Electromagnetic Nondestructive Evaluation of Wire Insulation and Models of Insulation Material Properties

Polymers have been widely used as wiring electrical insulation materials in space/air-craft. The dielectric properties of insulation polymers can change over time, however, due to various aging processes such as exposure to heat, humidity and mechanical stress. Therefore, the study of polymers used in electrical insulation of wiring is important to the aerospace industry due to potential loss of life and aircraft in the event of an electrical fire caused by breakdown of wiring insulation. Part of this research is focused on studying the mechanisms of various environmental aging process of the polymers used in electrical wiring insulation and the ways in which their dielectric properties change as the material is subject to the aging processes. The other part of the project is to determine the feasibility of a new capacitive nondestructive testing method to indicate degradation in the wiring insulation, by measuring its permittivity

Unsteady flamelet progress variable modeling of reacting diesel jets

Accurate modeling of turbulence/chemistry interactions in turbulent reacting diesel jets is critical to the development of predictive computational tools for diesel engines. The models should be able to predict the transient physical and chemical processes in the jets such as ignition and flame lift-off. In the first part of this work, an existing unsteady flamelet progress variable (UFPV) model is employed in Reynolds-averaged Navier-Stokes (RANS) simulations and large-eddy simulations (LES) to assess its accuracy. The RANS simulations predict that ignition occurs toward the leading tip of the jet, followed by ignition front propagation toward the stoichiometric surface, and flame propagation upstream along the stoichiometric surface until the flame stabilizes at the lift-off height. The LES, on the other hand, predicts ignition at multiple points in the jet, followed by flame development from the ignition kernels, merger of the different flames and then stabilization. The UFPV model assumes that combustion occurs in thin zones known as flamelets and turbulent strain characterized by the scalar dissipation rate modifies the flame structure. Since the flamelet is thinner than the smallest grid size employed in RANS or LES, the effect of the turbulence is modeled through probability distribution functions of the independent variables. The accuracy of the assumptions of the model is assessed in this work through direct numerical simulations (DNS) which resolves the flame. The DNS is carried out in turbulent mixing layers since the combustion in a diesel jet occurs in the fuel/air mixing layer surrounding the jet. ^ The DNS results show that the flamelet model is applicable but that its implementation in the UFPV model is flawed because the effects of expansion due to heat release and increase in diffusivity due to rise in temperature are not accounted for in the formulation of the scalar dissipation rate. A new diffusivity-corrected flamelet model is proposed which leads to an improved prediction of flame development. Furthermore, it is shown that the most commonly used approach to calculate the scalar dissipation rate in LES of reacting flows leads to large errors when the LES grid size is large. The DNS results are used to determine the best model for the filtered scalar dissipation rate and its PDF under diesel engine conditions. A new model is derived for the variance of the scalar dissipation rate. The DNS results are also used to compare the performance of the UFPV model with the Perfectly Stirred Reactor (PSR) model predictions. It is shown that the UFPV model performance is superior for turbulent intensities and grid sizes encountered in diesel engine application

Multiscale Modeling Of Thin Films In Direct Numerical Simulations Of Multiphase Flows.

Direct numerical simulations, where both the large and small scales in the flow are fully resolved, provide an excellent instrument to validate multiphase flow processes and also further our understanding of it. Three multiphase systems are studied using a finite difference/front-tracking method developed for direct numerical simulations of time-dependent system¬¬s. The purpose of these studies is to demonstrate the benefit in developing accurate sub-grid models that can be coupled with the direct numerical simulations to reduce the computational time. The primary reason to use the models is that the systems under consideration are sufficiently large that resolving the smallest scales is impractical. The processes that are examined are: (1) droplet motion and impact (2) nucleate boiling and (3) convective mass transfer. For droplet impact on solid walls and thin liquid films, the splash characteristics are studied. The collision of a fluid drop with a wall is examined and a multiscale approach is developed to compute the flow in the film between the drop and the wall. By using a semi-analytical model for the flow in the film we capture the evolution of films thinner than the grid spacing reasonably well. In the nucleate boiling simulations, the growth of a single vapor from a nucleation site and its associated dynamics are studied. The challenge here is the accurate representation of the nucleation site and the small-scale motion near the wall. To capture the evaporation of the microlayer left behind as the base of the bubble expands we use a semi-analytical model that is solved concurrently with the rest of the simulations. The heat transfer from the heated wall, the evolution of the bubble size and the departure diameter are evaluated and compared with the existing numerical results. The mass transfer near the interface, without fully resolving the layer by refining the grid is accommodated by using a boundary layer approximation to capture it. The behavior of the concentration profile is taken to be self-similar. A collection of potential profiles is tested and the accuracy of each of these models is compared with the full simulations

