### Browsing by Author "Bello-Ochende, Tunde"

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- ItemOpen AccessA numerical protocol for death-time estimation(2021) Mfolozi, Sipho; Malan, Arnaud George; Bello-Ochende, Tunde; Martin, Lorna JeanA body's axial temperature distribution at death was experimentally demonstrated by the author to predict the postmortem temperature plateau (PMTP), which is known to affect the measured core temperature value and hence death-time estimation. Yet today's methods of death-time estimation apply only a single-point approximation of a body's core temperature in life as well as a single-point measurement of a body's core temperature after death. Four studies were carried out to understand the relationship between a body's axial temperature distribution and the PMTP. The first study numerically approximated antemortem temperature distribution in an MRI-built, high-definition, anatomicallys egmented 3D computational human phantom consisting of several hundred tissues. Metabolic heat generation (QQmm) and blood perfusion (wwbb) parameters were applied to all thermogenic tissue using the Pennes BioHeat Model. The study demonstrated that the antemortem axial temperature distribution was nonlinear, that tissue temperature distribution was inhomogeneous, and that the position and size of the antemortem central isotherm was predicted by the size, shape and location of the most thermogenic internal organ in a given axial plane. Numerical approximation of a bodyâ€™s antemortem axial temperature distribution using this studyâ€™s materials and methods was proposed for death-time estimation. The second study examined postmortem axial heat transfer. The approximated antemortem axial temperature distribution constituted the initial condition. QQmm and wwbb were set to zero to simulate death. Postmortem cooling was simulated in still air, on a cold concrete floor and on a heated floor. The antemortem central isotherm that single-point core thermometry detects was the PMTP. Its size at death, body radius, axial thermometry-depth and length of the postmortem interval (PMI) all predicted PMTP length. The cold concrete floor shifted the central isotherm away from the floor, while the heated floor shifted it towards the floor. Ground temperature and material properties, along with the aforementioned PMTP predictors, result in variation in measured single-point core thermometry values, yet todayâ€™s death-time estimation methods do not measure, approximate or standardise them. This is a source of uncertainty. This study demonstrated that a bodyâ€™s postmortem axial thermal profile was very specific to the PMI at which it exists, including during the PMTP that single-point core thermometry detects. This study proposed a bodyâ€™s measured postmortem axial thermal profile for death-time estimation to reduce PMTP uncertainties. The study also proposed numerical modelling of the ground, its temperature and material properties. The third study proposed a multipoint axial thermometry (MAT) device to measure a bodyâ€™s postmortem axial thermal profile. The author designed the device prototype. Its fabrication was outsourced. Empiric and numerical MAT studies were conducted on a cooling dummy and 3D human phantom, respectively. MAT curves indicated a parabolic shape. The fourth study proposed a numerical protocol for death-time estimation that iteratively tested a MAT profile measured at an unknown PMI from a decedent using the proposed MAT device against MAT profiles predicted by numerical simulations of sequentially longer candidate PMIs. A candidate PMI whose MAT profile matched was considered the PMI estimated by the protocol. The proposed protocol applied the exact historical meteorological temperatures that existed during the final estimated PMI. Application of the protocol was demonstrated using a fictitious scenario in which a candidate PMI within 120s of the final estimated PMI was excluded. Potential sources of uncertainty of the proposed protocol were discussed and concluding remarks on future research were made.
- ItemOpen AccessEffects of thermal stresses on Pressurised Water Reactor nuclear containment vessels following a Loss of Coolant Accident with assimilated containment filtered venting system(2020) Hartnick, Angelo; Bello-Ochende, TundeIn a nuclear power plant, the last barrier under normal and accident operations is the containment building. This is normally constructed from concrete reinforced with steel bars, which are prestressed to enhance the overall capability to withstand thermodynamic stresses like over-pressurisation and high temperatures. The failure of this final barrier will lead to the release of radioactivity to the surrounding environment. To examine the effects of thermo-hydraulic stresses on PWR containment following a LOCA, a model is proposed with simulated scenarios performed at the Koeberg Nuclear Power Station as a case study. The accidents were simulated using the Koeberg engineering simulator to obtain the output data. The scenario for the proposed model correlates the critical mass flow from a double-ended guillotine break to the containment pressure and temperature increase. Different containment filtered venting systems (CFVS) are also investigated in this study as severe accident management systems. CFVS have historically been included in boiling water reactor (BWR) designs, but following the Fukushima Daiichi nuclear accident, they are being introduced as severe accident management systems to manage the threat of containment over-pressurisation in pressurised water reactors (PWR). Finally, the rate of change in containment pressure and temperature is analysed and compared to literature, with the incorporation of a simulated filtered venting system to the PWR containment building.
