L'intégration d'un accumulateur de chaleur dans les systèmes de refroidissement d'un véhicule permet d'optimiser la gestion thermique du groupe motopropulseur et ainsi de réduire la consommation et les émissions polluantes du véhicule. L'intérêt d'un tel accumulateur réside dans sa capacité à stocker / déstocker de l'énergie dans des matériaux à changement de phase (PCM) avec des puissances échangées en adéquation avec les besoins de l'automobile. La problématique scientifique concerne l'intensification, dans un volume restreint, des transferts thermiques dans le matériau de stockage. Le recours à des échangeurs compacts et l'optimisation de la géométrie des ailettes du côté du PCM permettent d'une part de maximiser la puissance échangée grâce à l'augmentation de la surface d'échange avec le PCM et d'autre part d'optimiser la capacité énergétique en améliorant le taux de remplissage en PCM. Le problème est abordé ici de deux façons : théorique, par le développement d'un modèle numérique d'optimisation, puis expérimentale, par la mise au point d'un banc d'essai et de prototypes. Le modèle numérique, validé à partir d'essais, a servi à optimiser la conception de l'accumulateur de chaleur. Enfin, l'intégration de ce composant dans un module de refroidissement pour améliorer la montée en température du groupe motopropulseur a montré des gains significatifs sur le temps de fonctionnement à froid du moteur, réduisant ainsi les émissions polluantes. / Integration of heat accumulator within engine cooling systems allows to optimize powertrain thermal management and to reduce vehicles consumption and pollutant emissions. Interest of such accumulators lies in their capacity to store and release energy within phase change materials (PCM) with powers in accordance with the automotive needs. Scientific problem concerns heat transfer enhancement, for a limited volume, in a phase change material. The use of compact heat exchangers filled with PCM and the optimization of fin design allow to maximize heat transfer thanks to extended heat transfer area with PCM. On the other hand, energy storage capacity is optimized by increasing PCM volume ratio. The problem is approached by two ways: theoretically, by the development of a numerical model of optimization, and experimentally, by the development of a test bench and several prototypes. The numerical model of heat accumulator, validated with test results, was used to run a parametric study to optimize the conception of the heat accumulator, in particular the fin design. Finally, integration of this new component within the cooling system in order to improve the warm-up of the powertrain has shown significant gains on the functioning time of engine during cold start, leading to reduced pollutant emissions.

No description available.

The thesis deals with the mathematical model for Stefan phase change problems. The model is then used in optimization procedures aimed at extremization of quantities describing the thermal behavior. The thesis also includes the derivation of the diferential heat equation, methods of energy accumulation and an introduction to phase change materials used for accumulation.

Práce je zaměřena na sestavení numerického modelu akumulace tepelné energie s fázovou přeměnou. Následně je tento model použit při tvarové optimalizaci, jejíž cílem je maximalizace uloženého tepla v tepelném výměníku. Kvůli komplexitě objektové funkce byl zvolen genetický algoritmus pro řešní úloh tvarové optimalizace. Práce obsahuje analýzu dvou problému týkající se tvarové optimalizace s následnou diskuzí nad obdrženými výsledky. Celý problém byl implementován v softwaru Matlab.

This thesis deals with the study of soils freezing in terms of phase change of water contained in the soil strata on green roofs. The aim of this work is to verify the effect of phase transformation of water on the course of temperature oscillation. First described the basic characteristics of soils generally, and subsequently described processes occurring during phase transformation of water in the soil and has been carried out experimental verification of the effect of moisture in the soil on the course of temperature oscillation. In the overall evaluation of the work, an analysis of the effect of phase change water in soil on the course of temperature oscillation and the resulting conclusion of work.

