A two-dimensional model for predicting the rate of radiation heat transfer for the interior of an industrial furnace is described. The model is two-dimensional due to the assumptions of the heat source as an infinite radiating plane and the heat sink as rows of parallel tubes that are both infinite in length and in number. A refractory back wall, located behind the tube rows, is also included in some of the model configurations. The optical properties for the heat source, heat sink, and refractory back wall are simplified by assuming the "black-body" case: all are treated as perfect absorbers and emitters of radiation. This assumption allows three different solution techniques-a graphical, crossed-string, and numerical method-to be used in solving for the radiant transfer rate. The numerical method, an innovative Monte Carlo technique, is the one employed in this study. Hottel used a graphical technique to solve the furnace model for a two row configuration in which the tubes are arranged on equilateral triangular centers. His results, along with those produced by the crossed-string method, are used in this work to validate the numerical technique. Having been validated, the numerical method was then employed to extend Hottel's work by adding more tube rows to the original equilateral triangular configuration and by generalizing the results to isosceles arrangements. Findings of this investigation are summarized in a table that lists the direct view factors for a ten tube row configuration arranged in an equilateral triangular array. Values from this table can be used to solve the transfer rate problem for twenty different cases by assuming a nonconducting refractory back wall. Results for twelve cases are represented graphically in this document The results are used to demonstrate the importance of a refractory back wall on overall radiation absorption. Examinations of the two row and five row cases for an isosceles triangular array indicate that the tabular values can be applied to any isosceles arrangement if the ratio of row separation distance to tube center-to-center distance is 0.7 or greater. / Graduation date: 1995

Graduation date: 1993

The implicit Monte Carlo (IMC) method [16] for radiative transfer, developed in 1971, provides numerical solutions to the tightly-coupled, highly-nonlinear radiative heat transfer equations in many physical situations. Despite its popularity, there are instances of overheating in the solution for particular choices of time steps and spatial grid sizes. To prevent overheating, conditions on teh time step size Δt have been sought to ensure that the implicit Monte Carlo (IMC) equations satisfy a maximum principle. Most recently, a discrete maximum principle (DMP) for teh IMC equations has been developed [32] that predicts the necessary time step size for boundedness given the spatial grid size. Predictions given by this DMP assumed equilibrium thermal initial conditions, was developed using pseudo-analytic and symbolic algebra tools that are computationally expensive, has only been applied to one-dimensional Marshak wave problems, and has not considered the evolution of the DMP predictions over multiple time steps. These limitations restrict the utility of the DMP predictions. We extend the DMP derivation to overcome these limitations and provide an algorithm that can be introduced into IMC codes with minimal impact on simulation CPU time. This extended DMP effectively treats non-equilibrium thermal initial conditions, decreases calculation time by using multigroup approximations in frequency, considers multiple spatial dimensions with an arbitrary number of neighboring sources, and overcomes inherent difficulties for the DMP in time-dependent problems. Disequilibrium in the initial conditions is introduced through a redefinition of existing terms from [32] to different radiation and material temperatures on the first time step. This results in a limiting DMP inequality similar in form to the original. Multifrequency approximations are then applied by assuming separation of variables. Energy deposition from multiple sources is assumed to follow linear superposition and the DMP from [32] is re-derived to incorporate multiple incident sources of energy in multiple dimensions. Lastly, an inherent flaw in the DMP resulting in poor predictions when temperature varies slowly over a region is overcome by developing a threshold temperature difference, above which the DMP operates. We have numerically implemented these improvements and validated the results against IMC solutions, showing the predictive capacity of the more general DMP algorithm. We find the disequlibrium conditions to be properly incorporated into the DMP, and multifrequency approximations to be accurate over a large range of time step and spatial grid sizes. The linear superposition assumption is generally very accurate, but infrequently leads to DMP predictions which are not conservative. We also demonstrate that the temperature difference threshold prevents inaccurate predictions by the DMP while preserving its functionality. / Graduation date: 2013

