Masters Degrees (Mechanical and Mechatronic Engineering)

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    Development of a calibration method for coupled CFD-DEM
    (Stellenbosch : Stellenbosch University, 2023-03) Wasserfall, Jacob Gabriel; Coetzee, Corne; Meyer, Chris; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: A fully coupled Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) model was developed for a granular material submerged in water. The granular material was divided into three distinct particle size distributions (PSD). The particle size and shape were measured and used to create multi-sphere (composite) particles in the model. The bulk density was measured and used to calibrate the particle density, which was also measured directly. The remaining input parameters were calibrated using a large-scale Draw Down Test (DDT) rig under dry, wet and submerged saturation conditions. Due to geometric constraints and the relatively large size of the particles (4 mm to 32 mm), momentum source smoothing had to be used to allow for the large particle-to-cell size ratios. This technique was implemented using the software Simcenter StarCCM+. The effects of the smoothing were investigated and it was found that while the use of smoothing improved model stability, the use of large smoothing lengths resulted in instability. Additionally, a drag coefficient modifier was implemented to improve fluid-particle interactions which were reduced due to the momentum source smoothing method. First, the granular material was calibrated for the dry and wet conditions and the input values for the particle-particle coefficient of friction and the coefficient of restitution determined. For the larger PSD, it was found that the change in saturation condition did not affect the results of the DDT which meant the same calibration parameters were found for both. For the smaller PSD, some change was seen with the increase in saturation but similar input parameters were found for the two conditions. The smallest PSD was influenced the most by the change in saturation which resulted in two distinct sets of input parameters being found for the two conditions. Secondly, the model was calibrated for submerged conditions using the Haider- Levenspiel drag model. The input parameter values were initially set equal to those calibrated for the dry/wet conditions. Under submerged conditions, results showed that the particle-particle coefficient of friction and the drag modifier had a significant influence on the results. Therefore these two parameters were calibrated. It was found that the drag modifier had to be calibrated, while the particle-particle coefficient of friction, calibrated under dry conditions, could be used for the submerged conditions. Thus, the particle-particle coefficient of friction calibrated under dry conditions, could be used for the wet and submerged conditions for the larger PSDs. Finally, a vertical suction pipe validation experiment was conducted. The suction pipe had a constant diameter, but the fluid velocity and the distance the pipe opening was held from the granular bed were varied. The amount of mass (particles) removed as well as the size of the cavity that formed in the material bed was measured and compared to model predictions. The results showed that using the parameter values calibrated in the DDT, too much material was removed (error of 30%). Removing the drag modifier (setting it equal to unity) significantly improved the results (error of 6%). It is concluded that due to the difference in flow mechanism (particle-induced in the DDT versus fluid-induced in the suction pipe), the DDT is not a suitable experiment to calibrate the input parameter values for a suction pipe. It is hypothesized that the flow mechanism and dynamics of the granular material and the fluid in the calibration experiment should be similar to that of the final application being investigated.
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    Performance analysis of a micro gas turbine engine using computational fluid dynamics
    (Stellenbosch : Stellenbosch University, 2023-03) Vass, Kane; Van der Spuy, S. J.; Van Eck, H.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: Micro Gas Turbines (MGTs) are typically made up of a centrifugal compressor, combustor, axial turbine and exhaust nozzle combination. The aim of this thesis was to develop a complete numerical model of the CAT 250 TJ micro gas turbine in order to determine the operational performance characteristics of the engine. Computational Fluid Dynamics (CFD) was used to determine the operational requirements and the performance of the MGT compressor and turbine stages. The boundary conditions from these simulations were used to determine the required temperature gain over the combustor. A model of the combustor was developed that incorporated a combination of heat sources to emulate the energy release from combustion without modelling the physical combustion process directly. The use of these heat sources was found to provide an adequate temperature rise over the combustor region. A model containing the combustor, turbine stage and nozzle was then developed using conditions from the compressor outlet as the combustor inlet boundary condition. Experimental tests were performed using the same MGT to determine the performance parameters over the engines speed range. These performance parameters were compared to the simulated engine performance. It was found that at the design speed of 115 000 rpm, the simulated thrust and mass flow rate was 0.7% and 2.67% lower than the corrected experimental values at this speed. It was found that the use of these heat sources provide an approximate temperature distribution within the MGT and can be used to model the overall engine performance. An obvious next step would be to improve the modelling of the combustion process, in order to obtain a more accurate temperature distribution within the engine.
