Η παρούσα διπλωματική εργασία αποτελείται από δυο μέρη. Στο πρώτο μέρος πραγματοποιήθηκε μεταβατική θερμική ανάλυση σε μια υγιή μηχανή και σε μια μηχανή με σπασμένη μπάρα και ακολούθησε σύγκριση των αποτελεσμάτων των δυο αναλύσεων. Στο δεύτερο μέρος έγινε σύγκριση των αποτελεσμάτων μιας μεταβατικής θερμικής ανάλυσης υγιούς μηχανής με αντίστοιχα πειραματικά δεδομένα. / Transient thermal analysis for healthy asychronous motor and for motor with broken bar to the motor.

Thermo-mechanical failure of components in planar-type solid oxide fuel cells (SOFCs) is a major obstacle on the path to bringing this technology to commercial viability. The probability of material degradation and failure in SOFCs depends strongly on the local temperature gradients at the interfaces of different materials. Therefore, it is of paramount importance to accurately predict and manage the temperature fields within the stack, especially near the interfaces. In this work we consider three effects in detail. First, we analyze radiative heat transfer effects within the semi-transparent solid electrolyte and compared them to thermal conduction. We also, present the modeling approach for calculation of surface-to-surface exchange within the flow channels and from the stack to the environment. The simplifying assumptions are identified and their carefully justified range of applicability to the problem at hand is established. This allows thermal radiation effects to be properly included in overall thermal modeling efforts with the minimum computational expense requirement. Second, we developed a series of reduced-order models for the transient heating and cooling of a cell, leading to a framework for optimization of these processes. The optimal design is one that minimizes heating time while maintaining thermal gradients below an allowable threshold. To this end, we formulated reduced order models (validated by rigorous CFD simulations) that yield simple algebraic design rules for predicting maximum thermal gradients and heating time requirements. Several governing dimensionless parameters and time scales were identified that shed light on the essential physics of the process. Finally, an analysis was performed to assess the degree of local thermal non-equilibrium (LTNE) within porous SOFC electrodes, and through a simple scaling analysis we discovered the parameter that gives an estimate of the magnitude of LTNE effects. We conclude that because of efficient heat transfer between the solid and gas in the microscale pores of the electrodes, the temperature difference between gas and solid is often negligible. However, if local variations in current density are significant, the LTNE effects may become significant and should be considered.

Dans le cadre des applications transports, et plus particulièrement de "l’avion plus électrique", avec une demande toujours plus présente de réduction d’encombrement et de poids, la tendance est à l’intégration de plus en plus poussée des convertisseurs statiques. L’augmentation de leur densité de puissance et celle des contraintes thermiques, induites par l’environnement dans lequel ces structures sont localisées, deviennent de plus en plus critiques. La gestion thermique de ces dispositifs est assurée par des systèmes de refroidissement sur lesquels sont montés les composants semi-conducteurs via un matériau d’interface thermique. Une gestion performante sera obtenue par la diminution de la résistance thermique globale entre les éléments dissipatifs et le milieu ambiant grâce en autre à l’amélioration du système de refroidissement et des propriétés thermiques des matériaux constituant le module. Or cette interface est un point délicat du transfert de chaleur car elle peut représenter plusieurs dizaines de pourcents de la résistance thermique globale. Elle nécessite donc une connaissance approfondie de son comportement aux sollicitations thermiques. Après un état de l’art sur les matériaux d’interfaces thermiques et les méthodes de caractérisation des propriétés thermophysiques des matériaux, nous proposons la mise en œuvre d’outils expérimentaux et mathématiques permettant de suivre l’éventuelle évolution de matériaux d’interfaces utilisés en électronique de puissance au cours d’un vieillissement par cyclage en température. Pour cela, deux méthodes sont présentées. La première repose sur la mesure de la résistance thermique des interfaces en régime stationnaire avec un transfert de chaleur monodimensionnel alors que la seconde, basée sur une caractérisation transitoire thermique d’un système, permet d’en identifier les constantes de temps et le réseau Résistance-Capacité du système testé. Des travaux de simulations numériques ont été menés sur les deux types de bancs expérimentaux, d’un côté pour pouvoir évaluer les pertes thermiques latérales du banc statiques, de l’autre côté pour montrer qu’il est bien possible de détecter une variation de la résistance thermique d’un matériau d’interface par l’analyse de l’impédance thermique. / In the context of transportation applications, and especially the "more electric aircraft", with an ever present demand for space and weight reduction, the trend is to integrate more extensive of static converters. The increase in power density and the thermal stresses induced by the environment in which these structures are located, are becoming increasingly critical. Thermal management of these devices is provided by cooling systems on which are mounted the semiconductor components via a thermal interface material. Effective management will be achieved by reducing the overall thermal resistance between the dissipative elements and the environment by improving the cooling system and thermal properties of the materials constituting the module. However, this interface is a delicate point of heat transfer because it can represent several tens of percent of the circuit total thermal resistance. It therefore requires a thorough knowledge of their behavior in thermal stresses. After a state of the art on the thermal interface materials and methods for characterizing thermophysical properties of materials, we propose the implementation of experimental and mathematical tools to monitor any change of interface materials used in power electronics during aging by temperature cycling. For this, two methods are presented. The first is based on the measurement of the thermal resistance of the interfaces with a steady one-dimensional heat transfer, while the second, based on a characterization of a transient thermal system, allows to identify the time constants and the resistor and capacitor network of the tested system. Numerical simulations were carried out on two types of experimental benches, on one side in order to assess the lateral heat losses from static bench, on the other side to show that it is possible to detect a change in the thermal resistance of a TIM with the analysis of the thermal impedance.

This thesis focuses on the comparison of numerical simulation and optical measurement of deformation of the turbinehousing made by Honeywell company. The numerical calculation performed by finite element method to simulate transient thermal load on the measured entity. Numerical result of the strain state of the body caused by temperature gradients. To verify the simulation is used an experimental digital correlation method VIC-3D. The thesis also mentioned the theoretical foundations of digital correlation methods and thermal analysis.

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Possibilities of modeling non-linear behavior of concrete within standard room temperatures and increased fire-load values using FEM software ANSYS are studied. Temperature dependences of material models are considered. Fire resistance of reinforced concrete and concrete-steel composite construction is analyzed. Fire loads are defined in accordance with relevant standards. Non-linear structural transient analyses are calculated after temperatures were determined by transient thermal analyses. Results obtained from analyses of simple reinforced concrete structure are compared with approach of isotherm 500 °C method.