Browsing by Author "Laubscher, Ryno"

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    Development aspects of a high temperature heat pipe heat exchanger for high temperature gas-cooled nuclear reactor systems
    (Stellenbosch : Stellenbosch University, 2013-03) Laubscher, Ryno; Dobson, R. T.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: High temperature heat sources are becoming an ever-increasing imperative in the process industry for the production of plastics, ammonia and fertilisers, hydrogen, coal-toliquid fuel and process heat. Currently, high temperature reactor (HTR) technology is capable of producing helium temperatures in excess of 950°C; however, at these temperatures, tritium, which is a radioactive contaminant found in the helium coolant stream, is able to diffuse though the steel retaining wall of the helium-to-steam heat exchanger. To circumvent this radioactivity problem, regulations require an intermediate heat exchange loop between the helium and the process heat streams. In this paper, the use of a uniquely designed sodium-charged heat pipe heat exchanger is considered, and has the distinct advantage of having almost zero exergy loss as it eliminates the intermediate heat exchange circuit. In order to investigate this novel heat pipe heat exchanger concept, a special intermediate-temperature (± 240°C) experimental heat pipe heat exchanger (HPHE) was designed. This experimental HPHE uses Dowtherm A as working fluid and has two glass windows to enable visual observation of the boiling and condensation two-phase flow processes. A high temperature air-burner supply simulates the high temperature stream, and the cold stream is provided by water from a constant-heat supply tank. This experimental apparatus can be used to evaluate the validity of steady-state and start-up transient theoretical models that have been developed. This paper will highlight the special design aspects of this HPHE, the theoretical model and the solution algorithm described. Experimental results will be compared with the theoretically calculated results. The theoretical model will then be used to predict the performance of a high temperature (sodium working fluid at 850°C) HPHE will be undertaken and conclusions and recommendation made.
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    Heat pipe heat exchanger for high temperature nuclear reactor technology
    (Global Digital Central, 2013) Dobson, Robert T.; Laubscher, Ryno
    ENGLISH ABSTRACT: When a high temperature nuclear reactor is used to supply process heat, nuclear regulators require an intermediate heat transfer loop between the primary reactor coolant stream and the secondary process heat stream. A sodium-charged heat pipe heat exchanger design is presented that circumvents the need for an intermediate coolant loop. This is done by physically separating the reactor coolant and secondary coolant by two pipe walls, and a vapour section and a liquid section. This tritium diffusion resistant design for the re-heater, preheater, steam generator and super-heater heat supply system of a 232 MW-electrical pebble bed modular reactor heat source for a superheated Rankine power cycle is presented. A theoretical model was then used to produce preliminary design sizes (tube sizes, tube lengths, working fluid, etc.) for a sodium-charged heat exchanger to be used with a high temperature gas-cooled reactor.
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    Utilization of artificial neural networks to resolve chemical kinetics in turbulent fine structures of an advanced CFD combustion model
    (Stellenbosch : Stellenbosch University, 2017-03) Laubscher, Ryno; Hoffmann, J. E.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: This work investigates an alternative chemistry integration approach to be used with the eddy dissipation concept (EDC) advanced combustion model for large-scale industrial applications where detailed or reduced mechanisms are utilised. The goal of the study was to reduce the computational resources required to solve reduced or detailed mechanisms using the EDC model. The unique approach uses artificial neural networks (ANNs) as a chemistry integrator for the reactions that take place in the fine structure regions created by the turbulence field. The ANNs are therefore utilised to predict the incremental species changes that occur in these fine structure regions as a function of the initial species composition, temperature and the residence time of the mixture in the fine structure regions. The ANN’s weights- and bias matrices were changed to minimise the network’s prediction error using the back-propagation algorithm and datasets generated using the results of separate ideal plug-flow reactor simulations. The effect that the ANN’s architecture has on its ability to accurately predict the temporal evolution of the species was also investigated and the best-performing configuration was selected. The novel chemistry integration approach for the EDC model was implemented to model a piloted methane/air turbulent jet diffusion flame (Sandia Flame D) at a Reynolds number of 22400. To prove the concept, a 5-step methane combustion mechanism was used to model the chemical reactions of the experimental flame. The results of the new approach were benchmarked against experimental data and the simulation results using the standard integration approaches in Fluent. It was shown that once the ANN is well-trained, it can predict the species mass fractions with relative accuracy in both a time and computer memory efficient manner compared to using traditional integration procedures.