- ItemOpen AccessExergy analysis of a Stirling cycle(2017) Wills, James Alexander; Bello-Ochende, TundeIn this dissertation the analysis of the Stirling engine is presented, this research topic falls within the category of thermal energy conversion. The research that was conducted is presented in three chapters of which the topics are: the effects of allocation of volume on engine performance, the GPU-3 (Ground Power Unit - developed by GM) Stirling engine analysis, and the optimisation of a 1000 cmÂ³ Stirling engine with finite heat capacity rates at the source and the sink. The Stirling engine has many advantages over other heat engines, as it is extremely quiet, has multi-fuel capabilities and is highly efficient. There is also significant interest in using Stirling engines in low to medium temperature solar thermal applications, and for waste heat recovery. To develop high-performance engines that are also economically viable, advanced mathematical models that accurately predict performance and give insight into the different loss mechanisms are required. This work aims to use and adapt such a model to analyse the effects of different engine parameters and to show how such a model can be used for engine optimisation using the Implicit Filtering algorithm. In the various analyses that are presented, the dynamic second order adiabatic numerical model is used and is coupled to equations that describe the heat and mass transfer in the engine. The analysis shows that the allocation of volume has a significant effect on engine performance. It is shown that in high-temperature difference (HTD) engines, increasing dead-volume ratio increases efficiency and decreases specific work output. In the case of low-temperature difference (LTD) and medium-temperature difference (MTD) engines, there is an optimal dead-volume ratio that gives maximum specific work output. It was also found that there are optimal swept volume ratios and that the allocation of heat exchanger volume has a negligible effect on engine performance - so long as the dead-volume ratio is optimal. The second order model with irreversibilities included was used to perform an exergy analysis of the GPU-3 Stirling engine. This model compared well with experimental results and the results from other models found in the literature. The results of the study show the two different approaches in modelling the engine losses and the effect that the various engine parameters have on the GPU-3 power output and efficiency. The optimisation of the 1000 cmÂ³ Stirling engine was performed using a model with finite heat capacity rates at the source and the sink, fixed number of heater and cooler tubes, and four different regenerator mesh types. The engine geometry was optimised for maximum work output using the implicit filtering algorithm, and the results show the dominant effect that the regenerator has on engine performance and the geometry that gives maximum work output. The critical insights obtained from this research are the importance of the dead-volume ratio in engine analysis, the merits of the novel Second law Stirling engine model, and the importance of regenerator mesh choice and geometry. The Implicit filtering algorithm is also shown to be a suitable choice of optimisation algorithm to use with Stirling engine mathematical models.
- ItemOpen AccessNumerical Design of a 3-Stage Cascaded Thermal Energy Storage System for Solar Application(2023) Oguike, Chimezie; Bello-Ochende, TundeThe analysis of a three-stage cascaded thermal energy storage is presented in this dissertation. Cascaded thermal energy storage systems has many advantages over conventional thermal energy storages, majorly it allows for maintaining of a nigh-constant temperature between the HTF and PCM during the charging and discharging cycles leading to improved performance of the system. This dissertation investigates the performance and transient response of a packed bed operating under high-temperature conditions with phase change materials in varying encapsulations (cascaded in a three-stage format) during charging and discharging cycle by employing computational numerical techniques via commercially available ANSYS Fluent software. The analysis was performed for nine different encapsulation geometries with increased surface area and constant volume in comparison to the base geometry (sphere) to determine the effects of each new encapsulation on the performance of the thermal energy storage (TES). The computational model used in the development of this work compares well with the experimental results by Raul [1]. Additionally, the effect of packing scheme/PCM layout is also investigated in this work. Comparative data analysis was performed on the TES with the various PCM encapsulation designs and the standard spherical PCM encapsulation to determine which geometry provides better performance during charging and discharging cycles. The results of this study show that the thermal performance of the cascaded thermal energy storage improves with each new encapsulation as evidenced by the decreases in charging and discharging times in comparison to the base encapsulation. This study also highlights which capsule design is most practical when considering the bed dimension increases/ decreases with in increasing thermal performance. This study's findings can serve as a benchmark for future optimization of cascaded thermal energy storage systems.
