<p>Heat dissipation requirements of next-generation power electronics in electric vehicles, high-performance computing, and radar systems will far exceed the capabilities of conventional heat sink technologies such as single-phase liquid cold plates and air-cooled heat sinks. The leading candidate technology that promises to meet these needs is microchannel flow boiling. Compared to conventional heat sink technologies, flow boiling provides some of the highest heat transfer coefficients available and can dissipate heat at a lower pumping power and with more uniform surface temperatures. However, there are unique challenges associated with flow boiling that currently prevent practical implementation of the technology, including limited modeling capabilities, inherent critical heat flux (CHF) limitations, and the presence of two-phase flow instabilities. This thesis is targeted primarily at addressing the impact of dynamic two-phase flow instabilities on heat transfer and CHF in microchannel heat sinks, in contrast with earlier literature that has focused on prediction and characterization of the flow dynamics.</p> <p><br></p> <p>Two dynamic instabilities of importance in microchannel heat sinks are pressure drop oscillations (PDO) and parallel channel instabilities, both resulting from an interaction between the inertia of a two-phase mixture within a heated channel and a source of compressibility outside of the channel. However, the individual impact of these instabilities on heat transfer performance has not been quantified. In this thesis, an experimental facility is developed to isolate the individual and combined impact of PDOs and parallel channel instabilities on surface temperature and CHF in single- and parallel-microchannels. This is achieved by introducing a measurable compressible volume directly upstream of the test section and isolating the test section from any unwanted compressibility within components throughout the rest of the system. Experiments are first performed targeting the investigation of PDOs in single channels and then targeting PDOs and parallel channel instabilities in multi-channel heat sinks. In the case of parallel channels, inlet restrictors are introduced to suppress channel-to-channel interactions and provide a baseline case of stable boiling. Throughout these experiments, only moderate increases in time-average surface temperature are observed (6 °C) and reduction of CHF is negligible, despite drastically different flow pattern observations when instabilities are present. These observations are in stark contrast with other cases in the literature, for which significant deterioration of surface temperatures and CHF have been attributed to the presence of PDOs. For example, significant temperature oscillations have been observed in the literature studying silicon-etched microchannel heat sinks experiencing PDOs. A predictive model is clearly required to understand and detect the conditions when dynamic instabilities should be considered in heat sink design.</p> <p><br></p> <p>To better understand the conditions when PDOs might have significant impact on heat transfer performance, an investigation of thermal capacitance is performed using a dynamic two-phase model and a targeted experimental approach in heat sinks having different thermal masses. The model reveals that, if thermal capacitance is low, PDOs become more severe, and the amplitude of temperature oscillations increase. These predictions are confirmed by experimental observations, and, in addition, premature CHF is observed in the heat sink with lower thermal mass. With sufficient thermal capacitance, the system recovers before triggering CHF, preventing deterioration of performance due to PDOs. Among the wide range of flow conditions considered in this thesis, the reduction of thermal mass resulted in the greatest impact on transient response of a heat sink during flow boiling instabilities. This reveals thermal capacitance as a critical parameter when determining if dynamic instabilities will deteriorate performance in a microchannel heat sink application. This allows engineers to make an informed judgement on whether adding features to suppress instabilities, at the cost of increased pumping power, is warranted. In order for the practical implementation of two-phase heat sinks to be realized, further development of dynamic modeling capabilities is required, and these models should be backed by systematic experimental investigations into conditions where instabilities should be considered.</p>

