<p> Design and optimization of aircraft thermal management systems (TMS) is typically conducted by considering a single system architecture at steady-state conditions, using per?formance metrics such as bleed air flow rate, fuel burn flow rate, or total system mass. However, when trying to increase the overall performance of a legacy system or analyzing new system architectures, it can be difficult to identify how individual component or sub?system changes will propagate throughout the overall TMS. In this thesis, new knowledge and tools are presented that will advance the use of exergy-based design techniques for next generation aircraft thermal management systems (TMS). This is motivated by the fact that exergy destruction is a quantity that can be calculated for any subsystem or component, regardless of energy domain or function. The relationship between exergy destruction min?imization (EDM) and conventional design metrics is investigated and quantified. This is performed through the use of a steady-state analysis and by leveraging a high fidelity model of a complex TMS. It is shown that exergy destruction is not only sensitive to individual component parameters in a manner consistent with conventional performance metrics, but that due to its generalizability, it also captures how changes in one subsystem propagate throughout the overall TMS. Specifically, through a design case study, it is shown that minimizing system-wide exergy destruction rate (without an engine model) yields a similar engine fuel burn rate as when fuel burn is minimized directly, but also results in a signif?icantly lower system mass. Building on these results, a transient design and analysis tool for TMS is developed using a graph theoretic approach. The tool is used on a case study of an air cycle machine (ACM) and on an architecture enumeration case study for a notional TMS. The transient exergy-based analysis is shown to provide insight into how efficiently energy is used at a component level, and captures the differences in thermal performance between architectures. </p>

The Swedish Armed Forces are investigating high-risk applications where lithium-ion batteries (LIB) can replace traditional lead-acid batteries. Understanding the potential safety risks and evaluating a battery's instability is crucial for military applications. This report aimed to identify critical safety parameters (temperature, potential, and impedance) in commercial batteries with NMC and LFP electrode chemistries, and to investigate how surrounding cells are affected when a battery suffers from thermal runaway (TR) in a battery module developed by FOI. Accelerated rate calorimetry (ARC) experiments on NMC-based Samsung SDI INR21700-40T and INR21700-50E and LFP-based A123 Systems ANR26650m1-B batteries were conducted to identify critical onset conditions of TR. ARC experiments were conducted with continuous electrochemical impedance spectroscopy (EIS) measurements to correlate thermal behavior with electrochemical changes in the cell impedance and voltage. The NMC-based batteries showed a distinct endothermic reaction between 116 °C and 121 °C, an onset temperature of exothermic self-heating at around 120 °C, which progressed to an explosive decomposition at about 170 °C and resulted in an adiabatic temperature rise of 250 °C to 290 °C. A significant increase in the cell’s impedance at around 100 °C indicated that the current interrupt device (CID) was triggered due to gas formation and critical pressure build-up within the cell. The LFP-based battery demonstrated improved thermal stability during ARC measurements and did not suffer from TR when heated to 300 °C. Thermal runaway propagation experiments were conducted in a battery module developed by FOI. The identified onset temperatures and electrochemical markers were then used to evaluate the stability of the module cells. Cell temperature increases between 16 °C and 48 °C was observed in cells directly adjacent to the trigger cell. Cells further from the trigger cell experienced uniform temperature increases of between 8 °C and 30 °C. EIS measurements of the module cells revealed no significant changes in their impedance spectra. The insulating polymer wrap around each cell was found to be crucial in preventing TR propagation. TR propagated from cell-to-cell in the module when the insulating wraps were removed, and cells were in direct contact with the thermally conductive heat sink. / Försvarsmakten undersöker högriskapplikationer där litiumjonbatterier kan ersätta traditionella blysyrabatterier. Att förstå säkerhetsrisker och utvärdera ett batteris instabilitet är särskilt viktigt för militära tillämpningar. Denna rapport syftar till att identifiera kritiska säkerhetsparametrar (temperatur, spänning och impedans) för kommersiella batterier med NMC- och LFP-elektrodkemier samt undersöka hur omkringliggande celler påverkas när ett batteri termiskt rusar (TR) i en batterimodul utvecklad av FOI. ARC-experiment genomfördes på NMC-baserad Samsung SDI INR21700-40T och INR21700-50E och A123 Systems ANR26650m1-B batterier för att karakterisera förloppet av termisk rusning (TR). ARC-experiment utfördes med kontinuerliga elektrokemisk impedansspektroskopi (EIS) för att korrelera termiskt beteende med elektrokemiska förändringar i cellimpedansen och spänningen. Det NMC-baserade batterierna uppvisade en tydlig endotermisk reaktion mellan 116 °C och 121 °C, exotermiska reaktioner påbörjades vid 120 °C och ledde till explosiv termisk rusning vid cirka 170 °C, vilket gav upphov till en adiabatisk temperaturökning på 250 °C till 290 °C. En signifikant ökning av cellens impedans vid cirka 100 °C indikerade att den inre säkerhetsventilen utlöstes på grund av gasbildning och kritisk tryckuppbyggnad i cellen. Det LFP-baserade batteriet visade förbättrad termisk stabilitet under ARC-mätningar och drabbades inte av TR vid uppvärmning till 300 °C. Termiska rusningsförsök genomfördes på en batterimodul utvecklad av FOI. De identifierade starttemperaturerna och elektrokemiska markörerna användes för att utvärdera modulcellernas stabilitet. Celltemperaturökningar mellan 16 °C och 48 °C observerades i celler direkt intill triggcellen. Celler längre från triggcellen upplevde likformiga temperaturökningar mellan 8 °C och 30 °C. EIS-mätningar av modulcellerna avslöjade inga signifikanta förändringar i deras impedansspektra. Det isolerande polymeromslaget runt varje cell var avgörande för att förhindra propagering av termisk rusning i modulen. Termisk rusning propagerade från cell till cell i modulen när de isolerande omslagen togs bort och cellerna var i direkt kontakt med den värmeledande kylflänsen.