Thermal energy storage (TES) modules are specifically designed to respond to transient thermal loading. Their dynamic response depends on the overall structure of the module, including module geometry and dimensions, the internal spatial distribution of phase change material (PCM) and conductive heat-spreading elements, and the thermophysical properties of the different materials composing the module. However, due to the complexity of analyzing a system’s dynamic thermal response to transient input signals, optimal design of a TES module for a particular application is challenging. Conventional design approaches are limited by (1) the computational cost associated with high fidelity simulation of heat transfer in nonlinear systems undergoing a phase transition and (2) the lack of model integration with robust optimization tools. To overcome these challenges, I derive reduced-order dynamic models of two different metal-PCM composite TES modules and validate them against a high fidelity CFD model. Through simulation and validation of both turbulent and laminar flow cases, I demonstrate the accuracy of the reduced-order models in predicting, both spatially and temporally, the evolution of the dynamic model states and other system variables of interest, such as PCM melt fraction. The validated models are used to conduct univariate and bivariate parametric studies to understand the effects of various design parameters on different performance metrics. Finally, a case study is presented in which the models are used to conduct detailed design optimization for the two HX geometries.

La thèse porte sur la caractérisation des transferts thermiques dans les milieux fluide-particules, notamment en proposant un modèle décrivant le changement de phase au sein de particules sous écoulement fluide. Les transferts thermiques sont modélisés en prenant en compte l'influence de la résistance aux transferts externes (échanges avec le fluide) et internes (conduction à travers la particule et sa paroi, convection naturelle dans la phase liquide de la particule, changement de phase) à la particule. Les échanges externes avec le fluide sont pris en compte à l’aide de corrélations liant un nombre de Nusselt externe aux nombres de Reynolds et de Prandtl. La conduction interne est décrite à l’aide de solutions analytiques. L’effet de la convection naturelle a été étudié sur une particule isolée soumise à un gradient de température sous différents nombres de Rayleigh et de Prandtl permettant son déclenchement. Les résultats obtenus ont permis d’établir une corrélation reliant un nombre de Nusselt interne aux nombres de Prandtl et Rayleigh de la particule. Cette corrélation permet de recalculer l’évolution temporelle de la température moyenne de la particule en prenant en compte l’effet de la convection naturelle. Le changement de phase est décrit grâce à un modèle local basé sur l’approche « Phase Field » moyenné sur l’ensemble de la particule et validé par comparaison avec des résultats numériques et expérimentaux issus de la littérature. Enfin, le modèle complet et l’influence des phénomènes pris en compte sont testés sur un lit fixe de particules à l’échelle moyennée (Discrete Element Method–Computional Fluids Dynamics). La conduction et la convection interne donnent une quantité totale d’énergie stockée relativement similaire dans le lit à nombre de Biot égal, mais dont la distribution est différente. Le changement de phase tend à grandement densifier le stockage. Une augmentation du nombre de Biot tend à augmenter la quantité d’énergie stockée. Enfin, il est montré que les transferts sont dépendants de la distribution de porosité. / This work intends to characterize heat transfer in fluid-particle flows, specifically when phase change occurs inside the particles. The proposed model takes into account the external heat resistance (heat transfer at the particle-fluid interface) and the internal heat resistance (conduction inside and at the wall of the particle, natural convection in the liquid phase of the particle, phase change). External transfer with the surrounding fluid is described by correlations linking an external Nusselt number to Reynolds and Prandtl numbers related to the surrounding fluid. Internal conduction is calculated thanks to analytical solutions. The influence of natural convection was studied on an isolated sphere for different combinations of Rayleigh and Prandtl numbers. A correlation between an internal Nusselt number, and particle Rayleigh and Prandtl numbers was established using these simulations. This correlation allows calculating the transient evolution of the average temperature of the particle when natural convection occurs. Phase change is taken into account by a Phase Field model averaged over the particle and validated by comparison with experimental and numerical studies from the literature. Finally, the whole model and the effects of the different phenomena it describes are tested on a fixed bed of particles at mesoscopic scale using a Discrete Element Method–Computional Fluids Dynamics (DEM-CFD) model. Internal conduction and natural convection gives similar quantities of total energy stored for the same Biot number, however heat transfer distribution is modified. Phase change greatly reduces the volume of storage. Increasing the Biot number leads to a greater amount of energy stored. Finally, heat transfer greatly depends on porosity distribution.

