Doctor of Philosophy with a Major in Mechanical Engineering

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Doctor of Philosophy with a Major in Mechanical Engineering

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https://hdl.handle.net/1853/73284

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Degree Series

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Organizational Unit

George W. Woodruff School of Mechanical Engineering

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Now showing 1 - 10 of 478

An Analytical Model for Oscillating Heat Pipe Performance and Experimental Testing of a Novel Helix-Shaped Design

(Georgia Institute of Technology, 2024-07-27) Pawlick, Maxwell

The research presented focuses on the development and assessment of a novel mechanistic model for oscillating heat pipes (OHPs), also known as pulsating heat pipes (PHPs) and the development of a novel helix-shaped OHP design inspired by insights gained from the model developed. OHPs are passive heat transfer devices with potential applications in fields such as electronics cooling, heat recovery systems, and hypersonic vehicles. Despite their potential, their adoption in industry has been slow due to the lack of reliable design tools. The complex physics governing OHP performance and the need for accurate modeling techniques have hindered the development of such tools. An OHP consists of a sealed capillary channel filled with alternating liquid slugs and vapor bubbles. When a temperature difference is present, evaporation and condensation cause fluid motion, leading to passive convective heat transfer. Traditional OHP modeling approaches, ranging from experimental correlations to complex 3D computational models, have had limited success in providing rapid and reliable performance predictions without experimental data. This research aims to develop a mechanistic model capable of predicting the performance of a basic closed-loop OHP design without experimental input. The model is intended to predict temperature profiles and performance trends, allowing designers to narrow down potential OHP designs for further analysis. Insights gained from the model were used to design a novel helix-shaped OHP, which was designed to leverage buoyancy-driven circulation flow for improved performance. The research establishes that analytical modeling methods can significantly enhance the understanding and prediction of OHP performance. The contributions of this study include a comprehensive evaluation of OHP literature, identifying various operating modes that influence performance, developing an analytical framework for understanding some of these modes, and using this framework to develope and test a novel OHP design. This operating mode framework classifies OHP operation based on liquid distribution, fluid motion type, and flow regime, providing a basis for comparing different OHP designs. The analytical model successfully predicted the temperature drop across multiple OHP datasets, although it has limitations in certain operating modes and complex geometries. To address these limitations, the research suggests augmenting the model with machine learning techniques, particularly for phenomena that are difficult to model analytically, such as oscillation amplitude in designs with flooded condensers. Experimental validation of the novel helix-shaped OHP demonstrated that the design generally improved the effective thermal conductivity and maximum heat transport capacity relative to a control design. Further studies on helix-shaped OHPs with different sizes and working fluids are recommended to extend the advantages of this design. Additionally, the insights gained from the model provide further opportunity for other novel designs that improve performance. This work represents significant progress in understanding and modeling OHP operation. The analytical model developed and the insights into OHP mechanisms provide a foundation for designing and optimizing OHPs for various applications. Further research that utilizes machine learning techniques to predict the most complex mechanisms in OHP operation is encouraged to increase the reliability and accuracy of the model. This work paves the way for broader and more effective use of OHPs in fields such as energy recovery and thermal management, which will contribute to a shift in how OHP technology is viewed and utilized in both academia and industry.
The interplay of length and force feedback in regulating joint and limb impedance and inter-joint coordination