The demand for more power and performance from electronic equipment has constantly been growing resulting in an increased amount of heat dissipation from these devices. Thermal management of high-density power control units for hybrid electric vehicles is one such application. Over the last few years, the performance of this power control unit has been improved and size has been reduced to attain higher efficiency and performance causing the heat dissipation as well as heat density to increase significantly. However, the overall cooling system has remained unchanged and only the heat exchanger corresponding to the power control unit (PCU) has been improved. This has allowed the manufacturing costs to go down. Efforts are constantly being made to reduce the PCU size even further and also to reduce manufacturing costs. As a consequence, heat density will go up (~ 200 – 250 W/cm2) and thus, a better high performance cooler/heat exchanger is required that can operate under the existing cooling system design and at the same time, maintain active devices temperature within optimum range (<120 – 125 °C) for higher reliability. The aim of this dissertation was to study the various cooling options based on jet impingement, mini-channel, ribbed mini-channel, phase change material and double sided cooling configurations for application in hybrid electric vehicle and other similar consumer products and perform parametric and optimization study on selected designs. Detailed experimental and computational analysis was performed on different cooling designs to evaluate overall performance. Severe constraints such as choice of coolant, coolant flow-rate, pressure drop, minimum geometrical size and operating temperature were required for the overall design. High performance jet impingement based cooler design with incorporated fin-like structures induced swirl and provided enhanced local heat transfer compared to traditional cooling designs. However, the cooling scheme could manage only 97.4% of the target effectiveness. Tapered/nozzle-shaped jets based designs showed promising results (~40% reduction in overall pressure drop) but were not sufficient to meet the overall operating temperature requirement. Various schemes of mini-channel arrangement, which were based on utilizing conduction and convection heat transfer in a conjugate mode, demonstrated improved performance over that of impingement cooling schemes. Impingement and mini-channel based designs were combined to show high heat transfer rates but at the expense of higher pressure drops (~5 times). As an alternate, mini-channel based coolers with ~1.5 mm size channels having trip strips or ribs were studied to accommodate the design constraints and to enhance local as well as overall heat transfer rates and achieve the target operating temperature. A step by step approach to the development of the heat exchanger is provided with an emphasis on system level design. The computational based optimization methodology is confirmed by a fabricated test bed to evaluate overall performance and compare the predicted results with actual performance. Additionally, one of the impingement based configuration (Swirl-Impingement-Fin) developed during the course of this work was applied to the internal cooling of a turbine blade trailing edge and was shown to enhance the thermal performance by at least a factor of 2 in comparison to the existing pin-fin technology for the conditions studied in this work. / Ph. D.

