Browsing by Author "Dobson, Robert Thomas"

Now showing 1 - 9 of 9
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    Deflection of Ag-atoms in a magnetic field
    (2011) Kheswa, Bonginkosi Vincent; Dobson, Robert Thomas
    ENGLISH ABSTRACT: An apparatus that generates and deflect silver-109 atoms with an inhomogeneous and homogeneous magnetic field was constructed. The experimental results found using this apparatus were used to make a recommendation, to the idea of removing silver-110 atoms from the helium fluid of Pebble Bed Modular Reactor (PBMR) with an inhomogeneous magnetic field based on the Stern-Gerlach principle. It is shown that experimental results corresponded well with the theoretical predictions. The apparatus was also used for a practical for a course for mechanical engineers in modern physics.
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    The design and testing of a secure surface tension driven water pump
    (2015) Fraser, J.; Dobson, Robert Thomas
    ENGLISH ABSTRACT: Water pumping is essential for life as we know it. The drive for alternative energies together with the need for water pumping has inspired the construction of a surface tension driven water pump (STDWP). The goal of this study was to construct and test a passive water pump inspired by tree water transport mechanisms. Here various experimental tests were done to determine the STDWP’s performance and behaviour under various environmental conditions and pumping heads. Theoretical mass transfer relations were used to model the evaporative rate from the “leaf” surface and compared to the experimentally measured results. It was found that the developed pump could pump water at a rate of nearly 400 mL/hr.m2, reaching pumping heads of 1.8 m, with a maximum functional lifespan of 13 days. The STDWP’s water collection efficiency was found to be 98% on average. The mechanistic causes of pump failure are addressed with recommendations for the STDWP improvement.
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    The development of a 1D-transient simulation model of a CO2 refrigeration system
    (2016) Garces de Gois, M.; Dobson, Robert Thomas
    ENGLISH ABSTRACT: Refrigeration is a highly essential part of modern day living. The drive for alternative cleaner technologies has renewed interest in designing refrigeration systems using CO2 as refrigerant. The goal of this project was to develop a numerical transient simulation model of a CO2 refrigeration cycle containing a capillary tube as expansion device. This simulation model can then be used to design CO2 refrigerators and provide insight into their operation. In this paper, the transient conservation equations are developed into forms that can be solved on a computer program. An algorithm for solving compressible flow equations is discussed. Lastly the use of the real gas equation of state for CO2 from Span and Wagner (1996) is discussed and methods are developed to calculate single phase and two-phase properties. It was found that the simulation model predicted evaporator temperature and phase-change processes with reasonable accuracy.
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    Experimental investigation and numerical simulation of instabilities in a multi-parallel channel two-phase natural circulation system
    (2015) Sangweni, L. S.; Dobson, Robert Thomas
    ENGLISH ABSTRACT: In the present study, two-phase natural circulation flow in a multi-parallel channel system is investigated using experimental and numerical modelling. The experimental model consisted, essentially, of four 25 mm diameter and two-meter long vertically orientated transparent polycarbonate pipes connected to a common manifold at the bottom and a relatively large steam drum at the top; three one-meter long electrical resistance heating elements were inserted into the lower end of three of the vertical pipes. Tests were conducted using different combinations of input power and as-defined and so-called open, closed and heat pipe system operating modes. A water-cooled condenser was placed in the upper portion of the steam drum and an expansion tank was connected to the lower manifold. For different power inputs and operating modes twelve temperatures and three flow rates as a function of time were recorded. In this way start-up transients and dynamical oscillatory responses were captured. So-called Type I instability were observed at low power inputs and open system operating modes (system open to the atmosphere). Type II instabilities and flashing instability were observed at medium and high-power excitations for the open system mode of operation. The fluid flow became more stable and less oscillatory at all power excitations for the closed system operation mode (system not open to the atmosphere). For the heat pipe mode of operation so-called geysering, followed by flashing-induced boiling was observed. After boiling had commenced downward single phase flow was invariable noted to occur in the central of the three hearted risers, even when all three heater power inputs were the same. Also, after boing had started a further increase in power input did not necessarily result in an increase in flow rate. The experimental system was discretized into a number of control volumes. The conservation equations, mass, momentum and energy were applied to each control volume and a set of time dependent temperature-coupled finite difference equations simulating the thermal-hydraulic behaviour of the system thus derived. This set of difference equations was solved using an explicit solution method. An encouragingly-good correspondence between the experimental and theoretical simulation model of the temperature and flow rate in the system was obtained.
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    Flow regime recognition in two-phase thermosyphon loops using pressure pulse analyses
    (Thermal-Fluids Central, 2012) Dobson, Robert Thomas; Ruppersberg, J. C.
    ENGLISH ABSTRACT: The nuclear industry finds application for natural circulation in the form of passive heat removal systems used under accident conditions as well as for the removal of parasitic heat loss during normal operation. A high temperature gas-cooled reactor containment cooling system for instance can take the form of a series of closed-loop two-phase natural circulation thermosyphon type heat pipes. In this paper a means of remotely predicting the flow patterns in the two-phase regions of a closed loop heat pipe is proposed. The process involves the measurement of the working fluid pressure response in the single-phase flow region where safe access to the loop is possible from a nuclear radiation hazard point-of-view. The pressure signal is then decomposed and a frequency spectrum generated in order to find a characteristic frequency for each of the flow phenomenon observed. To establish the validity of the analytical method a 2.2 m high rectangular loop was built using 25 mm diameter copper pipe. Experiments were performed and a comparison of the frequency spectrums shows the ability of the proposed analytical method to correctly identify the presence of different flow patterns. It is thus provisionally concluded that the procedure as outlined in this paper can be used to remotely characterising the flow pattern associated with the different flow regimes in a closed loop two-phase thermosyphon-type heat pipe.