(Georgia Institute of Technology, 2024-07-27) Govindaraj, Thendral

Neural feedback pathways arise from a variety of sensory receptors. The firing of muscle spindles is related to length and velocity, while Golgi tendon organs measure active contractile force. Understanding the functions of these pathways during voluntary movement is important because they become disrupted in Spinal Cord Injury (SCI) and stroke. Most spindle pathways are relatively localized, but some are inter-joint. These inter-joint pathways may play a role in regulating whole limb properties. Experiments have shown that force-dependent feedback can be widely distributed and asymmetric between a given muscle pair. Additionally, force feedback is modulated according to the task and condition, such as slope walking and SCI. Although the muscle-level distributions of force feedback in the feline hindlimb have been measured under different conditions, it is not known how these distributions regulate limb mechanics (impedance and inter-joint coordination). To investigate how inter-joint spinal reflex feedback influences joint and limb impedance and inter-joint coordination under locomotion-like conditions, we developed a novel computational modeling and analysis framework. Our hypothesis was that length and force feedback modulate joint and limb impedance in a task-dependent manner while maintaining inter-joint coordination. To address this hypothesis, we developed a set of novel computational models and an analysis framework. Our first model includes an infinitely thin rod with viscoelastic properties (intrinsic + reflex) incorporated into a single joint, and the analysis framework evaluates the impedance when a sinusoidal torque is applied to the joint. Using this model and analysis framework, the goal of aim 1 was to investigate the influence of muscle spindle and Golgi tendon organ feedback on the impedance regulation of a single joint. We found that different combinations of spindle and tendon organ gains can achieve the same impedance, even with 20% lower intrinsic impedance. In support of the stiffness regulation hypothesis, impedance and internal regulation can be controlled separately because changing the ratio of length to force feedback can modify the impedance without altering the compensation for fatigue. To test an extension of the stiffness regulation hypothesis to multi-joint systems, we developed a computational model with two infinitely thin rods, intrinsic viscoelastic properties incorporated into two joints, and reflexes represented at the joint level. The analysis framework evaluates the whole limb and joint apparent impedances and inter-joint coordination when a sinusoidal endpoint force is applied to the end of the distal segment. We varied the direction of the endpoint force to simulate different locomotion tasks and combinations of joints. The goal of aims 2 and 3 is to evaluate the influence of inter-joint length and force feedback on the regulation of whole limb impedance and on inter-joint coordination. As hypothesized, inter-joint length and force feedback modulate limb impedance in a task-dependent manner over part of a functionally relevant range of endpoint force directions, which has implications for rehabilitation after incomplete SCI. The two joint model and analysis framework can provide a template for the control of multi-joint exoskeletons. The results in this dissertation give insight into how reflex gains are modulated to achieve the impedance required for certain tasks and conditions in all animals with multi segmented limbs.
Digital Twin Design and Autonomous Control of Bioreactor Systems for Human Immune Cell Expansion

(Georgia Institute of Technology, 2024-07-27) Kanwar, Bharat

Immune cell therapy is a rapidly growing field with immense clinical potential for several indications, including regenerating tissue, immunomodulation, and engineered cells for disease removal. As a nascent industry, biomanufacturing of these cell therapies involves lengthy manual protocols which leads to increased risk of failed or inconsistent cell product. This work proposes a framework for designing digital-twin models for bioreactor platforms that are inherently designed to integrate novel sensors, imaging, process controls, and perfusion. This framework consists of a modular digital twin that can model the relevant fluid dynamics of convection, diffusion and osmosis and cell fluxes of the bioreactor platform. Given the sterility requirements for living cell expansion, measurement of important parameters during the process is often untenable. This work proposes methods to compute unmeasured states and parameters from measured ones with an Extended Kalman Filter and predictive models to explore the domain of critical process parameters to control and measure. This framework then proposes an optimal-cost Linear Quadratic Regulator control architecture to regulate nutrients and cell output of the bioreactor process and demonstrates bioreactor process control with improved hMSC expansion in a hollow fiber bioreactor and improved T cell expansion in a vertical wheel bioreactor.
Numerical Analysis of Fiber Orientation Kinetics

(Georgia Institute of Technology, 2024-07-26) Karahan, Dogukan Tugberk

Transport of fibrous matter is encountered in modern industrial applications such as papermaking, concrete reinforcement, and injection molding. The end-product quality in these applications is strongly dependent on flow properties and fiber orientation. The bulk deformation of the suspensions is generally modeled by non-Newtonian constitutive relations, and fiber orientation modeling is based on the Fokker-Planck equation. Using these ideas, this work presents a numerical analysis of fiber orientation kinetics for suspensions up to the semiconcentrated regime, where the effects of the flow and suspension on fiber orientation are represented on by a rotational diffusion coefficient. To this end, probabilistic measures for the fiber orientation, namely the fiber orientation probability density function (FOPD) and orientation tensors, are employed. The rheology of the suspension is modeled as a shear-thinning Herschel-Bulkley (HB) fluid. Flows of HB fluids are studied for laminar and turbulent flows in canonical geometries. An extensive statistical analysis with new data is presented to demonstrate the effects of yield stress and shear thinning on the flow characteristics and fiber orientation. Fiber orientation is obtained at a single point for simple flows and in contracting channels. For the former, a new solver is developed to obtain the FOPD. The results show significant improvements over existing results, and new ideas for the rotational diffusion coefficient for semiconcentrated suspensions are developed. For contracting channels, the governing equations for second-order orientation tensor are solved in order to obtain fiber orientation in practically relevant applications. A systematic analysis is presented to show the effect of the rheological properties and the rotational diffusion coefficient in these applications. It is demonstrated that the fiber orientation changes non-linearly in response to changes in the rheology and rotational diffusion coefficient.
Developing Blue and Green Ammonia Infrastructure: Insights into Operations, Economics, and Distribution

