Browsing by Author "Dr. Kevin Lyons, Committee Member"
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- Advancement of Online Systems in Engineering by Expert TA(2006-09-04) Morton, Jeremy Andrew; Dr. Thomas Honeycutt, Committee Member; Dr. Kevin Lyons, Committee Member; Dr. Larry Silverberg, Committee Chair; Dr. Eric Klang, Committee MemberThis dissertation introduces a new online system called Expert TA. The system was developed based on the hypothesis that expressions are key elements in engineering problems and that the treatment of expressions is critical to the advancement of online systems. This dissertation identifies ergonomic problems with expression entry that Expert TA overcomes through the use of a problem-customize integrated expression editor, called a palate. Then the dissertation shows, using an expression analyzer that operates in the background of Expert TA, that specific mathematical mistakes within an entered expression can now be located. Emulating standard instructional practices, detailed feedback pertaining to specific mistakes and grading on the basis of specific mistakes is now possible.
- Case Studies of Waste Heat Driven Industrial Heat Pumps from the North Carolina State University Industrial Assessment Center.(2007-04-16) Lewis, Nathaniel Bates; Dr. James Leach, Committee Chair; Dr. Herbert Eckerlin, Committee Member; Dr. Kevin Lyons, Committee MemberWaste heat driven heat pumps can produce useful heat streams for manufacturing facilities. A heat pump system that uses a waste heat stream as the low temperature source often can provide the subject facility heat more cheaply than conventional heat sources such as boilers. Heat pumps have long been studied for industrial use. Several case studies were used to show that waste heat driven heat pumps typically have payback periods of approximately 5 years. This payback period is independent of the type of system or the physical location of the installation. Case studies using clients from the North Carolina State University's Industrial Assessment Center also show payback periods between four and six years. Three of the four facilities examined had positive payback periods from the installation of a heat pump. The fourth facility, a large fiber optic cable manufacturer, could not readily replace a traditional heat source with the heat pump and therefore could not justify the installation of a heat pump system. The IAC case studies highlighted several things. First, a heat pump must have a steady source of waste heat. Second, the system must be installed in an area with a relatively low cost of electricity. Third, the system must be able to offset heat generated elsewhere in the facility. Finally, the system must be able to operate for a large portion of the working day.
- An Experimental Evaluation and Comparison of Four Daylighting Strategies for Schools in North Carolina(2006-12-08) Manning, Myra Ashley; Dr. Herbert M. Eckerlin, Committee Chair; Dr. Kevin Lyons, Committee Member; Dr. James Leach, Committee MemberA comprehensive research study on the performance of daylighting systems at four schools in the Research Triangle area of North Carolina has been conducted during the three-month period from August through October of 2006. To achieve operational control, the experiments were scheduled over weekends when the schools were not in session. On a typical test weekend, daylight sensors and dataloggers were set up around the classroom after the students had departed on Friday afternoon and had to be picked by 7:00 am on Monday morning. Data was recorded every five minutes during daylight hours on Saturday and Sunday. The experiments were designed to identify and quantify the positive and negative operating characteristics of each school and then compare their overall performance. In three of the four schools tested, the level of daylight available on a good solar day was found to be more than adequate for most of the day. However, one of the common problems at all schools was a lack of uniformity in daylight levels. This was particularly true for classrooms with south-facing glazing. Generally, the light levels at the front of the classrooms were too high and at the rear too low. This disparity in light levels is an issue that will have to be addressed in future designs. One of the pleasant surprises of this research was the excellent daylighting performance of classrooms with north facing glazing. Daylighting is an exciting new development for school design and holds great promise for reducing the cost of lighting and air conditioning in schools. However, much work remains to be done to achieve a greater balance between the front and rear of the classroom and at the same time lower the overall light level. Related to the latter is the persistent challenge of providing the teacher with the ability to darken the room sufficiently for audiovisual presentations.
