Theses and Dissertations (Mechanical Engineering)

Permanent URI for this collection

https://hdl.handle.net/10352/191

Browse

Now showing 1 - 20 of 29

Advanced modelling of porous screens in aerodynamic diffusers using variable resistance factors
(2014-12) Janse van Rensburg, Jacobus Johannes; Van Staden, M. P.; Jacobs, G. G.
Strict emission legislation has forced industry in general to seriously consider the negative impact it has on the environment, specifically concerning emissions from burning fossil fuel into the atmosphere. In cases where emission levels exceed the allowable limit, companies are forced to operate at lower operating conditions and these load losses can result in a significant loss of revenue. This has forced companies to improve their ash filtering capabilities by optimising electrostatic precipitation systems. One of the main factors impacting on the efficiency of such a system is the distribution of the flow across the collection plates. The design of the inlet diffuser plays a major role in the ultimate distribution of the flow through the precipitator. Porous screens are positioned in the diffuser in order to distribute the flow across the total flow area with the aim to achieve a uniform distribution of the flow. CFD is widely used in industry to simulate the flow through precipitators in order to optimise the flow distribution and thus increase the efficiency of the system. It was found however that the current methods used to simulate these screens in CFD models were not well researched and employed fixed resistance values that could not reliably compensate for changes in the resistance coefficient due to a change in the angle of incidence. This study investigates advanced numerical methods for the simulation of porous screens in applications where the angle of incidence changes continuously across the face of the screen. New methods are introduced where the resistance of the screen is calculated as a function of the changing angle of incidence. The methods currently used are also investigated and compared with results from the new methods. Extensive experimental work was required to supply empirical data for the validation of the numerical methods that are proposed. For this reason, the first part of this study focused on the design construction and commissioning of a low speed wind tunnel. Results are presented and discussed for flow profiles through wide-angle diffusers at different angles and also for a number of different screens positioned in the centre of the diffuser. This study also investigates the sensitivity of a CFD simulation code to factors such as numerical discretisation schemes, turbulence models and solution relaxation specifically for wide-angle diffusers. These factors were tested for diffusers at different angles and included tests on open diffusers and also with screens positioned inside the diffuser. It was concluded that the current methods used are not adequate to capture the true flow profiles for a range of different screen geometries. Although the proposed models did improve on the limitations of the current methods, it was found that the applicability of these models is still limited and that further research would be required to develop numerical methods that are valid for a wide range of applications.
An investigation of the damping response and structural strength of a fibreglass and rubber particle composite sleeper
(Vaal University of Technology, 2022-09-12) Mbatha, Abednigo Jabu; Maube, O., Dr.; Nkomo, N. Z., Dr.; Alugongo, Prof.
A railway sleeper is a supporting and dampening beam placed underneath the railway track and can be made of different materials. There are four main types of railroad sleeper materials: timber or wooden, steel, concrete and composite material. The railway structural material often suffers from aggressive loading and vibration in the locomotive industry, and the sleepers' current durability and their vibration properties are not sufficiently resilient to vibration. There is a need for a structural material that can withstand significantly higher static and dynamic loads as trains become heavier and faster. Tyre disposal is a global challenge to the environment, with approximately 1.5 billion tyre waste generated annually. Tyres are non-biodegradable, making their disposal extremely difficult.This study seeks to find a way to recycle the waste tyres in an environmentally friendly manner in accordance with Sustainable Development Goal 11, which focuses on sustainable cities and communities. The study aimed to optimize a hybrid composite sleeper using waste tyres ground into particles, fibreglass reinforcement and polyester resin to enhance the composites' structural strength while increasing the composite sleeper's damping. The specific objectictives were to characterized rubber particles of waste tyres and fabricate a composite railroad sleeper material using waste rubber particles, glass fibre, and polyester resin..Thereafter , evaluate the mechanical properties of the composite sleeper under loading conditions and damped vibration properties .Lastly , determine the optimal composite sleeper . The rubber particles were characterized through sieving, moisture analysis and SEM. Thereafter, the composite was fabricated following the full experimental design. After that composite was fabricated using the hand lay-up method where the rubber volume fraction of 5, 10, 15 and 20% were varied, and fibreglass volume fractions of 5, 6, 7 and 8 % were obtained. The UTM (universal testing machine) was used to carry out mechanical tests, which included tensile strength, compression strength and flexural strength. Then Leeb hardness was carried out, and the damping properties of composites were determined using a shaker table. Minitab software was used for the optimisation of the composite mix The ANOVA test showed the model's accuracy in predicting tensile strength, compression strength, flexural strength, and vibrational damping, as shown by R2 values of 60.69%, 86.60%, 60.05% and 81.41 %, respectively. However, the model was not reliable for hardness which had an R2 value of 37.87%. The optimisation model indicated that rubber particles of size 150 μm with 7.48% volume fraction of rubber particles and fibreglass volume fraction of 8% are optimum. The corresponding mechanical properties responses for the optimum are tensile strength of 13.3851 MPa, the compression strength of 36.0272 MPa, the flexural strength of 36.5865 MPa and Leeb hardness of 647.7510. The damping properties of composite gave a value of 0.1416. Thereafter, optimum results were validated experimentally, and the model was shown to represent the data accurately. The fabricated composite could help to absorb aggressive forces caused by heavily loaded trains. At the same time, maintain the composite's mechanical strength and eliminate pollution caused by tyres in our environment. Further investigation is required into the impact of using a variety of rubber particle sizes 75 m on the vibrational damping and mechanical characteristics of the composite railway sleeper. Studying the impacts of various synthetic and natural rubber kinds on composite characteristics is also necessary.
Characterisation and flowability of titanium grade 5 alloy powders
(Vaal University of Technology, 2013-09) Nziu, P. K.; Mendonidis, P., Prof.; Labuschagne, D., Dr.; Masu, L. M., Prof.
Flowability is one of the essential physical characteristics considered during the use of any powder in a manufacturing process. However, very little research on flowability of titanium powder has been conducted. To this end, this study dealt with global market survey of titanium powder manufacturers and suppliers. In addition, the effects of various physical parameters such particle size, shape, chemical analysis, density and soundness on flowability of titanium grade 5 alloys powder in additive manufacturing application were investigated. Twelve powder samples of titanium alloy grade 5 (Ti6Al4V) were sourced, tested and analyzed using various methods. The choice of the characterization method used depended on its accuracy, equipment availability and application. Particle size and shape were characterized using laser diffraction and scanning electron microscope techniques, respectively. Quantitative and crystallographic analyses were done to determine the chemical composition as well as alpha and beta phases. Shear cell and dynamic tests were performed to determine bulk density, stability, flow energy and flowability where as particle density was performed by a pcynometer. Research on potential manufacturers was conducted using questionnaires. It was established that high cost of titanium powder is partly driven by titanium powder firms that are not willing to disclose information about the product. It was observed that powder flowability is affected by particle size, shape, chemical composition, density and soundness. The particle density was found to be a function of chemical composition that is the alloying elements and impurities present in the powder. It was noted that bulk density, porosity, cohesion and agglomeration were affected by particle size. Soundness of the powder was also found to improve with sphericity of the particles. Among the physical parameters studied, particle size had the highest effect on powder flowability. The highest flowability was noted at particle size of 41 μm.
