Browsing by Author "Pierson, Mark A."

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    Application of Defocusing Technique to Bubble Depth Measurement
    Mugikura, Yuki (Virginia Tech, 2017)
    The thesis presents a defocusing technique to extract bubble depth information. Typically, when a bubble is out of focus in an image, the bubble is ignored by applying a filter or thresholding. However, it is known that a bubble image becomes blurred as the bubble moves away from the focal plane. Then, this technique is applied to determine the bubble distance along the optical path based on the blurriness or intensity gradient information of the bubble. Using the image processing algorithm, images captured in three different experiments are analyzed to develop a correlation between the bubble distance and its intensity gradient. The suggested models to predict the bubble depth are also developed based on the measurement data and evaluated with the measured data. When the intensity gradient of the bubble is lower or when a bubble is located farther from the focal plane, the model can predict the distance more accurately. However, the models show larger absolute and relative error when the bubble is near the focal plane. To improve the prediction in that region, another model should be considered. Also, depth of field analysis is introduced in order to compare three experimental results with different imaging setups. The applicability of the approach is analyzed and evaluated.
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    Coupled Field Modeling of Gas Tungsten Arc Welding
    Sen, Debamoy (Virginia Tech, 2012-07-03)
    Welding is used extensively in aerospace, automotive, chemical, manufacturing, electronic and power-generation industries. Thermally-induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Numerical simulation of weld pool dynamics is important as experimental measurements of velocities and temperature profiles are difficult due to the small size of the weld pool and the presence of the arc. From a structural integrity perspective of welded structures, it is necessary to have an accurate spatial and temporal thermal distribution in the welded structure before stress analysis is performed. Existing research on weld pool dynamics simulation has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Previous research has established that the weld pool depth/width (D/W) ratio and Heat Affected Zone (HAZ) are significantly altered by the weld pool dynamics. Hence, for a more accurate estimation of the thermally-induced stresses it is desired to incorporate the weld pool dynamics into the analysis. Moreover, the effects of microstructure evolution in the HAZ on the mechanical behavior of the structure need to be included in the analysis for better mechanical response prediction. In this study, a three-dimensional model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of thin stainless steel butt-joint plates has been developed. The model incorporates the effects of thermal energy redistribution through weld pool dynamics into the structural behavior calculations. Through material modeling the effects of microstructure change/phase transformation are indirectly included in the model. The developed weld pool dynamics model includes the effects of current, arc length, and electrode angle on the heat flux and current density distributions. All the major weld pool driving forces are included, namely surface tension gradient, plasma drag force, electromagnetic force, and buoyancy. The weld D/W predictions are validated with experimental results. They agree well. The effects of welding parameters (like welding speed, current, arc length, etc.) on the weld D/W ratio are documented. The workpiece deformation and stress distributions are also highlighted. The transverse and longitudinal residual stress distribution plots across the weld bead and their variations with welding speed and current are also provided. The mathematical framework developed here serves as a robust tool for better prediction of weld D/W ratio and thermally-induced stress evolution and distribution in a welded structure by coupling the different fields in a welding process.
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    Deposition of Newtonian Particles Entrained in a Turbulent Axisymmetric Free Jet
    Robertson, Zachary Burton Smith (Virginia Tech, 2012-04-23)
    In the past 10 years there has been a significant amount of research into two-phase particle transport. The terrorist events of September 11, 2001 sparked a series of studies analyzing particle entrainment and deposition in turbulent airflows. One area of research needing further attention has been the study of particles entrained in axisymmetric air jets. An experimental rig was designed and built to study entrainment properties and deposition of Newtonian particles, after injection into a turbulent axisymmetric free air jet. Newtonian spherical particles, ranging from 1mm to 6mm in diameter, were injected into a turbulent airstream and blown through a nozzle into a large, open space. As the particles fell out of the jet stream, their linear distances, from nozzle to initial-ground-contact, were recorded and analyzed. The experiments conducted indicated particle size and density to be significant factors when considering Newtonian particle entrainment. Additionally, particle deposition distribution revealed a consistent positive skewness, as opposed to an expected Gaussian form. The data presented in this paper provide a starting point for understanding entrainment of Newtonian spherical particles in jets. The simple experimental rig geometry and results also provide an opportunity for computational fluid dynamics models to be validated, answering a call from the 2006 Annual Review of Fluid Mechanics.