<p>Two-phase jet impingement combines the latent heat absorbed by boiling heat transfer with the strong forced convection of an impinging jet. It is a compact and highly effective heat transfer method that is capable of high heat transfer coefficients and high boiling critical heat flux limits. This makes it a suitable technology for electronics immersion cooling applications when configured as a submerged jet of a dielectric coolant. Previous studies have focused on the heat-flux-controlled nucleate boiling performance of an impingement jet up to the critical heat flux. Exploration of other boiling regimes that occur under temperature-controlled surfaces is of fundamental importance to fully understand the design space. It has been shown for free jets that a high and consistent heat flux can be dissipated over a wide range of surface superheats in the transition boiling regime when the surface is temperature-controlled. This effect is strongest in the stagnation zone, directly beneath the jet. Literature that studies this so-called “shoulder effect”, or heat flux shoulder, is scarce and almost completely focused on applications in metals processing using free jets of water as the coolant. It has been hypothesized previously that the impinging subcooled liquid delays and disrupts the start of film boiling, thereby dissipating heat flux levels comparable to that during nucleate boiling. To exploit operation in this unique transition boiling regime for potential applications in immersion cooling of electronics, the occurrence of this shoulder effect, as well as means for estimating the shoulder heat flux across different operating conditions, must be investigated for submerged jets and dielectric coolants. </p> <p>In this work, temperature-controlled submerged jet impingement is experimentally characterized using HFE-7100. A copper heater sized to be completely covered by the jet stagnation zone is increased in surface temperature throughout the transition boiling regime via a PID controller, which allows for steady-state temperature-controlled data to be acquired in this regime. The boiling curves, including critical heat flux and shoulder heat flux, are measured for jet velocities from 0.5-3 m/s and inlet subcooling from 5-30 K. The shoulder effect is shown to exist in these conditions. High-speed imaging is used to relate the flow behavior to the boiling thermal measurements and shows that the shoulder heat flux effect is an enhanced film heat transfer in the film-like mode of transition boiling. Trends and dependencies on inlet subcooling and jet velocity are measured and used to assess available predictive tools. It is observed that there is a proportionality between the critical heat flux and the shoulder heat flux. This implies a mechanistic similarity between the two effects. With further data to correlate, this similarity can potentially be used to predict the shoulder heat flux leveraging existing correlations for the critical heat flux, widening the design space of two-phase jet impingement systems. </p>

<p>  </p> <p>As the power density of computing devices increases, advanced liquid cooling thermal solutions offer an attractive thermal management approach. In particular, the low thermal resistance offered by microchannel heat sinks used in liquid cooling systems may enable increased total heat dissipation within fixed component temperature limits. There has been extensive work on the design of microchannel heat sinks, with many recent efforts to explore novel geometries and emerging manufacturing techniques. Of particular interest is additive manufacturing to allow for designs having complex, non-traditional internal geometries and package structures that cannot be made through conventional means. Despite the potential benefits for design and construction, additive manufacturing introduces new geometric uncertainties that could affect device performance. Direct metal laser sintering methods suitable for printing metal heat sinks typically produce a high internal roughness and other shape deviations in the flow paths of the final part. This extreme relative roughness and potential tortuosity in fluid flow through additively manufactured microchannels could lead to significant deviations in pressure drop and heat transfer predicted with traditional correlations and models. This work seeks to characterize the effects of high relative roughness on the friction factor and Nusselt number in additively manufactured microchannels having a rectangular cross section. Straight microchannel samples of 500 µm, 750 µm, and 1000 µm channel heights, and aspect ratios from 1 to 10 were manufactured to identify the design dimensions that resulted in visibly open channels, albeit with deviations in cross-sectional shape for submillimeter channel sizes and high internal roughness. Heat sink test samples were then printed with an array of these microchannels connected in parallel by inlet and outlet headers. Using water as the working fluid, the pressure drop and heat transfer performance of these sample heat sinks were characterized to explore how their behavior deviated from conventional predictions assuming smooth-walled channels. Flow through these additively manufactured microchannels displayed higher pressure drops than predicted, as well as a flow rate dependence of the hydrodynamic and thermal performance. These observed deviations are explored as effects of the physical conditions inside the channel as a result of additive manufacturing. Severe constriction of the channel would account for the difference in magnitude between the experimental and predicted results, while the introduction of flow redevelopment could lead to a flow rate dependence.  By further understanding the impact of these artifacts and deviations, these factors can be accounted for in the design and modelling of more complex additively manufactured heat sinks. </p>

