<div>Thermal management in next generation power electronic devices, radar applications and semiconductor packaging architectures is becoming increasingly challenging due to the need to reject localized high heat fluxes as well as large total powers. Air cooling has been considered as a simple and reliable method for thermal management compared to architectures that incorporate liquid cooling. However, air-cooled heat sinks typically require effective heat spreading to provide the requisite level of area enhancement to dissipate high heat fluxes. Compared to solid metallic heat spreaders, advanced heat sinks that incorporate two-phase heat transfer devices such as vapor chambers can significantly enhance the power dissipation capabilities in such configurations. Vapor chambers are devices that utilize evaporation/boiling processes within a sealed cavity to achieve efficient heat spreading. In high-heat-flux applications, boiling can occur within the internal wick structure of the vapor chamber at the location of the heat input (i.e., the evaporator). The maximum dryout heat flux and thermal resistance of the device is dictated by the resulting two-phase flow and heat transfer in the porous evaporator due to boiling. While various works in the literature have introduced new evaporator wick designs to improve the dryout heat flux during boiling, the enhancement is limited to small, millimeter scale hotspots or at a very high thermal resistance. In additixon, the effective design of such evaporator systems requires mechanistic models that can accurately predict the dryout limit and thermal performance. </div><div> This thesis first explores the usage of a novel ‘two-layer’ evaporator wick for passive high heat flux dissipation over large heater areas at a low thermal resistance. Moreover, a new mechanistic (first principles based) model framework is introduced for dryout limit and thermal performance prediction during boiling in capillary fed evaporators, by considering the resulting simultaneous flow of two phases (liquid and vapor) within the microscale porous media.</div><div> The novel two-layer wick concept uses a thick ‘cap’ layer of porous material to feed liquid to a thin ‘base’ layer through an array of vertical liquid-feeding ‘posts’. Vapor ‘vents’ in the cap layer allow for vapor formed during the boiling process (which is constrained to the base layer) to escape out of the wick. This two-layer structure decouples the functions of liquid resupply and capillary-fed boiling heat transfer, making the design realize high heat flux dissipation greater than 500 W/cm2 over large heat input areas of ~1 cm2. A reduced-order model is first developed to demonstrate the performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The fabrication of the proposed two-layer wick is then presented. The thermal performance of the fabricated wicks is characterized using a boiling test facility that utilizes high speed visualization to identify the characteristic regimes of boiling operation in the wicks. The performance is also benchmarked to conventional single-layer wicks. </div><div> It is observed that single-layer wicks exhibit an unfavorable boiling regime where the center of the heater area dries out locally, leading to a high value of thermal resistance. The two-layer wicks avoid local dryout due to the distributed feeding provided by the posts and enhance the dryout heat flux significantly compared to single-layer wicks. A two-layer design that consists of a 10 × 10 array of liquid feeding posts provided a 400% improvement in the dryout heat flux. Following a parametric analysis of the effect of particle size, two-layer wicks composed of 180 – 212 µm particles and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ~0.052 K/W. The effect of vapor venting and liquid-feeding areas is also experimentally studied. By understanding these effects, a parametrically optimized design is fabricated and shown to demonstrate an extremely high dryout limit of 512 W/cm2. We identify that the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than was previously possible in the literature.</div><div> The results from the characterization of two-layer wicks revealed that the overall performance of the design was limited by the boiling process in the thin base wick layer. A fundamental model-based understanding of the resulting two-phase flow and heat transfer process in such thin capillary-fed porous media was still lacking. This lack of a mechanistic model precluded the accurate prediction of dryout heat flux and thermal performance of the two-layer wick. Moreover, such an understanding is needed for the optimal design of advanced hybrid evaporator wicks that leverage capillary-fed boiling. Despite the existence of various experimental works, there are currently no mechanistic approaches that model this behavior. To fill this unmet need, this thesis presents a new semi-empirical model for prediction of dryout and thermal resistance of capillary-fed evaporator systems. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses. This physics–based modeling approach is then implemented into a vapor chamber model to predict the thermal performance limits of air-cooled heat sinks with embedded vapor chambers. The governing energy and momentum equations of a low-cost analytical vapor chamber modeling approach is coupled with the evaporator model to capture the effect of boiling in the evaporator wick. An example case study illustrating the usage of the model is demonstrated and compared to a purely evaporation-based modeling approach, for quantifying the differences in dryout limit prediction, signifying the need to account for boiling in the evaporator wick. </div><div> The understanding gained from this thesis can be utilized for the prediction of dryout and thermal performance during boiling in capillary limited evaporator systems. The work also suggests the usage of a universal relative permeability correlation for the two-phase flow configuration studied herein for capillary-fed boiling, based on a wide calibration to experimental data. The modeling framework can also be readily leveraged to find novel and unexplored designs of advanced evaporator wicks. From an application standpoint, the new vapor chamber model developed here can be used for the improved estimation of performance limits specifically when high heat fluxes are encountered by the device. This will enable better and informed design of air-cooled heat sink architectures with embedded vapor chambers for high performance applications. </div><div><br></div>

