Doctoral Degrees (Mechanical and Mechatronic Engineering)

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Adaptive digital image correlation using neural networks
(Stellenbosch : Stellenbosch University, 2023-03) Atkinson, Devan James; Becker, Thorsten Hermann; Neaves, Melody; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: Subset size selection is crucial to the accuracy and precision of digital image correlation (DIC) measured displacements. Increasing the subset size improves noise suppression (reducing random errors) at the cost of spatial resolution (ability to accurately measure complex displacement fields). The tradition of global correlation parameter assignment is suboptimal because the speckle pattern quality and displacement field complexity can vary spatially. Dynamic subset selection (DSS), which assigns location specific optimal subset sizes, is challenging because the metrological performance of correlation is dictated by complex interactions between correlation parameters (subset size and shape function) and image set properties (noise, speckle pattern and displacement field complexity). This dissertation uses an open-source DIC framework to investigate the potential of artificial neural networks (ANNs) for error prediction and DSS, prior to the DIC process, from purely image information. ANNs are capable of modelling complex relationships within noisy, incomplete data without imposing fixed relationships, inspiring their recent resurgence for DIC applications. Despite the plethora of open-source DIC algorithms available, none offer spatially and temporally independent assignment of correlation parameters. Subsequently, a modular, open-source DIC framework capable of such flexibility is developed. This framework is predominantly consistent with current state-of-the-art practices and performs on par with well-established open-source and commercial DIC algorithms. Drawing direct links between the well-documented theory of DIC and its nuanced practical implementation, bridges this gap in literature which has acted as a barrier to newcomers intending to develop the capabilities of DIC. This framework, implemented in 117 and 202 lines of MATLAB code for 2D and stereo DIC, respectively, is attractive as a starting point to further the capabilities of DIC. The feed-forward ANN developed using this DIC framework, predicts random errors based on the speckle pattern quality (contained within a subset) and standard deviation of image noise more accurately and precisely than established theoretical derivations. A DSS framework is developed which uses this ANN to appoint subset sizes, based on the local speckle pattern, that offer random errors consistent with a stipulated threshold value. Appropriate selection of the random error threshold offers a favourable compromise between noise suppression and spatial resolution for up to moderate displacement gradients. Consequently, in the presence of varying speckle pattern quality this framework outperforms the traditional approach of trialand- error global subset size selection for the same mean subset size. Speckle pattern characteristics outside the training scope reveal the generalisability limitations of the DSS method, and associated ANN, as it performs on par with the traditional global subset size approach, motivating the need to broaden its training scope. Investigation of convolutional neural networks for dynamic shape function selection is initiated, showing they are capable of quantifying displacement field complexity between image pairs to guide spatially and temporally independent shape function assignment. The dissertation reveals that ANNs are an attractive approach to model the correlation parameter assignment. Furthermore, such models facilitate dynamic correlation parameter assignment from purely image information such that they can operate as a pre-process to DIC.
Aerodinamiese eienskappe van lugverkoelde warmte-uitruilers
(Stellenbosch : Stellenbosch University, 1996) Duvenhage, Kobus; Kroger, D. G.; Du Toit, C. G.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The aim of this dissertation is to contribute in solving the problem, due to a lack of knowledge and information, concerning the aerodynamic features of a typical air-cooled heat exchanger (ACHE), numerically and experimentally. Only forced draught ACHE's are considered. The project consists mainly of numerical experimentation, using the general purpose code, PHEONICS.
Air-cooled condenser steam flow distribution and related dephlegmator design considerations
(Stellenbosch : Stellenbosch University, 2013-12) Owen, Michael Trevor Foxwell; Kroger, D. G.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The steam-side side operation of a practical air-cooled steam condenser is investigated using a combination of CFD, numerical, analytical and experimental methods. Particular attention is directed towards the vapor flow distribution in the primary condensers and dephlegmator performance. Analysis of the vapor flow in the distributing manifold, connecting the steam turbine exhaust to the air-cooled heat exchangers, highlights the importance of careful design of the guide vanes in the manifold bends and junctions. Improved guide vane design and configuration can reduce the steam-side pressure drop over the manifold and improve the vapor flow distribution, which may be beneficial to condenser operation. The vapor flow in the primary condensers is shown to exhibit a non-uniform distribution amongst the heat exchanger tubes. The vapor flow distribution is strongly linked to the distribution of tube inlet loss coefficients through the heat exchanger bundles. The non-uniform flow distribution places an additional demand on dephlegmator performance, over and above the demands of row effects in the case of multi-row primary condenser bundles. Row effects are shown to account for as much as 70 % of available dephlegmator capacity in this case. Simultaneously, inlet loss coefficient distributions can account for up to 30 % of dephlegmator capacity. In some situations then, the dephlegmator is fully utilized under ideal operating conditions and there is no margin of safety to cope with non-ideal operation of the primary condensers. The upstream regions of the primary condensers are therefore exposed to a high risk of undesirable noncondensable gas accumulation. Reduced dephlegmator capacity due to insufficient ejector performance may further compound this problem. Single-row primary condenser bundles eliminate row effects and thereby significantly reduce the demands on dephlegmator performance. The use of such bundles in the dephlegmator would also measurably reduce ejector loading. In light of the findings of this study, it is recommended that single-row bundles be considered as the primary option for future air-cooled condenser applications. A hybrid (dry/wet) dephlegmator concept is analysed and shown to be able to provide measurably enhanced dephlegmator performance when operating in wet mode, while consuming only a small amount of water. The enhanced dephlegmator cooling translates to an increase in total air-cooled condenser capacity of up to 30 % at high ambient temperatures in this case. The benefit of this enhanced cooling capacity to steam turbine output may be significant. The hybrid dephlegmator concept therefore offers a simple, cost-effective and sustainable solution to the issue of reduced air-cooled condenser performance during hot periods. Careful design of the first and second stage bundle configurations in the hybrid dephlegmator is necessary to avoid flooding in the first stage during wet operation of the second. Furthermore, the slightly poorer dry-operation performance of the hybrid dephlegmator results in increased risk of non-condensable gas accumulation in multi-row primary condensers. Again, single-row primary condenser bundles would lay rest to such concerns.