Modeling of Turbulent Sooting Flames

Modeling multiphase particles in turbulent fluid environment is a challenging task. To accurately describe the size distribution, a large number of scalars need to be transported at each time-step. Add to that the heat release and species mass fraction changes from nonlinear combustion chemistry reactions, and you have a tightly coupled set of equations that describe the (i) turbulence, (ii) chemistry, and (iii) soot particle interactions (physical agglomeration and surface chemistry reactions). Uncertainty in any one of these models will inadvertently introduce errors of up to a few orders of magnitude in predicted soot quantities. The objective of this thesis is to investigate the effect of turbulence and chemistry on soot evolution with respect to different soot aerosol models and to develop accurate models for simulating soot evolution in aircraft combustors. To investigate the effect of small scale turbulence time-scales on soot evolution, a partially-stirred reactor (PaSR) configuration is used and coupled with soot models from semi-empirical to detailed statistical models. Differences in soot property predictions including soot particle diameter and number density among the soot models are highlighted. The soot models will then be used to simulate the turbulent sooting flame in an aircraft swirl combustor to determine the large scale soot-turbulence-chemistry interactions. Highlights of this study include the differences in location of bulk soot mass production in the combustor using different soot models. A realistic aircraft combustor operating condition is simulated using a state-of-the-art minimally dissipative turbulent combustion solver and soot method of moments to investigate pressure scaling and soot evolution in different operating conditions. A separate hydrodynamic scaling is introduced to the pressure scaling, in addition to thermochemical scaling from previous studies. Finally, a Fourier analysis of soot evolution in the combustor will be discussed. A lower sooting frequency mode is found in the combustor, separate from the dominant fluid flow frequency mode that could affect statistical data collection for soot properties in turbulent sooting flame simulations.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147513/1/stchong_1.pd

Tuning the Growth and Mechanical Properties of Calcite Using Impurities: Insight from Molecular Simulation

Over many millions of years, evolution has provided living organisms with the tools to control the growth and properties of materials from the molecular scale upward. One of the many ways this is achieved is through the introduction of impurities into the solution in which these materials grow. A long-term goal of materials scientists is to harness nature's control mechanisms and apply them in the world of engineering. However, these mechanisms of growth control are highly complex, and understanding them requires insight into physical processes at the molecular scale. While experiments are so-far unable to offer such a high resolution, computer simulations can be used to directly model these physical process with no limit on the resolution. Throughout this thesis, an array of computational methodologies is applied to calcite in an attempt to understand how impurities are able to drive the growth process, and ultimately alter the mechanical properties of the crystal. A series of metadynamics simulations are applied to calcite kink sites, revealing a more complex growth mechanism in which kink-terminating ions do not initially occupy their crystal lattice sites, and only do so upon the adsorption of an additional solute. A combination of metadynamics and Kinetic Monte Carlo simulations are used to examine the adsorption free energies and growth inhibiting properties of amino acids and polyamines, the results of which are compared directly to experiment. This offers a robust insight into the molecular mechanisms that underpin how organic molecules are able to tune the growth of calcite. Simulations are also applied to two case studies of impure calcite. By examining lattice spacings, determining stress distributions and simulating a series of crack propagation events, insight into mechanisms through which biogenic crystals exhibit superior mechanical properties is found. Finally, the nature of non-Markovianity when using reaction coordinates -such as those used in rare event methodologies applied throughout this thesis- are investigated. By introducing non-Markovianity into the system, barrier crossing rates in a coarse-grained system more closely resemble those in the original two-dimensional system. Furthermore, we study the breakdown in rare-events sampling when a poor reaction coordinate is used, and identify which rare-events sampling techniques are more appropriate for detecting poor reaction coordinate choices