Local thermal management has important implications regarding comfort, energy consumption, and electronic device performance/lifetime. While organic thermoelectrics have emerged as promising materials for flexible thermoelectric energy harvesting devices, their potential as Peltier cooling element has been largely overlooked. Here, micro-thermoelectric coolers based on doped small molecule thin-films with a fast response time (around 25 µs) which is among the fastest micro-thermoelectric coolers reported are presented. This experimental cooling performance is supported by simulation using the finite-element method for thermal transport. The results show that organic thermoelectrics offer great potential for flexible and wearable micro-thermoelectric cooling applications.

Double-sided module exhibits electrical and thermal characteristics that are superior to wire-bonded counterpart. Such structure, however, induces more than twice the thermo-mechanical stress in a single-layer structure. Compressive posts have been developed and integrated into the double-sided module to reduce the stress to a level acceptable by silicon dice. For a 14 mm x 21 mm module carrying 6.6 mm x 6.6 mm die, finite-element simulation suggested an optimal design having four posts located 1 mm from the die; the z-direction stress at the chip was reduced from 17 MPa to 0.6 MPa. / Ph. D.

A novel cooling mechanism based on evaporation of thin liquid films is presented for thermal management of confined heat sources, such as microprocessor hotspots. The underlying idea involves utilization of thin nanoporous membranes for maintaining microscopically thin liquid films by capillary action, while providing a pathway for the vapor generated due to evaporation at the liquid-vapor interface. The vapor generated by evaporation is continuously removed by using a dry sweeping gas keeping the membrane outlet dry. This thesis presents a detailed theoretical, computational and experimental investigation of the heat and mass transfer mechanisms that result in dissipating heat. Performance analysis of this cooling mechanism demonstrates heat fluxes over 600W/cm2 for sufficiently thin membrane and film thicknesses (~1-5µm) and by using air jet impingement for advection of vapor from the membrane surface. Based on the results from this performance analysis, a monolithic micro-fluidic device is designed and fabricated incorporating micro and nanoscale features. This MEMS/NEMS device serves multiple functionalities of hotspot simulation, temperature sensing, and evaporative cooling. Subsequent experimental investigations using this microfluidic device demonstrate heat fluxes in excess of 600W/cm2 at 90 C using water as the evaporating coolant. In order to further enhance the device performance, a comprehensive theoretical and computational analysis of heat and mass transfer at micro and nanoscales is carried out. Since the coolant is confined using a nanoporous membrane, a detailed study of evaporation inside a nanoscale cylindrical pore is performed. The continuum analysis of water confined within a cylindrical nanopore determines the effect of electrostatic interaction and Van der Waals forces in addition to capillarity on the interfacial transport characteristics during evaporation. The detailed analysis demonstrates that the effective thermal resistance offered by the interface is negligible in comparison to the thermal resistance due to the thin film and vapor advection. In order to determine the factors limiting the performance of the MEMS device on a micro-scale, a device-level detailed computational analysis of heat and mass transfer is carried out, which is supported by experimental investigation. Identifying the contribution of various simultaneously occurring cooling mechanisms at different operating conditions, this analysis proposes utilization of hydrophilic membranes for maintaining very thin liquid films and further enhancement in vapor advection at the membrane outlet to achieve higher heat fluxes.