Controllability and stability of selectively wettable nanostructured membrane for oil/water separation
(Vaal University of Technology, 2019-12) Sob, Peter Baonhe; Alugongo, A. A., Prof.; Tengen, T. B., Prof.
Presently, the current membrane technologies used in oil/water separation are inefficient with poor controllability and stability during oil/water separation. The has led to the current problem of membrane fouling and degradation during oil/water separation. Several approaches have been used to modify or design a better wettable surface with limited success since the current problem of membrane fouling is persisting. It is, therefore, necessary for scientists, engineers, and researchers to come up with a new membrane technology that will be more efficient with stable wettability and controllability during oil/water separation. Membranes are made up of nanoparticles on their surface, which are both random in nature. Furthermore, the collection of membrane particles to form mesh membranes are made of pores with further ransom spatial distribution. Thus, it was necessary to use the tools of stochastic processes to theoretically characterize these parameters. These parameters affect both internal and external factors as well as characteristics of random membrane particle and pores on wettability like surface tension and surface energy were established in the current project. Design and production of the membrane material according to established relationships was by both low and high-pressure spay jet coating in a controlled laboratory environment, and microscopic characterization performed using SEM. TEM, EDS, statistical analysis, and Image J particle analyzer. The spread, orientation, morphology, spatial distribution, inter-separation distances, surface roughness, surface smoothness, contact angles, surface density of the particle, mean size of the coated nanoparticle on the membrane surface after different coating rounds were analyzed so as to establish conditions for optimal wettability. The testing of produced membranes under the application of external and internal factors was done. A centrifugal pump was used to pump contaminated oil and water mixture through the membrane under a steady flow rate of 10 L/s with a gauge pressure of 180 kPa at room temperature conditions. The membrane materials from different coating rounds were tested for their abilities to produce pure collected water or oil particles in the collected water. The separated water was analyzed using oil and grease analysis US EPA method 1664B with the SPE-DEX 1000 oil and grease system. As revealed theoretically and validated experimentally, it was found that the random natures of nanoparticle size, the spatial distribution of membrane channels, and their morphology have impacts on surface energy-driven separability of oil and water mixture. It was also observed that the scattering of nanoparticles on the membrane surface during coating lowered surface energy, which enhanced oil/water separation. It was also revealed that there is an optimal nanoparticle size, scattering, morphology, and spatial distribution of membrane channels that offer better separation of water from oil. From the microscopy analysis, different microstructures were revealed for glass, ceramics, and sediment during LP and HP coating. The microstructure characterization showed different surface densities of nanoparticles, mean particle sizes, surface roughness or smoothness, and nanoparticles inter-separation distances. It was also revealed that the materials, which were more stable and efficient with more controlled wettability were glass, sediment, and ceramic HP 3rd rounds of coating. Clusters were observed on the membrane surface during HP and LP coating rounds with more clusters observed in LP coating when compared with HP coating. These clusters increased surface energy, which negatively affected oil/water separation. It was concluded that to improved the wettability surface. membrane clusters must be minimized during coating rounds. This thesis contributed new knowledge to existing body knowledge of membrane technology used in oil/water separation in a number of ways by: (1) Designing a new membrane surface with a more controlled, efficient, and stable wettability process during oil/water separation. (2) Applying the logic of surface energy-driven separability, which has not been previously used extensively to study membrane wettability. (3) Establishing a model for the optimal membrane pore sizes that offer optimal membrane wettability during oil/water separation. (4) Establishing a model for optimal nanoparticle coating that offers optimal membrane wettability during oil /water separation. (5) A great attempt was made in characterizing nanoparticle surface densities, spread, particle coating, and nanoparticles intensity on a wettable membrane surface.
Development of a conducting multiphase polymer composite for fuel cell bipolar plate
(Vaal University of Technology, 2020-06) Alo, Oluwaseun Ayotunde; Pienaar, H. C. vZ., Prof.; Alugongo, A., Prof.; Otunniyi, I. O., Prof.
On account of their lightweight, low-cost, corrosion resistance, and good formability, conductive polymer composites (CPCs) are promising for the production bipolar plate (BP) for polymer electrolyte membrane fuel cell (PEMFC). However, a high conductive filler loading is needed to impart the required level of electrical conductivity to the insulating polymer matrix and as a consequence, the toughness of the plate deteriorates considerably. By using immiscible blend of polymers that have complementary hardness and ductility as matrix, with conducting multi-fillers of different morphologies, it is possible to optimize the matrix strength characteristics and favour the formation of conducting network to produce CPC meeting BP performance standards. Of course, a lot will depend on the formulation of the most favourable composition and production variables. In this regard, polypropylene-epoxy and polyethylene-epoxy blends, filled with zero- and two-dimensional carbon forms – graphite, carbon black (CB), and graphene (Gr) – were investigated over an extensive range of compositions and compression moulding pressures, in this study. Several compounding runs (using melt mixing), at different stages, followed by compression molding, were done. The goal is to obtain combination of composite formulation and processing conditions that will produce the most promising combination of properties for BP application. In the first stage of the investigations, by using thermogravimetric analysis, two-stage decomposition behavior of PP-epoxy and PE-epoxy blends was revealed, which confirms the immiscibility of PP and PE with epoxy resin. Scanning electron microscope (SEM) micrographs of the PP-epoxy and PE-epoxy blends revealed a co-continuous structure, which can be attributed to the close-to-symmetric composition of the blend and compatibilizers added. Preferential localization of synthetic graphite (SG), CB, and Gr in the polymer blends was also revealed by the SEM micrographs. This confirms the fact that CPCs based on PP-epoxy and PE-epoxy blends can be explored further. PP-epoxy and PE-epoxy blends filled with only SG, 30 – 80 wt %, were produced and characterized for their electrical conductivity and flexural properties. In-plane electrical conductivity ranged from 12.09 to 68.03 Scm-1 for PP-epoxy/SG and 11.68 to 72.96 Scm-1 for PE-epoxy/SG composites produced. These are higher than values reported for several single matrix polymer composites at similar filler loadings. With reference to the United States Department of Energy performance targets for BPs, PE-epoxy/SG composites performed better in terms of electrical conductivity, while PP-epoxy/SG composites exhibited better flexural properties. Thereafter, using SG and CB double filler, PE-epoxy/SG/CB composites performed better than PP-epoxy/SG/CB composites in terms of electrical conductivity, while PP-epoxy/SG/CB composites exhibited superior flexural properties than the PE-epoxy/SG/CB composites at similar filler loadings. However, with respect to the DOE targets, composites based on PP-epoxy blend exhibited a more promising combination of electrical conductivity and flexural properties than PE-epoxy blend matrix. PP-epoxy filled with SG/CB was studied further, by using graphene (Gr) as second minor filler. In-plane and through plane electrical conductivities as well as thermal conductivity and thermal diffusivity of the PP-epoxy/SG/CB/Gr composites increased as total filler content was increased from 65 to 85 wt%. It implies that more conductive networks between filler particles were formed. Also, flexural strength, flexural modulus, and impact strength decreased as the total filler content increased from 65 to 85 wt%. The reduced flexural properties could be due to increased agglomeration of CB and Gr, and poor filler wetting at higher filler loadings and low matrix material, which leads to the formation of microvoids and a reduction of the load bearing capacity of composites. With respect to the DOE targets, PP-EP/SG/CB/Gr composite with 80 wt% (i.e., PP/EP/73G/6.2CB/0.8Gr) filler has the best combination of properties. Further improvement in properties of the PP-EP/SG/CB/Gr composite with 80 wt% filler was achieved by molding at higher pressures. As molding pressure was increased from 4.35 to 13.05 MPa, in-plane electrical conductivity increased from 116.31 to 144.99 Scm-1, while flexural strength increased from 29.62 to 42.57 MPa, satisfying the performance requirement targets for bipolar plates.
