Master of Science / Department of Mechanical and Nuclear Engineering / Guoqiang Hu / Warren N. White / In recent years, the concern has risen to establish clean sources for electric power generation. In 2009, Kansas established an RPS (Renewable Portfolio Standard) mandating utilities acquire 20% of their electricity from renewable energy by 2020 [32]. One of the most prominent renewable energy sources is wind energy. Utility companies now are investing more in wind capture systems to comply with this mandate. This increase in the manufacture of wind turbines has caused researchers to investigate methods to improve the efficiency of captured wind energy and where improvements can be made. This thesis takes a control theory approach to maximizing the power capture of a wind turbine using the concepts of robust estimation, nonlinear control, and Lyapunov-based maximization. A two step control approach to optimize the power capture of a wind turbine is proposed. First, a robust controller is used to estimate unknown aerodynamic properties and regulate the wind turbine tip-speed ratio as it tracks a desired trajectory. Once the tip-speed ratio is regulated within a given tolerance, a Lyapunov-based control approach is developed to provide the robust controller with a desired trajectory to track. This is done by estimating the unknown coefficient of performance of the wind turbine. A discrete update law is then developed to alter the tip-speed ratio and the blade pitch of the wind turbine so that the coefficient of performance is maximized. A simulation is provided of this control strategy and tested under time varying wind conditions and measurement noise in order to demonstrate the controller’s performance. The system simulated is intended to emulate a commercial wind turbine operating in a realistic environment. A detailed discussion of the simulation model, control scheme, and results will be provided to supplement the theoretical controller development, as well as future work for this control application.

Master of Science / Department of Architectural Engineering and Construction Science / Julia A. Keen / Nanofibers are classified as fibers less than 1 micrometer in diameter. These fibers can be layered to form nanofibrous membranes, and these membranes offer great potential in the filtration industry. The membranes' smaller fiber diameters and pore sizes permit such filters to filter out more and smaller particulate. Additionally, antimicrobial agents can be incorporated into the membrane to inhibit fungal and bacterial growth on the membrane’s surface. This report evaluates nanofibrous membranes with antimicrobial agents and their potential in two specific locations: cleanrooms and protective environment rooms, where bacterial and fungal growth would have a detrimental effect on the process or occupant of the space.

Master of Science / Department of Mechanical and Nuclear Engineering / Guoqiang Hu / In some types of renewable energy systems such as wind turbines or solar power plants, the optimal operating conditions are influenced by the intermittent nature of these energies. This fact, along with the modeling difficulties of such systems, provides incentive to look for non-model based adaptive techniques to address the maximum power point tracking (MPPT) problem. In this thesis, a novel extremum seeking algorithm is proposed for systems where the optimal point and the optimal value of the cost function are allowed to be time varying. A sinusoidal perturbation based technique is used to estimate the gradient of the cost function. Afterwards, a robust optimization method is developed to drive the system to its optimal point. Since this method does not require any knowledge about the dynamic system or the structure of the input-to-output mapping, it is considered to be a non-model based adaptive technique. The proposed method is then employed for maximizing the energy capture from the wind in a variable speed wind turbine. It is shown that without any measurements of wind velocity or power, the proposed method can drive the wind turbine to the optimal operating point. The generated power is observed to be very close to the maximum possible values.