Air-cooled heat exchangers and cooling towers : thermal-flow performance evaluation and design
(Stellenbosch : Stellenbosch University, 2004-12) Kroger, Detlev G.; Von Backstrom, T. W.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: During the last 30 years I have been involved in the theory and practice of thermal engineering and in particular, in the areas of air-cooled heat exchangers and cooling towers for the power, refrigeration, process and petrochemical industries in South Africa and internationally. During this period, I have authored and co-authored more than 120 papers that were published in technical journals or presented at conferences nationally or internationally. Most of these papers are included in a manuscript entitled "Air-cooled Heat Exchangers and Cooling Towers", in which Ipresent a systematic approach to the thermal performance evaluation and design of industrial air-cooled heat exchangers and cooling towers. This original publication also includes the relevant practice applicable to the design of cooling systems, based on my experience as a consultant to industry. Design offices throughout the world presently follow our design methods, or at least employ many of our research results. Our work has furthermore contributed to the development of improved cooling system designs (e.g. new dephlegmator header designs), components (e.g. single-row flattened finned tubes) and product improvement and quality control (e.g. performance testing and measurement of thermal contact resistance between fin and tube during production). Many of our research findings have found application in the modification of existing cooling systems. The manuscript has also been used as reference work during the presentation of short courses to practising engineers and consultants in industry and to engineering graduates at the University of Stellenbosch. A two-volume edition of this manuscript was published by PennWell Corp., Tulsa, Oklahoma, USA in 2004.
Anti-Malarial Polymer-Peptide Conjugates
(Stellenbosch : Stellenbosch University, 2014-12) Reader, Paul William; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The primary aim of this study was to investigate an amphiphilic polyvinylpyrrolidone (PVP)-based drug delivery system for the treatment of the Plasmodium falciparum strain of malaria, using a known anti-malarial cylic decapeptide, tyrothricin. A triazole-based reversible addition-fragmentation chain-transfer (RAFT) agent, comprising an acetal-based R-group and a xanthate-based Z-group was synthesised. α-Acetal, ω-xanthate heterotelechelic PVP was synthesised via RAFT polymerisation and it was shown that the polymerisation was adequately controlled. A one-pot orthogonal chain-end functionality deprotection strategy was developed and conditions were established to conjugate a model targeting ligand and a model drug linker to the α- and ω-chain-ends, respectively. A micellar drug delivery system was developed by conjugating the aggregation-prone tyrothricin to PVP, via its ω-chain-end functionality through an acid-labile linker. The α-chain-end functionality of the PVP was sparsely conjugated to a targeting ligand and to a fluorescent marker. In aqueous media, the conjugate exhibited self-assembly into micelles. The tyrothricin formed the core of the micelle, stabilised via the hydrophilic PVP, decorated with the targeting ligands and fluorescent marker. The conjugate was shown to inhibit chloroquine-resistant P. falciparum strains of malaria in picomolar concentrations with virtually no haemolysis observed; a 700-fold improvement of the IC50 over tyrothricin alone was observed. In addition, the conjugates were found to vaccinate the erythrocytes against re-infection. The drug delivery system appears to be a promising candidate for further investigation as a treatment against drug-resistant strains of malaria.