- ItemOpen AccessNumerical investigation of the convective heat transfer coefficient of the human body using a representative cylindrical model(2017) Eferemo, Daniel; Bello-Ochende, Tunde; Malan, Arnaud GThe principal objective of this study is to investigate, develop and verify a framework for determining the convective heat transfer co-efficient from a cylindrical model that can easily be adaptable to more complex geometry - more specifically the human body geometry. Analysis of the model under forced convection airflow conditions between the transition velocity of about 1m/s - calculated using the Reynolds number - up until 12m/s were carried out. The boundary condition, however, also included differences in turbulence intensities and cylinder orientation with respect to wind flow (seen as wind direction in some texts). A total of 90 Computational Fluid Dynamic (CFD) calculations from these variations were analysed for the model under forced convective flow. Similar analysis were carried out for the model under natural convection with air flow velocity of 0.1m/s. Here, the temperature difference between the model and its surrounding environments and the cylinder orientation with respect to wind flow were varied to allow for a total of 15 CFD analysis. From these analysis, for forced convection, strong dependence of the convective heat transfer coefficient on air velocity, cylinder orientation and turbulence intensity was confirmed. For natural convection, a dependence on the cylinder orientation and temperature difference between the model and its environment was confirmed. The results from the CFD simulations were then compared with those found in texts from literature. Formulas for the convective heat transfer coefficient for both forced and natural convection considering the respective dependent variables are also proposed. The resulting formulas and the step by step CFD process described in this thesis provides a framework for the computation of the convective heat transfer coefficient of the human body via computer aided simulations. This framework can easily be adaptable to the convective heat transfer coefficient calculations of the human body with some geometric modelling adjustments, thus resulting in similar representative equations for a human geometric model.
- ItemOpen AccessOptical and Thermal Analysis of a Heteroconical Tubular Cavity Solar Receiver(2018) Maharaj, Neelesh; Bello-Ochende, TundeThe principal objective of this study is to develop, investigate and optimise the Heteroconical Tubular Cavity receiver for a parabolic trough reflector. This study presents a three-stage development process which allowed for the development, investigation and optimisation of the Heteroconical receiver. The first stage of development focused on the investigation into the optical performance of the Heteroconical receiver for different geometric configurations. The effect of cavity geometry on the heat flux distribution on the receiver absorbers as well as on the optical performance of the Heteroconical cavity was investigated. The cavity geometry was varied by varying the cone angle and cavity aperture width of the receiver. This investigation led to identification of optical characteristics of the Heteroconical receiver as well as an optically optimised geometric configuration for the cavity shape of the receiver. The second stage of development focused on the thermal and thermodynamic performance of the Heteroconical receiver for different geometric configurations. This stage of development allowed for the investigation into the effect of cavity shape and concentration ratio on the thermal performance of the Heteroconical receiver. The identification of certain thermal characteristics of the receiver further optimised the shape of the receiver cavity for thermal performance during the second stage of development. The third stage of development and optimisation focused on the absorber tubes of the Heteroconical receiver. This enabled further investigation into the effect of tube diameter on the total performance of the Heteroconical receiver and led to an optimal inner tube diameter for the receiver under given operating conditions. In this work, the thermodynamic performance, conjugate heat transfer and fluid flow of the Heteroconical receiver were analysed by solving the computational governing Equations set out in this work known as the Reynolds-Averaged Navier-Stokes (RANS) Equations as well as the energy Equation by utilising the commercially available CFD code, ANSYS FLUENTÂ®. The optical model of the receiver which modelled the optical performance and produced the nonuniform actual heat flux distribution on the absorbers of the receiver was numerically modelled by solving the rendering Equation using the Monte-Carlo ray tracing method. SolTrace - a raytracing software package developed by the National Renewable Energy Laboratory (NREL), commonly used to analyse CSP systems, was utilised for modelling the optical response and performance of the Heteroconical receiver. These actual non-uniform heat flux distributions were applied in the CFD code by making use of user-defined functions for the thermal model and analysis of the Heteroconical receiver. The numerical model was applied to a simple parabolic trough receiver and reflector and validated against experimental data available in the literature, and good agreement was achieved. It was found that the Heteroconical receiver was able to significantly reduce the amount of reradiation losses as well as improve the uniformity of the heat flux distribution on the absorbers. The receiver was found to produce thermal efficiencies of up to 71% and optical efficiencies of up to 80% for practically sized receivers. The optimal receiver was compared to a widely used parabolic trough receiver, a vacuum tube receiver. It was found that the optimal Heteroconical receiver performed, on average, 4% more efficiently than the vacuum tube receiver across the temperature range of 50-210â„ƒ. In summary, it was found that the larger a Heteroconical receiver is the higher its optical efficiency, but the lower its thermal efficiency. Hence, careful consideration needs to be taken when determining cone angle and concentration ratio of the receiver. It was found that absorber tube diameter does not have a significant effect on the performance of the receiver, but its position within the cavity does have a vital role in the performance of the receiver. The Heteroconical receiver was found to successfully reduce energy losses and was found to be a successfully high performance solar thermal tubular cavity receiver.
