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No description available.

There is growing interest in the use of microwaves to process materials. The main application of microwave processing of materials is in heating. The most important characteristic of microwave heating is {\it volumetric} heating, which is quite different from conventional heating where the heat must diffuse in from the surface of the material. Volumetric heating means that materials can absorb microwave energy directly and internally and convert it to heat. It is this characteristic that leads to advantages such as rapid, controlled, selective, and uniform heating. However, some problems hinder the widespread use of microwave energy. One of these problems is called thermal runaway, which is a type of thermal instability due to the interaction between the electromagnetic waves and materials. As thermal runaway occurs, the temperature of the heated material rises uncontrollably. The normal consequence of thermal runaway is the damage of the processed materials. The origins of thermal runaway are different under different processing conditions. When processing ceramic materials, thermal runaway is mainly due to the positive temperature dependence of dielectric loss of the material. These materials absorb more microwave energy as they are being heated. The most plausible explanation of this phenomenon is the so-called "S-curve" theory. However, prior to this work, no direct experimental evidence has been published to verify this theory. In this dissertation, we report the direct experimental evidence of the so-called "S-curve" by heating thermal runaway materials in a microwave resonant cavity applicator. A complete discussion of how the experimental results were achieved is presented. From the experimental results, we find that by the use of the cavity effects thermal runaway can be controlled. To explain the experimental findings, a theoretical model based on equivalent circuit theory is developed. Also, a coupled heat transfer and electromagnetic field model is developed to simulate the heating process. Both models give reasonably good comparison with our experimental results. Finally, a method to control thermal runaway is described. / Ph. D.

A microwave heat exchanger (MHE) is a device which converts microwave (MW) energy into usable form of heat energy. The working principle of the MHE is based on a collective effect of electromagnetic wave propagation, heat transfer and fluid flow, so the development of an efficient device requires complicated experimentation with processes of different physical nature. A peculiar phenomenon making the design of MHE even more challenging is extit{thermal runaway}, a nonlinear phenomenon in which a small increase in the input power gives rise to a large increase in temperature. Such high temperature may result in material damage through excessive thermal expansion, cracking, or melting. In this Thesis, we report on an initial phase in the development of a computational model which may help clarify complicated interaction between nonlinear phenomena that might be difficult to comprehend and control experimentally. We present a 2D multiphysics model mimicking operation of a layered MHE that simulates the nonlinear interaction between MW, thermal, and fluid flow phenomena involved in the operation of the MHE. The model is built for a triple layered (fluid-ceramic-fluid) MHE and is capable of capturing the S- and SS-profiles of power response curve which determines steady-state temperature solution as a function of incident power. The model is implemented on the platform of the COMSOL Multiphysics modeling software. We show that a MHE with particular thickness and dielectric properties of the layers can operate efficiently by keeping temperatures during thermal runaway under control. Overall temperatures increase rapidly as soon as the local maximum temperature reaches a critical value. This condition is held true both in absence and in presence of fluid flow. It is demonstrated that the efficiency of the MHE dramatically increases when thermal runaway is achieved. As the amount of heat energy, which is being transferred to the fluid from the heated dielectric, increases, incident power required to achieve thermal runaway also increases. It is also shown that, with appropriate length of the layered MHE, thermal runaway can be achieved at a lower power level. While the model developed in this Thesis studies the basic operation of a three layered MHE, it can further be developed to investigate optimum design parameters of the MHE of other structures so that maximum thermal efficiency is achieved.

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The benefits of applying microwave energy to material processing techniques have been well documented and studied. The potential benefits over conventional oven heating include faster processing times, more uniform heating, more consistent product quality, and the possibility of precise control. The actual implementation of microwave technology has been lacking and the benefits have gone largely unrealized. This is due in part to the temperature dependence of the dielectric loss of many industrial materials such as ceramics and polymers. These materials absorb more microwave energy as they heat, creating uncontrollable heating, often called 'thermal runaway'. The focus of this research is to address this challenge. The work described here is an experimental program for the microwave processing of specific ceramic rods and polymer tows. The objective of the program is to study the thermal runaway effect, and to provide data which will be used to verify numerical models. Accurate test data are essential to the development of precise, comprehensive models that can be used in applicator design and heating control strategies for thermal runaway materials. The experimental program explores the difficulties of microwave heating and offers solutions to more efficient systems. Successful measurements of power loss and control of thermal runaway are detailed for mullite, alumina, and nylon. / Master of Science