Dynamic modelling of a bolted disc rotor assembly
(2008) Blignaut, Gert; Roberts, Johan
A project investigating the behaviour of an assembled preloaded rotor was performed for an M-Tech qualification in the Mechanical Engineering Department. Pre-Stressing of mechanical structures is widely applied to improve their performance, and in this project the behaviour of an assembled preloaded rotor was investigated. An Impact Test was done on the structure to see if induced stresses originated by a set of bolts which keep the discs system together, would influence the natural dynamic response or the rotor. Tendencies in the natural response were investigated. Analytical models like the Finite Element Beam model and the Solid Finite Element model were studied in order to find a represntative description of this particular structure's dynamic behaviour after pre-tension. From the experimental results it was apparent that the slenderness of the pre-tensioned sector influences the natural frequency. The solid finite element model appears to be the most applicable model to present the assembled rotor-disk system as a continuous shaft. Furthermore, modelling and predictions for a typical rotor and similar assembled structures can be generated from the findings.
Effect of fault and transmission error on a spur gear meshing stiffness by vibration and time-frequency techniques
(Vaal University of Technology, 2021) Yakeu Happi, Kemajou Herbert; Tchomeni, B. X., Dr.; Alugongo, A. A., Prof.
To meet the ever-increasing demand for maintenance of gear systems, industrial companies have traditionally depended on the shutdown of the machines before processing the fault diagnosis. Nowadays, online monitoring has proven to be effective in terms of machine state analysis and fault prediction. However, the application of such a technique in the analysis of combined multiple nonlinear faults is still a subject of research. The vibration signature of a coexisting nonlinear crack and pit in two-stage gear system is unknown, it can be regarded as one of the most difficult problems to extract and diagnose. Additionally, incorporating both a crack and a pit into numerical models is a time-consuming process that demands a breadth of mechanical understanding. Diagnostics of faulty gears, on the other hand, can be performed in the time and frequency domain or in the Time-Frequency domain, depending on the complexity of the vibration. Non-linear and non-stationary phenomena (Features) occur when repeated pitting and cracking faults occur, reducing the reliability of standard signal processing methods (Gear displacement and Fast Fourier Transform). This thesis solves each of these shortcomings by developing an eight-degree-of-freedom (DOF) gear model with a breathing crack and multiple pitted gear teeth. The identified spur-gear model enabled the investigation of the crack and pitting signatures and their effect on the ensuing vibrations independently of the action of other system components. Additionally, when pitting and cracking coexist, the study was conducted to determine the influence of such a failure on the transmission system. Theoretical results indicated that the presence of pitting and crack in the tooth of the gear resulted in a decrease in mesh stiffness. Additionally, the influence of the breathing pitting and crack results in material fatigue, which results in the generation of a random term in the vibration signal. To corroborate the acquired results, several experimental tests on a spur-gear test rig with a defined pit and crack size range were undertaken under a variety of conditions. In comparison to the presented methodologies, theoretical and experimental results indicate that 3D Frequency-RPM analysis is the most sensitive and resilient method for the early detection and identification of pit and crack faults. Furthermore, when crack or pit faults are studied individually, the STFT analysis yields interesting results. The feature analysis revealed that, when using the Time-Frequency technique, the crack and pit combination in a two-stage gear system is a priori more efficient than the other options.
Enhancing wind turbine performance in cold climate through analysis of aerodynamic lift and drag
(Vaal University of Technology, 2022) Odiagbe, Franklin Oyakhilomen; Masu, L. M., Prof.; Alugongo, A. A., Prof.
Wind energy is one of the most economically sustainable energy, and power plays an important role in the diversification of energy security. Renewable energy resources such as wind energy are constantly replenished, and it is inexhaustible. Being one of the most environment-friendly and renewable, wind energy attracts enormous interest globally. However, wind energy production faces severe challenges in harshly cold climates or low-temperature conditions. Cold weather reduces the aerodynamic lift and increases aerodynamic drag, as ice accretes on the wind turbine blades. Some of the best sites for wind farms installation are cold regions, where the air density is favourable because of low-temperature conditions. Super-cooled droplets and precipitation affect the wind turbine operations and change the aerodynamic profile of the blade through ice accretion. This dissertation focuses on enhancing wind turbine performance in cold climate through ice accretion. this dissertation focuses on enhancing wind turbine performance in cold climate through analysis of aerodynamic lift and drags, using Computational Fluid Dynamics (CFD) and electromagnetic radiation principles. This is based on heat and mass transfer mechanisms of operation to prevent ice accretion on the turbine blades. To optimize the large wind turbines operation in ice prone cold regions, it is important to better understand the ice accretion behaviour and its effects on aerodynamic performance and power production losses. Numerical simulations on ice accretion for wind turbine aerodynamic were carried out for glaze and rime ice conditons using ANSYS. A multiphase based Computational Fluid Dynamic ANSYS was used to analyse electromagnetic radiation and the heat and mass transfer on the wind turbine. Results show that icing on a wind turbine can be mitigated using electromagnetic radiation and heat and mass transfer. Conservation of momentum and heat and mass transfer was applied to determine the effect of ice accretion on aerodynamic lift and drag. Ice mainly accretes along the leading edge of blade profile, which changes the aerodynamics profile by increasing surface roughness and heat fluxes during glaze and rime ice accretion. The effect of electromagnetic radiation on icing time and wind turbine rotation speed were analysed using the working principle of the infrared thermography ice sensing technique. Data collected by infrared sensors were used to retrieve features and parameters (temperature, icing time and wind turbine rotation speed) of the observed surface, without physical contact. The performance of electromagnetic radiation on wind turbine blades aerodynamic forces using Computational Fluid Dynamics (CFD) with Navier-Stokes equations and heat and mass transfer has been analysed and recommendations were drawn from the conclusions. The results from this investigation shows that electromagnetic energies are promising techniques for measuring critical parameters such as wind speed, icing time, and temperature of ice accretion. The electromagnetic energy (thermal infrared sensor) also detects the presence, type, location, thickness, and rate of the ice on a blade's surface. The combination of passing temperature gradients of fluid and solid requires heat transfer. The flow rate of passing temperature through the process (heat and mass transfer) has an essential impact on the application of Navier-Stroke equations for fluid around the wind turbine aerodynamic coefficients.
Formulation of nanocellulosic fibres and particle fillers and their mono and hybrid reinforced polymer composites
(Vaal University of Technology, 2022-03) Webo, Wilson Wachuli; Nziu, P. K., Dr.; Masu, L. M., Prof.