Application of a perforated shadow band to the decomposition of global solar irradiance
(Stellenbosch : Stellenbosch University, 2015-12) Brooks, Michael John; Von Backstrom, Theodor W.; Van Dyk, E. Ernst; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The earth’s atmosphere causes pronounced spatial and temporal variability in downwelling solar radiation at the planet’s surface. Since the characterisation of sun strength is important in solar resource assessment studies, and in the Earth sciences generally, more effective methods are sought to measure irradiance at ground stations. The general drive is towards greater spatial coverage, reduced instrument uncertainty, lower costs and higher temporal data resolution. This study investigates a new method of measuring the principle components of solar irradiance at 1-minute intervals using a single pyranometer and a novel shading structure. The perforated shadow band decomposes global horizontal irradiance (GHI) to obtain the diffuse horizontal and direct normal irradiance components (DHI and DNI). The design of the band and its positioning relative to the thermopile sensor of a radiometer are described. A ray trace-derived model of pyranometer exposure is presented as a function of the local hour angle. In operation, the band produces a composite output trace incorporating both global and diffuse fragments that require separation and reconstitution as independent time-series. DNI values can then be calculated from these components. Gaps between data fragments must be filled using appropriate interpolation techniques to lower statistical uncertainty. The structure of the trace is dependent on atmospheric turbidity and the nature of the prevailing cloud field. A test programme was run at the US National Renewable Energy Laboratory in Colorado to establish performance of the system relative to collocated reference instruments. The band functioned most effectively under clear sky conditions, where it produced GHI, DHI and DNI measurements with root mean square differences of 2.7%, 13.6% and 2.0% respectively. Mean bias differences were 0.1% for GHI, 7.9% for DHI and –0.3% for DNI. The presence of cloud introduces stochasticity to the perforated band output trace. In such a case the ray trace model of pyranometer exposure can be used to identify and separate GHI and DHI data. Uncertainties rise for GHI and DNI under partly cloudy conditions. As the inaugural study on perforated band performance, this work tested several approaches to filling measurement gaps, including numerical interpolation and data replacement by radiometric decomposition models. A key finding of the study is that uncertainties may be lowered by interpolating adaptively according to the prevailing clearness index. Tests run at a southern hemisphere ground station suggest that the system’s performance is not location-dependent. It may be concluded that the perforated shadow band system is most effective in sunny regions where the average daily clearness index remains above approximately 0.7. This would include large parts of continental Africa in the south-western and northern desert areas. The best potential for deploying the band is in existing sub-optimal measurement schemes utilising a single pyranometer, where it would enable the direct measurement of two radiometric components rather than one.
An architecture for the digital twin of a manufacturing cell
(Stellenbosch : Stellenbosch University., 2020-03) Redelinghuys, Anro Johannes Hermanus; Kruger, Karel; Basson, A. H.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The ongoing development of modern manufacturing technology contributes to the rise of the fourth industrial revolution, or Industry 4.0. The digital twin is considered to be key for interaction between the virtual and physical worlds. An important step towards the success of Industry 4.0 is the establishment of practical reference architectures. The dissertation presents the development, implementation and evaluation of the Six-Layer Architecture for Digital Twins with Aggregation (SLADTA). The development starts with the SLADT (excluding aggregation) for a single manufacturing system element, with an industry related case study. The SLADT provides the communication between the physical and digital twin, as well as between the digital twin and the outside world. The architecture is aimed at situations where the products of various vendors are used in the physical and digital twins, and for developing digital twins for newly designed and legacy manufacturing systems. Layers 1 and 2 of the SLADT form part of the smart connection level or physical twin. An Open Platform Communications Unified Architecture (OPC UA) server in Layer 3 provides a vendor-neutral communication interface between the physical twin and the other layers. The data-to-information conversion level, or IoT Gateway, is added as Layer 4 to add context to the data received from Layer 3 before passing the information to Layer 5. When information flows from higher levels to the physical twin, Layer 4 also converts the information to data that can be used by the physical twin. Layers 5 and 6 are the cognition level of the architecture. Layer 5 consists of cloud services that host historical information received from Layer 4. Layer 6 consists of simulation and emulation tools. This dissertation also extends the SLADT, by also providing for Aggregation (SLADTA) and evaluates it for a laboratory scale manufacturing cell that consists of a variety of physical twins. A hierarchical approach is considered for aggregating information from lower- to higher-level digital twins. This approach can also be considered as a digital twin of twins that reduces complexity by breaking a larger digital twin into smaller digital twins of encapsulated functionality. The OPC UA server (Layer 3) supports and simplifies the secure information flow between digital twins, while the IoT Gateway (Layer 4) supervises the information flow. The evaluation of the SLADTA considered its ability to acquire the physical twin state (Layers 1, 2, 3 and 4), maintain an information repository (Layer 5), and simulate and emulate operation (Layer 6). The evaluation further considered the data and information flow, configuration, and decision-making capabilities. Latencies between the OPC UA server (Layer 3) and the IoT Gateway (Layer 4) were identified during the SLADT case study evaluation and had a significant impact on the real-time communication. The latency considerations, between Layers 3 and 4, are evaluated in this dissertation. This dissertation concludes that the SLADTA provides a functional mechanism to implement digital twins. The layers in the SLADTA are not platform dependent and thus allow flexibility for integration into newly designed and legacy manufacturing systems.
An Architecture for the Integration of Human Workers into an Industry 4.0 Manufacturing Environment.