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    Development, Evaluation and Improvement of Correlations for Interphase Friction in Gas-Liquid Vertical Upflow
    Clark, Randy R. Jr. (Virginia Tech, 2015-10-15)
    In this study, liquid-vapor vertical upflow has been research with the intent of finding an improved method of modelling the interphase friction in two-phase vertical flow in nuclear thermal-hydraulic codes. An improved method of modelling interphase friction should allow for better prediction of pressure gradient, void fraction and the phasic velocities. Data has been acquired from several available published resources and analyzed to determine the interphase friction using a force balance between the liquid and vapor phases. Using the Buckingham Pi Theorem, a dimensionless interphase friction force was tested and refined before being compared against seven other dimensionless parameters. Three correlations have been developed that establish a dimensionless interphase friction force as a function of the Weber number, the Froude number and the mixture Froude number. Statistical analysis of the three correlations shows that the mixture Froude number correlation should be the most accurate correlation. The correlations have a weakness that makes them ineffective mostly for bubbly flow and some slug flow scenarios, while they should perform significantly better for annular flow cases. Comparisons have been made against the interphase friction calculations published in the manuals of RELAP5/MOD2, RELAP5/MOD3.3, RELAP5-3D and TRACE. The findings have generally shown that the equations in the manuals provide very inaccurate approximations of the interphase friction compared to the interphase friction that was found via force balance. When analyzing the source code of RELAP5/MOD3.3, several differences were noticed between the source code and manual, which have been discussed. Calculations with the source code equations reveal that the source code provides a modestly improved prediction of the interphase friction force, but still has significant errors. Despite the fact that the manual and source code equations indicate that RELAP5/MOD3.3 should perform poorly in modelling interphase friction, actual RELAP5/MOD3.3 model runs perform very well in predicting pressure gradient, void fraction, the liquid and vapor velocities and the interphase friction force. This is largely due to RELAP5/MOD3.3 being able to adjust parameters to converge to a solution that fits within the boundary conditions established in the input file. Modifications to the RELAP5/MOD3.3 code were first made with the three correlations developed using dimensionless parameters, and were tested with data points that the RELAP5/MOD3.3 flow regime map had predicted would be annular flow. While the mixture Froude number correlation has been analyzed to be the most statistically accurate of the three correlations, it was found that the Weber number correlation performed best when implemented into RELAP5/MOD3.3. In a parametric study of the Weber number correlation, it performed optimally at 150% of the original correlation, improving upon the original RELAP model in almost every metric examined. Additional investigations were performed with individual annular flow correlations that model specific physical parameters. Results with the annular flow physical models were inconclusive as no particular model provided a significant improvement over the original RELAP5/MOD3.3 model, and there was no clear indication that combining the models would provide significant improvement.
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    Guided Wave Structural Health Monitoring with Environmental Considerations
    Dodson, Jacob Christopher (Virginia Tech, 2012-04-09)
    Damage detection in mechanical and aerospace structures is critical to maintaining safe and optimal performance. The early detection of damage increases safety and reduces cost of maintenance and repair. Structural Health Monitoring (SHM) integrates sensor networks and structures to autonomously interrogate the structure and detect damage. The development of robust SHM systems is becoming more vital as aerospace structures are becoming more complex. New SHM methods that can determine the health of the structure without using traditional non-destructive evaluation techniques will decrease the cost and time associated with these investigations. The primary SHM method uses the signals recorded on a pristine structure as a reference and compares operational signals to the baseline measurement. One of the current limitations of baseline SHM is that environmental factors, such as temperature and stress, can change the system response so the algorithm indicates damage when there is none. Many structures which can benefit from SHM have multiple components and often have connections and interfaces that also can change under environmental conditions, thus changing the dynamics of the system. This dissertation addresses some of the current limitations of SHM. First the changes that temperature variations and applied stress create on Lamb wave propagation velocity in plates is analytically modeled and validated. Two methods are developed for the analytical derivative of the Lamb wave velocity; the first uses assumes a thermoelastic material while the second expands thermoelastic theory to include thermal expansion and the associated stresses. A model is developed so the baseline measurement can be compensated to eliminate the false positives due to environmental conditions without storage of dispersion curves or baseline signals at each environmental state. Next, a wave based instantaneous baseline method is presented which uses the comparison of simultaneously captured real time signals and can be used to eliminate the influence of environmental effects on damage detection. Finally, wave transmission and conversion across interfaces in prestressed bars is modeled to provide a better understanding of how the coupled axial and flexural dynamics of a non-ideal preloaded interface change with applied load.