Microwave processing of ceramics has seen a growth in research and development efforts throughout the past decade. One area of interest is the exploration of improved heating control through experiments and numerical modeling. Controlled heating may be used to counteract non-uniform heating and avoid destructive phenomena such as cracking and thermal runaway. Thermal runaway is a potential problem in materials with temperature dependent dielectric properties. As the material absorbs electromagnetic energy, the temperature increases as does its ability to absorb more energy. Controlled processing of the material may be achieved by manipulating the applied field. The purpose of this research is to model the interaction of the EM-field with a thin ceramic fiber to investigate possible mechanisms that may affect the heating process. The fiber undergoes microwave heating in a single-mode resonant applicator. Maxwell's equations for the fields within the cavity are solved using mode-matching techniques taking into account the field interaction of the fiber and an arbitrarily shaped coupling aperture. Effects of varying the aperture shape on the field distribution are explored. The coupled nature of the electromagnetic solution with the material's temperature-dependent properties, including an analysis of non-uniform heating, is also discussed. / Master of Science

Examensarbetet gjordes på Volvo Group Trucks Technology. Målet med arbetet var att studeravärmeutveckling i Li-jon cell för hybrid- och elbilar, HEV och EV. Battericeller undersöktesunder sina normala arbetsförhållanden och vid förstörande prov. Undersökningen baserades påcellernas yttemperatur. Arbetet beskrev cellernas beteende och syftade att vara ett underlag förkonstruktörer av batterisystem.En litteraturstudie gjordes för att studera faktorer som påverkar värmeutvecklingen. Sedananalyserades källor till samtliga faktorer. En moduleringsmetod för analys av cellensvärmeeffektivitet togs fram. Miljöpåverkan och ekonomiska aspekter av batterier undersöktes.Tre tester togs fram för att undersöka värmeutvecklingsfaktorer på fem celler. De flestafaktorerna var externa såsom laddning och urladdning, puls och kontinuerlig ström ochomgivningstemperatur. En infraröd kamera användes vid experimenten.Testerna visade hur olika faktorer påverkade cellernas temperatur. Vidare analys av källor visadekritiska områden i cellernas konstruktion.Förstörande värmeprov gjordes på tre par av celler. Dessa värmdes upp till 300°C vilketorsakade ”thermal runaway”. I vissa fall gick temperaturen över 600°C och celler fattade eld.Olika kemiska sammansättningar och uppbyggnad av cellerna gjorde att de betedde sig olika vidgenomförda tester.Testerna visade att olika celler presterade olika vid liknande testförhållanden. Därför är detviktigt att ta fram specifikationer för användningsförhållanden för att välja ut en cell för ettbatterisystem. Sedan kan prestandan av olika celler jämföras och effektivitet kan utvärderas församma belastningscyklar.Thermal Management System kan förhöja batteriets effektivitet och måste designas medanvändningsförhållanden i åtanke. Batteriernas säkerhet är väldigt viktig och människor får inteskadas av batterier. Därför måste säkerheten finnas i åtanke i alla steg av batteridesign.Arbetets resultat blev en sammanfattning av viktiga faktorer och specifikationer för batteridesignsom baserades på värmeutvecklingen. Samtliga riktlinjer sammanfattades i Appendix 5. / Master thesis work was done at Volvo Group Trucks Technology. Aim of the project was tostudy thermal behaviour of Li-ion battery for hybrid and electric vehicles, HEVs and EVs.Battery cells were tested in regular working conditions and abuse conditions. Surfacetemperature of cells was chosen for studying heat evolution.A literature study was conducted to research factors that influence cell temperature. Analysis ofsources of these factors was then performed. A modelling method for analyzing cell thermalefficiency was designed. Sustainability and economics aspects of batteries were also studied.When factors were established three tests were designed to study their effects. Five cells werestudied. Tests mainly examined external factors such as charge and discharge, pulse andcontinuous current, ambient temperature to name a few. An infrared camera was used.Study showed how different factors influenced cell temperature. Further analysis of sourcespointed out some hot spots of cell designs.Thermal abuse test were performed on three pairs of cells. Cells were heated up to 300°C andwent through thermal runaway which in some cases increased temperatures up to 660°C in lessthan a second and caused fire. Different cell chemistries and cell designs reacted differently tothe abuse conditions.A conclusion was reached that cells performed differently in similar test conditions. Whendesigning a battery system a set of specifications for usage conditions is crucial for choosing acell. When conditions and load cycles are known cells can be tested and their thermal andelectrical efficiency evaluated.Thermal Management System TMS can largely enhance cell efficiency and lifecycle. Suchsystem must also be designed according to usage conditions and particular cell’s performance.Battery safety showed to be a very important factor of designing a battery system. Humans shallnot be injured by systems with batteries which must be kept in mind during design.Work resulted in summary of important factors and specifications for designing a battery systembased on cell thermal behaviour. These guidelines are presented in Appendix 5.