(Stellenbosch : Stellenbosch University, 2021-04) Sparrow, DE; Kruger, K; Basson, AH; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: With the rise of Industry 4.0 and the development in technologies that contribute to this revolution in manufacturing, research has focused mainly on the machines and automated digital systems contributing to the manufacturing environment. Humans are still critical to manufacturing; offering unmatched ingenuity, robustness, and flexibility despite their apparent disadvantages in strength or precision. Many successful manufacturing firms still include humans in their manufacturing processes for these reasons, and it is critical that the integration of humans in an I4.0 manufacturing environment is given research attention. This dissertation first explores the requirements for the integration of human workers into an I4.0 environment. It was determined that the largest problem with human integration exists with data related to the human being digitised, managed, and communicated with other entities in processes that are identified as Administrative Logistics. It is identified that an administration shell similar to the RAMI4.0 administration shell concept is required to manage these Administrative Logistics on behalf of the human, and that a holonic systems approach is beneficial. The dissertation then proposes the concept of a Human Resource Holon Administration shell (HRH-AS). An architecture to implement such an administration shell is then developed, here named the BASE architecture. This administration shell facilitates the interfacing, data processing, and connectivity to other I4.0 components on behalf of the human, to aid in their integration to the digital factory environment around them. The BASE architecture addresses three identified responsibilities of such an administration shell, namely interfacing, digital data management, and delegation to other I4.0 components. BASE stands for Biography, Attributes, Schedule, and Execution, and is a time-based separation of concerns for key augmentations provided to the human worker. The BASE architecture separates value-adding and decision-making plugin components, which are specific to an application, from the core components, which are generic to any application. The BASE architecture led to the development of the 3SAL activity structure to facilitate the communication and management of industrial activities in a digital environment. With the help of an industry partner, two case studies were developed to evaluate an implementation of the BASE architecture. The company is an aerospace composites manufacturer and was chosen for the labour-intensive requirements of the composites industry. The case studies aimed to evaluate the architecture against the three identified administration shell responsibilities and determine if the human workers are elevated to resource holon status. The first case study aimed to show how BASE facilitates interfacing with humans in an I4.0 environment and also acted as a technology demonstrator for the second case study. The second case study evaluated the effect BASE had on the Administrative Logistics involved in the business processes workers were involved in. Together these case studies fully evaluate BASE’s ability to facilitate the integration of humans into an I4.0 manufacturing environment through identified responsibilities of the administration shell. The evaluation found that the BASE HRH-AS improves the effectiveness of Administrative Logistics of business processes the human workers were involved with, as well as opened new opportunities for decision making on the shop floor previously not possible. Value-adding, by means of the plug-in components of a BASE administration shell, has also been proven by the ability to do automated schedule management, automatic calculation of standard work and improved traceability using the 3SAL activity structure.
An artificial intelligence approach for biomass devolatilisation in an industrial CFD model with advanced turbulence-chemistry interaction
(Stellenbosch : Stellenbosch University, 2018-03) Du Toit, Philip C.; Meyer, Chris J.; Laubscher, Ryno; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: The ground work to include more detailed chemistry than global approaches in a combustion simulation was completed. A reduced-order model of the Biomass Chemical Percolation Devolatilisation model, ANN-Bio-CPD, was developed and implemented with artifcial neural networks in order to achieve ease of execution and computational cost reduction with regard to an industrial computational fluids dynamics application. ANN-Bio-CPD was validated with wire-mesh reactor and drop-tube furnace experiments from literature. Subsequently, the Eddy Dissipation Concept (EDC) turbulence-chemistry interaction model was implemented and validated with ANN-Bio-CPD in a bagasse- fired boiler simulation. The EDC model constants were adjusted to achieve the correct temperature and intermediate species results in combination with a two-step global reaction mechanism.
Aspects of distributed conceptual design support
(Stellenbosch : Stellenbosch University, 2002-04) Schueller, Andreas; Basson, A. H.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The findings of an investigation into the requirements of a support system for distributed conceptual design in small and medium enterprises are presented. Distributed conceptual design refers to the collaboration of spacially distributed design teams during the early stages of the product development process. Three main elements of a support system were identified. The first element, 'Design Methodology', places a framework for specification development, functional analysis, concept generation and concept evaluation at the designers' disposal. It systematically guides the users through the different steps of the design process. These steps can be performed either alone, or together with other team members. The users are also aided in documenting their steps in order to allow the team members or designers of followup projects to understand the decisions made. The second element, 'Communication and Information Transfer', co-ordinates the communication between the distributed designers and provides a platform for the exchange of design-related data, e.g. customer requirements, ideas, sketches, comments, and decisions. A case study was carried out to assess the use of various tools for communication and information transfer during synchronous and asynchronous collaboration. The case study is described and the results are presented and discussed. Both elements make use of a support service providing various 'Input Devices for Conceptual Design', the third element of a support system. While standard tools such as mouse and keyboard meet the requirements of subsequent stages of the design process, they are often impractical in creating or annotating sketches. Various low-cost input devices were investigated. A framework was developed to integrate the three elements into one support system. The 'Distributed Design Assistant', abbreviated as 'DiDeas', is an Internet-based system that allows simultaneous multi-user collaboration. A relational database is located on a central web-server and stores all design information entered into the system. The user interface was realized in the form of a collection of Microsoft Active Server Pages, which can be accessed platform-independently via a standard webbrowser. The development of the database structure and of the user interface is described in detail. A second case study was carried out to evaluate the Distributed Design Assistant. The case study is described and the results are presented and discussed. The low-cost system has proven to be a very useful tool for distributed conceptual design. The Distributed Design Assistant systematically guides novice and experienced designers through the stages of specification development and conceptual design. It facilitates the easy collection and the fast exchange of a large amount of textual and graphical information during these stages. Although the system is relatively simple, compared to professional product data management systems used in later stages of the design process, it strongly enhances the productivity of designers and distributed design teams. Keywords: Distributed Design, Conceptual Design, Systematic Product Development
Augmentation of the actuator-disk method for low-pressure axial flow fan simulation.