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    Hydraulic modeling of a confined gas recirculation mixing system for a CSTR anaerobic digester
    (2016) Vermaak, R. P.; Dobson, Robert Thomas
    ENGLISH ABSTRACT: Hydraulic numerical rnadelling and experimental werk was used to develop a mixing system, that is able to satisfy the biologica! requirement of micro bacteria and create a homogeneaus salution inside the digester. A review was conducted on the different mixing methods used commercially for CSTR's as well as the influence of mixing intensity and intervals on anaerobic digesters. The gas recirculation method, eperating in a two-phase plug-flow regime mixing method was selected and a one-dimensional explicit, transient numerical model was developed that could predict the liquid mass displaced by a gas plug. An experimental study foliowed the numerical rnadelling of the digester mixing system to capture the effect of various geometrical constructions of the plug-flow generator in two empirica! equations to predict the mass and frequency of plug released. With the numerical model and experimental results a full scale mixing system was developed for a 1 600 m3 digester. The benefits of the proposed mixing system over the mechanica! draft tube methad include the following: no moving parts inside the digester, tedious downtime of clogged rotating equipment is eliminated and only requires 7 % more power per unit volume.
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    Performance characterisation of a separated heat-pipe-heat-recovery-heat-exchanger for the food drying industry
    (2016) Thomas, N. S.; Dobson, Robert Thomas
    ENGLISH ABSTRACT: This paper presents the performance evaluation of a separated-HPHRHE for use in the food processing industry. The outside heat transfer coefficients were obtained by passing hot air over a HPHE filled with cold water and of similar geometry to the HPHE’s used for the separated-HPHE. The inside heat transfer coefficients for the separated-HPHE were determined with R600a, R134a and R123 as working fluids. The experiments were undertaken at various temperatures and flow rates. The results showed that R600a works the most effectively in the temperature range considered and this is expected since R600a is less dense and has a higher latent heat of vaporisation than both R134a and R123. As an example, the R600a charged separated-HPHE yielded heat transfer rates in the region of 9352 W compared to the 7017 W and 4555 W yielded for R134a and R123 respectively at an air temperature difference of 27 °C and mass flow rate of 0.841 kg/s. The as-tested separated-HPHRHE was shown to have worked effectively (recovering up to 90 % of the of the dryer exhaust heat) for typical food industry drying temperatures of between 25-80 °C. Additionally, the theoretical simulation models for the HPHRHE was validated in as much that its energy saving performance was within  12 % of the as-tested experimental models; and thus it was demonstrated that substantial energy cost saving could be realised using standard heat exchanger manufacturing technology. It is recommended that notwithstanding accuracies of roughly 22 % obtained by the theoretically predicted correlations to the experimental work, the heat exchanger design should be optimised to allow better refrigerant flow and various performance parameters such as liquid fill charge ratio and condenser/evaporator length dependencies should be further investigated.
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    Silver ion generation, deflection and deposition in vacuum
    (2011) Bam, L. C.; Dobson, Robert Thomas; Lindsay, R.
    ENGLISH ABSTRACT: The potential problems arising during the operation of a pebble bed modular type reactor has stimulated a number of research avenues in different fields. This study presents results of a first approach method of deflecting a silver ion beam, in vacuum. These results serve as a building block for the future research that needs to be conducted in helium. An experimental apparatus was constructed and it was found that theoretically predicted deflections corresponded to within about 20% of the experimental results. The test apparatus could also be used as a practical supplementing a Physic course.
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    Small scale organic Rankine cycle for solar applications
    (2015) Lombard, H. J.; Dobson, Robert Thomas
    ENGLISH ABSTRACT: The global demand for energy has risen dramatically in recent years and the cost of electricity has risen considerably. Local electricity sales prices followed the global trend, increasing by an average of 25% per annum from 2008 to 2014. This upward trend has opened the market for new technologies, such as renewable energy alternatives, better use of waste streams and low temperature power generation. The Organic Rankine Cycle is an example of a low temperature power generation cycle that can utilize a renewably energy source or recover waste heat. The Organic Rankine Cycle is in essence a Rankine Cycle that employs a different working fluid than water. The primary objective was to build a functioning ORC. The cycle utilized a low temperature (< 120°C) and pressure heat source to generate electricity. This temperature is easy achieved using solar collectors which will make the system suitable to operate using solar radiation as a heat source. To meet the temperature, pressure and global warming potential requirements, refrigerant R123 is used as a working fluid. The system consisted of a rotary vane pump, some plate heat exchangers, a scroll expander and a natural convection condenser. The scroll expander was a modified Copeland scroll compressor. The system demonstrated a cycle efficiency of 13% compared to the Carnot cycle efficiency of 25% and a modified Carnot efficiency of 14%. Due to excessive heat loss and poor conversion from mechanical to electrical energy the electrical output was 40 W. The conversion efficiency could be greatly improved if the electric generator was changed to a DC generator.