Despite the central importance of porous electrodes to any advanced electrochemical system, there is no clear answer to “<i>How to make the best electrode</i>?”. The source of ambiguity lies in the incomplete understanding of convoluted material interactions at smaller – difficult to observe length and timescales. Such mesoscopic interactions, however, abide by the fundamental physical principles such as mass conservation. The porous electrodes are investigated in such a physics-based setting to comprehend the interplay among structural arrangement and off-equilibrium processes. As a result, a synergistic approach exploiting the complementary characteristics of controlled experiments and theoretical analysis emerges to allow mechanistic insights into the associated mesoscopic phenomena. The potential of this philosophy is presented by investigating three distinct electrochemical systems with their unique peculiarities.

Understanding thermal transport across length scales lays the foundation to developing high-performance electronic devices. Although many experiments and models of the past few decades have explored the physics of heat transfer at nanoscale, there are still open questions regarding the impact of periodic nanostructuring and coherent phonon effects, as well as the interaction of strain and thermal transport. Thermomechanical effects, as well as strains applied in flexible electronic devices, impact the thermal transport. In the simplest kinetic theory models, thermal conductivity is proportional to the phonon group velocity, heat capacity, and scattering times. Periodic porous nanostructures impact the phonon dispersion relationship (group velocity) and the boundaries of the pores increase the scattering times. Strain, on the other hand, affects the crystal structure of the lattice and slightly increases the thermal conductivity of the material under compression. Intriguingly, applying strain combined with the periodic porous structures is expected to influence both the dispersion relation and scattering rates and yield the ability to tune thermal transport actively. But often these interrelated effects are simplified in models.<br><br>This work evaluates the combination of structure and strain on thermal conductivity by revisiting some of the essential methods used to predict thermal transport for a single layer of graphene with a periodic porous lattice structure with and without applied strain. First, we use the highest fidelity method of Non-Equilibrium Molecular Dynamics (NEMD) simulations to estimate the thermal conductivity which considers the impact of the lattice structure, strain state, and phononic band structure together. Next, the impact of the geometry of the slots within the lattice is interrogated with Boltzmann Transport Equation (BTE) models under a Relaxation Time Approximation. A Monte Carlo based Boltzmann Transport Equation (BTE) solver is also used to estimate the thermal conductivity of phononic crystals with varying pore geometry. Dispersion relations calculated from continuum mechanics are used as input here. This method which utilizes a simplified pore geometry only partially accounts for the effects of scattering on the pore boundaries. Finally, a continuum level model is also used to predict the thermal conductivity and its variations under applied strain. As acoustic phonon branches tend to carry the most heat within the lattice, these continuum models and other simple kinetic theories only consider their group velocities to estimate their impact on phonon thermal conductivity. As such, they do not take into account the details of phonon transport across all wavelengths.<br><br>By comparing the results from these different methods, each of which has different assumptions and simplifications, the current work aims to understand the effects of changes to the dispersion relationship based on strain and the periodic nanostructures on the thermal conductivity. We evaluate the accuracy of the kinetic theory, ray tracing, and BTE models in comparison to the MD results to offer a perspective of the reliability of each method of thermal conductivity estimation. In addition, the effect of strain on each phononic crystal with different pore geometry is also predicted in terms of change to their in-plane thermal anisotropy values. To summarize, this deeper understanding of the nanoscale thermal transport and the interrelated effects of geometry, strain, and phonon band structure on thermal conductivity can aid in developing lattices specifically designed to achieve the required dynamic thermal response for future nano-scale thermoelectric applications.

Heat transfer and solidification models were developed for use in a numerical model of a continuous caster to provide a means of predicting how the developing shell would react under variable operating conditions. Measurement data of the operating conditions leading up to a breakout occurrence were provided by an industrial collaborator and were used to define the model boundary conditions. Steady-state and transient simulations were conducted, using boundary conditions defined from time-averaged measurement data. The predicted shell profiles demonstrated good agreement with thickness measurements of a breakout shell segment – recovered from the quarter-width location. Further examination of the results with measurement data suggests pseudo-steady assumption may be inadequate for modeling shell and flow field transition period following sudden changes in casting speed. An adaptive mesh refinement procedure was established to increase refinement in areas of predicted shell growth and to remove excess refinement from regions containing only liquid. A control algorithm was developed and employed to automate the refinement procedure in a proof-of-concept simulation. The use of adaptive mesh refinement was found to decrease the total simulation time by approximately 11% from the control simulation – using a static mesh.

