O presente trabalho trata da modelagem da condução de calor transiente com geração de calor em meios heterogêneos, e tem o objetivo de desenvolver um modelo multiescala adequado a esse fenômeno. Já existem modelos multiescala na literatura relacionados ao problema proposto, e que são válidos para os seguintes casos: (a) o elemento de volume representativo tem tamanho desprezível quando comparado ao comprimento característico macroscópico (e como consequência, a microescala tem inércia térmica desprezível); ou (b) a geração de calor é homogênea na microescala. Por outro lado, o modelo proposto nesta tese, o qual é desenvolvido utilizando uma descrição variacional do problema, pode ser aplicado a elementos de volume representativos finitos e em condições em que a geração de calor é heterogênea na microescala. A discretização temporal (diferenças finitas) e as discretizações espaciais na microescala e na macroescala (método dos elementos finitos) são apresentadas em detalhes, juntamente com os algoritmos necessários para implementar a solução do problema. Nesta tese são apresentados casos numéricos simples, procurando verificar não só o modelo teórico multiescala desenvolvido, mas também a implementação feita. Para tanto, são analisados, por exemplo, (a) casos em que considera-se a microescala um material homogêneo, tornando possível a comparação da solução multiescala com a solução convencional (uma única escala) pelo método dos elementos finitos, e (b) um caso em um material heterogêneo para o qual a solução completa, isto é, modelando diretamente os constituintes no corpo macroscópico, é obtida, tornando possível a comparação com a solução multiescala. A solução na microescala para vários casos analisados nesta tese sofre grande influência da inércia térmica da microescala. Para demonstrar o potencial de aplicação do modelo multiescala, simula-se a cura de um elastômero carregado com negro de fumo. Embora a simulação demonstre que a inércia térmica não precise ser considerada para esse caso em particular, a aplicação da presente metodologia torna possível modelar a cura do elastômero diretamente sobre a microescala, uma abordagem até então não utilizada no contexto de métodos multiescala. Essa metodologia abre a possibilidade para futuros aperfeiçoamentos da modelagem do estado de cura. / This work deals with the modeling of transient heat conduction with heat generation in heterogeneous media, and its objective is to develop a proper multiscale model for this phenomenon. There already exist multiscale models in the literature related to this proposed problem, and which are valid for the following cases: (a) the representative volume element has a negligible size when compared to the characteristic macroscopic size (and, as a consequence, the microscale has a negligible thermal inertia); or (b) the heat generation is homogeneous at the microscale. On the other hand, the model proposed in this thesis, which is developed using a variational description of the problem, can be applied to finite representative volume elements and in conditions in which the heat generation is heterogeneous at the microscale. The time discretization (finite difference) and the space discretizations at both the microscale and the macroscale (finite element method) are presented in details, together with the algorithms needed for implementing the solution of the problem. In this thesis, simple numerical cases are presented, aiming to verify not only the theoretical multiscale model developed, but also its implementation. For this, it is analyzed, for instance, (a) cases in which the microscale is taken as a homogeneous material, making it possible the comparison of the multiscale solution with the conventional solution (one single scale) by the finite element method, and (b) a case in a heterogeneous material for which the full solution, that is, modeling all constituents directly on the macroscale, is obtained, making it possible the comparison with the multiscale solution. The solution at the microscale for several cases analyzed in this thesis suffers a large influence of the microscale thermal inertia. To demonstrate the application potential of the multiscale model, the cure of a carbon black loaded elastomer is simulated. Although the simulation shows that the thermal inertia does not have to be considered for this case in particular, the application of the present methodology makes it possible to model the cure of the elastomer directly at the microscale, an approach not used in multiscale methods context until now. This methodology opens the possibility for future improvements of the state of cure modeling.

