Doctoral Degrees (Mechanical and Mechatronic Engineering)
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Browsing Doctoral Degrees (Mechanical and Mechatronic Engineering) by browse.metadata.advisor "Dobson, R. T."
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- ItemOptimization of a supercritical carbon dioxide solar thermal power system(Stellenbosch : Stellenbosch University, 2020-12) Van der Westhuizen, Ruan; Groenwold, A. A.; Van der Spuy, A. A.; Dobson, R. T.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: A new procedure for the optimal design of a solar thermal power system that uses a supercritical carbon dioxide (sCO2) Brayton cycle is developed. The design procedure is compatible with different types of component models, solution methods and design constraints. The variables of the system, and the objectives and constraints of the system design, are managed through a comprehensive computational architecture. Multi-objective optimization of 23 thermodynamic-, geometric- and performance design variables of the system is achieved.The design procedure is based on a specific series of design decisions that continually reduce the design spaces of the turbo machinery and heat exchanger sub-systems, in such a way that Pareto-optimality ofthe final system design is ensured. For computational expediency, initial design decisions are made based on the analysis of a thermodynamic model. It is demonstrated that the optimal thermodynamic design of the system is influenced by the performance values of the turbo machinery and recuperator. Subsequent design decisions are made based on the independent analyses of detailed turbo machinery and heat exchanger models.The turbo machinery is modeled in Matlab® with a mean-line analytical approach that uses specified performance coefficients. Explicit constraints ensure that the turbo machinery designs are within established limits.The heat exchangers are modeled in Flownex® using a control-volume-based convection-diffusion approach that can accurately represent the internal pinch-point of the recuperator. All models make use of realistic thermodynamic properties for supercritical carbon dioxide and are extensively validated with published data.A formal derivation shows that there are two distinct operating regions for the heat exchangers of the system. A successful system design depends on the region in which the heat exchangers function. This region can be controlled by changing the value of the nominal flow area, which is considered the most important design variable of the system. Six designs of the same basic system, but with different objectives and constraints, are presented. These designs are evaluated and compared to each other through a detailed quantitative investigation that highlights which factors contribute most to the inefficiency of each design. The best design achieves a thermal-to-mechanical efficiency of 40% at a turbine inlet temperature of 550◦C. This efficiency is demonstrated to be near the practical maximum for an sCO2system that employs the recuperated cycle configuration with a linear solar receiver. Future developments of the design procedure could consider the addition of a financial model as well as constraints to account for the structural integrity of the system.
- ItemTheoretical simulation, manufacture and experimental evaluation of a free piston stirling engine electric generator.(Stellenbosch : Stellenbosch University, 2019-12) De La Bat, Jean Gerard; Harms, T. M.; Dobson, R. T.; Bell, A. J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: A Stirling engine is a suitable candidate for heat and electrical power co-generation when integrated with a parabolic-dish solar concentrator. Such solar systems are envisaged as being a viable option for micro off-grid co-generation using smart energy control strategies in developing countries rich in solar irradiance. This dissertation presents the theoretical simulation, fabrication and experimental evaluation of a novel prototype free-piston Stirling engine electric generator. The theoretical model was developed from first principles, by discretising the working fluid into a network of one-dimensional control volumes. By applying the conservation of mass, momentum and energy and applicable property functions to each control volume, a system of non-linear time-dependent partial differential equations was generated. These equations were solved sequentially using a fully-explicit numerical method with linear upwind-differencing and by employing a backwardsstaggered velocity scheme. A transient electromagnetic finite element analysis of the linear generator was performed and is represented in the theoretical simulation model through generator characteristic functions. A novel experimental engine prototype was manufactured that includes a linear motor attached to the displacer shaft. Finite element analyses were used to design the strength and safety-critical mechanical components of the engine. The electromechanical, flexure spring stiffness and displacer and piston mechanical friction characterising equations of the theoretical model were matched to the as-built engine by direct experimentation. Seven selected experimental test runs were used to establish whether the theoretical model is capable of emulating the behaviour of the as-manufactured engine, with hot-end temperatures varied between 300 and 600 C and working fluid charge pressures between 1.0 and 2.0MPa. Self-sustained, closed-circuit operation of the engine was achieved at a hot-end temperature of 470 C and charge pressure of 1.7MPa absolute, with a peak electrical output power of approximately 73W. A good correlation between experimental and simulation results was demonstrated for several operating conditions, thereby validating the theoretical simulation model. It is concluded that it is critical to ensure proper alignment of the moving parts so that internal sliding friction may be minimised. To achieve continuous and sustained operation without piston-casing collisions, it is recommended that an electronic feedback control system be integrated to the existing engine.