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A novel hybrid cooling scheme is proposed to remove non-uniform heat flux in real time from the microprocessor. It consists of a liquid cooled microchannel heat sink to remove the lower background heat flux and superlattice coolers to dissipate the high heat flux present at the hotspots. Superlattice coolers (SLC) are solid-state devices, which work on thermoelectric effect, and provide localized cooling for hotspots. SLCs offer some unique advantage over conventional cooling solutions. They are CMOS compatible and can be easily fabricated in any shape or size. They are more reliable as they don't contain any moving parts. They can remove high heat flux from localized regions and provide faster time response. Experimental devices are fabricated to characterize the steady-state, as well as transient performance, of the hybrid cooling scheme. Performance of the hybrid cooling scheme has been examined under various operating conditions. Effects of various geometric parameters have also been thoroughly studied. Heat flux in excess of 300 W/cm² has been successfully dissipated from localized hotspots. Maximum cooling at the hotspot is observed to be more than 6 K. Parasitic heat transfer to the superlattice cooler drastically affects its performance. Thermal resistance between ground electrode and heat sink, as well as thermal resistance between ground electrode and superlattice cooler, affect the parasitic heat transfer from to the superlattice cooler. Two different test devices are fabricated specifically to examine the effect of both thermal resistances. An electro-thermal model is developed to study the thermal coupling between two superlattice coolers. Thermal coupling significantly affects the performance of an array of superlattice coolers. Several operating parameters (activation current, location of ground electrode, choice of working fluid) affect thermal coupling between superlattice coolers, which has been computationally as well as experimentally studied. Transient response of the superlattice cooler has also been examined through experiments and computational modeling. Response time of the superlattice cooler has been reported to be less than 35 µs.

A novel complementary metal oxide semiconductor (CMOS) compatible microchannel heat sink is designed, fabricated, and tested for electronic cooling applications. The proposed microchannel heat sink requires no design change of the electronic circuitry underneath. Therefore, microchannels can be fabricated on top of the finished CMOS wafers by just adding a few more steps to the fabrication flow. Combining polymer (parylene C) and metal (copper) structures, a high performance microchannel heat sink can be easily manufactured on top of the electronic circuits, forming a monolithic cooling system. In the design stage, computer simulations of the microchannels with several different dimensions have been performed. Microchannels made of only parylene showed poor heat transfer performance as expected since the thermal conductivity of parylene C is very low. Therefore an alternative design comprising structural parylene layer and embedded metal layers has been modeled. Copper is selected as the metal due to its simple fabrication and very good thermal properties. The results showed that the higher the copper surface area the better the thermal performance of the heat sinks. Based on the modeling results, the final test structures are designed with full copper sidewalls with a parylene top wall. Several different microchannel test chips have been fabricated in METU-MEMS Research &amp / Application Center cleanroom facilities. The devices are tested with different flow rates and heat loads. During the tests, it was shown that the test devices can remove about 126 W/cm2 heat flux from the chip surface while keeping the chip temperature at around 90&deg / C with a coolant flow rate of 500 &mu / l/min per channel.

