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A new methodology for the accurate and efficient determination of steady state thermal hydraulic parameters for prismatic high temperature gas reactors is developed. Two conceptual reactor designs under investigation by the nuclear industry include the General Atomics GT-MHR and the Department of Energy MHTGR-350. Both reactors use the same hexagonal prismatic block, TRISO fuel compact, and circular coolant channel array design. Steady state temperature, pressure, and mass flow distributions are determined for the base reference designs and also for a range of values of the important parameters. Core temperature distributions are obtained with reduced computational cost over more highly detailed computational fluid dynamics codes by using efficient, correlations and first-principles-based approaches for the relevant thermal fluid and thermal transport phenomena. Full core 3-D heat conduction calculations are performed at the individual fuel pin and lattice assembly block levels. The fuel compact is treated as a homogeneous medium with heat generation. A simplified 1-D fluid model is developed to predict convective heat removal rates from solid core nodes. Downstream fluid properties are determined by performing a channel energy balance down the axial node length. Channel exit pressures are then compared and inlet mass flows are adjusted until a uniform outlet pressure is reached. Bypass gaps between assembly blocks as well as coolant channels are modeled. Finite volume discretization of energy, and momentum conservation equations are then formed and explicitly integrated in time. Iterations are performed until all local core temperatures stabilize and global convective heat removal matches heat generation. Several important observations were made based on the steady state analyses for the MHTGR and GT-MHR. Slight temperature variation in the radial direction was observed for uniform radial powers. Bottom-peaked axial power distributions had slightly higher peak temperatures but lower core average temperatures compared to top and center-peaked power distributions. The same trend appeared for large bypass gap sizes cases compared to smaller gap widths. For all cases, peak temperatures were below expected normal operational limits for TRISO fuels. Bypass gap flow for a 3 mm gap width was predicted to be between 10 and 11% for both reactor designs. Single assembly hydrodynamic and temperature results compared favorably with those available in the literature for similar prismatic HTGR thermal hydraulic, computational fluid dynamics analyses. The method developed here enables detailed local and core wide thermal analysis with minimal computational effort, enabling advanced coupled analyses of high temperature reactors with thermal feedback. The steady state numerical scheme also offers a potential for select transient scenario modeling and a wide variety of design optimization studies.

For semiconductor processors temperature increases leakage current, which in turn in- creases the temperature of the processor. This increase in heat is seen by other parts of the processor since heat is diffusive across a processor die. In this way, cores are thermally coupled to one another such that when the temperature of one core increases, the temperatures of all cores on the same die can also increase. This increase in temperature and power consumption is not accompanied by any increase in performance. Cores on a chip can also be performance coupled to one another since cores can share data between them. These interactions between cores present new challenges to microarchitects who seek to optimize the energy consumption of a chip multiprocessor (CMP) comprised of multiple symmetric or asymmetric processing cores. This thesis seeks to understand and model the impact of thermal coupling effects between adjacent cores in a chip multiprocessor starting with measurements with a commercial multi-core processor. The hypothesis is that the thermal coupling of compute cores will be influenced by the adjacent core’s performance characteristics. Specifically, we expect thermal coupling is related to the nature of the workloads, e.g. compute intensive workloads will increase coupling over memory intensive workloads. However, we find that simpler parameters such as frequency of operation have more impact on coupling behaviors than the workload behaviors such as memory intensity or instruction retirement rates. A model is developed to capture thermal coupling effects and enable schemes to mitigate its impact.

In the last few years a new method for surface preparation has evolved, namely thermo-abrasive blasting. This technique utilises a high enthalpy thermal jet to propel abrasive particles. The thermo-abrasive blasting gun, also called a thermal gun, is based on the principles of High Velocity Air Fuel (HVAF) processes. Nozzles used for thermo-abrasive blasting are subjected to thermal loading, wear and mechanical stresses. Therefore, the nozzle geometry and materials are critical for reliable performance of a thermo-abrasive system. In this investigation, the thermal stresses developed in the nozzle materials for thermo-abrasive blasting were analysed. The analytical and the computational models of the thermo-abrasive gun and the nozzle were developed. The computational fluid dynamics, thermal and structural finite element analyses have been employed in this study. The nozzle materials investigated were tungsten carbide, hot pressed silicon carbide, nitride-bonded cast silicon carbide and SIALON. The simulation and experimental results show that the highest thermal stresses occur during the first two minutes from the start of the thermal gun. However, thermal stresses are also high after the system is shut off. The nozzle geometry was optimised, which provided high cleaning rates with evidence of improved thermal loading, based on the experimental results. A new design of the thermal gun and the ignition method associated with a HVAF system were developed in this study. It is also concluded that the computation fluid dynamic and the finite element technique can be used to optimise the design of thermo-abrasive blasting nozzles. / Thesis (Ph.D. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2004.