La montée en puissance des systèmes de production de l'énergie électrique, à partir de centrales éoliennes ou photovoltaïques, a fait apparaître un besoin aigu en moyens de stockage de cette énergie. Les technologies d'accumulateurs électrochimiques sont à même de répondre à cette problématique, en particulier les batteries sodium chlorure de nickel, une technologie d'accumulateur à haute température. Mais afin de gérer au mieux les performances de ces batteries, il est nécessaire d'avoir une connaissance approfondie du comportement électrothermique des cellules unitaires les composant. Ce travail de thèse présente en détail le développement d'un modèle multiphysique 2D d'une cellule Na-MCl2 commerciale, à même de simuler son fonctionnement en décharge à courant constant. Ce modèle a été construit sur une étude approfondie des mécanismes électrochimiques à l'œuvre dans ce type de technologie, et des effets thermiques associés. Il repose également sur la synthèse et la critique des modèles proposés précédemment dans la littérature. Le modèle inédit proposé dans ce manuscrit permet de prendre en compte deux réactions électrochimiques simultanées, ainsi que le comportement thermique de la cellule. Le modèle a été validé en confrontant les résultats de simulations aux performances électriques réelles d'une cellule commerciale Na-MCl2 (incluant une caractérisation des phénomènes électrochimiques par voltammetrie cyclique). Ce modèle permet de simuler avec succès des décharges à courant constant à différents régimes, et à partir de différentes températures. Il est également capable de prévoir les effets électrothermiques qui seraient consécutifs à un changement de design de la cellule (design structurel et de constitution). / The growing production capacity of wind turbines or photo-voltaic plants has revealed an acute need for electric energy storage systems. Electrochemical accumulator technologies are able to address this issue, in particular sodium nickel chloride batteries, a high temperature battery system. But to enhance the energy management of these batteries, it is necessary to have a deep knowledge of the electro-thermal behavior of its unit cell component. This thesis presents the development of a comprehensive multiphysics 2D model of a commercial Na-MCl2 cell, able to simulate its operation in constant current discharge. This model was built on a thorough study of electrochemical mechanisms at work with this type of technology, and its associated thermal effects. It is also based on the synthesis of existing models, and their critique. The new model proposed in this manuscript can take into account two simultaneous electrochemical reactions, as well as the thermal behavior of the cell. The model was validated by comparison between simulation results and the actual electrical performances of a commercial Na-MCl2 cell (including the characterization of electrochemical phenomena by cyclic voltammetry). This model successfully simulates constant current discharges at different rates, and from different temperatures. It is also capable of predicting the electro-thermal effects consecutive to design changes of the cell (structural and relevant to the active material).

Les travaux présentés dans cette thèse concernent l'étude thermique des batteries Li-ion en vue d'une application de gestion thermique pour l'automobile. La compréhension des phénomènes thermiques à l'échelle accumulateur est indispensable avant de réaliser une approche de type module ou pack batterie. Ces phénomènes thermiques sont mis en évidence à partir d'une modélisation thermique globale de deux accumulateurs de différentes chimies, en décharge à courant constant. La complexité du caractère résistif de l'accumulateur Li-ion a mené au développement d'un modèle prenant en compte l'interaction entre les phénomènes électrochimiques et thermiques, permettant une approche prédictive de son comportement. Enfin la réalisation de deux boucles expérimentales, de simulation de systèmes de gestion thermique d'un module de batterie, montre les limites d'un refroidissement classique par air à respecter les critères de management thermique. En comparaison, le second système basé sur l'intégration innovante d'un matériau à changement de phase (MCP) se montre performant lors de situations usuelles, de défauts ou encore lors du besoin d'une charge rapide de la batterie. / This work relates to the thermal study of Li-ion batteries in order to develop an optimized battery thermal management system. The understanding of thermal phenomena at cell scale is essential before to undertake an approach of the battery module or pack. Galvanostatic discharges of two kind of Li ion cells are modeled to highlight thermal phenomena. The complexity of the resistive behavior of Li-ion cell led to the development of an electrochemical-thermal coupled model to get a predictive approach. Then, two experimental tests benches were designed so as to compare two battery thermal management systems (BTMS). Restrictions of air cooling highlight its disability to achieve thermal management criteria. Innovative integration of a phase change material (PCM) was then tested under several uses of the battery module. This new BTMS showed really promising performances during intensive driving cycles, failure tests, and when a fast charge is needed.