Submitted by JORGE ANDRES ARROYAVE ORTEGON (jaarroyaveo@unal.edu.co) on 2018-08-20T15:11:59Z No. of bitstreams: 1 Dissertação_Jorge_Arroyave_Versão_Final_FC(1).pdf: 6142009 bytes, checksum: 185f73f2530ec7bc30607d8a9e004919 (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-08-20T20:10:47Z (GMT) No. of bitstreams: 1 arroyaveortegon_ja_me_ilha.pdf: 6142009 bytes, checksum: 185f73f2530ec7bc30607d8a9e004919 (MD5) / Made available in DSpace on 2018-08-20T20:10:47Z (GMT). No. of bitstreams: 1 arroyaveortegon_ja_me_ilha.pdf: 6142009 bytes, checksum: 185f73f2530ec7bc30607d8a9e004919 (MD5) Previous issue date: 2018-04-27 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / A energia solar pode ser aproveitada como fonte de energia térmica para aquecimento de água, por exemplo, em coletores solares ou como fonte de energia elétrica usando sistemas de células fotovoltaicas. Entretanto, as células fotovoltaicas, geralmente, de custos relativamente altos, têm algumas restrições relacionadas a altas temperaturas de operação e distribuições de temperatura não homogêneas levando a redução da vida útil e eficiência elétrica de tais sistemas. Essas limitações têm sido o foco de pesquisas, a fim de melhorar as eficiências elétricas, regular as temperaturas de operação e reduzir os materiais necessários para fabricação das células. Assim, este projeto de pesquisa tem como objetivo avaliar o desempenho de um dissipador de calor, baseado em microcanais retangulares paralelos, no resfriamento de uma célula fotovoltaica de alta concentração (HCPV-High Concentration Photovoltaic Cell), utilizando-se de análise teórica (modelo térmico), simulação numérica (usando o software comercial CFD ANSYS® Fluent v15) e de uma bancada experimental. Neste trabalho, foram consideradas as condições de máxima radiação (denominado de pior cenário, quando a célula não gera eletricidade e todo o calor deve ser dissipado) e de radiação média ao longo do período considerado. Os dados climatológicos foram obtidos do site Canal Clima - UNESP, com dados historicos do clima na região noroeste paulista. Foi realizada uma revisão do estado da arte a fim de compreender como os sistemas de geração elétrica fotovoltaica podem ser otimizados pelo uso de concentradores solares e materiais mais eficientes (células de junção-múltipla). A influência da temperatura nestes sistemas e como sistemas de resfriamento podem melhorar seu desempenho também foram analisados. Uma bancada experimental permitiu validar os resultados teóricos e numéricos obtidos. Comprovou-se que o uso de dissipador de calor baseado em microcanais pode permitir um controle efetivo da temperatura da célula HCPV, melhorando sua eficiência de conversão de energia solar em energia elétrica. O dissipador de calor foi avaliado sob condições de fluxo de calor constante, variando-se a velocidade mássica, G, no intervalo de 300 kg/m2s a 1500 kg/m2s. Assim, foi possível manter a superfície da célula a uma temperatura de 40°C, aproximadamente, para uma queda de pressão de, em média, 6 kPa. Os resultados das três análises apresentaram comportamentos similares e a concordância entre eles foi razoável, considerando as limitações de cada abordagem. / Solar energy can be used as a source of thermal energy in solar collectors, for example, or as a source of electricity using photovoltaic cell systems. However, photovoltaic cells requires high investments having some restrictions related to high operating temperatures and nonhomogeneous temperature distributions, leading to a reduction in the useful life and electrical efficiency. These limitations have been the focus of researches in order to improve electrical efficiencies, to regulate operating temperatures, and to reduce required materials in the cells. Thus, this research project aims to evaluate the performance of a heat sink based on parallel rectangular microchannels for cooling of a high concentration photovoltaic cell (HCPV), using theoretical analysis (thermal model), numerical simulation (using commercial software CFD ANSYS® Fluent v15) and an experimental bench. In this work, it was considered the conditions of maximum radiation (named worst scenario, when the cell does not generate electricity and all the heat must be dissipated) and the average radiation over the period considered. These climatological data were obtained from the Canal Clima – UNESP site, in the northwestern region of São Paulo state. A review on the subject was carried out in order to understand how solar photovoltaic systems can be optimized using solar concentrators and more efficient materials (multiple-junction cells). The influence of temperature and cooling systems were analyzed. An experimental bench was built, which allowed the validation of the theoretical and numerical results. The use of microchannel heat sinks can allow an effective temperature control of the HCPV cell, improving its efficiency of converting thermal energy into electrical energy. The heat sink was evaluated for different heat flux values and for mass velocity, G, in a range of 300 kg/m2s to 1500 kg/m2s. It was possible to maintain the high concentration cell at 40 °C with a pressure drop of 6 kPa, for the worst scenario. The three analyzes presented similar behavior and the agreement between them was reasonable, considering the approaches limitations. / FAPESP 2013/15431-7 / CNPq 458702/2014-5

