Browsing by Author "Dobson, R. T."
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- ItemExperimental evaluation of a Ranque-Hilsch vortex tube as a particle separator(South African Institution of Mechanical Engineering, 2012) Burger, A.; Dobson, R. T.The Pebble Bed Modular Reactor (PBMR) is a Generation IV graphite-moderated helium cooled nuclear reactor developed in South Africa from the German Arbeitsgemeinschaft Versuchreaktor (AVR). After decommissioning of the AVR plant, radioactive isotopes of silver 44Ag110 as well as graphite particles were found in the primary helium coolant loop of the reactor. Hence the main objective of this work was to evaluate the efficiency of the Ranque-Hilsch vortex tube (RHVT) as a separation device for removing graphite particles from a helium coolant stream. This objective was accomplished by designing and building an experimental test apparatus and measuring the particle separation efficiency of the RHVT under different operating conditions. It was found that the RHVT is a very efficient particle separator, which may, however, not easily be incorporated into the PBMR system.
- ItemA heat pipe heat recovery heat exchanger for a mini-drier(Energy Research Centre, University of Cape Town, 2006) Meyer, A.; Dobson, R. T.This paper considers the thermal design and the experimental testing of a heat pipe (thermosyphon) heat exchanger for a relatively small commercially available mini-drier. The purpose of the heat exchanger is to recover heat from the moist waste air stream to preheat the fresh incoming air. The working fluid used was R134a and the correlations are given for the evaporator and condenser inside heat transfer coefficients as well as for the maximum heat transfer rate. The theoretical model and computer simulation program used for the thermal design calculations are described. The validity of the as-designed and manufactured heat exchanger coupled to the drier is experimentally verified. The theoretical model accurately predicted the thermal performance and a significant energy savings and a reasonable payback period was achieved.
- ItemInside-pipe heat transfer coefficient characterization of a thermosyphon-type heat pipe suitable for the reactor cavity cooling system of the Pebble Bed Modular Reactor(2011) Dobson, R. T.; Sittmann, I.ENGLISH ABSTRACT: The feasibility of a closed loop thermosyphon for the Reactor Cavity Cooling System of the Pebble Bed Modular Reactor has been the subject of many research projects. One of the difficulties identified by previous studies is the hypothetical inaccuracies of heat transfer coefficient correlations available in literature. This article presents the development of an inside-pipe heat transfer correlation, for both the evaporator and condenser sections, that is specific to the current design of the RCCS. A one-third-height-scale model of the RCCS was designed and manufactured using copper piping and incorporating several strategically placed sight glasses, allowing for the visual identification of two-phase flow regimes and an orifice plate to allow for forward and reverse flow measurement. Twelve experiments, lasting at least 5 hours each, were performed with data logging occurring every ten seconds. The experimental results are used to mathematically determine the experimental inside-pipe heat transfer coefficients for both the evaporator and condenser sections. The experimentally determined heat transfer coefficients are correlated by assuming that the average heat flux can be described by a functional dependence on certain fluid properties, the average heat flux is directly proportional to the heat transfer coefficient and that the heat transfer coefficient is a function of the Nusselt number. The single-phase inside-pipe heat transfer coefficients were correlated to 99% confidence intervals and with less than 30% standard deviation from experimental results. The generated correlations, along with identified and established two-phase heat transfer coefficient correlations, are used in a mathematical model, with experimental mass flow rates and temperatures used as input variables, to generate theoretical heat transfer coefficient profiles. These are compared to the experimentally determined heat transfer coefficients to show that the generated correlations accurately predict the experimentally determined inside-pipe heat transfer coefficients.
- ItemInvestigation of two-stage electrostatic precipitation for silver removal from a helium stream(2010) Steyn, H. J.; Dobson, R. T.ENGLISH ABSTRACT: In high temperature gas cooled nuclear reactors various unwanted particles have been found to plate out onto the surfaces of the primary cicuit components. A simple deterministic deflection model is outlined in this paper to describes the deflection of charged silver nanoparticle particles in helium using the principle of electrostatic precipitation. An apparatus, consisting of a charging region and collection region, as in the case of two-stage electrostatic precipitators, was built to validate the applicability of the model. Not withstanding its simplicity, it showed that it is able to favourably capture the experimentally determined particle deflection trajectories.
- ItemModelling and testing a passive night-sky radiation system(University of Cape Town, Energy Research Centre, 2017) Joubert, G. D.; Dobson, R. T.ENGLISH ABSTRACT: The as-built and tested passive night-sky radiation cooling/heating system considered in this investigation consists of a radiation panel, a cold water storage tank, a hot water storage tank, a room and the interconnecting pipework. The stored cold water can be used to cool a room during the day, particularly in summer. A theoretical time-dependent thermal performance model was also developed and compared with the experimental results and it is shown that the theoretical simulation model captures the experimental system performance to within a reasonable degree of accuracy. A natural circulation experimental set-up was constructed and subsequently used to show that under local (Stellenbosch, South Africa) conditions the typical heat-removal rate from the water in the tank is 55 W/m2 of radiating panel during the night; during the day the water in the hot water-storage tank was heated from 24 °C to 62 °C at a rate of 96 W/m2. The system was also able to cool the room at a rate of 120 W/m3. The results thus confirmed that it is entirely plausible to design an entirely passive system, that is, without the use of any moving mechanical equipment such as pumps and active controls, for both room-cooling and water-heating. It is thus concluded that a passive night-sky radiation cooling/heating system is a viable energy-saving option and that the theoretical simulation, as presented, can be used with confidence as an energy-saving system design and evaluation tool.