This study aimed to focus on the state of knowledge and practice on natural fibres, extract cellulose, and subsequent formation of nanocellulosic fibres and particles from selected natural fibres. Moreover, both mono and hybrid composites were fabricated and modeled using ABAQUS software. After that, the formulation of the optimal mix ratio of both the fibres and the matrix for different mechanical applications was determined using the Minitab software version 2017. Sisal and rice husk were selected for this purpose due to their ease of availability. An experimental solution was used to extract cellulose, and, subsequently, nanofibres and nanoparticles were formed. These were then used to fabricate composites. The possibility of using new processing technology for modeling both sisal and rice husk nanocomposites in their mono and hybrid forms through the use of Finite Element Analysis (FEA) is a novel idea that has been explored in this study. Finite Element Analysis (FEA) using ABAQUS/CAE software version 2018 was used to develop novel models of mono and hybrid nanocomposites and to determine their mechanical properties of strength and stiffness. The finite element analysis method incorporates the effects of nonlinearities which are very common in composite fabrication. Furthermore, this study sought to formulate the optimal combination mix ratio of fibres and matrix for different mechanical applications using the Minitab software version 2017. A total of 198 finite element part models were created and analyzed. The Finite Element Analysis (FEA) models developed predicted correctly the flexural and tensile properties of the nanocellulosic composites that had been modelled. The experimental method was used to validate the FEA results. Moreover, analysis of variance (ANOVA) was performed for each property of the mono and hybrid composites in this study. The tensile and flexural properties of the mono and hybrid composites were found to gradually increase with an increase in fibre volume fraction up to a certain optimum point, beyond which they gradually began to reduce with more fibre additions. The analysis of variance of properties indicated that, for all the tensile and flexural properties under study, the F statistic > F critical, and the P-value < 0.05, implying that the tests were significant and there was a difference between the means of the groups. In the thermal analyses, the Thermogravimetric Analysis(TGA) graph exhibited three distinct sections: An initial flat section, then a section with a constant slope and finally a flat section. In the Dynamic Scanning Calorimetry (DSC) graphs, all the samples exhibited a glass transition temperature of between 50ºC and 75ºC. Furthermore, all the samples exhibited a melting temperature of between 350ºC and 400ºC. Overall, the hybrid nanocomposites had better tensile, flexural and thermal properties than the mono composites. Regarding the formulation of the optimal combination mix of the fibres and the matrix for use in different mechanical applications, this study established that: a ratio of fibres to a matrix of 4:1 is suitable for mechanical applications where all the flexural and tensile properties of the mono and hybrid composites are maximized. For applications that require the flexural properties of mono and hybrid nanocellulosic composites to be maximized, while the tensile properties of the mono and hybrid nanocellulosic composites are minimized, the ratio was found to be 1:2. A similar ratio was found to be suitable for applications that require the tensile properties of mono and hybrid nanocellulosic composites to be maximized while the flexural properties are minimized. For applications that required a target of 10 MPa for the tensile and flexural strength of mono and hybrid nanocellulosic composites and 10 GPa for the tensile and flexural stiffness of mono and hybrid nanocellulosic composites, this ratio was found to be 2:3.
Identification and analysis of steam temperature maldistribution in superheater tubes via measured and derived parameters
(Vaal University of Technology, 2019-08) Vilakazi, Lethukuthula Nokwazi; Rousseau, Pieter, Prof.; Alugongo, Alfayo, Prof.
Superheater and reheater heat exchangers in power plant boilers can experience temperature excursions and gradients significantly above design values due to cyclic operations. This may result in accelerated life consumption of these components. To understand better the influence of different operating conditions, research is ongoing to develop detailed thermo-fluid process models of the various boiler heat exchangers, and real-plant data are required in the validation of these models. In this study, the final superheater of a 620 MW coal-fired power plant unit was analysed based on real plant measurements taken during steady state operation at 100, 80 and 65 percent of the current boiler capacity. Process parameters routinely measured via the plant distributed control system (DCS), such as the steam temperatures, pressures and mass flow rates, were used as input data to derive other unmeasured parameters using the mass and energy balance (MEB) methodology. Thermocouples were installed previously on the inlet and outlet final superheater stub boxes as well as the outlet manifolds. Thermocouple data were collected from a data logger at the corresponding dates and times of the DCS MEB inputs. Measurement uncertainties were determined by considering instrument and statistical uncertainties, which were then propagated through the MEB calculation to the derived parameters. The MEB methodology was applied to determine the flue gas temperature and flow rates at different operating loads (65, 80 and 100 percent). The good comparison obtained between the values calculated with the MEB and those of the C-schedule for the 100 percent boiler maximum continuous rating (BMCR) provided confidence in the validity of the MEB. The MEB was also compared to real plant data of flue gas temperature. The comparison provided a difference that is less than 26℃. Identification of the measurement uncertainties provided a detailed analysis on each instrument and or measurement and how certain I could be about each measurement. Uncertainties of parameters derived using the MEB methodology were determined. This was achieved by uncertainty propagation through the MEB model. Uncertainty propagation also provided a sensitivity percentage relative to the propagated uncertainties. The extent of temperature maldistribution was determined based on the measured outside tube metal temperatures. The results from the thermocouple measurements on the steam pipes connected to the final super heater inlet and outlet manifold headers show that there is temperature maldistribution between the inlet headers of the four legs. There is also significant maldistribution at the outlet headers resulting in noticeable local temperature gradients. It can also be concluded that the low load of 65 percent resulted in the highest temperature maldistribution compared to the higher loads, of 100 and 80 percent. Super heater tube metal temperatures are exposed to high temperatures at low loads which may lead to tube leaks.
Impact of equal channel angular pressing operational parameters on the mechanical properties of characterized Titanium-Based powders
(Vaal University of Technology, 2020-03) Nhlapo, Mirriam; Machio, C., Dr.; Masu, L., Prof.
The powder metallurgy technique Equal Channel Angular Pressing is a severe plastic deformation method. Where desired microstructure and texture can be developed by applying temperature, changing the orientation of the billet through a number of successive passes. It has been extensively researched as a tool for processing solid metal, where it has been shown to provide components with ultra-fine grains that impart superior mechanical properties. The ECAP of titanium-based powder has the potential to greatly aid the cheaper production of titanium-based components. The main objective of this study was to investigate the fundamental interaction between titanium powder characteristics and ECAP process parameters to establish a one on one relationship between powder characteristics, ECAP process parameters and the mechanical properties of the green compacts. Six different titanium powders were used in this study to determine major powder characteristics such as particle size distribution, morphology, density, flowability and chemical composition. The powder was compacted using ECAP technique to produce billets which are also called green compacts. The green compacts were produced based on selective ECAP parameters which were considered effective. Thereafter the green compacts were analysed to check any improvement on mechanical and physical properties. The particle size distribution test results obtained agreed with the supplier’s particle sizes. Four of the powder’s compositions were found to be cohesive, the other two powders, one was freeflowing, the other one flowability could not be measured, due to large particle size distribution. The test results revealed that the morphology for all the powders was irregular with some powders showing angularity, others were dendritic. The tests revealed that the interstitial elements were within the required limits for all the compositions. After the ECAP process, it was found that particle size distribution alone has some effect on the mechanical properties of components. But the morphology, density, flowability and chemical composition, have major effect on mechanical properties of ECAP samples. The relative density was measured after ECAP process, the free-flowing and cohesive powders yielded a relative density of 90% and above, after ECAP first and second pass. The microhardness of the new ECAP billets was found to match that of steels and wrought iron. It was found that application of temperature, backpressure and great number of passes improved the mechanical and physical properties of the billets.