(Stellenbosch : Stellenbosch University, 2024-02) Venter, AJ; Owen, Michael ; Muiyser, Jacques; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Actuator-disk rotor models are an invaluable simulation tool for cost-effective turbomachinery simulation. Actuator-disk models implicitly represent turbomachine rotors as momentum sources where the source term magnitude is determined from classical two-dimensional blade-element theory (BET) force calculations. Actuator-disk models accordingly require appropriate lift and drag coefficients as input to complete the force calculations. Conventional actuator-disk models utilize standard two-dimensional airfoil coefficient data, but this limits the accuracy of the models to only a small operating window where the bulk of the flow over the rotor itself is principally two-dimensional. This, consequently, limits the application of traditional actuator-disk models in industrial system analyses where complex flow environments prevail. This study considers the particular example of low-pressure axial flow fans, widely applied in thermoelectric air-cooled condenser (ACC) systems. ACCs are a key water-conservative cooling solution to the thermoelectric power industry, yet their operation is beset by inefficiencies and corresponding high operating costs. Given the scale of ACC systems, numerical investigations are forced to rely on simplified implicit fan models like actuator-disk rotor models, which provide limited approximations of actual ACC fan performance over a wide range of flow conditions. Expanding the usable window of actuator-disk axial fan models is therefore vital to providing an enhanced capacity to robustly analyse and ultimately improve ACC systems (and other industrial cooling fan systems alike). To realize this enhanced analysis capability, a new means of appropriately defining the actuator-disk model input coefficient data is required. The input coefficient data needs to appropriately reflect actual fan blade behaviour in a three-dimensional rotating context. Physical fan blade behaviour, however, has not been comprehensively investigated, and the multi-dimensional effects of rotation and blade solidity remain somewhat obscure. This study therefore sets out to define generalizable axial fan behaviour and to use the newly acquired insight to fabricate new coefficient formulations. This study constitutes a numerical analysis in which two low-pressure axial flow fans are both explicitly (full, solid rotating fan geometry) and implicitly simulated. Novel insights into generalizable aerodynamic behaviour of axial flow fans at off design operating conditions are presented and key details on the underlying phenomena are uncovered. Furthermore, this study rigorously explores the feasible potential of the actuator-disk method for axial flow fan simulation and ultimately proposes its revised coefficient formulation. The augmented actuator-disk method (AADM) is shown to more accurately simulate axial fan performance compared to existing model variants, and to resolve flow fields that are more representative of the physical case – an important feature for ACC and other industrial heat exchanger system analyses. Over a wide range of axisymmetric operating conditions (and across both considered fan types), the AADM is shown to approximate reference static pressure rise results with a maximum error of 10%, shaft power results within 8% and blade force magnitudes within 10%, thus offering a marked improvement in comprehensive accuracy relative to existing models.
Characteristics of gas-liquid counterflow in inclined ducts with particular reference to reflux condensers
(Stellenbosch : Stellenbosch University, 1997-12) Zapke, Albert; Kroger, D. G.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering; Jacobs, Ivan
ENGLISH ABSTRACT: An experimental investigation on gas-liquid counterflow in inclined rectangular ducts is conducted. The pressure drop across the sharp-edged gas inlet and the pressure gradient inside the duct are measured. Combinations of water, methanol, propanol, air, argon, helium and hydrogen are tested. The duct height and width are varied from 50 mm to 150 mm and 10 mm to 20 mm respectively. The emphasis is on high void fraction flow, i.e. low liquid flow rates as encountered in air-cooled reflux condensers. At low to moderate gas flow rates the pressure gradient is gas Reynolds number related while it becomes dependent on the superficial densimetric gas Froude number as the gas flow is increased. According to experiment the hydraulic diameter is the required length dimension in the gas Reynolds number while the duct height becomes the characteristic dimension in the Froude number regime. Flooding curves are generated for duct inclinations from close to the horizontal to the vertical. The data correlate in terms of the phase Froude numbers and a dimensionless liquid property parameter containing the hydraulic diameter, density, surface tension and the viscosity. The flooding gas velocity is found to be strongly dependent on the duct height, the phase densities and the duct inclination. The liquid viscosity has a stronger effect than the surface tension. Both these properties however playa secondary role. Flooding is not related to the gas Reynolds number. A theoretical model, based on the phenomenological findings of the adiabatic counterflow investigation, is derived to evaluate the performance of an air-cooled reflux condenser. Field tests are conducted on a full scale reflux condenser and the measured performance is compared to the model prediction. The reflux condenser is found to achieve only 60% of the predicted heat rejection rate due to the existence of so-called cold or dead zones. Indications are that excessive entraiment in the bottom header and the subsequent accumulation of condensate in the finned tubes causes a maldistribution of the steamside flow. In the process noncondensable gases accumulate and form dead zones, causing ineffective performance. Flooding as found in single-ducts does not appear to contribute to the formation of the dead zones.