Orientador: Elaine Maria Cardoso / Resumo: A energia solar pode ser aproveitada como fonte de energia térmica para aquecimento de água, por exemplo, em coletores solares ou como fonte de energia elétrica usando sistemas de células fotovoltaicas. Entretanto, as células fotovoltaicas, geralmente, de custos relativamente altos, têm algumas restrições relacionadas a altas temperaturas de operação e distribuições de temperatura não homogêneas levando a redução da vida útil e eficiência elétrica de tais sistemas. Essas limitações têm sido o foco de pesquisas, a fim de melhorar as eficiências elétricas, regular as temperaturas de operação e reduzir os materiais necessários para fabricação das células. Assim, este projeto de pesquisa tem como objetivo avaliar o desempenho de um dissipador de calor, baseado em microcanais retangulares paralelos, no resfriamento de uma célula fotovoltaica de alta concentração (HCPV-High Concentration Photovoltaic Cell), utilizando-se de análise teórica (modelo térmico), simulação numérica (usando o software comercial CFD ANSYS® Fluent v15) e de uma bancada experimental. Neste trabalho, foram consideradas as condições de máxima radiação (denominado de pior cenário, quando a célula não gera eletricidade e todo o calor deve ser dissipado) e de radiação média ao longo do período considerado. Os dados climatológicos foram obtidos do site Canal Clima - UNESP, com dados historicos do clima na região noroeste paulista. Foi realizada uma revisão do estado da arte a fim de compreender como os sistemas de... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Solar energy can be used as a source of thermal energy in solar collectors, for example, or as a source of electricity using photovoltaic cell systems. However, photovoltaic cells requires high investments having some restrictions related to high operating temperatures and nonhomogeneous temperature distributions, leading to a reduction in the useful life and electrical efficiency. These limitations have been the focus of researches in order to improve electrical efficiencies, to regulate operating temperatures, and to reduce required materials in the cells. Thus, this research project aims to evaluate the performance of a heat sink based on parallel rectangular microchannels for cooling of a high concentration photovoltaic cell (HCPV), using theoretical analysis (thermal model), numerical simulation (using commercial software CFD ANSYS® Fluent v15) and an experimental bench. In this work, it was considered the conditions of maximum radiation (named worst scenario, when the cell does not generate electricity and all the heat must be dissipated) and the average radiation over the period considered. These climatological data were obtained from the Canal Clima – UNESP site, in the northwestern region of São Paulo state. A review on the subject was carried out in order to understand how solar photovoltaic systems can be optimized using solar concentrators and more efficient materials (multiple-junction cells). The influence of temperature and cooling systems were analyzed. An exp... (Complete abstract click electronic access below) / Mestre

Doubling transistor count for every two years in a computer chip, transmitter and receiver (T/R) module of a phased-array antenna that demands higher power with smaller dimensions are all results of miniaturization in electronics packaging. These technologies nowadays depend on improvement of reliable high performance heat sink to perform in narrower volumes. Employing microchannels or open cell metal foam heat sinks are two recently developing promising methods of cooling high heat fluxes. Although recent studies especially on microchannels can give a rough estimate on performances of these two methods, since using metal foams as heat sinks is still needed further studies, a direct experimental comparison of heat exchanger performances of these two techniques is still needed especially for thermal design engineers to decide the method of cooling. For this study, microchannels with channel widths of 300 &micro / m, 420 &micro / m, 500 &micro / m and 900 &micro / m were produced. Also, 92% porous 10, 20 and 40 ppi 6101-T6 open cell aluminum metal foams with compression factors 1,2, and 3 that have the same finned volume of microchannels with exactly same dimensions were used to manufacture heat sinks with method of vacuum brazing. They all have tested under same conditions with volumetric flow rate ranging from 0,167 l/min to 1,33 l/min and 60 W of heat power. Channel height was 4 mm for all heat sinks and distilled water used as cooling fluid. After experiments, pressure drops and thermal resistances were compared with tabulated and graphical forms. Also, the use of metal foam and microchannel heat sinks were highlighted with their advantages and disadvantages for future projects.