Improving the performance of membrane backwash system for efficient and stable wettability process during oil/water separation
(Vaal University of Technology, 2022-10) Mopeli, Motebang Josias; Alugongo, A. A., Prof.; Sob, P. B., Dr.; Tengen, T. B., Prof.
Membrane technology has enormous potential for oil/water separation applications. However, membrane performance is hampered by the ongoing fouling issue. Membrane fouling does not only affect the water permeability and separation efficiency but it also reduces the membrane lifespan. Numerous studies on backwash optimization have been conducted to reduce the fouling effect. However, it's worth mentioning that none of the studies in membrane backwash on oil/water separation applications has attempted to improve the cleaning procedure by identifying critical operation conditions through numerical model simulation and experimentation. In the past, accurate modelling of the backwash flow to dislodge the foulants found on the membrane pores or surfaces (concentration polarization) in pressure-driven membrane processes was hindered by complex couplings between the flow equations and the variable operating properties. However, the developed backwash model in this study is based on Navier Stokes laws which govern the entire flow field that incorporate the backwash media flow domain, and oil droplet dislodgement on the membrane domain. The varying nature of flow necessitates different modelling methodologies to help predict the behaviour of backwash flow. As such, Navier Stokes's laws governing the fluid flow were the obvious choice, due to their high accuracy and diversity in describing a whole set of flow phenomena:, from laminar to turbulent under Newtonian flow. Additionally, incorporating computational fluid dynamics (CFD) ANSYS Fluent numerical model simulation, as a preliminary evaluation tool to improve the backwash cleaning efficiency for oil/water separation application has shown to be an effective approach. This developed backwash model depends heavily on the backwash critical operating parameters such as temperature, driving back-pressure, and the subsequent backwash flow velocity. The theoretical numerical simulation model and experimental results were in good agreement that oil droplets can be dislodged effectively, only if the critical backwash operating conditions for oil/water separation application are identified and utilized. These critical operating parameters are identified to improve the backwash system, such as the thermal forces applied to lower the oil viscosity (critical temperature 0 65 c) and the critical pressure (190kpa) was subsequently utilized to loosen the interfacial tension force(adhesive forces). Consequently, the oil droplet blockage was easily dislodged by the backwash flowmedia (backwash velocity). Ultimately, this study investigated the dynamic relationship of the proposed critical operating parameters (temperature, pressure, and the subsequent backwash flow velocity) with membrane material capable of withstanding this proposed intense backwash procedure. The evaluation criteria were focused on permeate flux recovery, thermal stability, and the ability of the membrane to withstand harsh operating conditions during the backwash procedure. Consequently, this study developed an improved backwash cleaning procedure in relation to membrane material selection for efficient wettability which has proven to be an effective approach to control fouling during oil/water separation. According to the results obtained, the use of critical backwash pressure resulted in efficient fouling removal. In addition, the thermal stability of ceramic membrane permits the use of high temperature by backwash parameter to lower the oil droplets viscosity, subsequently allowing easy dislodgement of foulants found on the membrane structure. Consequently, this attempt to exploit the research gap in membrane backwash has led to this dissertation contributing to advancing new knowledge of membrane technology for application in oil/water separation. The contributions of the project includes: 1. Establishing the most efficient backwash process, through the identification of critical operating conditions (temperature, pressure, and the subsequent backwash flow velocity) using Navier Stokes laws under fluid flow modelling. 2. Designing a numerically simulated model by utilizing computational fluid dynamics (CFD) ANSYS Fluent software tool, as a preliminary evaluation measure, to validate an improved backwash procedure 3. Establishing an improved backwash cleaning procedure in relation to membrane material design for efficient wettability to mitigate fouling during oil/water separation.
Lateral-torsional stability for curved 6061-T6 structural aluminium alloys
(Vaal University of Technology, 2020-12-02) Tebo, E-P. T.; Nziu, P. K., Dr.; Masu, L. M., Prof.
Though aluminium (Al) is justifiably described as a green metal with an increasing rate of application in structures, designers still restrain themselves from its applications as a load-bearing skeleton in structure due to insufficient design guidelines. This insufficient information is more with channel sections that might experience lateral-torsional buckling (LTB) when used as a load-bearing skeleton in structures. This study investigates the effects on imperfections on LTB load-carrying stability for 6061-T6 Al alloy channel section arches and proposed design guidelines. The case study focused on freestanding circular fixed end arches subjected to a transverse point load at the shear centre. The software package Abaqus was used to study a total of 110 arch models from three separate channel sections with an additional 16 arch models for validation. Sixty-six channel arches were developed at a constant length, while the remaining 44 arches were formed at constant slender ratios using 11 discrete included angles. The FE analyses methods used for the investigation were validated with existing analytical methods and showed good agreement, despite the assumptions of the bilinear curve used for material nonlinearity, initial geometric imperfections and residual stresses that presented the imperfections of the models. The different investigated factors include slender ratios, change in cross-section area, imperfections, and angles. These factors were found to have substantial impacts on the prebuckling state, which turns to impact LTB behaviour and load-carrying capacity. From arches developed at constant span length, the arches with moderately included angles (50°≤2𝛼≤90°) were found suitable for the designs against LTB, followed by the shallow (2𝛼<50°) and deep arches (90°<2𝛼≤180°) respectively. For arches developed at constant slender ratios, the deep arches were found to be more suitable in the design against LTB, followed by the moderate and shallow arches, respectively. In addition, it was realised that the change in web-flange thickness, section depth and slender ratios, had significant effects on the LTB loads magnitudes and very insignificant effects on the general behaviour across the included angles. The same occurrence was also observed on the prebuckling analyses. All the investigated channel section arches showed the imperfections to have significant impacts on the LTB loads. Arches developed at constant span length showed the maximum elastic LTB loads to have overestimated the expected real LTB loads by approximately 48 percent. While the maximum elastic LTB loads of arches developed at 𝑆𝑟𝑥⁄= 60 and 90 showed that the real LTB loads were overestimated by about 39 and 14 percent, respectively. That said, the elastic LTB loads on average overestimated the real LTB loads by over 50 percent for the arches developed at the constant span length and by only 18 percent for arches developed at the constant slender ratios.
Mechanical and crystallisation properties of polyetheretherketone polymer from dry solid lubricants
(Vaal University of Technology, 2023-06) Ladipo, Taiwo Lolade; Nziu, P. K., Dr.; Masu, L. M., Prof.