Advanced Electrodes And Electrolytes For Long-Lived And High-Performance Lithium-Sulfur Batteries

ABSTRACT ADVANCED ELECTRODES AND ELECTROLYTES FOR LONG-LIVED AND HIGH-PERFORMANCE LITHIUM-SULFUR BATTERIES by DEEPESH GOPALAKRISHNAN August 2020 Advisor: Dr. Leela Mohana Reddy Arava Major: Mechanical Engineering Degree: Doctor of Philosophy Lithium – Sulfur (Li-S) batteries have received much attention and considered as a promising candidate for next generation energy storage devices because of their high theoretical energy density (≈2600 Wh kg‒1) and environmental friendliness. However, the uncontrollable growth of lithium dendrites in the lithium metal anode and the fatal effect of polysulfide shuttle hinder their practical applications. The formation of dendrites during repeated Li plating/stripping processes results in: reduced Li availability for the electrochemical reactions, disruption in Li transport through the interface causing rapid capacity decay and increased safety concerns due to short circuiting. Polysulfide shuttle is a common phenomenon in Li-S batteries where the soluble intermediate polysulfide species (Li2Sx, 4 ≤ × ≤ 8) are inevitably produced and shuttled between cathode and anode, and react with the Li-metal to form insoluble Li2S and Li2S2 on the surface of anode, resulting in surface passivation of Li metal anode, fast self-discharge and rapid capacity fading in Li–S batteries. Thus, the major problems from both anode and cathode side are needed to be addressed, preferably by employing effective strategies. These issues can be addressed only when we have a better mechanistic understanding about chemical and electrochemical processes occurring in the Li-S battery. In the past decade, several strategies have been developed around the world and recently, our group demonstrated utilization of electrocatalyst to improve the polysulfides reaction and trap them inside the cathode of Li−S battery[28, 29, 77]. The electrocatalyst reduces the energy barrier of electrochemical reaction and also act as an anchor for polysulfides and confide them to the cathode reducing their shuttle effect. Herein, we carried out fundamental electrochemical studies on the sulfur -electrocatalyst interface to develop a suitable catalytic cathode. The potentiodynamic and potentiostatic methodologies are used to infer diffusional, adsorption and the kinetics behavior of polysulfides with respect to catalytic and non-catalytic interfaces. In this context, we evaluated the kinetics of sulfur redox chemistry on different electrocatalytic surfaces such as Pt, WS2 and NbS2 and their influences on reaction kinetics at different stages. Also, we have demonstrated the influence of catalyst on solid-to-liquid & liquid-to-solid polysulfides reaction kinetics and their effect on Li2S nucleation ending up in gaining of high capacity during the discharge process. In addition, we have explained in detail the impact of catalytic interface on cathode surfaces as well as on the reversibility of sulfur redox chemistry. We studied the synergistic effect of electrocatalyst NbS2 and conductive carbon substrate in Li-S battery performance. The other issue that we address in the thesis is lithium dendrite formation in Li-S batteries. Though the dendrite formation is one of the oldest issues, fundamental understanding about how the interfacial chemistry and Li deposition is correlated, how anode overpotential affect the cell characteristics etc. are still have no answers which are essential to address the dendrite formation. Here, we demonstrate a novel strategy using a special class of ionic liquids (ILs) with liquid crystal properties called Ionic Liquid Crystals (ILCs) as electrolyte cum pseudo-separator to detain the dendrite growth with their anisotropic nature controlling the Li-ion mass transport. The thermotropic ILC with two-dimensional Li-ion conducting pathways have been synthesized and well characterized. Detailed microscopic and spectroscopic analysis elucidates that the ILC is formed with Smectic A phase and can be utilized for wide temperature window operation. The electrochemical results corroborate the efficacy of ILC electrolytes in mitigating dendrites formation even after 800 hrs. and further substantiated by numerical simulation and deduced the mechanism involved in dendritic suppression. Thus, the research combines experimental development, characterization of the Ionic liquid crystals (ILCs) and analysis of their potential as electrolyte for improving Li battery performance supported by the numerical models