Boiling is used in a wide variety of industries, including electronics cooling, distillation, and power generation. Fundamental studies on the boiling process are needed for effective implementation. Key performance characteristics of boiling are the heat transfer coefficient, which determines the amount of heat flux that can be dissipated for a given superheat, and critical heat flux(CHF), the failure point that occurs when vapor blankets the surface. The wettability of a surface is one of the key parameters that affects boiling behavior. Wetting surfaces(e.g., hydrophilic surfaces), typically characterized by a static contact angle below 90°,have better critical heat flux due to effective rewetting, but compromised heat transfer coefficients due to increased waiting times between nucleation of each bubble. Meanwhile, nonwetting surfaces (e.g., hydrophobic surfaces), characterized by static contact angles greater than 90°, have better heat transfer coefficients due to improved nucleation characteristic, but reach critical heat flux early due to surface dry out. However, recent studies have shown that the static contact angle alone offers and incomplete, and sometimes inaccurate, description of this behavior, which is instead governed entirely by the dynamic wettability. Specifically, the receding contact angle impacts the size and contact area of bubbles forming on a surface during boiling, while the advancing contact angle determines how the bubble departs. With this more complete set of wettability descriptors, three characteristic wetting regimes define the boiling behavior: hygrophilic surfaces having advancing and receding contact angles both under 90°; hygrophobic surfaces having both these dynamic contact angles over 90°;and ambiphilic surfaces having a receding contact angle less than 90°, but an advancing contact angle greater than 90°.The goal of this thesis is to experimentally characterize and compare the behavior of boiling surfaces in each of these regimes, observe the contact line behavior, and explain the mechanisms for their differences in performance.

<div>A microheater and resistance temperature detector (RTD) are designed and fabricated for various applications. First, a hierarchical manifold microchannel heatsink with an integrated microheater and RTDs is demonstrated. Microfluidic cooling within the embedded heat sink improves heat dissipation, with two-phase operation offering the potential for dissipation of very high heat fluxes while maintaining moderate chip temperatures. To enable multi-chip stacking and other heterogeneous packaging approaches, it is important to densely integrate all fluid flow paths into the device. Therefore, the details of heatsink layouts and fabrication processes are introduced. Characterization of two-phase cooling as well as reliability of the microheater/RTDs are discussed. In addition, another application of microheater for mining particle detection using interdigitated capacitive sensor. While current personal monitoring devices are optimized for monitoring microscale particles, a higher resolution technique is required to detect sub-micron and nanoscale particulate matters (PM) due to smaller volume and mass of the particles. The detection capability of the capacitive sensor for sub-micron and nanoparticles are presented, and an incorporated microheater improved stable capacitive sensor reading under air flow and various humidity. </div><div>This paper also introduces the characterization of nanomaterials such as metallic nanowires (NWs) and single layer graphene. First, the copper nanowire (CuNW)/graphene hybrid networks for transparent conductors (TC) is investigated. Though indium tin oxide (ITO) has been widely used, demands for the next generation of TC is increasing due to a limited supply of indium. Thus, the optical and electrical properties of CuNW/graphene hybrid network are compared with other transparent conductive materials including ITO. Secondly, silver nanowire (AgNW) growth technique using electrodeposition is introduced. A vertically aligned branched AgNW arrays is made using a porous anodic alumina template and the optical properties of the structure are discussed.</div><div><br></div>

<p> With new technologies taking advantages of the rapid miniaturization of devices to microscale across emerging industries, there is an unprecedented increase in the heat fluxes generated. The relatively low phase-change thermal resistance associated with boiling is beneficial for dissipating high heat flux densities in compact spaces. However, for boiling heat transfer, a high degree of geometrical confinement significantly alters two-phase interface dynamics which affects the flow pattern, wetting dynamics, and moreover, the heat transfer rate of the boiling processes. Hence, it is crucial to have a deeper understanding of the mechanistic effects of confinement on two-phase heat dissipation and carefully examine the applicability of boiling correlations developed for unconfined pool boiling to predict and optimize design of extremely compact two-phase thermal management solutions. This dissertation develops and demonstrate a fundamental understanding of the impact of confinement on pool boiling. To elucidate the mechanisms that impact confined boiling, this study experimentally evaluates boiling characteristics through the quantification of boiling curves and high-speed visualization across a range of gap spacing smaller than the capillary length of the working fluid. </p> <p><br></p> <p> This work reveals the existence of two distinct boiling regime uniquely observed in boiling in confined configurations (namely, intermittent boiling and partial dryout). In contrast to pool boiling where the maximum heat transfer coefficient occurs below the critical heat flux limit, the intermittent boiling regime demonstrates the highest heat transfer coefficient in confined boiling. Then, this study provides a mechanistic explanation for the enhanced heat transfer rate due to geometrical confinement. Mainly, small residual pockets of vapor, termed ‘stem bubbles’ herein, remain on the boiling surface through a pinch-off process. These stems bubbles act as seeds for vapor growth in the next phase of the boiling process without the need for active nucleation sites. Furthermore, this dissertation develops a more accurate, mechanistic-based model for the phenomena that occur at CHF in confined configurations. The newly developed mechanistic understanding and model provides guidance on new directions for designing extremely compact two-phase thermal solutions.</p>

