebruik om die vloei oor twee ogee-vormige afvoergeute wat in die hidrouliese labrotorium van die Universiteit van Stellenbosch ge-installeer is, te ondersoek. Ten einde hierdie vloei, wat die interaksie tussen water en lug insluit, te simuleer, is die vloeigedrag deur ”volume van vloeistof” (VOF) en die "𝑘−𝜀" turbulensie numeriese modules, gemodeleer. Die VOF en "𝑘−𝜀" numeriese modules simuleer onderskeidelik die vry oppervlakte vloei van die twee-fase vloei en turbulente vloei. Die ontwerp van ’n tipiese ”ogee”-tipe dam oorloop word bespreek, gevolg deur die beskrywing van die geometrie van die fisiese modelle, die toetsprosedure en die eksperimentele toetsresultate. Vir die CFD modellering is die CFD pakket, Ansys-Fluent, gebruik. Vir die simulering van die fisiese model is die Reynolds-gemiddeld Navier-Stokes vergelykings tesame met die k-ε eddy-viskositeit geslote module gebruik. Die proses van CFD ontwikkeling en die onderliggende teorie daarvan word bespreek. Verskillende toets-scenario’s wat 2D en 3D simulasies insluit, uitgevoer. Ten einde die toepaslike berekeningsrooster grootte vir die numeriese model te verkry, is sensitiewiteitstoetse uitgevoer op die twee- en drie-dimensionele numeriese modelle. Laastens is die CFD numeries gesimuleerde drukke en die watervlakke met die van die fisiese modelle vergelyk om die akkuraatheid van die CFD resultate te verkry. Die uitkomstes het getoon dat CFD modelle gebruik kan word om die gedrag van albei vloei fases te simuleer aangesien dit goed vergelyk het met die uitkomstes van die fisiese modellering. Daar was wel klein verskille in die druk waardes, maar die tendense in drukverspreiding was ooreenstemmend.

McDonalds Lime Ltd, situated in Otorohanga, New Zealand, operate two dry process rotary lime kilns producing burnt and hydrated lime for a range of industries including agriculture, roading, water treatment, gold mining and steel making. The following Technology in Industry Fellowship (TIF) funded Masters Project is structured around investigating the combustion characteristics of Kiln Two at McDonald’s Lime Ltd using Computational Fluid Dynamics (CFD). Numerical results obtained using the commercial CFD code FLUENT were first validated against experimental data from the International Flame Research Foundation’s (IFRF) Furnace No.1. The validation study focussed on comparing the finite rate and mixture fraction/PDF approaches to combustion chemistry, as well as different methods for defining coal particle size distributions. Numerical modelling of Kiln Two at McDonald’s Lime Ltd began with full three-dimensional simulations, however due to their complexity and large computational times, two-dimensional axisymmetric models were primarily used for investigations. Comparisons were made between the two approaches. Investigations into the original pulverised coal fired system focussed on how the kiln aerodynamics and heat transfer properties were affected by changes to the coal and air inlet properties. The performance of a recently installed waste oil firing system was also investigated, with results showing that firing the kiln with a 25% thermal substitution of oil is the most efficient mode of operation. As the investigations focussed on the combustion characteristics the effects of the reacting limestone bed were ignored in all simulations. CFD modelling of the combustion characteristics within a large scale rotary kiln proved to be an extremely complex task. The work presented in this thesis has however provided some promising results which will ultimately assist McDonalds Lime Ltd in reducing their operating costs and environmental impact. Futhermore, the project has laid the foundation for further investigations.