Transport phenomena in high porosity open-cell fibrous structures have been the focus of many recent industrial and academic investigations. Unique features of these structures such as relatively low cost, ultra-low density, high surface area to volume ratio, and the ability to mix the passing fluid make them excellent candidates for a variety of thermofluid applications including fuel cells, compact heat exchangers and cooling of microelectronics. This thesis contributes to improved understanding of thermal transport phenomena in fuel cell gas diffusion layers (GDLs) and metal foams and describes new experimental techniques and analytic models to characterize and predict effective transport properties. Heat transfer through the GDL is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity as well as the thermal contact resistance (TCR) associated with the interface between the GDL and adjacent surfaces/ layers. The effective thermal conductivity significantly differs in through-plane and in-plane directions due to anisotropy of the GDL micro-structure. Also, the high porosity of GDLs makes the contribution of TCR against the heat flow through the medium more pronounced. A test bed was designed and built to measure the thermal contact resistance and effective thermal conductivity in both through-plane and in-plane directions under vacuum and ambient conditions. The developed experimental program allows the separation of effective thermal conductivity and thermal contact resistance. For GDLs, measurements are performed under a wide range of compressive loads using Toray carbon paper samples. To study the effect of cyclic compression, which may happen during the operation of a fuel cell stack, measurements are performed on the thermal and structural properties of GDL at different loading-unloading cycles. The static compression measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The outcomes of the cyclic compression measurements show a significant hysteresis in the loading and unloading cycle data for total thermal resistance, TCR, effective thermal conductivity, thickness, and porosity. It is found that after 5 loading/unloading cycles, the geometrical, mechanical, and thermal parameters reach a“steady-state”condition and remain unchanged. A key finding of this study is that the TCR is the dominant component of the GDL total thermal resistance with a significant hysteresis resulting in up to a 34 % difference between the loading and unloading cycle data. Neglecting this phenomenon may result in significant errors in evaluating heat transfer rates and temperature distributions. In-plane thermal experiments were performed using Toray carbon paper samples with different polytetrafluoroethylene (PTFE) content at the mean temperature of 65−70◦C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. Results show that the in-plane effective thermal conductivity remains approximately constant, k ≈ 17.5W/mK, over a wide range of PTFE content, and it is approximately 12 times higher than the through-plane conductivity. Using the test bed designed for the through-plane thermal conductivity measurement, the effective thermal conductivity and thermal contact resistance of ERG Duocel aluminum foam samples were measured under varying compressive loads for a variety of porosities and pore densities. Also, an experimental program associated with an image analysis technique is developed to find the size and distribution of contact spots at different compressive loads. Results show that the porosity and the effective thermal conductivity remain unchanged with the variation of pressure in the range of 0 to 2 MPa; but TCR decreases significantly with pressure due to an increase in contact area. Moreover, the ratio of contact area to cross-sectional area is 0-0.013, depending upon the compressive force, porosity, and pore density. This study clarifies the impact of compression on the thermal and structural properties of GDLs and metal foams and provides new insights on the importance of TCR which is a critical interfacial transport phenomenon. / Graduate

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When a fire reaches the point of flashover the hot gases inside the burning room ignite resulting in furnace-like conditions. Thereafter, the building frame experiences temperatures sufficient to compromise its structural integrity. Physical and mathematical models help to predict when this will happen. This thesis looks at both the thermal and structural aspects of modelling a frame exposed to a post-flashover fire. The temperatures in the frame are calculated by solving a 2D heat conduction equation over the cross-section of each beam. The solution procedure uses the finite element method with automatic mesh generation/adaption based on the Delaunay triangulation process and the recovered heat flux. With the Euler-Bernoulli assumption that the cross-section of a beam remains plane and perpendicular to the neutral line and that strains are small, an error estimator, based on the work of Bank and Weiser [9], has been derived for finite element solutions to small-deformation, thermoelastic and thermoplastic frame problems. The estimator has been shown to be consistent for all finite element solutions and asymptotically exact when the solution involves appropriate higher degree polynomials. The asymptotic exactness is shown to be closely related to superconvergence properties of the approximate solution in these cases. Specifically, with coupled bending and compression, it is necessary to use quadratic approximations, instead of linear, for the compression and twisting terms to get a global O(h2) rate of convergence in the energy norm, some superconvergence properties and asymptotic exactness with the error estimator.

Thermal analysis of buildings was carried out using simplified design tools, prior to the widespread use of computers. Since the early 1980's, the rapid growth of computational power has lead to the introduction of many building dynamic thermal simulation software programs. The accurate performance of many of these programs has lead to the view that manual calculation methods should only be used as indicative design tools. The CIBSE admittance method is based on the fundamentals of building heat transfer, its calculations procedures being simplified for use on hand held calculators. Manual calculation methods must be developed for use on more powerful calculators, if greater accuracy is required. Such calculators are available in the form of computer spreadsheet programs. The computational power of the computer spreadsheet program, combined with suitable mathematical thermal modelling techniques, has thus far, remained unexploited. This thesis describes the development of a powerful manual thermal design method, for application on a computer spreadsheet program. All the modes of building heat transfer are accurately modelled. Free-running or plant-controlled spaces can be simulated. In the case of a single zone, the accuracy of the new manual dynamic thermal model is comparable with commercially available software programs. The level of mathematical modelling complexity is limited only by computer power and user ability. The Iterative Frequency Domain Method (IFDM) and the Adiabatic Iterative Frequency Domain Method (AIFDM) are alternative mathematical simulation techniques developed to form the core of the Thermal Analysis Design Method. In the IFDM and AIFDM, the frequency domain and numerical iteration techniques have been integrated to produce a thermal simulation method that can model all non-linear heat transfer processes. A more accurate formulation of sol-air temperature, a window sol-air temperature and an accurate reduced internal long-wave radiant exchange model is a sample of further innovations in the thesis. Many of the developments described in the thesis, although designed for the computer spreadsheet environment, may also be employed to enhance the performance of some of the current dynamic thermal models of buildings.