With increased focus on miniaturization and high performance in electronics, thermal management is a very important area of research today. In multiple applications such as portable electronics, consumer electronics, military applications, automobile, power electronics, high performance computing, etc. innovative thermal management strategies are necessary. In this work, two novel approaches to dissipate redundant heat better- first by novel carbonaceous-nanoparticle additives to develop thermal interface materials with superior performance and the second by using advanced metal additive manufacturing techniques to design and analyze metal-lattice based heat sinks are presented. Thermal Interface Materials with multiple carbon-based nanoparticle fillers such as coal-derived Multi Layered Graphene (MLG), standard reduced Graphene Oxide (rGO), Multi-Walled Carbon Nano Tubes (MWCNTs), and Graphene Nano-Platelets (GNPs) in thermal paste were synthesized and seen to have superior heat dissipation properties. Also, graphene was synthesized from coal through an in-house, facile, scalable and cost-effective process. The enhancement in thermal conductance varies from ~70% in the coal-MLG to ~14% in MWCNTs-based TIMs. Noteworthy is ~3.5 times larger enhancement in thermal performance with the in-house coal-derived-MLG as compared to the commercially available g-MLG. At a 3% wt. fraction of coal-MLG, enhancement in thermal conductance was almost 120% higher compared to the base thermal grease. In the second part, metal lattice-based heat sinks are designed for additive manufacturing for use in passive cooling of high-flux thermal management. A parametric optimization based on the lattice geometry, thickness, and height subject to additive manufacturing constraints is conducted. Intricate metal lattices with low mass based on the Simple Cubic, Octet, and Voronoi structures were generated by implicit modelling in nTopology® and their thermal performance was analyzed through numerical analysis using commercial CFD packages. The Voronoi lattice performed best with a significant improvement in thermal performance (~18% reduction in junction temperature difference with respect to ambient) as compared to a standard baseline Longitudinal heat Sink (LHS), while reducing the mass of the heat sink by ~2.1 times. Such optimized metal lattice-based heat sinks can lead to significant downsizing, reduction in overall mass and cost in applications where thermal management is critical with a need for low mass. We believe that such novel scalable materials and processes suited for mass production could be critical in meeting the material, design and product development needs to tackle the thermal management challenges of the future. / Master of Science / With increase in demand of high power and performance in electronics, there is a concurrent increase in redundant heat that needs to be dissipated. With enhanced focus and push towards electric vehicles, defense, consumer electronics, datacenter and supercomputing applications, electronics cooling is a critical area of research today. There are two primary resistances to heat- as it is removed from electronics package to the surrounding atmosphere – due to the thin layer of a material called Thermal Interface Material (TIM) at the interface between the heat sink and the package, and the resistance offered by the heat sink itself. In this work, a two-pronged approach for better cooling in electronics is presented. Firstly, carbon-based nano-sized particles are used to synthesize novel TIMs that provide superior heat transport capabilities as compared to a standard baseline. In the second approach, complex metal-lattice based heat sinks are designed for manufacturing with advanced techniques such as metal 3D printing. Multiple carbon-based nano-particle additives such as Multi Layered Graphene synthesized from coal (MLG), standard commercially available reduced Graphene Oxide (rGO), Multi-Walled Carbon Nano Tubes (MWCNTs), and Graphene Nano-Platelets (GNPs) are dispersed in thermal paste and all of the resulting composites were found to remove heat better from electronics packages. The improvement in this ability varies from ~70% in the coal-MLG to ~14% in MWCNTs-based TIMs. Noteworthy is ~3.5 times larger enhancement in the heat transport ability with the use of in-house coal-derived-MLG as compared to the commercially available g-MLG. At an 3% wt. fraction of coal-MLG, there was a 1.2x increase in thermal performance as compared to the base thermal grease. Also, it is significant to mention that MLG was synthesized from coal through an in-house, facile scalable and cost-effective process. In the second part, metal lattice-based heat sinks designed for metal 3D printing for use in passive cooling of electronics was investigated. Multiple geometric parameters such as the lattice type, thickness, and height subject to additive manufacturing constraints were studied. Intricate metal lattices with low mass based on three structures- Simple Cubic, Octet, and Voronoi were generated by implicit modelling, and their thermal performance was predicted by computer based-simulations using commercial CFD packages. The Voronoi lattice performed best with a significant reduction (~18%) in junction temperature difference with the surrounding atmosphere- as compared to a standard baseline rectangular heat sink design, while simultaneously reducing the mass of the heat sink by ~2.1 times. Such optimized metal lattice-based heat sinks can lead to significant reduction in overall mass, size, and cost in weight sensitive applications. We believe that such novel scalable materials, designs, and processes suited for mass production could be critical in meeting the material, design and product development needs to tackle the thermal management challenges of the near future.