This thesis investigates the application of CFD techniques to the aerodynamic analysis of a U-shaped JETC. Investigations were carried out to determine the flow patterns present at a number of locations within the structure of a full U-shaped JETC. The CFD solutions produced in these investigations used recommendations from the literature in the set-up of the CFD solver, and provided the computational component towards problem-specific validation of the CFD techniques used. A structured series of CFD-aided investigation and design processes were then performed. These processes were based around a series of analyses that evaluated the influence of a number of cell parameters in terms of cell airflow efficiency and velocity distortion. Four cell components; the inlet and exhaust stack baffle arrangements, the turning-vanes, the rear of the working section and augmenter entrance, and the lower exhaust stack, including the BB, were investigated in individual analyses. Throughout the investigations the value of CFD as a design tool was constantly assessed. Overall, the findings suggest that aerodynamic optimisation of the baffle arrangements would provide the greatest gains to cell airflow efficiency. As some cells contain as many as three baffle arrangements, the potential increases made to cell airflow capacity are sizable. Through implementing the findings of the baffle arrangement investigations, static pressure loss across the five-row baseline arrangement was reduced by 79%. For low levels of velocity distortion in the upstream region of the working section, the need to design the inlet stack baffles in the turning-vane arrangement was highlighted. Mid-baffle vane alignment, consistent flow channels, and sufficiently low chord to gap ratios should be incorporated into a turning-vane design to maximise flow uniformity. The need for the baffle and vane components to combine with the geometry of the cell to limit adverse pressure gradients was found as a requirement to minimise inner corner separation, and the downstream threat it creates to a safe testing environment. CFD proved to be a valuable analysis tool throughout the investigations performed in this thesis. The number of design iterations analysed, and the detail of data that could be extracted, significantly exceeded what could have been achieved through an isolated experimental testing programme.

This thesis details a 1-d model of a lymphatic vessel, developed from a model by Reddy. Some additions to the modelling techniques were found to be necessary to prevent numerical phenomena not found in experiment. Furthermore the details of the wall and valve were important to the mechanics of the system. This developed model presents flow characteristics which are not represented in the existing lumped parameter or 1-d models of the lymphatic system. Additional terms allow more realistic representation of some modes of flow such as those occurring during collapse. The model was validated using Poiseuille flow calculations and experimental work. Features found in experiment were reproduced in the model. Such as the shark tooth shape of the radius time graph. A study of the sensitivity of the model to experimental parameters was performed. Features that increased flow included: increased compliance of the vessel, a larger diameter, amplitude of contraction or frequency, or a faster contraction wave. A lumped parameter model, relating the radius directly to the pressure, was investigated but this did not reproduce flow features such as the shark tooth shaped radius with time relationship or the radius peak at the beginning of a contraction or passive relaxation of the vessel. In the 1-d model the time constant of this passive relaxation increased with the magnitude of contraction. This value may have physiological relevance.

Many developed nations depend on oil for the production of gasoline, diesel, and natural gas. Meanwhile, oil shortages progress and bottlenecks in oil productions continue to materialize. These and other factors result in an energy crisis, which cause detrimental social and economic effects. Because of the impending energy crisis, various potential energy sources have developed including solar, wind, hydroelectric, nuclear, and biomass. Within the biomass sector for renewable energy sources, algae-based biofuels have become one of the most exciting, new feedstocks. Of the potential plant biofuel feedstocks, microalgae is attractive in comparison to other crops because it is versatile and doesn’t pose a threat to food sources. Despite its many advantages, the process to convert the microalgae into a biofuel is very complex and inefficient. All steps within the algae to biofuel production line must be optimized for microalgal biofuel to be sustainable. The production of biofuels from algae begins with selecting and cultivating an algae strain and giving it all the necessities to grow. The algae is then harvested and processed for specific uses. It is the harvesting or lysing step, which includes the extraction of the algal lipids, which is the biggest hindrance of algae being used as a cost effective energy source. The lysing step within the microalgal biofuel processing is of particular interest and will be the focus of this work. This work discusses the optimization of the biofuel production from microalgae biomass through computational and experimental approaches. With atomic force microscopy (AFM), a key mechanical property that would aid in the computational modeling of mechanical lysis in the in-house computational fluid dynamics (CFD) code, Particle-Surface Analysis Code (P-STAC), was determined. In P-STAC, various flow patterns were modeled that would most effectively lyse microalgal cells based on the shear stresses placed on the cells, which will be compared against microfluidic experiments using lipid specific dyes. These results would be influential in developing an energy-efficient method of processing microalgae for biofuel.