- ItemNumerical modelling of a loop-type heat pipe suitable for the reactor cavity cooling system of the Pebble Bed Modular Reactor(2011) Dobson, R. T.; Sittmann, I.ENGLISH ABSTRACT: The feasibility of a closed loop thermosyphon for the Reactor Cavity Cooling System (RCCS) of the Pebble Bed Modular Reactor has been the subject of many research projects. One of the difficulties identified by previous studies is the precise applicability of appropriate heat transfer coefficient correlations available in literature. This article presents the numerical modelling of the current design of the RCCS, incorporating single phase inside-pipe heat transfer coefficient correlations developed by the author. A one-third-height-scale model of the RCCS was designed and manufactured on which twelve experiments, lasting at least 5 hours each, were performed. The experimental results obtained, were used to verify the theoretical model.
- ItemTheoretical modelling and experimental verification of the heat transfer behaviour of a water-charged closed-end loop pulsating heat pipe(2001) Swanepoel, G.; Dobson, R. T.; Taylor, A. B.ENGLISH ABSTRACT: In this paper the theoretical modelling of a Pulsating Heat Pipe (PHP) is presented. In this particular model the flow of the working fluid is modelled as discrete liquid plugs moving back and forth because of the evaporation and condensation processes taking place inside of the PHP. A PHP was manufactured to determine the heat transfer rate of the PHP experimentally and to compare the experimental results with the theoretical results. The average heat transfer rate predicted by the theoretical model in the top heat mode was 62 W compared to the experimental value of 60 W. In the bottom heat mode the average theoretical predicted heat transfer rate was 86 W compared to the experimental determined value of 65W.
- ItemThermal modelling of a heat block and heat sink connected by heat pipes using finite element methods(2001) Groenewald, A.; Dobson, R. T.ENGLISH ABSTRACT: This article considers the heat transfer between a simple heat block and a heat sink, using heat pipes as the energy transport medium. Both the heat block and heat sink are made from carbon steel. The heat pipes have copper containers and uses water as the working fluid, with a phosphor bronze mesh wick. The source, sink and heat pipe combination were modelled using finite elements. The program used was Nastran 2.0 for Windows, although the finite element application would be universal. The heat pipe was assigned constant properties, namely density, specific heat capacity and also conductivity. The purpose of the research was to investigate the merits of using constant properties for heat pipes in a finite element simulation. Using constant properties is a significant simplification of the complex internal thermo-fluid processes of a heat pipe, and will result in much faster calculation times. The results indicated though that this type of simplification may lead to inaccuracies. It would be beneficial if a heat pipe property can be developed and assigned to a three dimensional finite element. Another (and maybe more realistic) option would be to rather do modelling of heat pipes with a finite element based program that is able to incorporate non-linear boundary conditions, with which the heat pipe’s boiling and condensation processes can be modelled.
- ItemThermally driven natural circulation water pump(2015) Hobbs, K.; Dobson, R. T.ENGLISH ABSTRACT: The water utilized by passive air-conditioning systems in buildings is typically required at higher elevations. The thermally driven natural circulation water pump (TDNCWP) is a passively driven pumping system for delivering water from ground level against gravity to a higher elevation. The TDNCWP is shown by theoretical and experimental work to provide water at varied elevations using non-mechanical, passive means. Experiments were conducted on two experimental TDNCWP set-ups of different cross-sectional areas to evaluate the pump design and the theoretical model. A temperature difference of 9 to 12.5 °C between the heating and cooling sections induced an average velocity of 0.4 to 0.6 m/s for a duct cross section of 100 mm2. For a larger cross section of 400 mm2, a temperature difference of 2 to 5 °C induced an average velocity of 0.25 to 0.3 m/s. An asymmetrical velocity profile was observed which varied at different points in the loop. A water delivery rate of 1.2 to 7.5 L/day was experimentally determined. This compares well to the passive air-conditioning water requirements of a small building. The theoretical model over-predicted the delivery rate at increased duct cross-sectional areas but fared well when compared to the smaller experimental model results. Further refinement of the numerical model and the TDNCWP design is required, and recommendations were made regarding this. It is clear however that the TDNCWP provides an alternative to a conventional water pump for low-volume water pumping requirements.
- ItemTransient model and simulation of a single Effect water/lithium-bromide vapour absorption system(2016-12) Jeggels, D. H.; Dobson, R. T.; Mechanical and Mechatronic Engineering
- ItemUse of passive cooling systems in generation IV nuclear reactors for core decay heat removal and containment cooling(2011) Dobson, R. T.; Sittmann, I.ENGLISH ABSTRACT: The Pebble Bed Modular Reactor (PBMR) concept evolved from a German high temperature helium-cooled reactor design with ceramic spherical fuel pebbles. The removal of parasitic heat between the reactor core and concrete citadel is facilitated through the Reactor Cavity Cooling System (RCCS). The RCCS primary function is to passively maintain the cavity temperature within a required range. This is in order to provide protection to the concrete structures surrounding the reactor and also, during loss of coolant accident operating conditions, to transport parasitic heat from the reactor to the environment. Several Generation IV reactor designs incorporate passive safety systems. The main objective of this study is to familiarise the reader with specific “innovative” nuclear reactor designs and discuss the different passive safety systems employed in these designs for core decay heat removal and containment cooling systems. A table is given comparing the type, thermal efficiency, fuel, coolant and passive safety systems employed by each reactor to those of the PBMR.