Master of Science / Department of Mechanical and Nuclear Engineering / Zhongquan Zheng / This study investigates a method using computational fluid dynamics (CFD) to model low-elevation atmospheric conditions. There are three goals in this research: to analyze the wind behavior downwind from buildings and trees, to validate the accuracy of the simulations by comparing wind measurements to the simulation for a specific site, and to find a relationship between the wind speed and the power output of a small-scale wind turbine. The first goal is to define a proper CFD model for buildings and trees. The trends in the Strouhal number are found to correlate to changes in building height and the wind resistance of a tree as supported in literature, with minor differences with the addition of a tree. The second goal of this study is to model an actual low-elevation environment to compare the energy output predictions for a small-scale wind turbine versus traditional methods. The simulations are compared to on-site wind measurements at a suburban wind turbine, recorded by the rotor and two anemometers installed on the wind turbine tower. The measurements and simulations presented in this study show an improvement in the accuracy in the estimation of the energy output of a wind turbine versus using traditional methods involving high-elevation wind maps. The third goal is to provide a relationship between the wind speed and the power output of a small-scale wind turbine. To accomplish this task, system identification is implemented. The traditional auto-regressive model with exogenous input variables (ARX), its moving average counterpart (ARMAX), and the output error (OE) model are compared in this study. It is found that the transfer function provided by the ARX model most sufficiently estimates the power output of the studied wind turbine, with power output accuracies of 83%. With all three goals addressed, the feasibility of small-scale wind turbines in different low-elevation environments is assessed. In accomplishing these tasks, the siting of a small-scale wind turbine can be optimized qualitatively and quantitatively.

Master of Science / Department of Mechanical and Nuclear Engineering / Kirby S. Chapman / The increased emissions monitoring requirements of industrial gas turbines have created a demand for less expensive emissions monitoring systems. Typically, emissions monitoring is performed with a Continuous Emissions Monitoring System (CEMS), which monitors emissions by direct sampling of the exhaust gas. An alternative to a CEMS is a system which predicts emissions using easily measured operating parameters. This system is referred to as a Parametric Emissions Monitoring System (PEMS). A review of the literature indicates there is no globally applicable PEMS. Because of this, a PEMS that is applicable to a variety of gas turbine manufacturers and models is desired. The research presented herein includes a literature review of NOx reduction techniques, NOx production mechanisms, current PEMS research, and combustor modeling. Based on this preliminary research, a combustor model based on first-engineering principles was developed to describe the NOx formation process and relate NOx emissions to combustion turbine operating parameters. A review of available literature indicates that lean-premixed combustion is the most widely-used NOx reduction design strategy, so the model is based on this type of combustion system. A review of the NOx formation processes revealed four well-recognized NOx formation mechanisms: the Zeldovich, prompt, nitrous oxide, and fuel-bound nitrogen mechanisms. In lean-premixed combustion, the Zeldovich and nitrous oxide mechanisms dominate the NOx formation. This research focuses on combustion modeling including the Zeldovich mechanism for NOx formation. The combustor model is based on the Siemens SGT-200 combustion turbine and consists of a series of well-stirred reactors. Results show that the calculated NOx is on the same order of magnitude, but less than the NOx measured in field tests. These results are expected because the NOx calculation was based only on the Zeldovich mechanism, and the literature shows that significant NOx is formed through the nitrous oxide mechanism. The model also shows appropriate trends of NOx with respect to various operating parameters including equivalence ratio, ambient temperature, humidity, and atmospheric pressure. Model refinements are suggested with the ultimate goal being integration of the model into a PEMS.