- ItemOpen AccessOptimisation of feedwater heaters and geothermal preheater in fossil-geothermal hybrid power plant(2019) Nsanzubuhoro, Christa; Bello-Ochende, Tunde; Malan, ArnaudSufficient energy supply is a fundamental necessity for the stimulation of socio-economic advancement. However, the current rapid rise in urbanisation has resulted in the significant increase in energy demands. Consequently, the current conventional energy supply systems are facing numerous challenges in meeting the world's growing demand for energy sustainably. Thus, there is an urgent and compelling need to develop innovative, more effective ways to integrate sustainable renewable energy solutions into the already existing systems or better yet, create new systems that all together make use of renewable energy. This research aims to investigate and establish the optimum working conditions of a feedwater heater and geothermal preheater in a power plant that makes use of both renewable and non-renewable energy resources, where renewable energy (geothermal energy) is used to boost the power output in an environmentally sustainable way. Henceforth, a simplified model of a Rankine cycle with single reheat and regeneration and another model with a geothermal preheater substituting the low-pressure feedwater heater were designed. The Engineering Equations Solver (EES) software was used to perform an analysis of the thermodynamic performance of the two models designed. The models were used to analyse the energetic and exergetic effects of replacing a low-pressure feedwater heater with a geothermal preheater sourcing heat from a low temperature geothermal resource (temperature generally < 150Â°C). The results of this research work reveal that the replacement of the low-pressure feedwater heater with a geothermal preheater increases the power generated since less heat is bled from the low-pressure turbine (allowing more heat energy from the steam to be converted into mechanical energy in the turbine). Applying the principle of the Second Law of thermodynamics analysis, the Number of Entropy Generation Units (EGU) and Entropy Generation Minimisation (EGM) analysis were employed to optimise the designed hybrid system. The feedwater heaters and geothermal preheater were modelled as counter-flow heat exchangers and a downhole co-axial heat exchanger, respectively. The feedwater heaters were optimised by means of the method of Number of Entropy Generation Units whereas the geothermal preheater was optimised by means of the Entropy Generation Minimisation analysis method. Owing to the optimisation of these components, the operating conditions of the boiler and turbines were secondarily improved. Overall, this research emphasises the impact renewable energy has on major power plant systems that are in operation and run on non-renewables.
- ItemOpen AccessSensitivity analysis of the secondary heat balance at Koeberg Nuclear Power Station(2021) Boyes, Haydn; Bello-Ochende, TundeAt Koeberg Nuclear Power Station, the reactor thermal power limit is one of the most important quantities specified in the operating licence, which is issued to Eskom by the National Nuclear Regulator (NNR). The reactor thermal power is measured using different methodologies, with the most important being the Secondary Heat Balance (SHB) test which has been programmed within the central Koeberg computer and data processing system (KIT). Improved accuracy in the SHB will result in a more accurate representation of the thermal power generated in the core. The input variables have a significant role to play in determining the accuracy of the measured power. The main aim of this thesis is to evaluate the sensitivity of the SHB to the changes in all input variables that are important in the determination of the reactor power. The guidance provided by the Electric Power Research institute (EPRI) is used to determine the sensitivity. To aid with the analysis, the SHB test was duplicated using alternate software. Microsoft Excel VBA and Python were used. This allowed the inputs to be altered so that the sensitivity can be determined. The new inputs included the uncertainties and errors of the instrumentation and measurement systems. The results of these alternate programmes were compared with the official SHB programme. At any power station, thermal efficiency is essential to ensure that the power station can deliver the maximum output power while operating as efficiently as possible. Electricity utilities assign performance criteria to all their stations. At Koeberg, the thermal performance programme is developed to optimize the plant steam cycle performance and focusses on the turbine system. This thesis evaluates the thermal performance programme and turbine performance. The Primary Heat Balance (PHB) test also measures reactor power but uses instrumentation within the reactor core. Due to its location inside the reactor coolant system, the instrumentation used to calculate the PHB is subject to large temperature fluctuations and therefore has an impact on its reliability. To quantify the effects of these fluctuations, the sensitivity of the PHB was determined. The same principle, which was used for the SHB sensitivity analysis, was applied to the PHB. The impact of each instrument on the PHB test result was analysed using MS Excel. The use of the software could be useful in troubleshooting defects in the instrumentation. A sample of previously authorised tests and associated data were used in this thesis. The data for these tests are available from the Koeberg central computer and data processing system.