Condensation of low-pressure steam in inclined air-cooled tubes
(Stellenbosch : Stellenbosch University, 1991) Bellstedt, Michael Otto; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The subject of this dissertation is the analysis, prediction and description of the various types of flow that may occur within a vertical or inclined air cooled condenser tube. A literature survey on the subjects of condensation heat transfer, two-phase pressure drop and the occurrence of flooding during the counterflow of liquid and gas in a tube is undertaken.
A critical evaluation and refinement of the performance prediction of wet-cooling towers
(Stellenbosch : University of Stellenbosch, 2003-12) Kloppers, Johannes Christiaan; Kroger, D. G.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
The thermal performance prediction of wet-cooling towers is critically analyzed and refined. Natural draft counterflow towers and mechanical draft counterflow and crossflow towers are considered. The Merkel, Poppe and e-NTU heat and mass transfer methods of analysis are derived from first principles, as these methods form the cornerstone of wet-cooling tower performance evaluation. The critical differences between these methods, when applied to fill performance analyses and cooling tower performance evaluations, are highlighted. The reasons for these differences are discussed with the aid of psychrometric charts. A new extended empirical relation for the loss coefficient of fills is proposed where the viscous and form drag effects are accounted for as well as the buoyancy, momentum and fill height effects. The empirical equation for the transfer characteristic of fills is extended to include the effects of fill height and the inlet water temperature. Empirical equations to predict the temperature inversion profile, height of the temperature inversion and the height from which air is drawn into the cooling tower are developed. The influence of temperature and humidity inversions on the performance of wet-cooling towers is subsequently investigated. A comprehensive analytical computer program is developed to predict and optimize the performance of wet-cooling towers. Computer programs are also developed to generate cooling tower performance curves, analyze fill performance test data and plot psychrometric charts.
Design analysis of a lomolding machine
(Stellenbosch : University of Stellenbosch, 2007-12) Goussard, Charl Leonard; Basson, A. H.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
This dissertation describes the design analysis of a lomolder (a machine similar to an injection moulding machine). It focuses on key design aspects that will drive the purchase cost of the machine and that will also influence the maintenance and operating cost. The main objective of the study is to provide an understanding of the key factors that influence the cost of a lomolder as well as the factors that contributes to a quality manufactured part. A semi-analytical flow model was developed to predict cavity pressure drops for a range of part sizes. This model was necessary to eliminate time consuming numeric simulations required for machine optimisation. Numerous machine concept designs were developed and a final layout design chosen. A parametric CAD model was built for the lomolder. Layout designs for different sized lomolders can be generated with this model. The dissertation concludes with a cost study that focuses on the purchase cost of a lomolder unit. Key elements such as choice of actuator and piston to part area ratio are described.
Design and development of a novel wave energy converter
(Stellenbosch : Stellenbosch University, 2013-12) Joubert, James Rattray; Van Niekerk, J. L.; Retief, G. de F.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The design, development and evaluation of a novel wave energy converter (WEC) device, called the ShoreSWEC, in a South African port development is presented. Based on the device requirements, site selection criteria were specified and applied to identify a suitable deployment location. A wave modeling procedure was developed to determine the operational wave conditions and available wave power resource at the selected location. The site was found to have a low mean annual average resource of approximately 2.3 kilowatt per meter wave crest (kW/m) due to its relatively sheltered location. The wave model was further used to determine design storm conditions and a structural stability analysis of the device was conducted. Experimental tests were performed to evaluate the hydrodynamic conversion efficiency of a single chamber of the device at its most conservative orientation, under a variety of wave energy conditions. The effect of a floor incline and an additional chamber on the performance of the system was investigated. The incline improved efficiency for low wave heights, making it ideal for the low wave power resource conditions of the site, whilst the multi-chamber system experienced increased performance at high wave periods. A comparison between the ShoreSWEC and a conventional oscillating water column (OWC) WEC showed that the OWC extracted 72% more energy, highlighting the sensitivity of performance on device orientation. A three-dimensional (3D) numerical model of the experimental setup was developed. The numerical model provided comparable water surface elevations inside the flume and chamber, yet predicted significantly higher internal chamber pressures and overall efficiency. The electricity generation potential of a 10 chamber ShoreSWEC at the specified location, approximated from the experimental results and 11 years of hindcast wave data, was found to be 6 kW on average for a 15 kW capacity system. Results of this study highlighted the need for greater understanding of the hydrodynamic characteristics of a full length device. Experimental tests in a 3D wave basin on a scaled full length ShoreSWEC model are therefore recommended. Once conducted, South Africa will be one step closer to the deployment of the full scale SWEC device.