(Georgia Institute of Technology, 2024-07-19) Fernandez Otero, Carlos Arturo

Developing blue and green ammonia infrastructure is essential to meet the rising demand for fertilizers while minimizing the environmental impacts of fertilizer production. Traditional ammonia production methods, such as the Haber-Bosch process, are energy-intensive and heavily reliant on fossil fuels, leading to significant carbon emissions. Alternatives, such as blue ammonia, which involves carbon capture and storage, and green ammonia, which utilizes electrification with renewable energy, offer more sustainable options. This dissertation investigates the operations, economics, and distribution of blue and green ammonia infrastructure, integrating thermodynamic, economic, environmental, and social metrics to evaluate low-carbon ammonia production technologies. The aim is to identify optimal deployment strategies for these technologies, considering future energy markets and geographic resource availability. Chapter 1 discusses the significance of ammonia in synthetic fertilizers and global food production, and the future of ammonia as an energy vector. The current Haber-Bosch process, which is centralized and fossil fuel-dependent, results in high CO2 emissions. Blue and green ammonia provide viable pathways to decarbonize ammonia production. Chapter 2 is a literature review that covers various ammonia production technologies, including gray, blue, and green Haber-Bosch processes, and emerging electrochemical methods. It also discusses the techno-economics, renewable integration, and ammonia storage and distribution methods. Chapter 3 estimates projections for future ammonia and nitric acid markets, emphasizing electrochemical nitrogen and nitrate reduction technologies. By predicting the market size and value for ammonia and nitric acid by 2050, Chapter 3 highlights the need for green alternatives to reduce carbon emissions. Chapter 4 expands the analysis by including energy consumption, costs, and emissions associated with green and blue ammonia production under different future energy market scenarios. Chapter 4 underscores the importance of renewable energy sources, carbon capture, and policy in reducing the carbon footprint of ammonia production. Chapter 5 presents an analysis of decentralized ammonia production using renewable energy sources. Chapter 5 presents an optimization model of production and distribution networks using techno-economic models and multi-objective optimization. The findings suggest that integrating renewable energy with ammonia production can significantly lower emissions and costs, especially when production is decentralized.
Development and Deployment of Orbital Angular Momentum Generating Acoustic Arrays

(Georgia Institute of Technology, 2024-05-02) Kelly, Mark Edward

The use of vortex waves in multiple environments is of increasing interest for numerous applications, including underwater acoustic communications and power ultrasonics. The underwater acoustic communication environment is both complex and challenging. Acoustic orbital angular momentum (OAM) has been demonstrated as an additional degree of freedom that may be used to alleviate communications bottlenecks, though the development of arrays capable of implementing OAM-based communications systems in underwater acoustic communications channels is largely unexplored. Ray tracing tools are employed to simulate the performance of communications arrays in various complex environments over multiple ranges and explore the array processing implications of these systems. Additionally, acoustic OAM has been shown to impact cavitation-based phenomena. Medical applications have found success in the treatment of tumors and blood clots; however, industrial cleaning applications have not been explored. This study addresses the challenges of developing these OAM-based systems. Prototype systems are developed and demonstrated.
Thermophysical and Molten Salt Corrosion Behavior of Structural Materials for Next-Generation Clean Energy Systems