Polyetheretherketone (PEEK) polymer suffers high viscosity during fused filament fabrication (FFF). Adding solid lubricants as fillers to PEEK should reduce its viscosity. However, very little research has been conducted on the tribological properties of PEEK printed using FFF. This study dealt with filament-making, tensile properties, crystallinity, and tribological characterisation of FFF-printed PEEK impregnated with dry solid lubricants. PEEK, graphite, and Molybdenum disulphide (MoS2) were sourced, tested, and analysed. Three active functional groups were found in the PEEK: oxy, phenyl, and carbonyl. While the MoS and graphite powder indicated sulfide and Carbon functional groups. The PEEK and both lubricants had an average particle diameter of 100 microns. Three weight ratios of each solid lubricant were mechanically blended into PEEK powder. Seven samples were prepared using a tabletop extruder. Each filament was 3D printed into 35 dog bones and seven discs for ultimate tensile testing, X-ray diffraction analysis (XRD), and disc on-pin testing. The diagrammatic Hermans-Weidinger approach with XRD analysis was used to evaluate the degree of crystallinity. It was established that the MoS2-filled PEEK is better than the graphite-filled PEEK. MoS2-filled PEEK reinforces as the weight content of MoS2 increases up to 104 MPa at 10 wt%. This reinforcement suggests perfect adhesion between PEEK and MoS2. On the other hand, the graphite-filled PEEK decreased in tensile strength to 36 MPa due to agglomeration at 10 wt% filling of graphite. No existing planar peaks were destroyed by introducing MoS2 and graphite, but a new peak was formed per solid lubricants and intensified on the increase of the solid lubricants. Generally, the MoS2-filled PEEK has the best-recorded crystallinity level at about 70% for 5 wt.% while the lowest recorded value was 3 wt.% of graphite valued at 31%. The tribological observation showed that the average response time for pure 3D-printed PEEK is about 950 seconds, indicating that FFF-printed PEEK has a weak capacity to minimise friction independently. Impregnating PEEK with MoS2 and Graphite decreases the response time by about 73%. However, the coefficient of friction of graphite-filled PEEK was initially reduced but started increasing after 5 wt.% due to agglomeration. The MoS2-filled PEEK has a minimum of 30 % decrease in the wear rate, while the graphite-filled PEEK initially decreases to 9% but later spikes to about 6% due to agglomeration. The agglomeration of Graphite in PEEK at a high weight fraction makes graphite a non-perfect choice for FFF compared to MoS2.
Mechanical shock values applied in condition monitoring of bearings operating under variable speed and load conditions
(2014-08) Olivier, Allan Andre; Alugongo, A. A.; Masu, L. M.
Monitoring the condition of equipment in industry is very important to prevent unplanned breakdowns and to prolong their life. This is necessary, since it is not always economically viable to stop equipment at regular intervals to do maintenance. Failure on machines can lead to high repair costs and production losses. It is thus of paramount importance that early failure symptoms be identified by means of condition monitoring. This study in the field of condition monitoring is performed to determine if the mechanical shock values induced in defect bearings could be used to measure the condition of a bearing while operating under variable speed and variable load. Variable speed and variable load is becoming more popular in industry because variable speed drives applications ensure effective process control. Variable speed application, cause fault frequencies to fluctuate and therefore vibration applications for constant speed applications, which are speed-dependent, can no longer apply. Vibration-monitoring techniques that have applied for many years have now become obsolete in these variable speed applications. Methods such as Short Time Fourier Transformation (STFT), time scale like wavelet transform, and Order tracking has been applied in variable speed applications with some success. These methods analyses the vibration phases on the signal buy compensating for the speed changes. In this thesis, the Shock pulse method is selected as the analyses tool to measure the mechanical shock. Shock pulse monitoring does not focus on the vibration phases but measures in a small-time window when mechanical shocks are induced in the bearing material before the vibration phase. There is very little documented research in the field of mechanical shock pulse monitoring for conditions of variable speed and variable loads, and therefore this research focuses on recording these mechanical shock values by empirical tests. The tests were performed on a bearing with an induced defect on the outer race. The rolling element of the bearing strikes the defect and the mechanical shock value (dBsv) is measured. The mechanical shock is measured with the Shock pulse method in a small-time window before vibration occurs. In this time window, the dBsv is recorded over time to provide diagnostic information of the bearing during acceleration, deceleration and various loading conditions. These mechanical shocks are elastic waves that mirror the impact-contact-force's time function and the Shock pulse monitoring accelerometer, which is tuned to 32 kHz, will respond to the elastic wave fronts with transient amplitudes proportional to the square of the impact velocities. The mechanical shock values were analysed and reoccurring fault levels were identified on each empirical test. These recurring events from the empirical tests were used as primary data for analysis in this research. These tests were performed on a bearing with an induced failure and it was found that the dBsv measured over time could not be used to monitor the condition of the bearing under variable speed applications. This was because the dBsv changed as the speed increased. To overcome this problem Sohoel’s theory was applied and the initial mechanical shock value (dBi) was calculated for the bearing. The dbi value was subtracted from the dBsv and a value called the maximum mechanical shock value (dBm) was obtained. The dBm values stayed constant for the duration of the test and this allowed the condition of the bearing to be measured under variable speed and variable load conditions with some exception. The exception to the findings was that the dBm values stayed constant during acceleration phases, but during the deceleration phases the values were erratic and scattered. At speed below 200rpm the dBm values did not stay constant and therefore it was concluded that the dBm value recorded the best results only when thrust on the bearing was maximum. The other exception was under no-load conditions. The values were erratic and scattered, and therefore the results were not a true reflection of the bearing condition. The third exception was that the results on bearings with various loads remained constant during increased load changes unless the loading was erratic. During erratic load changes, the results were affected. The results also indicated that the larger the defect on the bearing raceway, the higher the dBm values were. Multipil defects on the bearing race ways were not part of this thesis and this gives an opertunity for futher research. The Shock pulse monitoring technique was 100% successful in monitoring the bearing condition only while the speed of the bearing was increasing. The results obtained in this work demonstrated that the condition of bearings can be monitored in applications of variable speed and variable load if the exception are eliminated and to obtain conclusive results the mechanical shock pulses should be measured over time and not be used as once-off value.
Modelling stain rate sensitive nanomaterials' mechanical properties: the effects of varying definitions
(Vaal University of Technology, 2016-06) Sob, Peter Baonhe; Alugongo, A. A. Prof.; Tengen, T. B., Prof.