Design and optimization of hydrofoil-assisted catamarans
(Stellenbosch : Stellenbosch University, 2002-03) Migeotte, Gunther; Hoppe, K. G.; Thiart, G. D.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: This work is concerned with the hydrodynamic design of hydrofoil-assisted catamarans. Focus is placed on the development of new and suitable design methods and application of these to identify the most important geometric parameters of catamaran hulls and hydrofoil configurations that influence efficiency and performance. These goals are pursued by firstly gaining a thorough understanding of the governing hydrodynamic principles involved in the design process. This knowledge is then applied to develop new and improved experimental techniques and theoretical methods needed for design. Both are improved to the extent where they can be applied as design tools covering the important semi-displacement and semi-planing speeds, which are the focus of this study. The operational speed range of hydrofoil-assisted catamarans is shown to consist of three distinct hydrodynamic phases (displacement, transition and planing) and that different hydrodynamic principles govern vessel performance in each phase. The hydrodynamics are found to differ substantially from that of conventional high-speed craft, primarily due to the interaction between the hull and the hydrofoils, which is found to vary with speed and results in the need for more complex experimental procedures to be followed if accurate predictions of resistance are to be made. Experimental predictions based on scaled model tests of relatively small hydrofoilassisted catamaran models are found to be less accurate than that achievable for conventional ships because of the inability to correct for all scaling errors encountered during model testing. With larger models scaling errors are encountered to a lesser degree. The most important scale effect is found to be due to the lower Reynolds number of the flow over the scaled foils. The lower Reynolds number results in higher drag and lower lift coefficients for hydrofoils compared with those achieved at full scale. This effect can only be partially corrected for in the scaling procedure using the available theoretical scaling methods. Presently available theoretical methods commonly used for the design of conventional ships were found to be ill adapted for modeling the complex hydrodynamics of hydrofoil-assisted catamarans and required further development. Vortex lattice theory was chosen to model the flow around hydrofoil-assisted catamarans as vortex theory models the flow around lifting surfaces in the most natural way. The commercial code AUTOWING is further developed and generalized to be able to model the complex hull-hydrofoil interactions that change with speed. The method is shown to make good predictions of all hydrodynamic quantities with accuracies at least as good as that achievable through model testing and therefore fulfills the requirements for a suitable theoretical design tool. The developed theoretical and experimental design tools are used to investigate the design of hydrofoils for hydrofoil-assisted catamarans. It is found that the main parameter needing consideration in the hydrofoil design is selection of a suitable hydrofoil lift fraction. A foil lift fraction in the order of 20-30% of the displacement weight is needed if resistance improvements using hydrofoil assistance are to be obtained over the hull without foils. It is often more favorable to use higher foil lift fractions (50%+) as the resistance improvements are better, although careful attention should then be given to directional and pitch-heave instabilities. The Hysuwac hydrofoil system patented by the University of Stellenbosch is found to be hydrodynamically optimal for most hullforms. The hullform and in particular the curvature of the aft buttock lines of the hull are found to have an important influence on the achievable resistance improvements and behaviour of the hydrofoil-assisted hull at speed. Hull curvature is detrimental to hydrodynamic performance as the suction pressures resulting from the flow over the curved hull counter the hydrofoil lift. The hullform best suited to hydrofoil assistance is found to be one with relatively straight lines and hard chine deep- V sections. The main conclusion drawn from this study is that hydrofoil-assistance is indeed suitable for improving the performance and efficiency of catamarans. The design and optimization of such vessels nevertheless requires careful consideration of the various resistance components and hull-foil interactions and in particular, how these change with speed. The evaluation of resistance for design purposes requires some discipline between theoretical analysis and experimental measurements as the complexity of the hydrodynamics reduce the accuracies of both. Consideration of these factors allows hulls and hydrofoils to be designed that are efficient and also free of dynamic instabilities.
A design framework for aggregation in a system of digital twins
(Stellenbosch : Stellenbosch University, 2022-04) Human, Carlo; Basson, AH; Kruger, K; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: The digital twin (DT) concept has become a popular means of capturing and utilising data related to physical systems and has been applied in many domains. The data provided within DTs allow for the integration of services and models to improve understanding and decision-making related to the physical system. Through aggregation, multiple DTs can be combined to represent larger, more complex system, while maintaining the separation of concerns. The design framework presented in this dissertation aims to enable systematic, effective decisions when designing a system of DTs to represent a complex physical system. In particular, this framework adopts hierarchical aggregation as one of its primary enablers and it considers the use of a services network, such as a service-oriented architecture, as well. The design framework is intended to be broadly applicable, by remaining vendor-neutral, and it enables traceability of design choices. The approach starts with an analysis of physical system complexity to identify key needs related to managing complexity. A suitable requirements classification is then introduced to help translate the needs into requirements that the system of DTs should satisfy. Hierarchical aggregation is also introduced as a primary architectural approach to manage complexity. Hierarchical aggregation allows for the separation of concerns, computational load distribution, incremental development and modular software design. The design framework is arranged in six steps: 1) needs and constraints analysis, 2) physical system decomposition, 3) services allocation, 4) performance and quality considerations, 5) implementation considerations and 6) verification and validation. The dissertation then introduces a general reference architecture that combines a system of DTs (which follows hierarchical aggregation principles) with a services network to allow for reliable and adaptable service provisioning. The design framework is then discussed in the context of the general reference architecture. The design steps of the design framework are then moulded into six design patterns, which simplify the design process by focussing of key quality attributes. The quality attributes considered for the respective design patterns are performance efficiency, reliability, maintainability, compatibility, portability and security. The use of the design framework and design patterns are then demonstrated and validated through three case studies, two high-level case studies and one detailed case study. The high-level case studies consider a water distribution system and a smart city, respectively. The detailed case study considers a heliostat field. The dissertation concludes that the design framework, as well as the design patterns, enable a systematic approach to designing a system of DTs. The design framework can also be applied to numerous and varying domains, such as the case studies considered.
