The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

Hydraulic tomography is a cost -effective technique for characterizing the heterogeneity of hydraulic parameters in the subsurface. During hydraulic tomography surveys, a large number of hydraulic heads (i.e., aquifer responses) are collected from a series of pumping or injection tests in an aquifer. These responses are then used to interpret the spatial distribution of hydraulic parameters of the aquifer using inverse modeling. In this study, we developed an efficient sequential successive linear estimator (SSLE) for interpreting data from transient hydraulic tomography to estimate three-dimensional hydraulic conductivity and specific storage fields of aquifers. We first explored this estimator for transient hydraulic tomography in a hypothetical one-dimensional aquifer. Results show that during a pumping test, transient heads are highly correlated with specific storage at early time but with hydraulic conductivity at late time. Therefore, reliable estimates of both hydraulic conductivity and specific storage must exploit the head data at both early and late times. Our study also shows that the transient heads are highly correlated over time, implying only infrequent head measurements are needed during the estimation. Applying this sampling strategy to a well -posed problem, we show that our SSLE can produce accurate estimates of both hydraulic conductivity and specific storage fields. The benefit of hydraulic tomography for ill -posed problems is then demonstrated. Finally, to affirm the robustness of our SSLE approach, we apply the SSLE approach to transient hydraulic tomography in a hypothetical two- dimensional aquifer with nonstationary hydraulic properties, as well as a hypothetical three-dimensional heterogeneous aquifer.

No description available.

No description available.

Carbon nanotubes are carbon based structures with outstanding electronical and mechanical properties. They are used in a wide range of applications, usually embedded in polymer in the form of composites, in order to affect the electronic behavior of the matrix material. However, as the nanotubes properties are directly dependent on their intrinsic structure, it is necessary to select specific nanotubes depending on the application, which can be a complicated and inefficient process. This makes it attractive to be able to reduce the amount of material used in the composites. In this thesis, focus is placed on the electrical properties of the composites. A simple patterning method is presented which allows the use of extremely low amounts of nanotubes in order to increase the electrical conductivity of diverse polymers such as polystyrene (PS) or poly(3-hexylthiophene) (P3HT). This method is called nanoimprint lithography and uses a flexible mold in order to pattern composite films, leading to the creation of conducting nanotube networks, resulting in vertically conducting samples (from the bottom of the film to the top of the imprinted patterns). In parallel, X-ray diffraction measurements have been conducted on thin P3HT polymer films. These were prepared on either silicon substrate or on graphene, and the influence of the processing conditions as well as of the substrate on the crystallinity of the polymer have been investigated. The knowledge of the crystalline structure of P3HT is of great importance as it influences its electronic properties. Establishing a link between the processing conditions and the resulting crystallinity is therefore vital in order to be able to make opto-electronic devices such as transistor or photovoltaic cells.

An important criteria for optimising hydraulic systems is their size. Especially for tanks and heat exchangers oil parameters as heat capacity and thermal conductivity have a big influence on the size. Additionally, various oils differ in their parameters. Accordingly, the heat capacity and thermal conductivity need to be known. However, little research has been done. Data-sheets usually do not provide any thermal data. In this paper, the thermal conductivity is measured for varying types of hydraulic oils. The thermal conductivity is determined by a newly designed test-rig measuring the radial temperature difference in a tube at a quasi-static state using a constant heat flux. Thus, an overview over the thermal conductivity of different oils is achieved. Based on the results, a comparison between different types of fluid is made.

The accurate measurement of multiphase flows of oil/water/gas is a critical element of oil exploration and production. Thus, over the last three decades; the development and deployment of in-line multiphase flow metering systems has been a major focus worldwide. Accurate measurement of multiphase flow in the oil and gas industry is difficult because there is a wide range of flow regimes and multiphase meters do not generally perform well under the intermittent slug flow conditions which commonly occur in oil production. This thesis investigates the use of Doppler and cross-correlation ultrasonic measurements made in different high gas void fraction flow, partially separated liquid and gas flows, and homogeneous flow and raw slug flow, to assess the accuracy of measurement in these regimes. This approach has been tested on water/air flows in a 50mm diameter pipe facility. The system employs a partial gas/liquid separation and homogenisation using a T-Y junction configuration. A combination of ultrasonic measurement techniques was used to measure flow velocities and conductivity rings to measure the gas fraction. In the partially separated regime, ultrasonic cross-correlation and conductivity rings are used to measure the liquid flow-rate. In the homogeneous flow, a clamp-on ultrasonic Doppler meter is used to measure the homogeneous velocity and combined with conductivity ring measurements to provide measurement of the liquid and gas flow-rates. The slug flow regime measurements employ the raw Doppler shift data from the ultrasonic Doppler flowmeter, together with the slug flow closure equation and combined with gas fraction obtained by conductivity rings, to determine the liquid and gas flow-rates. Measurements were made with liquid velocities from 1.0m/s to 2.0m/s with gas void fractions up to 60%. Using these techniques the accuracies of the liquid flow-rate measurement in the partially separated, homogeneous and slug regimes were 10%, 10% and 15% respectively. The accuracy of the gas flow-rate in both the homogeneous and raw slug regimes was 10%. The method offers the possibility of further improvement in the accuracy by combining measurement from different regimes.

Increasing urbanization is often accompanied by problematic changes in watershed hydrology. Decreasing surface permeability can lead to increased overland flow volumes, which may spread surficial contaminants and increase the strain on municipal stormwater infrastructure. This study examines a mixed-use property in the Proctor Creek watershed in Atlanta, Georgia, to better understand how land-management practices influence soil overland flow potential. Field saturated hydraulic conductivity (Kfs) measurements were collected from soils 1) subjected to compaction, 2) in urban agricultural use, and 3) under common lawn maintenance. Mean values were 9.1E-7 cm/s, 2.2E-4 cm/s, and 9.0E-6 cm/s respectively. Measurements were collected in-situ with the use of the Aardvark constant-head permeameter. Statistical analyses indicated a substantial difference in Kfs based on land-management practices and that urban farming can increase soil Kfs and limit overland flow. Additional analysis revealed no significant difference in grain-size distributions suggesting land-management practices controlled Kfs, not soil texture.

The work presented in this thesis covers an investigation into the use of metal nanoclusters in nanoelectronics design. Initial studies explored the interactions of the dodecanethiol passivated gold nanocluster, held in solution with toluene, and the native oxide covered silicon surface. Deposition of the clusters is achieved by pipetting u-litre quantities of the solution onto the surface, and allowing the solvent to evaporate leaving the clusters as residue. Patterning of the surface with micron scale photoresist structures prior to cluster exposure, led to the selective aggregation of cluster deposits along the resist boundaries. An extension of this technique, examined the flow of the cluster solution along photoresist structures which extended beyond the solution droplet. Investigation into the electronic properties of nanocluster arrays generated non-linear current-voltage curves, which are explained in terms of two very simple models. These results cast doubt over the suitability of the lateral approach to nanocluster device fabrication, and led to the exploration of vertical device design. Vertical devices, based around -50nm diameter silicon nanopillars with nanoc1usters on top, afford the necessary level of control over all aspects of nanocluster positioning; deposition of a single cluster layer is confined laterally to the pillar cross-section. Initial results of vertical device fabrication, show the considerable promise of this approach to cluster based electronic systems.