Presently there exist a lot of controversies about the mechanical properties of nanomaterials. Several convincing reasons and justifications have been put forward for the controversies. Some of the reasons are varying processing routes, varying ways of defining equations, varying grain sizes, varying internal constituent structures, varying techniques of imposing strain on the specimen etc. It is therefore necessary for scientists, engineers and technologists to come up with a clearer way of defining and dealing with nanomaterials’ mechanical properties. The parameters of the internal constituent structures of nanomaterials are random in nature with random spatial patterns. So they can best be studied using random processes, specifically as stochastic processes. In this dissertation the tools of stochastic processes have been used as they offer a better approach to understand and analyse random processes. This research adopts the approach of ascertaining the correct mathematical models to be used for experimentation and modelling. After a thorough literature survey it was observed that size and temperature are two important parameters that must be considered in selecting the relevant mathematical definitions for nanomaterials’ mechanical properties. Temperature has a vital role to play during grain refinement since all severe plastic deformation involves thermomechanical processes. The second task performed in this research is to develop the mathematical formulations based on the experimental observation of 2-D grains and 3-D grains deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing. The experimental observations revealed that grains deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing are elongated when observed from the rolling direction, and transverse direction, and equiaxed when observed from the normal direction. In this dissertation, the different experimental observations for the grain size variants during grain refinement were established for 2-D and 3-D grains. This led to the development of a stochastic model of grain-elongation for 2-D and 3-D grains. The third task was experimentations and validation of proposed models. Accumulative Roll-Bonding, Equal Channel Angular Pressing and mechanical testing (tensile test) experiments were performed. The effect of size on elongation and material properties were studied to validate the developed models since size has a major effect on material’s properties. The fourth task was obtaining results and discussion of theoretical developed models and experimental results. The following facts were experimentally observed and also revealed by the models. Different approaches of measuring grain size reveal different strains that cannot be directly obtained from plots of the corresponding grain sizes. Grain elongation evolved as small values for larger grains, but became larger for smaller grains. Material properties increased with elongation reaching a maximum and started decreasing as is evident in the Hall-Petch to the Reverse Hall-Petch Relationship. This was alluded to the fact that extreme plastic straining led to distorted structures where grain boundaries and curvatures were in “non-equilibrium” states. Overall, this dissertation contributed new knowledge to the body of knowledge of nanomaterials’ mechanical properties in a number of ways. The major contributions to the body of knowledge by his study can be summarized as follows: (1) The study has contributed in developing a model of elongation for 2-D grain and 3-D grains. It has been generally reported by researchers that materials deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing are generally elongated but none of these researchers have developed a model of elongation. Elongation revealed more information about “size” during grain refinement. (2) The Transmission Electron Microscopy revealed the grain shape in three directions. The rolling direction or sliding direction, the normal direction and the transverse direction. Most developed models ignored the different approaches of measuring nanomaterials’ mechanical properties. Most existing models dealt only with the equivalent radius measurement during grain refinement. In this dissertation, the different approaches of measuring nanomaterials’ mechanical properties have been considered in the developed models. From this dissertation an accurate correlation can be made from microscopy results and theoretical results. (3) This research has shown that most of the published results on nanomaterials’ mechanical properties may be correct although controversies exist when comparing the different results. This research has also shown that researchers might have considered different approaches to measure nanomaterials’ mechanical properties. The reason for different results is due to different approaches of measuring nanomaterials’ mechanical properties as revealed in this research. Since different approaches of measuring nanomaterials’ mechanical properties led to different obtained results, this justify that most published results of nanomaterials’ mechanical properties may be correct. This dissertation revealed more properties of nanomaterials that are ignored by the models that considered only the equivalent length. (4) This research has contributed to the understanding of nanomaterials controversies when comparing results from different researchers.
Modification of ceramic membrane surface by nanoparticle coating for improved wettability during oil-water separation
(Vaal University of Technology, 2022-03-07) Maome, Tshepo G.; Alugongo, Alfayo A., Prof.; Sob, Peter B., Prof. (Assistant); Tengen, Thomas B., Prof.
The developed oil-water separation membranes used in membrane technology are currently inefficient due to their poor morphological and topographical properties during nanoparticle coating. Researchers have developed different wettable membrane surfaces using jet spray coating. Most of these developed membranes are inadequate due to poor morphological and topographical properties normally observed as clusters, creating a rougher membrane surface that hinders wettability. This has resulted in the existing membrane fouling and degradation during the oil/water separation process and again due to different responses to corrosion and rusting. In the current study, membrane clusters were minimised on the ceramic membranes to create a smoother surface, improving membrane wettability. These clusters were minmised at optimal coating force, optimal coating distance and optimal coating angle. Part one of the study was to model and simulate different parameters that decreased clusters using the jet-spray coating. A theoretical model was derived from the first principles and all the external and internal forces that impact membrane clusters were considered during the model derivation. These forces are the force due to applied pressure from the spray gun, the force of nano-particles, the force of viscosity, the upward force on solid wall due to nanoparticles, the downward force on solid wall due to nanoparticles and the reaction force on the solid wall due to nanoparticles. The tools of stochastic theory and the concept of fluid dynamics were used in the modelling process. The total coating force from the jet spray gun nozzle was increased from 0,2x107 kN to 2,4x107 kN, which gave optimal coating force. The coating distance from the jet spray gun nozzle to the membrane surface was increased from 10 mm to 24 mm, which gave optimal coating distance. The jet spray angle in the spray region was also increased from 1⁰ to 9⁰ with reference from the vertical axis to the membrane surface, which gave optimal coating angle. This lead to optimal spread of nanoparticles on the membrane surface thus resulting to optimal cluster minimisation during the coating process. This decreased cluster sizes during nanoparticle coating, resulting in a smooth membrane surface, thus leading to lowered surface energy on the membrane. Part two of the study was to fabricate the ceramic membrane with fewer clusters on the surface for improved wettability using the jet-spray coating. It was important to produce the ceramic membrane surfaces with minimised membrane clusters by considering the optimal parameters revealed to minimise these membrane clusters during coating. Nanoparticle coating was performed under a controlled laboratory environment, and the optimal parameters that were studied to minimise membrane clusters were revealed. These parameters are coating force, coating distance and coating angle. More coating rounds were applied on ceramic samples and clusters were minimised during these coating rounds. The coated samples were analyzed by a scanning electron microscope and the nanoparticles on the membrane surfaces were characterised for optimal performance during oil-water separation. The scattering, orientation, morphology, spatial distribution, surface roughness, surface smoothness, contact angles, surface density of the particles, pore size network, mean size of the coated nanoparticle on the membrane surface after different coating rounds were characterised and analysed to minimise membrane cluster during nanoparticle coating. It was shown that more clusters were observed in 1st LP, 2nd LP, 3rd LP and 4th LP coating rounds when compared to 1st HP, 2nd HP, 3rd HP and 4th HP coating rounds. It was also shown that material surface roughness increased the formation of clusters in membrane surface as more clusters were observed in rough membrane surface when compared to the smooth membrane surface. The microstructure revealed a smoother membrane surface where membrane clusters were minimised. Part three of the study was to compare the newly designed ceramic membrane with the previously designed ceramic membrane from previous the literature. The correlation was done on the experimental results obtained in this study with the experimental results obtained from the previous literature. Different coating rounds were performed from the current study and the previous literature to design nanostructured ceramic membranes with fewer clusters on the surface. The results in the last coating round in this study, revealed a smooth membrane with a homogeneous substrate with fewer clusters and small sizes compared to other coating rounds.
Optimal geometric configuration of a cross bore in high pressure vessels.
(Vaal University of Technology, 2018-04) Nziu, P. K.; Masu, L. M., Prof.