Doctor of Philosophy

dissertationEnergy generation through combustion of hydrocarbons continues to dominate as the most common method for energy generation. In the U.S., nearly 84% of the energy consumption comes from the combustion of fossil fuels. Because of this demand, there is a continued need for improvement, enhancement, and understanding of the combustion process. As computational power increases, and our methods for modelling these complex combustion systems improve, combustion modelling has become an important tool in gaining deeper insight and understanding of these complex systems. The constant state of change in computational ability leads to a continual need for new combustion models that can take full advantage of the latest computational resources. To this end, the research presented here encompasses the development of new models which can be tailored to the available resources, allowing one to increase or decrease the amount of modelling error based on the available computational resources and desired accuracy. Principal component analysis (PCA) is used to identify the low-dimensional manifolds which exist in turbulent combustion systems. These manifolds are unique in there ability to represent a larger dimensional space with fewer components, resulting in a minimal addition of error. PCA is well-suited for the problem at hand because of its ability to allow the user to define the amount of error in approximation, depending on the resources at hand. The research presented here looks into various methods which exploit the benefits of PCA in modelling combustion systems, demonstrating several models, and providing new and interesting perspectives for the PCA-based approaches to modelling turbulent combustion

Experimental and Computational Approaches to Optimizing Bovine Gamete Cryopreservation

Cryopreservation uses freezing to suspend the metabolic activity of biological specimens for increased longevity of biotic materials like gametes. Cryopreservation has been known to affect both the functionality (performance of activities) and the viability (survival) of biological specimens during the freezing and thawing process due to four types of damage: (1) thermal, (2) ice, (3) osmotic stress, and (4) cytotoxic. Cryoprotective agents (CPAs) are known to reduce thermal and ice damage but cause osmotic stress and cytotoxic damage. Osmotic stress occurs when the addition of CPAs causes a rapid expulsion of water from the cell as the extracellular environment has become hypertonic. Cytotoxic damage occurs when a cell is exposed for too long to CPAs that may be damaging to the cell at high temperatures, but aid in preservation at low temperatures. The purpose of my project is to minimize osmotic stress in bovine embryos and cytotoxic damage in bovine sperm caused by CPAs using novel algorithmically guided techniques. To minimize osmotic stress in bovine embryos, I aim to facilitate the equilibration of embryos with cryoprotective agents isochorically (constant volume). Isochoric cryoprotectant equilibration, requires a feedback control system that in our case will use real-time image analysis developed in this thesis to estimate current embryo volume and then adjusts the concentration of CPAs being administered to the system. I implemented a colour-based image analysis software that was able to process images of bovine embryos as they were exposed to CPAs at a sub-second rate. The sub-second processing rates include cell volume estimates that are comparable to manual cell volume estimates. To minimize cytotoxic damage in bovine sperm, I optimized cryopreservation media (CPM) composition to maximize post-thaw motility. The composition of CPM can contain many ingredients that have the potential to interact and are infeasible to test only empirically. Here, I combined empirical experiments, data-driven optimization algorithms, and machine learning to optimize the composition of CPM. I used differential evolution and Gaussian process regression to optimize CPM composition that are on par with commercial media after 9 iterations. During the optimization process I determined that Gaussian process regression model was superior to artificial neural networks when predicting post-thaw motility for a given CPM composition. By optimizing these cryopreservation processes, cellular damage can be reduced, improving functionality and viability of gametes used in assisted reproductive technology that can be applied across animal husbandry and biomedical fields