This thesis deals with the synthesis and characterization of Ge-Te-Sb (GST) thin films. The films were deposited using a Pulsed Laser Deposition (PLD) method and mainly characterized with XRD, SEM, AFM and TEM. For amorphous and polycrystalline films, un-etched Si(100) was used. The amorphous films showed a similar crystallization behavior as films deposited with sputtering and evaporation techniques. When depositing GST on un-etched Si(100) substrates at elevated substrate temperatures (130-240°C), polycrystalline but highly textured films were obtained. The preferred growth orientation was either GST(111) or GST(0001) depending on if the films were cubic or hexagonal. Epitaxial films were prepared on crystalline substrates. On KCl(100), a mixed growth of hexagonal GST(0001) and cubic GST(100) was observed. The hexagonal phase dominates at low temperatures whereas the cubic phase dominates at high temperatures. The cubic phase is accompanied with a presumed GST(221) orientation when the film thickness exceeds ~70 nm. Epitaxial films were obtained with deposition rates as high as 250 nm/min. On BaF2(111), only (0001) oriented epitaxial hexagonal GST films are found, independent of substrate temperature, frequency or deposition background pressure. At high substrate temperatures there is a loss of Ge and Te which shifts the crystalline phase from Ge2Sb2Te5 towards GeSb2Te4. GST films deposited at room temperature on BaF2(111) were in an amorphous state, but after exposure to an annealing treatment they crystallize in an epitaxial cubic structure. Film deposition on pre-cleaned and buffered ammonium fluoride etched Si(111) show growth of epitaxial hexagonal GST, similar to that of the deposition on BaF2(111). When the Si-substrates were heated directly to the deposition temperature films of high crystal-line quality were obtained. An additional heat treatment of the Si-substrates prior to deposition deteriorated the crystal quality severely. The gained results show that PLD can be used as a method in order to obtain high quality epitaxial Ge-Sb-Te films from a compound target and using high deposition rates.

This dissertation presents the development of two unique measurement platforms. <br><br>The first system is a high-temperature Z-Meter. This system is designed to simultaneously measure the electrical resistivity, Seebeck coefficient, and thermal conductivity of a thermoelectric sample to accurately determine the figure of merit, ZT, for that material. It is designed to operated at sample temperatures of up to 1000C, and with temperature gradients on the order of 500C across the sample. This system also provides <i>in situ</i> load monitoring for contact pressure and allows the user to adjust loading during the experiment. <br><br>The second part of this dissertation focuses on the development of enhanced composite phase change material (PCM) heat sinks to improve passive thermal management in mobile electronics. We present a new design for a composite PCM heat sink and utilize off-the-shelf PCMs to show characterize the performance. In order to accurately investigate the performance enhancement of these designs, we develop a turn-key thermal management evaluation platform to allow the user complete control over the power profiles and cycling applied to the test chip, as well as providing <i>in situ</i> temperature monitoring within the chip. The proposed package designs show significant improvement in the length of time extended before reaching the cut-off temperature within the heatfluxes tested, 6 - 14 W/cm^2, and accomplish this while weighing less than the equivalent sensible heat storage design.<br><br><br><br>

This study has been focusing on a school located in the municipality of Laholm, South of Sweden. Employing an energy balance of the last five (5) years, a proposal for measures is made in terms of performance for comparison with the baseline of the current consumption trend. This comparison allowed us to narrow down the alternatives for the renovation with the potential to have a great impact on the school's energy use and indoor environment, but also on the preserved characteristics without any violation of the laws and regulations. A complementary analysis was used to analyze important variables for decision-making and implementation of improvements. This analysis consists of an exergy analysis which was utilized as a pre-design tool for an optimized building renovation proposal. Exergy losses were calculated to assess the performance of the systems. The study revealed that in relation to the use of new technologies and materials, aerogel and vacuum insulation panels bring relevant savings as their insulation mechanisms are the most efficient for such a building in a climate like Laholm. As for the heating system, it was evident that the use of a geothermal heat pump associated to PV panels brings considerable energy benefits when compared to the current oil boiler system and given that the village does not yet have a local district heating system. When the proposed measures are applied during 2022- 2023, the results will show that also older buildings can be energy efficient which is demanded of the buildings stock throughout the European Union.