Due to the aggressive downscaling of the CMOS technology, power and current densities are increasing inside the chip. The limiting current conduction capacity(106 Acm−2)and thermal conductivity(201Wm−1K−1 for Al and 400 Wm−1K−1 for Cu) of the existing interconnects materials has given rise to different electro-thermal issues such a shot-spot formation, electromigration, etc. Exploration of new materials with high thermal conductivity and current conduction has thus attracted much attention for future integrated circuit technology. Among all the elemental materials, carbon nanomaterials (graphene and carbon nanotube) possess exceptionally high thermal (600-7000 Wm−1K−1) and current( ~108 -109 Acm−2)conduction properties at room temperature, which makes them potential candidate for interconnect materials. At the same time development of efficient energy harvesting techniques are also becoming important for future wireless autonomous devices. The excess heat generated at the hot-spot location could be used to drive an electronic circuit through a suitable thermoelectric generator. As the See beck coefficient of graphene is reported to be the highest among all elementary semiconductors, exploration of thermoelectric properties of graphene is very important. This thesis investigates the electrothermal and thermoelectric properties of metallic single walled carbon nanotube (SWCNT) and single layer graphene (SLG) for their possible applications in thermal management in next generation integrated circuits. A closed form analytical solution of Joule-heating equation in metallic SWCNTs is thus proposed by considering a temperature dependent lattice thermal conductivity (κ) on the basis of three-phonon Umklapp, mass-difference and boundary scattering phenomena. The solution of which gives the temperature profile over the SWCNT length and hence the location of hot-spot(created due to the self-heating inside the chip) can be predicted. This self-heating phenomenon is further extended to estimate the electromigration performance and mean-time-to-failure of metallic SWCNTs. It is shown that metallic SWCNTs are less prone to electromigration. To analyze the electro-thermal effects in a suspended SLG, a physics-based flexural phonon dominated thermal conductivity model is developed, which shows that κ follows a T1.5 and T−2 law at lower(<300 K) and higher temperature respectively in the absence of isotopes(C13 atoms). However in the presence of isotopic impurity, the behavior of κ sharply deviates from T−2 at higher temperatures. The proposed model of κ is found to be in excellent match with the available experimental data over a wide range of temperatures and can be utilized for an efficient electro-thermal analysis of encased/supported graphene. By considering the interaction of electron with in-plane and flexural phonons in a doped SLG sheet, a physics-based electrical conductance(σ) model of SLG under self-heating effect is also discussed that particularly exhibits the variation of electrical resistance with temperature at different current levels and matches well with the available experimental data. To investigate the thermoelectric performance of a SLG sheet, analytical models for See beck effect coefficient (SB) and specific heat (Cph) are developed, which are found to be in good agreement with the experimental data. Using those analytical models, it is predicted that one can achieve a thermoelectric figure of merit(ZT) of ~ 0.62 at room temperature by adding isotopic impurities(C13 atoms) in a degenerate SLG. Such prediction shows the immense potential of graphene in waste-heat recovery applications. Those models for σ, κ, SB and Cph are further used to determine the time evolution of temperature distribution along suspended SLG sheet through a transient analysis of Joule-heating equation under the Thomson effect. The proposed methodology can be extended to analyze the graphene heat-spreader theory and interconnects and graphene based thermoelectrics.

Le travail de thèse présenté ici apporte des éléments de compréhension sur l'influence de la gestion de l'eau et de la gestion thermique d'une PEMFC sur ses performances électriques. Un modèle bidimensionnel représentant les transferts de matière et de chaleur dans l'épaisseur d'une cellule élémentaire et le long des canaux d'alimentation est mis en place. Une partie spécifique de l'étude concerne la modélisation microscopique des agglomérats réactionnels et l'influence de l'eau présente dans ces zones actives sur les performances électriques locales et globales de la pile. Dans l'optique de réaliser un modèle pouvant s'intégrer à un système de contrôle, les transports couplés de l'eau liquide et vapeur, de la chaleur et des charges sont résolus analytiquement. Quelles que soient les conditions de fonctionnement, le modèle permet de calculer quel est l'état interne de la pile en terme d'hydratation, de production de courant et de chaleur. Le diagnostic de l'état interne de la pile en fonction des conditions opératoires permet de connaître précisément comment adapter les paramètres d'alimentation de la pile pour obtenir des performances électriques optimales. Les distributions locales d'eau, de courant et de température sont présentées en fonction de la stratégie d'alimentation en gaz de la pile et de l'orientation du circuit de régulation thermique. Une partie consacrée au diagnostic de fonctionnement d'une pile illustre en quoi l'utilisation du modèle et la connaissance de l'état interne de la pile apporte des informations primordiales lors de l'utilisation d'une pile à combustible / The present work contributes to the understanding of water management and thermal management of a PEM fuel cell influences on the electrical performances. A bidimensional model representing mass and heat transfer in the cell thickness and along distribution channels is done. A specific part of the study concerns a microscopic representation of reactive agglomerates. This part presents the impact of the liquid water presence on local and global current densities. In te vein of integrating this model in a total fuel cell control system, simplifications have been done and coupled mass, heat and charge transfers are solved analytically. For any operating condition, the model gives a complete view of the hydric, thermal and electric inner situation of the cell. This diagnosis of the inner state of the cell leads to find the operating conditions giving to the optimal electric performances. Water, current and temperature distributions are presenting for different gas feeding strategies and different thermal management configurations. A specific part is dedicated to illustrate how this local diagnosis of the cell state can be used to estimate its global electric performances