The focus of the world is on the reduction of greenhouse gases, such as carbon dioxide, which contribute to the global warming currently experienced. Because most of the carbon dioxide emitted into the atmosphere is from fossil fuel combustion, alternative energy sources were developed and others are currently under study to see whether they will be good alternatives. One of these alternative sources of energy is the combustion of wood instead of coal. The advantages of wood are that it is a neutral carbon fuel source and that currently installed infrastructure used to combust coal can be retrofitted to combust wood or a mixture of wood and coal in an attempt to reduce the carbon dioxide emissions. Spent nuclear fuel has to be cooled so that the decay heat generated does not melt the containment system, which could lead to the unintentional release of radioactive material to the surroundings. The heat transfer mechanisms involved in the cooling have historically been analysed by assuming that the fluid and solid phases are at local thermal equilibrium (LTE) in order to simplify the analysis. The exergy destruction of the combustion of pine wood in an adiabatic combustor was investigated in this thesis using analytical and computational methods. The exergy destruction of the combustion process was analysed by means of the second law efficiency, which is the ratio of the maximum work that can be achieved by a Carnot engine extracting heat from the combustor, and the optimum work of the combustor. This was done for theoretical air combustion and various excess air combustions, with varied inlet temperatures of the incoming air. It was found that the second law efficiency reached an expected maximum for theoretical air combustion, and this held true for all varying air inlet temperatures. However, it was found that as the air inlet temperature was increased more and more, the maximum second law efficiency was the same for all excess air combustions, including the theoretical air combustion. It was also found that the results of the analytical and commercial computational fluid dynamics code compared well. Another analysis was conducted of irreversibilities generated due to combustion in an adiabatic combustor burning wood. This was done for a reactant mixture varying from a rich to a lean mixture. A non-adiabatic non-premixed combustion model of a numerical code was used to simulate the combustion process where the solid fuel was modelled by using the ultimate analysis data. The entropy generation rates due to the combustion and frictional pressure drop processes were computed to eventually arrive at the irreversibilities generated. It was found that the entropy generation rate due to frictional pressure drop was negligible when compared with that due to combustion. It was also found that a minimum in irreversibilities generated was achieved when the air-fuel mass ratio was 4.9, which corresponded to an equivalent ratio of 1.64, which was lower than the respective air-fuel mass ratio and equivalent ratio for complete combustion with theoretical amount of air of 8.02 and 1. Studieswere conducted to numerically analyse irreversibilities generated due to combustion in an adiabatic combustor burning wood. The first study analysed the effect of changing the incoming air temperature from 298 K to 400 K. The second study analysed the effect of changing the wall condition of the combustor from adiabatic to negative heat flux (that is heat leaving the system) for an incoming air temperature of 400 K. The irreversibilities generated in the combustor were calculated by computing the entropy generation rates due to the combustion, heat transfer and frictional pressure drop processes. For the first part of the study, it was found that for the minimum irreversibilities generated in the adiabatic combustor, the optimal air-fuel ratio (AF) corresponding to minimum irreversibilities slightly reduced from 4.9 to 4.8. In the second part of the study, it was found that by changing the wall condition from adiabatic to heat flux on the combustor, the AF corresponding to the minimum irreversibilities increased from 4.8 to 6. For the third part of the study, the combustor with a heat flux wall condition and a wall thickness simulated at an AF of 6, the sum of twice the wall thickness and the optimum diameter always added up to 0.32 m, resulting in the minimum irreversibilities. An analytical model was developed to minimise the thermal resistance of an air-cooled porous matrix made up of solid spheres with internal heat generation. This was done under the assumption of LTE. It was found that the predicted optimum sphere diameter and the minimum thermal resistance were both robust in that they were independent of the heat generation rate of the solid spheres. Results from the analytical model were compared with those from a commercial numerical porous model using liquid water and air for the fluid phase, and wood and silica for the solid phase. The magnitudes of the minima of both the temperature difference and the thermal resistance seemed to be due to equal contribution from the thermal conduction heat transfer inside the solid spheres and heat transfer in the porous medium. Because the commercial numerical porous model modelled only the heat transfer occurring in the porous medium, it expectedly predicted half of the magnitudes of the minima of the temperature difference and thermal resistance of those by the analytical model. / Thesis (PhD)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Yes / In this study, the thermal performance analysis of porous fin with temperature-dependent thermal conductivity and internal heat generation is carried out using Haar wavelet collocation method. The effects of various parameters on the thermal characteristics of the porous fin are investigated. It is found that as the porosity increases, the rate of heat transfer from the fin increases and the thermal performance of the porous fin increases. The numerical solutions by the Haar wavelet collocation method are in good agreement with the standard numerical solutions.