<p>A battery assembly experiences both mechanical and thermal loadings during its operation. It is critical to perform the thermomechanical analysis to propose a novel design for the highest efficiency.In this study,two main goals include mechanical characterization and deformation responses for a battery cell and assembly, as well as air-cooled concepts design and analysis.Initially, the cell dimensions were measured by cell-sectioning method, and then the mechanical properties were empirically measured by both 3-point flexural, and nanoindentation experiments. Moreover, three pairs of experiments and simulations were conducted to study mechanical behaviors on both a single cell and a battery assembly. They include (1) point-force loading for single, open cell; (2) internal pressurization for single, sealed cell; and (3) internal pressurization for battery assembly.Additionally, both parametric and experimental studies were executed to design, analyze,and validate air-cooled concepts based on the idea of microchannel heatsink. The proposed concepts have the features, which are integrated into the battery cell for generating the cooling channels. A series of thermomechanical simulations and a forced convection testbed were built for computationally and empirically analyzing the performances of the concepts. The results from the mechanical characterization showed a significant difference between the actual and nominal values of both cell dimensions and mechanical properties. Therefore, the effect of the manufacturing process to such values must be considered before inputting for analyzing the deformation responses. From the thermomechanical analyses, it was found that the mechanical loading might negatively influence the thermal performance if there were not enough mechanical supports from the air-cooling structure. The impact was minimal in the tapered-channel battery assembly. This configuration also significantly reduced the temperature difference on the cell compared with other concepts and the reference design.<br></p>

<p>Advancements in future technologies such as artificial intelligence, electric vehicles, and renewable energy create a consistent need for more powerful and smaller electronic devices and systems. As a result, thermal management components such as heat sinks need to remove higher heat loads from more compact spaces to keep electronics within their operational temperature limits. Constraints imposed by conventional manufacturing processes restrict the design of heat sinks to simple geometries with limited cooling performance. Recent widespread commercialization of metal additive manufacturing (AM) tools offers new potential for leveraging the design freedom of these manufacturing technologies to design and fabricate heat sinks with improved performance. </p> <p>In AM, three dimensional parts are created through layer-by-layer depositing of materials, which allows fabrication of complex geometries that would be impossible or too costly using conventional subtractive methods. Many novel heat sink geometries have been proposed in literature which incorporate features such as manifolds, flow mixers, and curved channels using engineering intuition to reduce pressure drop or enhance heat transfer. Although such designs have been shown to offer improved performance, mathematical design algorithms such as topology optimization (TO) have been shown to outperform engineering intuition. Topology optimization optimizes the material distribution within a given design space, guided by physics-based simulations, to achieve a user-defined objective such as minimization of thermal resistance. Previous TO approaches have used penalization methods to ensure the final designs are composed of macroscopic and non-porous features due to the past precedent of fabrication capabilities. This traditional penalization approach is well-suited to the constraints of conventional manufacturing methods; however, microstructures and porous features are easily fabricable with additive manufacturing. There is a need to develop TO approaches that are better suited for leveraging AM for the design of heat sinks. In this thesis, a homogenization approach to topology optimization is proposed wherein the material distribution is represented as parametrized microstructures. This formulation allows design of thermal management components that have sub-grid features and leverages AM for fabrication. The focus of this thesis is the development of the homogenization approach for TO of heat sinks, as well as the exploration of the design problems it can address, the performance benefits made available, and the two-phase flow physics that it uniquely allows to be incorporated into the topology optimization process.</p> <p>A topology optimization algorithm using the homogenization approach is developed by representing the material distribution as arrays of pin fins with varying gap sizes. To this end, the pin fins are modeled as a porous medium with volume-averaged effective properties. Height-averaged two-dimensional flow and non-equilibrium thermal models for porous media are developed for transport in the pin fin array. Through multi-objective optimization, TO designs are generated for an example case involving a hotspot over a uniform background heat input. The resulting topologies have porous-membrane-like designs where the liquid is transported through a fractal network of open, low-hydraulic-resistance manifold pathways and then forced across tightly spaced arrays of pin fins for effective heat transfer. The TO designs are revealed to offer significant performance improvements relative to the benchmark straight microchannel (SMC) heat sink with features optimized under the same multi-objective cost function. A series of microchannel heat sinks are fabricated using direct metal laser sintering to investigate the printing capabilities and to