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    Hydrodynamics and Transient Heat Transfer Characteristics in Fluidized and Spouted Beds
    Brown, Steven Lewis (Virginia Tech, 2012-06-21)
    Hydrodynamics and heat transfer characteristics found in fluidization were studied in a small scale laboratory fluidized bed. Experiments were designed to capture field data on multiple slit jet gas distributor systems for the validation of computational models. Localized data was quantified through the use of several novel non-intrusive experimental measurement techniques. The analyses provide a unique study that connects full field-of-view multiphase flow dynamics with transient heat transfer distributions. The gas-solid hydrodynamics were captured through three non-invasive measurement techniques, viz. Particle Image Velocimetry (PIV), Digital Image Analysis (DIA), and pressure drop spectral analysis. The effects of inlet gas flowrate, Geldart B and D classified particle types, and the number inlet gas slit jets were investigated. Frequency analysis of a differential pressure signal resulted in the classification of four difference flow regimes. The coupling of PIV with DIA captured particle velocity, solid circulation rates, average cycle times, dead zone sizes, jet merging effects, gas void fraction distributions, and maximum expanded bed heights. The heat transfer in fluidized and spouted beds containing a heated inlet gas source was studied through transient heat transfer measurements and analyses. Innovative experimental procedures were introduced to quantify bed-to-wall and gas-to-particle heat transfer characteristics. Two techniques were developed to overcome the spatial, time varying, and instrumental intrusive limitations often found in multiphase flow heat transfer studies. Infrared thermography was utilized along with derived discrete differential equations, and an inverse heat conduction analysis to solve for transient localized heat flux profiles and heat transfer coefficient distributions. As a result new data containing increased spatial resolution is presented on gas, wall, and particle temporal maps. Computations based from the thermal gradients quantified bed-to-wall heat flux profiles, gas-to-particle heat transfer coefficients, and localized rates of energy stored.
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    Modeling Two-Phase Flow in the Downcomer of a Once-Through Steam Generator using RELAP5/MOD2
    Clark, Randy Raymond (Virginia Tech, 2011-12-07)
    The purpose of this study is to develop an accurate model of the downcomer of the once-through steam generator (OTSG) developed by Babcock & Wilcox, using RELAP5/MOD2. While the physical model can be easily developed, several parameters are left to be adjusted to optimally model the downcomer and match data that was retrieved in a first-of-a-kind (FOAK) study conducted at Oconee Unit I in Oconee, South Carolina. Once the best-fit set of parameters has been determined, then the model must be tested for power levels exceeding that for which the steam generator was originally designed, so as to determine the power level at which a phenomenon known as flood-back becomes a concern. All known previous studies that have been conducted using RELAP5/MOD2 have shown that RELAP over-predicts interphase friction. However, all of those studies focused on heated two-phase upflow, whereas the downcomer is modeled as adiabatic two-phase downflow. In this study, it is found that the original slug drag model for RELAP5/MOD2 developed by Idaho National Engineering Laboratory (INEL) under-predicts the interphase friction between the liquid and vapor phase within the downcomer. Using a modified version of the original slug drag model created by Babcock & Wilcox (B&W), an optimum multiplier is found for each power level. An increase of 1181% in interphase friction over the INEL slug drag model, which equals an increase of 4347% for the default B&W model provides the most accurate results for all power levels studied. Emphasis is also placed on modeling the orifice plate of the OTSG downcomer which has been added to stabilize pressure fluctuations between the downcomer and tube bundle of the OTSG. While several different schemes are explored for modeling the orifice plate, a branch connection with an inlet area 14.22% of that of the downcomer is used to model the orifice plate along with the volume that transitions the two-phase downflow to horizontal flow into the tube nest of the OTSG. Power levels exceeding that for which the steam generator was designed are tested in RELAP using the slug drag multiplier to determine at which power level a liquid level would occur and would flood-back become a concern. In this study, it is determined that a liquid level would form at 135% power and that at any higher power level, flood-back would be of concern for any user of the steam generator.