The rapid electrification of vehicles has neccessitated advancements in battery technology, impacting various components within the car battery pack. This project focuses on the development of thermal barriers, which serve as crucial components that prevent potential fires from thermal runaway incidents in the battery pack from reaching the vehicle's interior compartment. Currently, thermal barriers are predominantly composed of mica. The objective of this project is to identify a more sustainable alternative material to mica that possesses similar properties while considering economic, social, and environmental factors.  Following the product development process, a comprehensive background study and benchmark analysis were conducted. Subsequently, the specific requirements for the new material were defined, and potential suppliers providing alternative materials were identified. Torch tests and dielectric strength tests were performed to assess thermal conductivity, fire resistance, and dielectric properties, which were deemed critical attributes. The test results unveiled a novel material comprised of a combination of two distinct materials from different suppliers. One material was a composite consisting of glass fiber and carbon, exhibiting exceptional thermal properties. The other material was an electrical-grade paper boasting excellent electrical properties. When combined, these materials formed a composite that paralleled the properties of mica. The composite material had a total thickness of 2 mm, slightly thicker than the current implementation of mica. To explore the feasibility of a thinner alternative, an estimation was conducted, suggesting that a thinner sheet of the composite material could be possible. In conclusion, this study demonstrates the feasibility of replacing mica with an alternative material. Further testing and investigation are required to determine the optimal implementation and minimum thickness of the new material. However, based on the successful test results and similarity in properties, it can be affirmed that the new material has the potential to replace mica as a sustainable thermal barrier in the battery pack. / Denna avhandling fokuserar på utvecklingen av hållbara termiska barriärer som ersättning för mica i batteripacken i elektriska bilar, vilket har blivit nödvändigt på grund av den snabba elektrifieringen av fordon. Syftet med studien är att identifiera ett mer hållbart alternativ till mica och samtidigt ta hänsyn till ekonomiska, sociala och miljömässiga faktorer. Genom att följa produktutvecklingsprocessen har en omfattande bakgrundsstudie och benchmark-analys genomförts. Därefter definierades specifika krav för det nya materialet, och potentiella leverantörer av alternativa material identifierades. Väsentliga attribut som termisk ledningsförmåga, brandsäkerhet och elektriska egenskaper utvärderades genom att utföra flam-tester och tester för den elektriska ledningsförmågan. Resultaten av testerna resulterade i ett innovativt material bestående av en kombination av två olika material från olika leverantörer. En komposit av glasfiber och kol visade exceptionella termiska egenskaper, medan ett elektriskt kvalitetspapper uppvisade utmärkta elektriska egenskaper. När dessa material kombinerades bildade de en komposit som liknade micas egenskaper. Det komposita materialet hade en total tjocklek på 2 mm, något tjockare än mica. Genom en uppskattning undersöktes möjligheten att använda en tunnare variant av det komposita materialet. Sammanfattningsvis visar denna studie att det är genomförbart att ersätta mica med det föreslagna alternativa materialet. Vidare tester och undersökningar krävs för att optimera implementeringen och fastställa den minsta möjliga tjockleken på det nya materialet. Baserat på de positiva testresultaten och likheten i egenskaper kan det konstateras att det föreslagna materialet har potential att vara en hållbar termisk barriär i batteripaket.

Electromagnetic (EM) heat exchangers are devices that absorb EM radiation and convert its energy to thermal energy for a specific purpose such as to power a turbine. They have recently been of growing interest, yet the field is predominantly studied with thermal resistance network models and is in need of more rigorous continuum modeling. Homogenization has been used in low and high frequency electromagnetics to describe macroscopic behavior of traveling waves. While dielectric material parameters vary with temperature, coupling the energy equation with Maxwell’s equations, little effort has been made toward homogenization techniques that capture the effects of this dependence, which is necessary to accurately model porous medium heat exchangers. Firstly, we have examined the effect the wave-geometry interactions of high-frequency illumination has on a triple-layer laminate, which approximates the unit cell of a homogenization problem. Secondly, we develop an extension to a high-frequency homogenization (HFH) method developed for photonics. The extension is made by developing a three-dimensional vector-valued HFH of Maxwell’s curl-curl equation that includes dielectric loss. It is validated for a one-dimensional geometry where the exact solution to the scattering problem is known by implementing the Transfer Matrix Method. The HFH model produces perturbation approximations to the dispersion curves showing the nonexistence of band gaps and generates low attenuation outside the band gap regions.