This study experimentally investigated the effect of hydrophobic and hydrophilic surfaces characteristics on nucleate boiling heat transfer performance for the application of direct immersion cooling of electronics. A dielectric liquid, HFE – 7100 was used as the working fluid in the saturated boiling tests. Twelve types of 1-cm2 copper heater samples, simulating high heat flux components, featured reference smooth copper surface, fully and patterned hydrophobic surface and fully and patterned hydrophilic surfaces. Hydrophobic samples were prepared by applying a thin Teflon coating following photolithography techniques, while the hydrophilic TiO2 thin films were made through a two step approach involving layer by layer self assembly and liquid phase deposition processes. Patterned surfaces had circular dots with sizes between 40 – 250 μm. Based on additional data, both hydrophobic and hydrophilic surfaces improved nucleate boiling performance that is evaluated in terms of boiling incipience, heat transfer coefficient and critical heat flux (CHF) level. The best results, considering the smooth copper surface as the reference, were achieved by the surfaces that have a mixture of hydrophobic/hydrophilic coatings, providing: (a) early transition to boiling regime and with eliminated temperature overshoot phenomena at boiling incipience, (b) up to 58.5% higher heat transfer coefficients, and (c) up to 47.4% higher CHF levels. The studied enhanced surfaces therefore demonstrated a practical surface modification method for heat transfer enhancement in immersion cooling applications.

Growing need for application of electronics at temperatures beyond their rated limit, (usually > 150 °C) and the non availability of high temperature compatible electronics necessitates thermal management solutions that should be compact, scalable, reliable and be able to work in environments characterized by high temperature (150 -250 °C), mechanical shock and vibrations. In this backdrop the proposed research aims at realization of an adsorption cooling system for evaporator temperatures in the range of 140 °C-150 °C, and condenser temperature in the range of 160 °C-200 °C. Adsorption cooling systems have few moving parts (hence less maintenance issues), and the use of Thermo-Electric (TE) devices to regenerate heat of adsorption in between adsorbent beds enhances the compactness and efficiency of the overall 'ThermoElectric-Adsorption' (TEA) system. The work presented identifies the challenges involved and respective solutions for high temperature application. An experimental set up was fabricated to demonstrate system operation and mathematical models developed to benchmark experimental results. Also, it should be noted that TEA system comprises TE and adsorption chillers. A TE device can be a compact cooler in its own right. Hence a comparison of the performance of TEA and TE cooling systems has also been presented.

Comprehensive molecular design was used to identify new heat transfer fluids for direct immersion phase change cooling of electronic systems. Four group contribution methods for thermophysical properties relevant to heat transfer were critically evaluated and new group contributions were regressed for organosilicon compounds. 52 new heat transfer fluids were identified via computer-aided molecular design and figure of merit analysis. Among these 52 fluids, 9 fluids were selected for experimental evaluation and their thermophysical properties were experimentally measured to validate the group contribution estimates. Two of the 9 fluids (C6H11F3 and C5H6F6O) were synthesized in this work. Pool boiling experiments showed that the new fluids identified in this work have superior heat transfer properties than existing coolant HFE 7200. The radiative forcing and global warming potential of new fluids, calculated via a new group contribution method developed in this work and FT-IR analysis, were found to be significantly lower than those of current coolants. The approach of increasing the thermal conductivity of heat transfer fluids by dispersing nanoparticles was also investigated. A model for the thermal conductivity of nanoparticle dispersions (nanofluids) was developed that incorporates the effect of size on the intrinsic thermal conductivity of nanoparticles. The model was successfully applied to a variety of nanoparticle-fluid systems. Rheological properties of nanofluids were also investigated and it was concluded that the addition of nanoparticles to heat transfer fluids may not be beneficial for electronics cooling due to significantly larger increase in viscosity relative to increase in thermal conductivity.

<p>Management of incoming solar radiation and use of the sky as an ultimate heat sink are technological imperatives as climate change shifts our reliance from fossil fuels to sustainable sources.  Selective solar absorbers are a possible route for solar harvesting as they collect the incoming radiation for process heat or space heating.  Here, improvement in the performance of selective solar absorbers via photon recycling is investigated using a stepped index rugate filter.  The final proposed filter when integrated with a high vacuum selective solar absorber could see an improvment in solar-thermal conversion efficiency from 13% to 30.6%. Then, a frequency selective optical filter is fabricated with uses including improvement of radiative coolers.  The measured optical characteristics are compared with simulation data and found to match well.</p> <p><br></p> <p>The shift to sustainable sources of electricity will require an expansion of the electrical grid.  The backbone of the grid for converting high voltage AC to DC, and vice versa, is power electronics.  The current state-of-the-art technology is GaN HEMTs, but GaN MISHEMTs are poised to replace them since MISHEMTs reduce the gate leakage current; a deficiency of the GaN HEMT architecture.  First, time dependent dielectric breakdown in GaN MISHEMTs is investigated using concurrent electrical and thermoreflectance methods.  A susceptibility in the MISHEMT architecture is found and possible solutions are proposed.  Then, liquid cooling of GaN HEMT PAs is explored by demonstrating integration of an X-band front end module, printed circuit board, and fluid manifold.  The integration shows great promise as two-phase cooling performance improved with increasing power dissipated, while single-phase cooling performance degraded.</p>