- Growth of Binary Alloyed Semiconductor Crystals by the Vertical Bridgman-Stockbarger Process with a Strong Magnetic Field.(2004-12-01) LaPointe, Stephen James; Dr. Richard Johnson, Committee Member; Dr. Nancy Ma, Committee Chair; Dr. Kevin Lyons, Committee MemberThis thesis presents a model for the unsteady species transport for the growth of alloyed semiconductor crystals during the vertical Bridgman-Stockbarger process with a steady axial magnetic field. During growth of alloyed semiconductors such as germanium-silicon (GeSi) and mercury-cadmium-telluride (HgCdTe), the solute's concentration is not small so that density differences in the melt are very large. These compositional variations drive compositionally-driven buoyant convection, or solutal convection, in addition to thermally-driven buoyant convection. These buoyant convections drive convective transport which produces non-uniformities in the concentration in both the melt and the crystal. This transient model predicts the distribution of species in the entire crystal grown in a steady axial magnetic field. The present study presents results of concentration in the crystal and in the melt at several different stages during crystal growth.
- Liquid-encapsulated Czochralski Growth of Compound Semiconductor Crystals with Steady and Rotating Magnetic Fields(2006-07-13) Yang, Mei; Dr. Nancy Ma, Committee Chair; Dr. Kevin Lyons, Committee Member; Dr. Zhilin Li, Committee Member; Dr. Tarek Echekki, Committee MemberIntegrated circuits and optoelectronic devices are produced on surfaces of thin wafers sliced from a photonic or compound semiconductor crystal. The growth of compound semiconductor crystals is critically important because viable substrates which are compositionally uniform both within a wafer and from wafer to wafer are needed. A dopant is an element that is added to the melt during growth to give the semiconductor crystal specific electrical and/or optical properties. More and better compound semiconductor crystals are needed for advanced optoelectronic devices. This investigation is focused on developing mathematical and numerical models to understand transport phenomena during bulk growth of compound semiconductor crystals. Since molten semiconductors are electrical conductors, magnetic fields can be used to control the melt motion in order to control the crystal's dopant distribution. Compound semiconductor crystals can be grown from the melt by the liquid-encapsulated Czochralski (LEC) process with a steady magnetic field. During this process, the molten semiconductor (melt) is covered with a layer of liquid encapsulant in order to prevent the escape of the volatile component. In this dissertation, we treat several different problems. We investigate the coupling of free convections in the melt and liquid encapsulant in a rectangular enclosure with steady vertical and horizontal magnetic fields, and find that these flows are coupled and the competition between these flows determines the direction of the horizontal velocity of the encapsulant-melt interface. We also investigate the dopant transport during the LEC process with a steady axial magnetic field, and find that both the radial and axial homogeneity of the crystal improves as the magnetic field strength decreases. With magnetic stabilization alone, however, the radially-inward flow below the crystal-melt interface does not become large enough to produce acceptable levels of segregation. A transverse magnetic field which rotates around the centerline of the melt can provide an electromagnetic stirring of the melt, and may represent a promising means to produce a crystal with good homogeneity. We investigate LEC growth with a combination of steady and rotating magnetic fields, and find that a rotating field can increase the magnitude of the radially-inward flow below the crystal-melt interface.