Master of Science / Department of Mechanical and Nuclear Engineering / Kevin Lease / Carbon fiber reinforced composites (CFRP’s) are a mainstay in many industries, including the aerospace industry. When composite components are damaged on an aircraft, they are typically repaired with a composite patch that is placed over the damaged material and cured into the existing composite material. This curing process involves knowledge of the curing time necessary to sufficiently cure the patch. The inexact nature of curing composites on aircraft causes a significant waste of time and material when patches are unnecessarily redone. Knowing how differences in cure cycle affect the strength of the final material could reduce this waste. That is the focus of this research. In this research, the interfacial shear strength (IFSS) of carbon fiber/epoxy composites was investigated to determine how changes in cure cycle affect the overall material strength. IFSS is a measure of the strength of the bond between the two materials. To measure this, the microbond method was used. In this method, a drop of epoxy is applied to a single carbon fiber. The specimen is cured and the droplet is sheared from the fiber. The force required to debond the droplet is recorded and the data is analyzed. The IFSS of AS4/Epon828, T650/Epon828, and T650/Cycom 5320-1 composites were evaluated. For the former two material systems, a cure cycle with two steps was chosen based on research from others and then was systematically varied. The final cure time was changed to determine how that parameter affected the IFSS. It was found that as the final cure time increased, so did the IFSS and level of cure achieved by the composite to a point. Once the composite reached its fully cured state, increasing the final cure time did not noticeably increase the IFSS. For the latter material system (T650/Cycom 5320-1), the two cure cycles recommended by the manufacturer were tested. These had different initial cure steps and identical final cure steps. Although both cure cycles caused high IFSS, the cycle with the higher initial temperature, but shorter initial cure time achieved a higher level of cure than that with a longer time, but shorter temperature.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Youqi Wang / Textile fabrics and textile composite materials demonstrate exceptional mechanical properties, including high stiffness, high strength to weight ratio, damage tolerance, chemical resistance, high temperature tolerance and low thermal expansion. Recent advances in weaving techniques have caused various textile fabrics to gain applications in high performance products, such as aircrafts frames, aircrafts engine blades, ballistic panels, helmets, aerospace components, racing car bodies, net-shape joints and blood vessels. Fabric mechanical properties are determined by fabric internal architectures and fabric micro-geometries are determined by the textile manufacturing process. As the need for high performance textile materials increases, textile preforms with improved thickness and more complex structures are designed and manufactured. Therefore, the study of textile fabrics requires a reliable and efficient CAD/CAM tool that models fabric micro-geometry through computer simulation and links the manufacturing process with fabric micro-geometry, mechanical properties and weavability. Dynamic Weaving Process Simulation is developed to simulate the entire textile process. It employs the digital element approach to simulate weaving actions, reed motion, boundary tension and fiber-to-fiber contact and friction. Dynamic Weaving Process Simulation models a Jacquard loom machine, in which the weaving process primarily consists of four steps: weft insertion, beating up, weaving and taking up. Dynamic Weaving Process Simulation simulates these steps according to the underlying loom kinematics and kinetics. First, a weft yarn moves to the fell position under displacement constraints, followed by a beating-up action performed by reed elements. Warp yarns then change positions according to the yarn interlacing pattern defined by a weaving matrix, and taking-up action is simulated to collect woven fabric for continuous weaving process simulation. A Jacquard loom machine individually controls each warp yarn for maximum flexibility of warp motion, managed by the weaving matrix in simulation. Constant boundary tension is implemented to simulate the spring at each warp end. In addition, process simulation adopts re-mesh function to store woven fabric and add new weft yarns for continuous weaving simulation. Dynamic Weaving Process Simulation fully models loom kinetics and kinematics involved in the weaving process. However, the step-by-step simulation of the 3D weaving process requires additional calculation time and computer resource. In order to promote simulation efficiency, enable finer yarn discretization and improve accuracy of fabric micro geometry, parallel computing is implemented in this research and efficiency promotion is presented in this dissertation. The Dynamic Weaving Process Simulation model links fabric micro-geometry with the manufacturing process, allowing determination of weavability of specific weaving pattern and process design. Effects of various weaving process parameters on fabric micro-geometry, fabric mechanical properties and weavability can be investigated with the simulation method.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Liang-Wu Cai / X. J. Xin / Thin walled structures such as plates and shells have application in many fields of engineering because these structures are light weight and can support large loads when designed suitably. In real world, loads may cause these structures to vibrate which can be undesirable causing fatigue and failure of the structure. Such undesirable vibrations need to be reduced or eliminated. In this work, analytical studies of flexural wave propagation for idealized geometries are conducted and finite element method (FEM) is used to explore the effects of composite panel designs of finite size for the reduction of vibration transmission. In the analytical studies, the influence of the material properties on the reflection and transmission characteristics are explored for an infinite bi-material plate, and infinite plate with a strip inhomogeneity. In the analytical study of an infinite thin plate with a solid circular inclusion, the far and near field scattering characteristics are explored for different frequencies and material properties. All the analytical studies presented here and reported in the literature consider infinite plates to characterize the flexural wave propagation. Obtaining closed form solutions to characterize the flexural wave propagation in a finite plate with inclusions is mathematically difficult process. So, FEM is used to explore the composite panel designs. The understanding gained about the material properties influence on the flexural wave propagation from analytical studies helped with the choice of materials for FEM simulations. The concept of phononic crystals is applied to define the design variations that are effective in suppressing vibration transmission. Various design configurations are explored to study the effects of various parameters like scatterer’s material properties, geometry and spatial pattern. Based on the knowledge gained through a systematic parametric study, a final design of the composite sandwich panel is proposed with an optimum set of parameters to achieve the best vibration reduction. This is the first study focused on reducing vibration and wave transmission in composite rotorcraft fuselage panels incorporating the concept of phononic crystals. The optimum sandwich panel design achieved 98% vibration transmission reduction at the frequency of interest of 3000 Hz.