No description available.

<p dir="ltr">Due to advances in 3D integration and miniaturization of chips, the power density and number of hotspots within electronic packages have increased rapidly. A major bottleneck in the chip-to-coolant thermal resistance pathway is the interfacial resistance at solid-solid contacts and, therefore, thermal interface materials (TIMs) are employed to minimize interfacial thermal resistance. To improve heat dissipation, thermal grease (a type of TIM) is generally employed to reduce the overall thermal resistance from a heat-generating component to the heat sink. However, these materials degrade throughout their lifetime and the process is not well understood.</p><p dir="ltr">The first part of this dissertation focuses on investigating the degradation behavior of thermal greases using traditional and accelerated reliability techniques. The performance of a thermal grease often worsens with time due to the thermomechanical cycling driven by the coefficient of thermal expansion mismatch between the substrates via pumpout (material moves out of the interface) and dryout (phase separation of the composite material) phenomena. I isolate the effect of thermal cycling (from mechanical cycling) on the degradation of thermal greases by subjecting them to power cycling while holding the bond line thickness constant. In addition to thermocouples in the system, a high-resolution temperature mapping of the thermal grease is leveraged using an infrared microscope for evaluation of local degradation <i>in situ</i>. The results demonstrate a novel pathway for evaluating thermal grease performance by showcasing the importance of the viscosity-temperature hysteresis. However, traditional reliability testing methods such as thermal cycling have long testing periods, often of the order of days or months. Therefore, to accelerate the degradation analysis of thermal greases, I propose adding mechanical cycling while maintaining a constant heat flow rate. The reliability of thermal greases is investigated at different mechanical oscillation amplitudes and squeezing pressures using a novel custom-designed and machined experimental rig. The results uncover that the mechanical reliability of thermal greases depends on the ratio of elastic modulus to viscosity, with higher ratios being more desirable. Meanwhile, the thermal reliability depends upon the synergy of material properties with higher elastic modulus and higher thermal conductivity, resulting in a lesser increase in thermal resistance over the lifetime of thermal greases. </p><p dir="ltr">The second part of this dissertation focuses on the characterization of the rheology of the thermal greases and the associated uncertainty. Thermal greases have complex rheological properties that impact the performance over their lifetime. I perform rheological experiments on thermal greases and observe both stress relaxation and stress buildup regimes, which are not captured by steady shear-thinning models. Instead, a thixo-elasto-visco-plastic and a nonlinear-elasto-visco-plastic constitutive model characterizes each of the observed regimes. I use the models within a data-driven approach based on physics-informed neural networks (PINNs) to solve the inverse problem of determining the rheological model parameters from the dynamic response in experiments. Further, from a microscopic point of view, these rheological behaviors and associated uncertainties arise from the microstructure rearrangements due to particles' inhomogeneous mixing or separation/settling over time. However, this model calibration approach does not address parameter uncertainty arising due to epistemic (limited rheological data) and aleatoric (randomness of rheological experiments) sources. The last part of this dissertation addresses this limitation and quantifies uncertainties arising in the model calibration process. A hierarchical Bayesian inference methodology is used to obtain distributions of the rheological parameters. The uncertainty is further propagated to shear stress distributions and thermal resistances of thermal grease to demonstrate that the rheological models considered are suitable representations of the experimentally observed regimes. </p><p dir="ltr">Therefore, the current dissertation addresses the thermomechanical degradation behaviors and associated complex rheological characteristics of thermal greases. Understanding the degradation and rheology of thermal greases can help design better thermal greases which are degradation-resistant and hence can improve the reliability of electronic packages.</p>