(Georgia Institute of Technology, 2024-05-01) Brankovic, Sonja

As next-generation clean energy technologies like concentrated solar power (CSP) and molten salt reactor (MSR) systems operate at ever higher temperatures to increase efficiency and thermal energy storage capabilities, using molten chloride salt as a heat transfer and energy storage fluid can provide many benefits, including high-temperature operation, a low operating pressure, extended storage time, and increased safety. In this extreme environment, it is essential to understand the temperature-dependent thermophysical properties and molten salt corrosion behavior of candidate structural alloys and aluminosilicate refractories for salt storage tanks, piping, and heat exchangers. Though these types of materials have been used in established applications (for example, aerospace and gas turbine engine components in the case of Ni-alloys, hot-face furnace liners for aluminosilicate refractories), corrosion studies of these types of alloys are not easily comparable in the literature; for several alloys and many of the refractories studied in this project, published corrosion data does not exist. High-temperature thermophysical data of the candidate alloys and refractories are more widely available, though not consistently in the temperature range of interest (600–800°C). The purpose of this thesis project is first to characterize the high-temperature thermophysical properties (thermal diffusivity, specific heat, and thermal conductivity) of the candidate materials. This data, combined with published results from the literature, is then used to down-select materials for temperature-dependent immersion corrosion testing. Twelve Ni- and Fe-based alloys and seven aluminosilicate refractories were initially selected for experimental testing and sourced from commercial manufacturers. The temperature-dependent thermal diffusivity and specific heat of these candidate alloys and refractories were determined via light flash analysis (LFA) and differential scanning calorimetry (DSC), respectively. These experimental results were compared with available manufacturer data of the materials’ high-temperature thermophysical properties. A subset of high-performing and commercially viable alloys and refractories, with the addition of two alumina-forming alloys, were selected for molten chloride salt corrosion testing. Samples were immersed in purified 45.98 MgCl2–38.91 KCl–15.11 NaCl wt% salt for 100 hours at 650°, 725°, and 800°C. Corrosion rates were calculated based on nominal sample densities and measured weight changes after the immersion test; comparisons of pre- and post-test surface elemental and phase compositions were performed using X-ray fluorescence (XRF) and X-ray diffraction (XRD), respectively. A more detailed cross-section analysis was performed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). For the Ni-based alloys, the measured specific heat and thermal diffusivity were approximately linear as a function of temperature (which is commonly seen in manufacturer data sheets) but did see some evidence of second-order phase changes in the DSC data. In situ HT-XRD testing of several down-selected alloys showed that the alloys’ crystal structure was expanding as a function of temperature in a roughly linear manner, though there was no clear appearance of new phases or decrease in material stability. The aluminosilicate refractories exhibited no obvious phase changes in the DSC or LFA runs; this was confirmed by the in situ XRD tests. After the 100-hour immersion testing, uniform corrosion was visible on many of the samples’ surfaces and increased as a function of temperature, based on the measured mass loss of each sample. The temperature-dependent increase was most apparent in the alloys with a significant base iron content. This trend was confirmed by SEM imaging and EDS linescans of the sample cross-sections. XRF testing of the corroded alloys’ surfaces showed several compositional changes that are commonly seen in molten halide corrosion, including depletion of active metals like iron and chromium, and a corresponding enrichment in more noble elements like nickel and molybdenum. For several of the alloys, XRD testing of the corroded surfaces showed some evidence of oxide contamination. For the pre-oxidized alloys, no significant difference in performance was observed compared to the bare alloys; the developed oxide layer provided no measurable corrosion protection after 200 hours of chloride salt corrosion testing. Corrosion testing of the aluminosilicate refractories revealed no consistent, temperature-dependent trend in mass gain after 200 hours of chloride salt immersion at three temperature points. However, at higher test temperatures (725° and 800°C), vaporized chloride salt penetrated the refractories’ surface above the immersion line. A “transition line” was also observed, marking the highest level of the molten salt; this line was darker than the vapor and immersed regions of the refractories, indicating that any residue or contaminants floated on the surface of the molten salt. This thesis work is significant because it provides a broad, high-temperature thermophysical characterization of candidate alloys and aluminosilicate refractories for the next-generation solar and nuclear industries. Compared to the provided manufacturer data, the temperature-dependent runs from this thesis work provides a much finer dataset and elaborates on trends that are more subtle in the published archive. The results from the immersion molten chloride salt testing of down-selected alloys and refractories contribute important data for these same industries where understanding material corrosion resistance is critical for safe and economic performance.
Rayleigh-Bénard Convection at High Ra – Facility Design for the Exploration of the Existence of an Ultimate Regime

(Georgia Institute of Technology, 2024-04-29) Johnston, Stephen Ross

Turbulent thermal convection is a crucial part of heat transport in several important natural and engineering flows. Large scale natural systems such as the Earth's atmosphere---its oceans as well as the interior---and the interior of stars such as the Sun, are all affected to various degrees by thermal convection. The simplified physical model used to understand this ubiquitous heat transport mechanism is the Rayleigh-Bénard convection (RBC), which is a fluid flow driven by a temperature difference between the top and bottom plates of an experimental cell with adiabatic sidewalls. Despite the long history of the subject and the recent progress in theoretical, numerical and experimental domains, many questions remain unresolved. A fundamental question concerns the heat transfer scaling in highly turbulent convective flows, and particularly the existence of a possible transition to an asymptotic regime of enhanced heat transfer. Multiple theories predict a variety of possible asymptotic regimes, some of which literature disagreeingly claims to observe. The current study explores RBC at extreme parameters to help elucidate the true nature of convection in this regime. A modular facility was developed for conducting natural convection experiments at Rayleigh numbers exceeding 10^17. The study employs cryogenic nitrogen as the working fluid, taking advantage of its thermophysical property variations following the saturation curve from atmospheric pressure up to its critical point. This inaugural experimental campaign reveals a consistent heat transfer scaling relationship of Nu∝Ra^0.306 for the aspect ratio 1 test cell configuration over the entire parameter range (10^9
Atomistic Modeling and Machine Learning for the Rational Design of Organic Energy Storage Materials