Master of Science / Department of Mechanical and Nuclear Engineering / Mohammad H. Hosni / Byron W. Jones / Gas turbine engine oil is contaminating the bleed air of an aircraft with enough frequency and intensity that health concerns are of public interest. While previous work measured micro particles and used only a simulator, this work mainly consists of measurements in the nanoparticle and ultrafine range using both the simulator and two different gas turbine engines. No previous research has been conducted using working jet engines to simulate a bleed air system and characterize the oil particulate contamination. Oil was injected into a bleed air simulator and an Allison 250 CC18 turbine engine in order to observe the particle size distributions resulting from thermal degradation and was measured with three particle sizing counters and an FTIR. The aerosol size distributions are given for various temperature and pressure ranges consistent with the process conditions associated with the bleed air in a commercial aircraft. Particle sizes of approximately 80nm to 100nm were observed at temperatures over 200°C while particles similar to injection distributions and smaller than measureable size were observed at lower power settings. Temperature is thought to be the controlling factor affecting particle size above 200°C while blade shear is likely the dominant factor for lower temperatures. The bleed air simulator produced results similar to the gas turbine engine results at higher temperatures, but did not replicate the size characteristics at lower temperatures. The observed particles are ultrafine and situated in the size range that may impact health safety more than larger particles.

Master of Science / Department of Mechanical and Nuclear Engineering / Amy R. Betz / The purpose of this research was to test if biphilic surfaces mitigate frost and ice formation. Frost, which forms when humid air comes into contact with a surface that is below the dew point and freezing temperature of water, hinders engineering systems such as aeronautics, refrigeration systems, and wind turbines. Most previous research has investigated increasingly superhydrophobic materials to delay frost formation; however, these materials are dependent on fluctuating operating conditions and surface roughness. Therefore, the hypothesis for this research was that a biphilic surface would slow the frost formation process and create a less dense frost layer, and water vapor would preferentially condense on hydrophilic areas, thus controlling where nucleation initially occurs. Preferential nucleation can control the size, shape, and location of frost nucleation. To fabricate biphilic surfaces, a hydrophobic material was coated on a silicon wafer, and a pattern of hydrophobic material was removed using photolithography to reveal hydrophilic silicon-oxide. Circles were patterned at various pitches and diameters. The heat sink was comprised of two parts: a solid bottom half and a finned upper half. Half of the heat sink was placed inside a polyethylene base for insulation. Tests were conducted in quiescent air at room temperature, 22 °C, and two relative humidities, 30% and 60%. Substrate temperatures were held constant throughout all tests. All tests showed a trend that biphilic surfaces suppress freezing temperature more effectively than plain hydrophilic or hydrophobic surfaces; however, no difference between pattern orientation or size was noticed for maximum freezing temperature. However, the biphilic patterns did affect other aspects such as time to freezing and volume of water on the surface. These effects are from the patterns altering the nucleation and coalescence behavior of condensation.