The following research contributes original concepts to the fields of deterministic neutron transport modeling and reactor power excursion simulation. A deterministic neutron transport code was created to assess the value of new methods of determining neutron current, fluence, and flux values through the use of view factor and average path length calculations. The neutron transport code is also capable of modeling the highly anisotropic neutron transport of deuterium-tritium fusion external source neutrons using diffusion theory with the aid of a modified first collision source term. The neutron transport code was benchmarked with MCNP, an industry standard stochastic neutron transport code. Deterministic neutron transport methods allow users to model large quantities of neutrons without simulating their interactions individually. Subsequently, deterministic methods allow users to more easily couple neutron transport simulations with other physics simulations. Heat transfer and thermoelastic mechanics physics simulation modules were each developed and benchmarked using COMSOL, a commercial heat transfer and mechanics simulation software. The physics simulation modules were then coupled and used to simulate reactor pulses in fast burst and externally driven nuclear assemblies. The coupled system of equations represents a new method of simulating reactor pulses that allows users to more fully characterize pulsed assemblies. Unlike older methods of reactor pulse simulation, the method presented in this research does not require data from the operational reactor in order to simulate its behavior. The ability to simulate the coupled neutron transport and thermo-mechanical feedback present in pulsed reactors prior their construction would significantly enhance the quality of pulsed reactor pre-construction safety analysis. Additionally, a graphical user interface is created to allow users to run simulations and visualize the results using the coupled physics simulation modules. / text

No description available.

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A dynamic phantom for use in investigating hyperthermia heating systems has been designed, constructed, and tested. A computer controlled the flow rate of 80% Ethanol to each of 4 preserved in vitro canine kidneys which acted as the phantom material. The flow rates were regulated with stepper motor controlled valves and measured with flow meters by the computer. This provided a flexible system for adjusting the perfusion as desired. The system was tested with step and ramp changes in perfusion under constant power ultrasound and with a temperature controlled perfusion algorithm, all of which yielded repeatable results. The dynamic phantom developed in this work shows potential for expediting investigations of hyperthermia controllers, temporal blood flow patterns, and inverse problems. Its computer based nature gives it great flexibility which would lend itself well to automated testing procedures.

No description available.

The Loop Heat Pipe (LHP) is a passive, two-phase heat transfer device used, most commonly, for thermal management of aerospace and aeronautical electronic equipment. A unique feature is a porous wick which generates the necessary capillary action required to maintain circulation between the heat source and the heat exchanger. What differentiates LHP devices from traditional heat pipes, which also work through the use of a wick structure, is the constrained locality of the wick, placed solely in the evaporator, which leaves the remainder of the piping throughout the device as hollow. This provides the LHP with a number of advantages, such as the ability to transport heat over long distances, operate in adverse gravitational positions and to tolerate numerous bends in the transport lines. It is also self-priming due to the use of a compensation chamber which passively provides the wick with constant liquid access. These advantages make LHPs popular in aerospace and aeronautical applications, but there is growing interest in their deployment for terrestrial thermal management systems. This research had two aims. Firstly, to create and validate a robust mathematical model of the steady-state operation of an LHP for terrestrial high heat flux electronics. Secondly, to construct an experimental LHP, including a sintered porous wick, which could be used to validate the model and demonstrate the aforementioned heat exchange and gravity resistant characteristics. The porous wick was sintered with properties of 60% porosity, 6.77x10-13 m2 permeability and an average pore radius of 1μm. Ammonia was the chosen working fluid and the LHP functioned as expected during horizontal testing, albeit at higher temperatures than anticipated. For safety reasons the experimental LHP could not be operated past 18 bar, which translated into a maximum saturated vapour temperature of 45°C. The heat load range extended to 60 W, 50 W and 110 W for horizontal, gravity-adverse and gravity-assisted operation respectively. Because of certain simplifying assumptions in the model, the experimental results deviated somewhat from predicted values at low heat loads. Model accuracy improved as the heat load increased. The experimental LHP behaved as expected for 5° and 10° gravity-assisted and gravity-adverse conditions, as well as for transport line variation, in which performance was assessed while the total tubing length was increased from 2.5 m to 4 m. Overall, the construction of the LHP, particularly of the porous wick, its operation and the modeling of the constant conductance mode of operation proved to be successful. The variable conductance mode of operation was not accurately modeled, nor was expected behaviour in the elevation testing encountered, although the reasons for these results are suggested. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2013.

No description available.