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    An investigation into using fan blade loading as an axial flow fan performance and scaling parameter
    (Stellenbosch : Stellenbosch University, 2023-03) Van Jaarsveld, Nicolaas Vlok; Meyer, C. J.; Van der Spuy, S. J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: Air-cooled condensers (ACCs) utilise axial flow fans to produce airflow over heat exchangers. The size of these fans makes it challenging to determine their performance characteristics experimentally. Current methods of determining a fan's installed operating point exist, but each method contains significant inaccuracies. A new metric was investigated with the goal of improving the prediction of a fan’s installed operating point. The fan under investigation is the M-fan (7.31 m in diameter) designed for application in ACCs installed at concentrated solar power plants. Stellenbosch University has the facilities to test the model M-fan in a standardised testing facility and a large M-fan in its installed environment. Fan static pressure measurements are advantageous for use in fan characterisation due to the steep slope of the fan static pressure characteristic. The generation of thrust by an axial flow fan blade is the cause for the rise in static pressure across the fan, leading to the expectation that a proportional relationship exists between the axial fan blade loading (blade thrust) and the fan static pressure rise. The use of fan blade thrust to determine a fan’s installed operating point, in conjunction with fan power, was therefore investigated. During investigation of a model-scale system, the use of fan blade loading to determine a fan’s operating point was deemed successful. When compared to the fan characteristic curve, the measured fan power corresponded to a volume flow rate of 16.21 m3/s, whereas the flap-wise strain corresponded to a volume flow rate of 15.96m3/s. The difference between the two predicted volume flow rates is 1.55%. Following an extensive experimental analysis, the use of fan blade loading was deemed successful for use in large-scale fan performance prediction through scaling with the square of the geometrical scaling factor (GF2). A difference in fan power and fan blade loading was, however, observed between the scaled model fan characteristics and the large-scale test results. The scaling laws underpredicted the fan power by an average of 9.54 %. A further slight decrease in fan power was observed at high volume flow rates. The former shows a near-perfect correlation between an increase in blade setting angle, whereas the latter represents a similar trend to that of distorted inflow conditions.
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    The numerical investigation of wave slamming on a polar supply and research vessel
    (Stellenbosch : Stellenbosch University, 2023-03) Van der Westhuizen, Alexander; Bekker, Anriette; Meyer, Christiaan J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: The SA Agulhas II is a polar supply and research vessel owned by the Department of Forestry, Fisheries and the Environment of South Africa. She facilitates and supports research in the Southern Ocean and Antarctica. The SA Agulhas II was designed to polar class 5 and experiences harsh seas and ice conditions during her voyages. The ship was designed with a raised transom which results in significant slamming during open water navigation. This can cause local damage to the hull as well as increased fatigue damage to the structure of the vessel. The ship was designed to manage ice passage which may be sub-optimal during open water navigation due to the increased slamming which results from the raised transom. An investigation into a monitoring scheme with the use of full-scale measurements and simulation software is instantiated to detect slamming pressures experienced by the hull of the vessel. The full-scale slam load measurement estimation is established with the use of plate theory and strain measurements on sections of the hull plating at the transom. The strain measurements are used to determine an estimate of the pressure loads experienced on the hull during slamming events. The results show promise in the technique used after some improvements in noise attenuation and error adjustment due to simplifying assumptions. A computational fluid dynamic model and simulation procedure is developed to analyse the validity of using simulation software to predict loading and vessel responses as well as inform hindsight investigations of the vessel responses to her environment. The simulation software does not capture the response of the vessel well in some seaways and hydrodynamic pressure estimates are not determined well for small sea states.
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    A parametric study of the cable positioning of Tensairity beam members using experimental and finite element methods
    (Stellenbosch : Stellenbosch University, 2023-03) Trafford, Nicholas John; Venter, M. P.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: A parametric study into the arrangement of reinforcing cables of a Tensairity beam girder was conducted. A 1500mm long, 140mm diameter beam was constructed with low-cost and freely available materials and fabrication methods. Three-point bending tests were performed to find cable arrangement parameters responsible for decreasing the central deflection of the beam. The reference test used in the study contained one pair of cables, with each cable forming a helical shape of one revolution around the bladder in a symmetrical orientation. Comparisons with the reference test were confirmed with two-tailed, pooled, statistical t-tests based on a 95% confidence interval. In comparison to the reference test, using one pair of cables with two or three revolutions decreased the stiffness of the beam by 49.7% and 55%, respectively. Adding a second pair of cables with either two or three revolutions statistically increased the stiffness of the beam by 8.5% in the worst case and by 18.6% in the best case scenario. The shortening of each pair of cables by 0%, 1%, 2% and 3%, respectively, was also investigated. The increase in stiffness was found for cables shortened by at least 1%. However, there was no added stiffness past a 1% decrease in length. The cable shortening affected the beam's shape more significantly than the stiffness. A non-linear Finite Element (FE) model of the Tensairity beam was also produced with linear-elastic material models derived from material testing of two components of the beam. The beam deflection calculated by the FE model, agreed within 5% of the experimental testing. The model concluded that a no-slip, no-roll assumption between the bladder and cables is viable for future simulation of Tensairity beams with shorted cables. Experimental testing also demonstrated viable use of a proportional method for maintaining air-pressure, which can be tested on full-scale models in future studies.