(Georgia Institute of Technology, 2024-04-27) Allam, Omar Adel Youssef

The development of environmentally sustainable and cheaper alternatives to conventional inorganic materials used in ion batteries is critical for addressing both resource limitations and the environmental impact of battery production. This dissertation investigates the development of electrochemically active organic materials, which offer the potential for improved sustainability, cost-effectiveness, and fine-tuned performance. A framework integrating quantum mechanical calculations and machine learning is developed to facilitate the large-scale identification of organic materials with tailored electrochemical properties. Insights gained from this framework guide the design of carbon quantum dots exhibiting enhanced alkali-ion storage capabilities through the modulation of their functional group composition. The dissertation also investigates temperature-dependent reaction mechanisms in glyme electrolytes in CO2-containing Li-O2 batteries, offering insights into how temperature affects their stability and reactivity. Further, this work investigates strategies for prolonging the cycling stability of organic cathode materials. Specifically, to inhibit the dissolution of organic cathodes in the electrolyte, the development of novel solid polymer electrolytes is investigated. Employing molecular dynamics simulations with high-accuracy forcefields derived from quantum mechanical calculations, the effect of molecular design on nanophase morphology and ion transport are studied. This aims to establish design principles for optimizing polymer electrolytes for enhanced performance and battery stability.
Investigation of a helium-cooled modular divertor with multiple jets using a reversed heat flux approach

(Georgia Institute of Technology, 2024-04-27) Musa, Shekaib Ahmad

The divertor, an important plasma-facing component in future long-pulse magnetic fusion energy (MFE) reactors, is vital in sustaining fusion reactions by removing fusion products, impurities, and debris from the core plasma. This thesis focuses on the helium-cooled modular divertor with multiple jets (HEMJ) design. An HEMJ module employs an array of impinging jets to cool the inner surface of an endcap brazed to the plasma-facing surface, a tungsten tile. Originally proposed for the European demonstration DEMO fusion reactor, this concept has been experimentally shown to remove steady-state incident heat fluxes of at least 10 MW/m^2. Individual HEMJ “fingers” are initially assembled into bundles, or units, of nine fingers with a common inlet and outlet to form the divertor with a plasma-facing area of O(100 m^2). To date, most of the studies and models of the HEMJ are based on a single finger, and there are few studies of even a single HEMJ unit. The main objective of this thesis was therefore to evaluate the thermofluids characteristics of a representative HEMJ bundle to verify the results obtained from a single HEMJ finger can be used for a multi-finger bundle. Unlike a single finger outer shell, the outer shell of the HEMJ unit in the center of the bundle doesn’t have sidewalls and therefore lacks conduction paths to the manifold. The experimental studies presented here use a reversed heat flux (RHF) approach, whereby heat is removed (rather than added) at the plasma-facing surface, thereby reducing the operating temperatures for the test section. This approach was first successfully validated in experimental studies of a single HEMJ finger by comparing RHF and normal heat flux tests for dimensionless heat transfer coefficient, or Nusselt number Nu, and dimensionless pressure drop, or pressure loss coefficient K_L. Experimental studies were then conducted on a seven-finger HEMJ bundle where a central finger was surrounded by six outer fingers using the RHF method at the prototypical He pressure of 10 MPa, He temperatures as great as 300 °C and incident heat flux magnitudes as great as 5.9 MW/m^2. For this purpose, a larger helium loop with a mass flow rate as great as 100 g/s was designed and built. The results indicate that the Nu correlation developed from single-finger HEMJ studies is applicable to the seven-finger HEMJ unit at the prototypical mass flow rate of 6.8 g/s. Baring the anomalous pressure drops in the latter half of the experiments, the K_L results are higher than the single-finger HEMJ due to the inclusion of the inlet and outlet chambers in the manifold. Computational fluid dynamics (CFD) studies of a seven-finger HEMJ unit were carried out with a commercial software package ANSYS and used to design the seven-finger test section and clarify some of the experimental results, including the impact of partial blockage of the jet exit holes on Nu and K_L.