An investigation was conducted to determine the response of landfills to the operation of a vertical ground source heat pump (i.e., heat extraction system, HES). Elevated landfill temperatures, reported various researchers, impact the engineering performance of landfill systems. A numerical model was developed to analyze the influence of vertical HES operation on landfills as a function of climate and operational conditions. A 1-D model of the vertical profile of a landfill was developed to approximate fluid temperatures in the HES. A 2-D model was then analyzed over a 40 year time period using the approximate fluid temperatures to determine the heat flux applied by the HES and resulting landfill temperatures. Vertical HES configurations simulations consisted of 15 simulations varying 5 fluid velocities and 3 pipe sizes. Operational simulations consisted of 26 parametric evaluations of waste placement, waste height, waste filling rate, vertical landfill expansions, HES placement time, climate, and waste heating. Vertical HES operation in a landfill environment was determined to have 3 phases: heat extraction phase, transitional phase, and ground source heat pump phase. During the heat extraction phase, the heat extraction rate ranged from 0 to 2550, 310 to 3080, and 0 to 530 W for the first year, peak year, and last year of HES operation, respectively. The maximum total heat energy extracted during the heat extraction phase ranged from 163,000 to 1,400,000 MJ. The maximum difference in baseline landfill temperatures and temperatures 0 m away from the HES ranged from 5.2 to 43.2°C. Climate was determined to be the most significant factor impacting the vertical HES. Trends pertaining to performance of numerous variables (fluid velocity, pipe size, waste placement, waste height, waste filling rate, vertical landfill expansions, HES placement time, climate, and waste heating) were determined during this investigation. Increasing fluid velocity until turbulent flow was reached increased the heat extraction rate by the system. Once turbulent flow was reached, the increase in heat extraction rate with increasing fluid velocity was negligible. An increase in the heat extraction rate was caused by increasing pipe diameter. Wastes placed in warmer months caused an increase in the total heat energy extracted. Increasing waste height caused an increase in the peak heat extraction rate by 43 W/m waste height. Optimum heat extraction per 1 m of HES occurred for a 30 m waste height. Increasing the waste filling rate increased the total heat energy extracted. Heat extraction rates decreased as time between vertical landfill expansions increase. Total heat energy extracted over a 35 year period decreased by approximately 21,500 MJ/year for every year after the final cover was placed until HES operation began. For seasonal HES operation, the total heat energy obtained each year differs and the fourth year of operation yielded the most energy. Wet Climates with higher heat generating capacities yielded increased heat extraction rates. Maximum temperature differences in the landfill due to the HES increased by 16.6°C for every 1 W/m3 increase in peak heat generation rate. When a vertical HES was used for waste heating, up to a 13.7% increase in methane production was predicted. Engineering considerations (spacing, financial impact, and effect on gas production) for implementing a vertical HES in a landfill were investigated. Spacing requirements between the wells were dependent on maximum temperature differences in the landfill. Spacing requirements of 12, 12, 16, and 22 m are recommended for waste heating, winter-only HES operation, maximum temperature differences in the landfill less than 17°C, and maximum temperature differences in the landfill greater than 17°C, respectively. A financial analysis was conducted on the cost of implementing a single vertical HES well. The energy extracted per cost ranged from 0.227 to 0.150 $/MJ for a 50.8 mm pipe with a 1.0 m/s fluid velocity and a 50.8 mm pipe with a 0.3 m/s fluid velocity, respectively. A vertical HES could potentially increase revenue from a typical landfill gas energy project by $577,000 per year.