Waste heat recovery is a multi-billion-dollar industry with a compound annual growth rate of 8.8% assessed between 2016 to 2024 and low-grade waste heat (< 230^oC ± 20^oC) makes up 66% of this ubiquitous resource. Thermoelectric generators are preferred for the recovery process because they are cheap and are well suited for this temperature range. They generate power by converting thermal potential to electric potential, known as the Seebeck effect. Since they have no moving parts, they are inherently immune to mechanical failure or an intermittent need for maintenance. However, the challenge has been to effectively harvest waste heat with these modules to generate power, using passive processes. This work is focused on designing a device for optimized harvesting of waste energy from the ambient with a custom, evaporatively-cooled heat sink. This heat sink is designed to passively handle the cooling of the other side of the thermoelectric module so as to enable the attainment of a minimum of 5V, which is the minimum voltage required to power small mobile devices. The heat sink model is similar to a loop heat pipe but engineered for compactness. To ensure this level of efficacy is attained, several studies are made to optimize the wick. Non-metal wicks were considered as they do not contribute to an increase in temperature of the compensation chamber in loop heat pipes. A non-metal wick integrated with nanoparticles is tested and results show a clear thermal management enhancement over similar but virgin non-metal wicks, at over 16%. The heat source section of the device is optimized for energy-harvesting in low grade temperature regimes by incorporating a near-black body coating on the metal heat source section. Experimental results show that both the heat source and sink sections were able to induce sufficient thermal potential for the thermoelectric modules to passively generate up to 5V using eight 40mm by 40mm Bismuth Telluride modules in 3.5 minutes. The prototype is relatively cheap, inherently reliable and presents the possibility of passively harvesting low-grade waste heat for later use, including powering small electronic devices.

<div>A growing global population and improved standard of living in developing countries have resulted in an unprecedented increase in energy demand over the past several decades. While renewable energy sources are increasing, a huge portion of energy is still converted into useful work using heat engines. The combustion process in diesel and petrol engines releases carbon dioxide and other greenhouse gases as an unwanted side-effect of the energy conversion process. By improving the efficiency of internal combustion engines, more chemical energy stored in petroleum resources can be realized as useful work and, therefore, reduce global emissions of greenhouse gases. This research focused on improving the thermal efficiency of opposed-piston engines, which, unlike traditional reciprocating engines, do not use a cylinder head. The cylinder head is a major source of heat loss in reciprocating engines. Therefore, the opposed-piston engine has the potential to improve overall engine efficiency relative to inline or V-configuration engines.</div><div><br></div>The objective of this research project was to further improve the design of opposed-piston engines by using computational fluid dynamics (CFD) modeling to optimize the engine geometry. The CFD method investigated the effect of intake port geometry and exhaust piston lead angle on the scavenging process and in-cylinder turbulence. After the CFD data was analyzed, scavenging efficiency was found insensitive to transfer port geometry and exhaust piston lead angle with a maximum change of 0.61%. Trapping efficiency was altered exclusively by exhaust piston lead angle and changed from 18% to 26% as the lead angle was increased. The in-cylinder turbulence parameters of the engine (normalized swirl circulation, normalized tumble circulation, and normalized TKE) experienced more complex relationships. All turbulence parameters were sensitive to changing transfer port geometry and exhaust piston lead angle. Some examples of trends seen during the analysis include: an increase in normalized swirl circulation from 0.01 to 4.45 due to changes in swirl angle, a change in normalized tumble circulation from -28.52 to 21.11 as swirl angle increased, and an increase in normalized tumble circulation from 14.20 to 33.68 as exhaust piston lead angle was increased. Based on the present work, an optimum configuration was identified for a swirl angle of 15°, a tilt angle of 10°, and an exhaust piston lead angle of 20°. Future work includes expanding the numerical model’s domain to support a complete cylinder-port configuration, adding combustion products to the diffusivity equation in the UDF, and running additional test cases to describe the entire input space for the sensitivity analysis.<br>