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    Pneumatic Particulate Collection System for an Unmanned Ground Sampling Robot
    Couch, Michael Robert (Virginia Tech, 2010-11-19)
    The design of unmanned material collection systems requires a great deal of foresight and innovative design on the engineer's part in order to produce solutions to problems operators may encounter in the field. In this thesis, the development of a particulate collection system for use onboard a lightweight, helicopter deployable ground robot is presented. The Unmanned Systems Laboratory at Virginia Tech is developing a ground sampling robot to be carried in the payload pod of a Yamaha RMAX unmanned aerial vehicle. The robot's ultimate objective is to collect material samples from a hazardous environment. The pneumatic system presented here is a novel design developed to collect particulate without draining the resources of the robot. Vacuum samplers have been developed in the past, but they are large and cumbersome and require large amounts of electrical energy to operate. The pneumatic particulate collection system utilizes the kinetic energy from the release of compressed air to transport the particulate to a collection chamber. Consideration is given to the drop in pressure of the air supply tank as it empties, and a feasible air supply tank design is presented. Two forms of particulate collection are investigated experimentally: jet impingement and particle entrainment (i.e. steep attack angle and parallel flow). Turbulent, free jet characteristics and critical velocities of particles are studied. Ultimately, a final design is presented that effectively collects particulate material from the top 5/8" layer of both thick and thin particle beds.
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    Reduction of Solid Uranium Dioxide in Calcium Salts
    Karakaya, Nagihan (Virginia Tech, 2022-07-01)
    Nuclear energy has gained crucial importance since it has a minor impact on climate change and greenhouse gas releases; additionally, the other energy sources are insufficient to reach the world's energy needs without nuclear energy. Another sign that the Generation IV International Forum (Kelly, Gen IV International Forum: A decade of progress through international cooperation, 2014) has pointed out is to utilize uranium resources to the maximum and recycle spent nuclear fuel through burn-up in the Generation IV reactor designs, one of which is the molten salt reactor (MSR). Therefore, the MSR can use the spent nuclear fuel as a fresh fuel when the actinides recycle. That reprocessing of spent fuel could be one of the opportunities to contribute to future nuclear energy goals. This study aims to develop a modified pyroprocessing method to prepare molten salt fuels for MSR from spent oxide nuclear fuel that was burned in light water reactors (LWRs). The process diagram illustrated as (1) spent fuel treatment, (2) chopping and voloxidation of spent oxide fuel, (3) oxide reduction of spent fuel, and then depending on the fuel structure and composition for the MSR, it continues by one or two of the following; – electrorefining, – chlorination, and – fluorination. The subject of this study focused on oxide reduction in two categories: chemical reduction and electrochemical reduction. The system designs have been optimized in calcium salts since they have high calcium metal and calcium oxide solubility. The significant results indicated that both methods would substantially reduce the solid uranium dioxide pellet. The chemical reduction will reduce the total solid pellet at 850oC in the composition of 55.73mol%CaCl2-12.37mol%CaF2-26.58mol%Ca-5.32mol%UO2 over 12 hours. The total reduction in the electrochemical test is seen at 850oC during 12 hours with a salt composition of 79mol%CaCl2-17mol%CaF2-4mol%CaO. These oxide reduction mechanisms are convenient ways to reprocess spent oxide fuel from LWRs to utilize in the MSR. Additionally, the reduced fuel is also applicable to using other next-generation reactors. The prospect of this research is the explicit comparison between chemical and electrochemical methods in calcium salts.
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    Theory and Application of a Class of Abstract Differential-Algebraic Equations
    Pierson, Mark A. (Virginia Tech, 2005-04-25)
    We first provide a detailed background of a geometric projection methodology developed by Professor Roswitha Marz at Humboldt University in Berlin for showing uniqueness and existence of solutions for ordinary differential-algebraic equations (DAEs). Because of the geometric and operator-theoretic aspects of this particular method, it can be extended to the case of infinite-dimensional abstract DAEs. For example, partial differential equations (PDEs) are often formulated as abstract Cauchy or evolution problems which we label abstract ordinary differential equations or AODE. Using this abstract formulation, existence and uniqueness of the Cauchy problem has been studied. Similarly, we look at an AODE system with operator constraint equations to formulate an abstract differential-algebraic equation or ADAE problem. Existence and uniqueness of solutions is shown under certain conditions on the operators for both index-1 and index-2 abstract DAEs. These existence and uniqueness results are then applied to some index-1 DAEs in the area of thermodynamic modeling of a chemical vapor deposition reactor and to a structural dynamics problem. The application for the structural dynamics problem, in particular, provides a detailed construction of the model and development of the DAE framework. Existence and uniqueness are primarily demonstrated using a semigroup approach. Finally, an exploration of some issues which arise from discretizing the abstract DAE are discussed.