Ce travail de recherche traite de l’optimisation et de la modélisation de nanofibres obtenues par filage électrostatique pour des applications techniques en tant que, a) cellules résistives pour génération de chaleur et b) couche ultra filtrante pour système de protection respiratoire. Afin d’intégrer ces matériaux nanofibreux aux applications visées, deux procédés de production ont été mis en oeuvre, à savoir mono-aiguille et multijets. Une étude statistique a été réalisée pour modéliser et optimiser les non-tissés de nanofibres pour des productions à échelle laboratoire et semi-industrielle. L’outil statistique c’est révélé pertinent pour anticiper la morphologie des matériaux produits et assurer une homogénéité optimale. Les non-tissés présentant les caractéristiques morphologiques souhaitées ont été sélectionnés, testés, et les résultats ont révélé leur fort potentiel pour les champs applicatifs visés. Ces travaux valident deux pistes de recherche qui déboucheraient sur une intégration concrète de ces matériaux innovants. / Optimization and modelling of electrospun nanofibrous nonwovens and their technical applications, i-e heat generation and respiratory protection, were studied in this work. For utilization in these applications, nanowebs were statistically modelled and optimized using different electrospinning techniques i-e needle and needleless setups based on significance of these techniques for lab and bulk scale production of nanowebs. Moreover, quantitative impact of different electrospinning parameters was also observed. Statistical analysis was found to be a useful tool for study of electrospinning process and production of nanowebs with minimum defects. The optimized nanowebs were used for selected applications and based on results it was concluded that they can be a potential material for both, heat generation and respiratory protection. These observations are expected to initiate more focused studies in both the fields.

<p>This project deals with the potential of membrane stratified solar ponds which consist of two water layers, where one is a salt solution here, and a separating translucent membrane. An experimental pond was set up to study the thermal behaviour of such collector systems. The input is mainly solar radiation, sometimes when the ambient temperatures are higher than the pond temperatures also heat from the environment is transferred into the pond.</p><p>The measured temperatures of the pond, the ambient temperature, the global radiation and wind speed were the basis data for thermal calculations which showed that the pond was working well as a solar collector and thermal storage system all in one. Heat was not extracted from the pond however, only the losses to the environment were studied.</p><p>It was found out that the pond temperatures were higher than the ambient temperature over the whole measurement period of 12 days, and insulation and pollution problems as well as future prospects and suggestions for further studies are discussed at the end of this paper.</p>

This project deals with the potential of membrane stratified solar ponds which consist of two water layers, where one is a salt solution here, and a separating translucent membrane. An experimental pond was set up to study the thermal behaviour of such collector systems. The input is mainly solar radiation, sometimes when the ambient temperatures are higher than the pond temperatures also heat from the environment is transferred into the pond. The measured temperatures of the pond, the ambient temperature, the global radiation and wind speed were the basis data for thermal calculations which showed that the pond was working well as a solar collector and thermal storage system all in one. Heat was not extracted from the pond however, only the losses to the environment were studied. It was found out that the pond temperatures were higher than the ambient temperature over the whole measurement period of 12 days, and insulation and pollution problems as well as future prospects and suggestions for further studies are discussed at the end of this paper.