experimentally demonstrate the performance of topology optimized designs. Advantages of the homogenization approach over the penalization approach can be summarized as follows: (1) reduced computational costs due to its ability to create sub-resolution features, (2) intrinsically fabricable parts using available metal AM tools, and (3) easier to use due to significantly reduced number of hyperparameters (e.g., penalization factors) that are controlled by the user. </p> <p>Topology optimization has been applied to thermal management methods involving single-phase flows such as natural convection, forced air cooling, and pumped liquid cooling. Compared to these conventional heat sink technologies, flow boiling offers very high heat transfer coefficients and effective heat capacities, making it a promising candidate for future cooling electronics applications. The final goal of this thesis is to enable topology optimization of flow boiling heat sinks. However, TO of flow boiling heat sinks has been avoided due to difficulties in modeling the boiling phenomena; of note, there are no examples of TO being applied to the design of heat sink under flow boiling throughout the literature. Multi-dimensional two-phase flow models require prior knowledge of friction factor and heat transfer coefficients. Correlations are available in literature but are not universal and depend significantly on channel/fin geometries, surface roughness, and operating conditions. Given that traditional penalization-based TO approach results in fin and channel geometries with unknown shapes, dimensions, and alignment before the optimization is completed, this prohibits their use for optimization of flow boiling heat sinks. However, the homogenization approach to topology optimization developed in this thesis enables the optimization of flow boiling heat sinks. As it relies on user-defined microstructures with known shapes, alignments, and ranges of geometric dimensions, a universal correlation for flow boiling in microchannels is not needed. Instead, correlations for the user-defined microstructures are sufficient to simulate flow boiling in TO designs generated using the homogenization approach. To this end, a predefined microstructure geometry is chosen for which two-phase flow correlations exist and therefore topology optimization can be performed. Topology optimized heat sink designs under flow-boiling are generated and investigated at various heat inputs, topology optimization grid sizes, and maximum vapor quality constraints. Topology optimized heat sinks designed for single-phase versus two-phase flow are compared.  There are significant differences in hydraulic and thermal responses of the single-phase and two-phase designs due to high effective heat capacity rates and high heat transfer coefficients of flow boiling. The algorithm demonstrated in this work extends the capabilities of topology optimization to two-phase flow physics, and thereby enables the design of various two-phase flow components such as evaporators, condensers, heat sinks, and cold plates.</p> <p>The flow and heat transfer of the TO algorithm for microchannel heat sinks under flow boiling use a two-phase mixture model featuring an effective porous medium formulation. However, closure of the governing equations requires empirical correlations for pressure drop and heat transfer that are specific to the operating conditions, microstructure geometry, and surface finish. Therefore, it must be demonstrated these available correlations can be successfully calibrated over a range of microstructural variations present within the homogenization framework, so as to attain the required prediction generality and accuracy needed to ensure the resulting designs achieve Pareto-optimality. To this end, a set of uniform pin fin calibration samples are additively manufactured and experimentally tested under flow boiling at various flow rates and heat inputs for model calibration. All of the unknown/free coefficients in the adopted correlations are determined by minimizing the error between the model predictions and the experimental measurements using gradient-based optimization. The calibrated topology optimization algorithm is then used to generate a Pareto-optimal set of heat sinks optimized for minimum pressure drop and thermal resistance during flow boiling. Experimental characterization of these additively manufactured heat sinks, unseen during the model coefficient calibration process, reveals that the measured Pareto optimality curve matches that predicted by the topology optimization algorithm. Lastly, a heat sink design is generated for a design space involving multiple hot spots and background heating to showcase the capability of the experimentally calibrated two-phase topology optimization algorithm at handling complex boundary conditions. The optimized heat sink intelligently distributes an adequate amount of coolant flow to each of the heated regions to avoid local dry-out. This work demonstrates a complete framework for two-phase topology optimization of heat sinks through experimental calibration of flow boiling correlations to the porous medium used by the homogenization approach. </p> <p>The major contribution of this thesis is the development of a homogenization approach for TO of additively manufactured microchannel heat sinks under single- and two-phase flows. Not only does the homogenization approach provide several advantages over the traditional penalization approaches such as reduced computational costs, intrinsic fabricability using AM, and ease of use, but it also enables TO of heat sinks under flow boiling and potentially TO of other two-phase thermal management components. The work discussed in this thesis serves a comprehensive end-to-end guide on TO of microchannel heat sinks using the homogenization approach with experimental demonstrations for validation.</p>