- ItemOpen AccessThermal analysis of the internal climate condition of a house using a computational model(2020) Knutsen, Christopher; Bello-Ochende, TundeThe internal thermal climatic condition of a house is directly affected by how the building envelope (walls, windows and roof) is designed to suit the environment it is exposed to. The way in which the building envelope is constructed has a great affect on the energy required for heating and cooling to maintain human thermal comfort. Understanding how the internal climatic conditions react to the building envelope construction is therefore of great value. This study investigates how the thermal behaviour inside of a simple house reacts to changes made to the building envelope with the objective to predict how these changes will affect human thermal comfort when optimising the design of the house. A three-dimensional numerical model was created using computational fluid dynamic code (Ansys Fluent) to solve the governing equations that describe the thermal properties inside of a simple house. The geometries and thermophysical properties of the model were altered to simulate changes in the building envelope design to determine how these changes affect the internal thermal climate for both summer and winter environmental conditions. Changes that were made to the building envelope geometry and thermophysical properties include: thickness of the exterior walls, size of the window, and the walls and window glazing constant of emissivity. Results showed that there is a substantial difference in indoor temperatures, and heating and cooling patterns, between summer and winter environmental conditions. The thickness of the walls and size of the windows had a minimal effect on internal climate. It was found that the emissivity of the walls and window glazing had a significant effect on the internal climate conditions, where lowering the constant of emissivity allowed for more stable thermal conditions within the human comfort range.
- ItemOpen AccessThermodynamic design optimisation of an open recuperative twin-shaft solar thermal Brayton cycle with combined or exclusive reheating and intercooling(2017) Meas, Matthew Robert; Bello-Ochende, TundeThe Gouy-Stodola Theorem implies that the net power output of a system can be maximised by synchronously sizing the components, thus minimising the cumulative entropy generation rate. The resulting optimal design is related to, and therefore characteristic of, the cycle configuration, since the entropy generation rates in the individual components are interdependent. In this work, optimal design of three common open solar thermal Brayton cycle variants is investigated and compared using principles of the second law of thermodynamics and the method of entropy generation minimisation. The basic cycle, modified accordingly to construct the reheated, intercooled and combined cases, comprises a modified cavity receiver, a counter-flow plate-type recuperator, and a pair of proprietary automotive turbochargers configured to operate as micro-turbines. An additional modified cavity receiver and cross-flow plate heat exchanger constitute the reheater and intercooler, respectively. Net power output is expressed in terms of the temperature and pressure fields in each case, defined in terms of geometric variables characteristic of the components. Heat addition is calculated using the receiver sizing algorithm developed by Stine and Harrigan. Maximum constraints are applied to the recuperator and intercooler lengths and to the surface temperatures of the receiver and reheater absorber tubes. The dynamic-trajectory method is implemented to optimise the variables such that the net power output is maximised. An array of inputs are considered and compared, including 22 micro-turbine models, eight concentrator diameters ranging from six to 20 meters, and both circular and rectangular absorber tube profiles. The influence of receiver inclination, concentrator optics, environmental conditions and design constraints are investigated and the optimisation subroutines validated in the Flownex simulation environment. Results show the optimised power output, operating conditions and design parameters. The intercooled case demonstrates both the highest ratio of total irreversibility to heat input and the highest power output per unit collector surface area. The combined and reheated cases follow. Temperature differences across the components are identified as the primary cause of entropy generation. The optimised heat exchanger lengths are shown to lie on their maximum constraints, and the channel cross-sections found to decrease in size with increasing mass flow rate such that the heat transfer area is maximised and the heat transfer effectiveness improved. As such, plate counts in the optimised heat exchangers are found to be relatively high, and investigation of various compact heat exchanger designs, and regenerative- as opposed to recuperative heat exchangers, is recommended for future work on this topic. The receiver and reheater geometric parameters are found to change such that the absorber tube surface temperatures are kept below the maximum constraint. Trends in the data obtained for circular section absorber tubes are found to be less smooth than the trends in the data obtained for absorber tubes of rectangular section, indicating that the geometric constraints required to maintain the receiver shape offer greater design flexibility for rectangular section absorber tubes than for absorber tubes of circular section. It is concluded that the increases in the compressor and turbine outlet temperatures with mass flow rate and compressor pressure ratio drive the changes in the temperature differences across the heat exchangers, and thus the component entropy generation rates. The entropy generation rates must in turn be distributed during the optimisation procedure such that the cumulative rate is less than the power output, and all of the constraints are met.