Design of a tip appendage for the control of tip leakage vortices in axial flow fans
(Stellenbosch : Stellenbosch University, 2021-03) Meyer, Thomas Oliver; Van der Spuy, S. J.; Meyer, Chris J.; Corsini, Alessandro; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The design of a tip appendage for the passive control of tip leakage vortices inaxial flow fans is investigated. A periodic three-dimensional numerical model of a1.542 m (casing) diameter subsonic axial flow fan, designed for use in air-cooledheat exchangers (ACHEs), is developed using the open-source CFD libary Open-FOAM (OF) v1806. The implemented meshing strategy, which uses a combina-tion of the cross-platform libraries cfMesh and SnappyHexMesh (SHM), is found to sufficiently validate against experimental data with a root-mean-square error(RMSE) of 3.8 Pa in total-to-static pressure rise, 129.6 W in fan shaft power consumption, and 1.5 percentage points in total-to-static efficiency for the datum fan case. Steady and unsteady numerical computations are used to determine the effect of tip clearance on fan performance characteristics. An increase of 15.6 percent intotal-to-static pressure rise and a 5.1 percentage point increase in total-to-static efficiency is found with a two-thirds reduction in tip clearance at the fan’s design point flow rate. Normalized helicity contours are used in the visualization and quantification of the flow near the blade tip. The findings show a reduction in the relative tip leakage flow (TLF) velocity,WTLF, and tip leakage vortex (TLV) exit trajectory angle, βTLV, along the blade chord with a reduction in tip clearance. These two flow characteristics are numerically investigated with the aim of con-trolling them through implementation of two tip appendage designs, these being a constant thickness (CT) end-plate and novel trailing edge (TE) end-plate de-sign. A numerical comparison between the two designs indicates that the new TE end-plate concept improves the fan performance characteristics through exhibiting control of the relative TLV exit trajectory angle. Consequently, this design is chosen with the aim of further improving the fan aerodynamic performance characteristics through optimization. The Design and Analysis of Computer aided Experiments (DACE) method is used in the construction of the Kriging based surrogate model’s database. The resulting database is coupled with an Efficient Global Optimization (EGO) algorithm which completes the workflow of the multi-objective multi-point (MOMP) optimization process. The Pareto-front of non-dominated solutions is used to guide the optimal design selection, on which the experimental evaluations are based. The results of the optimized design indicate improved fan performance characteristics at greater than peak efficiency flow rates. This design is found to increase the datum fan’s design point performance characteristics by a value of 32.90 percent in total-to-static pressure rise and a 7.66 percentage point increase in total-to-static efficiency at the fan’s design speed of 722 rpm.
The design, kinematics and error modelling of a novel micro-CMM parallel manipulator
(Stellenbosch : Stellenbosch University, 2014-04) Rugbani, Ali Milud; Schreve, K.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The research presented in this dissertation establishes a micro-CMM parallel manipulator as a viable positioning device for three degree of freedom micro measurement applications. The machine offers the advantages associated with parallel kinematic manipulators, such as light carrying weight, high stiffness and no accumulation of errors, while avoiding some of the traditional disadvantages of parallel manipulators such as the associated effects of angular errors (Abbé error), singularity problems, work space limitation and the extensive use of spherical joints. In this dissertation, the direct position kinematic solution is developed analytically and the solution of the inverse position kinematic is solved numerically. A workspace analysis has been performed. A fully functional prototype demonstrator is fabricated to demonstrate this machine. While the demonstrator was not intended to achieve submicron accuracy, it was intended to validate the error models. Computer controlled measurement is developed and used to position the probe and to record measurements. A reliable kinematic error model based on the theory of error propagation is derived analytically. A numerical method is used to verify the analytical results. Comparison shows that the results of the error model, both analytical and numerical, represent a very good match and follow the same trend. The kinematic position model is validated using a conventional CMM. Results show that an average difference of less than 0.5 mm over a set of 30 points is achieved. This result of the micro-CMM demonstrator measurements falls within the error budget of approximately 0.75 mm estimated by the proposed analytical error model.

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