The purpose of this study was to develop analytical and numerical solutions to be used in the design of thick walled high pressure vessels for optimal location of a cross bore. In addition, the effects of internally applied combined thermo-mechanical loading on Stress Concentration Factor (SCF) on these vessels, was also evaluated. An analytical solution, to predict principal stresses on radial circular cross bore, was developed. The developed analytical solution was verified using finite element analysis methods. An optimisation process, using finite element analysis, was further done to determine the optimal combination of the major cross bore geometry that affect stress concentration. The cross bore geometries that were studied included the size, shape, location, obliquity and thickness ratio. The geometrically optimised cross bore was then subjected to combined thermo-mechanical loading to determine the resulting stress concentration effects. A total of 169 finite element part models were created and analysed. Seven thick walled cylinders having either circular or elliptical shaped cross bore positioned at radial, offset or and inclined were investigated. The analytical solution developed correctly predicted all the radial stresses at the intersection of the cross bore and main bore. However, out of 35 studied models, this analytical solution predicted the magnitude of hoop stresses in 9 models and that of axial stresses in 15 models correctly. The lowest SCF given by the radial circular cross bore was 2.84. Whereas, the SCF due to offsetting of the same cross bore size reduced to 2.31. Radial elliptical shaped cross bore gave the overall lowest SCF at 1.73. In contrast, offsetting of the same elliptical shaped cross bore resulted in tremendous increase in SCF magnitude exceeding 1.971. Additionally, the magnitudes of SCF were observed to increase whenever the circular offset cross bores were inclined along the RZ axis of the cylinder. The hoop stress due to internally applied combined thermo-mechanical loading increased gradually with increase in temperature until it reached a maximum value after which it began to fall sharply. In contrast, the corresponding SCF reduced gradually with increase in temperature until it reached a uniform steady state. After which, any further increase in temperature had insignificant change in stress concentration factor. The optimal SCF magnitude due to combined thermo-mechanical loading was 1.43. This SCF magnitude was slightly lower than that due to the pressure load acting alone.
Optimal geometric configuration of a cross bore in thick compound cylinders
(Vaal University of Technology, 2021-09) Kiplagat, N.; Nziu, P. K., Dr.; Masu, L. M., Prof.
The purpose of this research was to develop optimal numerical solutions that can be employed during the design of cross bored thick-walled compound cylinders. The geometric design parameters of a cross bored compound cylinder that were optimized include shrinkage pressure, cross bore size, shape, location, and obliquity. Finite Element Analysis (FEA) modeling software called Abaqus version 2019 was used to generate numerical solutions. A total of 48 different part models were created and analyzed in this work. The generated FEA results from these models were validated using analytical solutions developed from Lame’s theory. The effects of shrinkage pressure on hoop stresses and Stress Concentration Factor (SCF) were studied to determine the optimal conditions. The optimum shrinkage pressure obtained was henceforth used for further analysis in this work. In addition, using one factor at time optimization technique, an optimization process was carried out to determine the optimal combination of the cross bore configuration geometry that gives minimum SCF. These parameters of cross bore configuration geometry include different sizes of either circular or elliptical-shaped cross bore, positioned at radial, offset, and/or inclined. The analyses of the effects of shrinkage pressure ranging from 4.4733 to 223.662 MPa on 11 different part models, established that the shrinkage pressure of 89.464 MPa generated the minimum SCF magnitude of 3.02. After analyzing 8 different circular cross bore size ratios ranging from 0.1 to 0.8, at the radial position, it was established that the hoop stress increases with an increase in a cross bore size. The smallest cross bore size ratio of 0.1 gave the lowest hoop stress and minimum SCF of 3.02. Whereas the highest stress was developed at the cross-size ratio of 0.8 with an SCF magnitude of 6.75. The minimum magnitude of SCF translates to a reduction of the pressure carrying capacity of the compound cylinder by 67% than a similar plain compound cylinder. Generally, offsetting of the circularly shaped cross bore from the radial position, led to a reduction of the magnitude of SCFs. For instance, from the 8 offset positions analyzed, the minimum SCF occurred at the offset position of 0.006 m with a magnitude of 2.50. This SCF magnitude indicated a reduction of pressure carrying capacity of 60% in comparison to a similar plain compound cylinder. Evaluation of 12 different diameter ratios of elliptical-shaped cross bore ranging from 0.5 to 10, at the radial position, established the lowest SCF magnitude of 1.33 that occurred at a diameter ratio of 5. Henceforth, this optimum diameter ratio was used for further analysis. This aforesaid SCF magnitude translated to a reduction of the pressure carrying capacity of the compound cylinder by 24.81% when compared to a similar plain compound cylinder. Besides, offsetting of elliptically shaped cross bore generally decreased the magnitudes of SCFs. Therefore, for elliptically shaped cross bore, the lowest SCF occurred at radial position with magnitude of 1.33. A general comparison between the effects of circular and elliptical cross bore, established that the elliptical-shaped cross bores generated both lower hoop stresses and SCFs than those of circularly shaped cross bores. On the other hand, oblique elliptical offset cross bores along the Z-axis of the compound cylinder led to an increase in SCFs. As the oblique angle increased from 0 0 to 75 0, the SCFs also increased progressively, however, there was a significant increase in SCF when the inclination angle increased from 60 0 to 75 0. The lowest and highest SCF magnitude was 1.52 and 1.92 at 15 0 and 6.19 at 75 0, respectively. Overall, the optimum geometric configuration of a cross bore in a thick compound cylinder was found to be elliptically shaped, offset at radial position which is an obliquity angle of 0 0 having a diameter ratio a/b of 5.
Optimal strength of carbon fibre overwrapped composite high-pressure vessels
(Vaal University of Technology, 2021-12-08) Numbi, M. N.; Nziu, P. K., Dr.; Masu, L. M., Prof.
The purpose of this study was to design a composite overwrapped pressure vessel by combining the best optimal structural options. This study investigated the effects of constituents such as fibre and shell thickness, on the bursting strength. Thereafter, these constituents were combined in order to achieve optimization of strength for an improved sustainable composite pressure vessel. The analytical method was carried out using the Tsai-wu failure theorem. The developed analytical equations were solved with Matlab 2016 software to determine composite fibre and shell thickness. With variation of the vessel’s liner, a total of 56 parts were created on two different profiles with purpose of generating of vessels resistant to bursting failure. Henceforth, the structural integrity of fibre imparted into the design was optimally analyzed at an angle of 55⁰, through the negative and positive directions. The shell thickness overwrapping the liner, being as well an influential factor to this optimization, was, therefore, analyzed on symmetrical and asymmetrical lamination patterns. The optimal fibre and shell thickness range were thereafter determined on a first ply failure and hoop stress threshold approach. Additionally, the identified optimal range of pressure vessel constituents were numerically validated, on Abaqus/CAE software, to have a degree of reassurance on the result generated, using Hashin failure criteria. Optimal design with improved strength and weight factor was therefore achieved by combining the generated optimal vessel constituents yielded from Minitab software version 2016. The generated results of the study revealed no change on the fibre thickness determined with respect to direction. For shell thickness on the other hand, asymmetrical pattern was identified as the desired sequence of lamination. In addition, with two profiles considered in the research, the composite constituents were found for a p value of 0.066, to be optimal on profile 1 at 0.0048 mm of liner, 0.0005 mm of fibre and 0.0027 mm of shell. The profile 2 on the other hand, revealed optimization of liner at 0.0095 mm, fibre at 0.0021 mm and shell at 0.0055 mm. Through combination of these ultimate constituents the response optimizer on Minitab software generated optimal bursting strength with factor of 4% improvement with a weight reduction of 33% compared to the stainless steel vessel. It was, therefore, concluded that profile 1 was the most optimal with hoop strength of 123.43 MPa, Von Mises of 178.56 MPa and Tresca of 179.48 MPa.

Browse

Browsing Theses and Dissertations (Mechanical Engineering) by Title

Results Per Page

Sort Options