- A Model-Based Closure Approach for Turbulent Combustion using the One-Dimensional Turbulence Model(2007-03-21) Ranganath, Bhargav Bindiganavile; Dr. Tarek Echekki, Committee Chair; Dr. William Roberts, Committee Member; Dr. Kevin Lyons, Committee Member; Dr. Zhilin Li, Committee MemberA new model-based closure approach for turbulent combustion using the One-Dimensional turbulence model (ODT) is developed and validated in context to a turbulent jet diffusion flame. The interaction of turbulence and chemistry provides interesting finite rate chemistry effects including the phenomena of extinction and re-ignition. The ODT model resolves both spatially and temporally all the scales in a turbulent reaction flow problem, thus, combining the accuracy of a DNS like solver with efficiency by reduction in the number of dimensions. The closure approach is based on identifying the mechanisms responsible for the above mentioned effects and parameterizing the ODT results with a minimum set of scalars transported in the coarse grained solvers like the Reynolds-Averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES). Thus, the closure from ODT is based on a "one-way" coupling between the coarse grained solvers and ODT. Two approaches for closure are developed in the present work with respect to a RANS solver; however, they can be easily extended to LES. The first approach relies on ODT to provide the history effects associated with the geometry, which represent the interactions of turbulence and chemistry, by tabulating scalar statistics (first and second moments) on two parameters measuring, the extent of mixing, the radial mean mixture fraction, and the extent of entrainment, the centerline mean mixture fraction. However, based on the above parameterization, the approach is limited to jet diffusion flame geometry. Furthermore, the closure requires a one to one correspondence between the flames simulated in the coarse grained solver and ODT. As a second approach, the results from ODT are parameterized based on general representative scalars; mixture fraction, which specifies the mixedness of the mixture and temperature, which specifies the reactedness of the mixture. The history effects associated with the flow geometry are provided by the RANS solver in the form of probability distribution functions (PDFs). Two classes of turbulent jet diffusion flames; hydrogen⁄air (Flame H3) and piloted methane/air (Sandia flames D and F), are considered for validation of the above ODT-based closure approaches. The piloted methane air flames, owing to higher turbulence, exhibit severe extinction in the near field followed by re-ignition around the flame height. Good comparisons of the conditional statistics for temperature and reactive scalars with the experiments are obtained for both the flames. Good predictions of entrainment as well as mixing for both the flames, as seen in the comparisons of Favre averaged axial and radial profiles, are obtained. Furthermore, the correct trends of extinction and re-ignition are predicted successfully for the piloted methane/air flames. Thus, the results show the capability of ODT to address the closure needs for a turbulent combustion problem both at molecular length scales (conditional profiles) and integral length scales (Favre averaged axial and radial profile) successfully. Refinements in terms correct representation of the PDFs for the second closure approach can be recognized, whereas, a robust "two-way" coupling of RANS and ODT may yield good results.
- Numerical Investigation of the Mechanisms of Local Extinction Using Flame Kernel-Vortex interactions.(2006-11-14) Kolera-Gokula, Hemanth; Dr. Nancy Ma, Committee Member; Dr. William Roberts, Committee Member; Dr. Tarek Echekki, Committee Chair; Dr. Kevin Lyons, Committee MemberThe response of premixed flames to unsteady stretch is studied via kernel-vortex interactions. In this configuration a spark ignited kernel interacts with a vortex pair of variable strength. Both detailed and simple chemistry approaches are explored. In the detailed chemistry effort a dilute Hydrogen-air mixture is used. The vortex causes significant distortion of the kernel topography. Two distinct regimes; "Breakthrough" and "Extinction" are observed. A continuous increase in flame area and volumetric reaction rate values are observed throughout interactions in the breakthrough regime. However, corresponding consumption speed values are lower than 1-D laminar flame speed values. Detailed chemistry analysis of downstream interaction at the leading edge is carried out. During intermediate stages of the interaction, the mixture in between the interacting flames shows rich burning conditions. As the interaction proceeds the pool of products expands against the counter velocity gradient imposed by the vortex. The decrease in the temperature causes a steady decrease in the rates of reaction of the chain branching reactions causing. The behavior of various reaction layers is dictated to a large extent by their arrangement across the region of interaction. A simple two-step global reaction mechanism is formulated for lean methane combustion. These simple chemistry computations are carried out in an axis-symmetric configuration in a spherical frame of reference. Four distinct regimes of interaction: 1) the no-effect regime, 2) the wrinkling regime 3) the break-through regime, and the 4) global extinction regime are observed. Interactions in the no-effect regime show only minor deviations from unperturbed kernel values. Vortices in the wrinkling regime impose substantial stretch on the kernel causing major deviations from unperturbed kernel values. A sharp drop in the flame surface area and the integrated reaction rate is observed during breakthrough. The primary mechanism governing global extinction is downstream flame-flame interaction. A turbulent combustion diagram was derived for kernel-vortex interactions. Predominance of the breakthrough regime was observed. The turbulent combustion diagram represents an important contribution of this work.