Fernwärme ist aufgrund der verstärkten Förderung der Bundesregierung in den letzten Jahren wieder stärker in den Fokus der Energiewirtschaft gerückt. In Hinblick auf einen wirtschaftlich und energetisch effizienten Betrieb von Fernwärmesystemen ist nicht nur der Einsatz der Wärmeerzeuger mittels einer Einsatzplanung sorgfältig zu organisieren, sondern auch das Betriebsverhalten des Wärmeverteilnetzes selbst zu berücksichtigen. So führen ständig auftretende Änderungen der thermischen Last, der Vorlauftemperatur am Einspeisepunkt oder der Rücklauftemperaturen der Abnehmer zu instationären Betriebszuständen im Fernwärmenetz. Die damit verbundene zeitliche Entkopplung der zentralen Wärmeeinspeisung von der dezentralen Wärmeentnahme induziert eine Speicherung thermischer Energie in dem in den Rohrleitungen befindlichen Wasser und in den Rohren selbst. In den seltensten Fällen wird dieser Vorgang der Wärmespeicherung aktiv genutzt, er tritt vielmehr als eher unerwünschter Nebeneffekt auf. Bei Kenntnis der thermodynamischen Zusammenhänge hingegen lässt sich das Fernwärmenetz durch zielgerichtete Steuerung der Vorlauftemperatur analog einem thermischen Heißwasserspeicher als Wärmespeicher nutzen. Dies ist wiederum für die Betriebs- und Gewinnoptimierung interessant, da so weitere Speicherkapazitäten nutzbar gemacht werden können, um Lastspitzen zu verschieben und KWK-Anlagen zu betreiben ohne zusätzliche Investitionen tätigen zu müssen. In dieser Arbeit wird eine Methode vorgestellt, in der die Netzspeicherleistung mit Hilfe eines thermo-hydraulischen Simulationsprogrammes bestimmt wird, wobei dynamische Effekte wie die veränderliche Vorlauftemperaturen am Einspeisepunkt oder die stark schwankenden Lastanforderungen der Abnehmer in der Berechnungsmethodik realitätsnah abgebildet werden. Für eine aktive Nutzung des Fernwärmenetzes als Wärmespeicher muss der Verlauf der Vorlauftemperatur an die jeweilig gewünschte Netzspeicherleistung angepasst werden. Dazu wird in dieser Arbeit ein vereinfachter Funktionsansatz für die Netzspeicherleistung bestimmt, der zunächst mittels Regressionsanalyse aus den Ergebnissen gezielter thermo-hydraulischer Simulationen ermittelt und anschließend so in eine Einsatzplanung integriert wird, sodass die Vorlauftemperatur als zu optimierende Variable in die Gesamtoptimierung einfließt. Anhand eines realen Anwendungsfalls werden die Möglichkeiten und Grenzen der aktiven Nutzung der Speicherfähigkeit von Fernwärmenetzen aufgezeigt. Insbesondere werden monetäre Gewinnpotentiale bei Anwendung der aktiv gesteuerten Netzspeicherung in durch KWK-Anlagen versorgten Fernwärmenetzen bei gleichzeitigem Stromhandel auf dem Spotmarkt abgeschätzt.

Fernwärme ist aufgrund der verstärkten Förderung der Bundesregierung in den letzten Jahren wieder stärker in den Fokus der Energiewirtschaft gerückt. In Hinblick auf einen wirtschaftlich und energetisch effizienten Betrieb von Fernwärmesystemen ist nicht nur der Einsatz der Wärmeerzeuger mittels einer Einsatzplanung sorgfältig zu organisieren, sondern auch das Betriebsverhalten des Wärmeverteilnetzes selbst zu berücksichtigen. So führen ständig auftretende Änderungen der thermischen Last, der Vorlauftemperatur am Einspeisepunkt oder der Rücklauftemperaturen der Abnehmer zu instationären Betriebszuständen im Fernwärmenetz. Die damit verbundene zeitliche Entkopplung der zentralen Wärmeeinspeisung von der dezentralen Wärmeentnahme induziert eine Speicherung thermischer Energie in dem in den Rohrleitungen befindlichen Wasser und in den Rohren selbst. In den seltensten Fällen wird dieser Vorgang der Wärmespeicherung aktiv genutzt, er tritt vielmehr als eher unerwünschter Nebeneffekt auf. Bei Kenntnis der thermodynamischen Zusammenhänge hingegen lässt sich das Fernwärmenetz durch zielgerichtete Steuerung der Vorlauftemperatur analog einem thermischen Heißwasserspeicher als Wärmespeicher nutzen. Dies ist wiederum für die Betriebs- und Gewinnoptimierung interessant, da so weitere Speicherkapazitäten nutzbar gemacht werden können, um Lastspitzen zu verschieben und KWK-Anlagen zu betreiben ohne zusätzliche Investitionen tätigen zu müssen. In dieser Arbeit wird eine Methode vorgestellt, in der die Netzspeicherleistung mit Hilfe eines thermo-hydraulischen Simulationsprogrammes bestimmt wird, wobei dynamische Effekte wie die veränderliche Vorlauftemperaturen am Einspeisepunkt oder die stark schwankenden Lastanforderungen der Abnehmer in der Berechnungsmethodik realitätsnah abgebildet werden. Für eine aktive Nutzung des Fernwärmenetzes als Wärmespeicher muss der Verlauf der Vorlauftemperatur an die jeweilig gewünschte Netzspeicherleistung angepasst werden. Dazu wird in dieser Arbeit ein vereinfachter Funktionsansatz für die Netzspeicherleistung bestimmt, der zunächst mittels Regressionsanalyse aus den Ergebnissen gezielter thermo-hydraulischer Simulationen ermittelt und anschließend so in eine Einsatzplanung integriert wird, sodass die Vorlauftemperatur als zu optimierende Variable in die Gesamtoptimierung einfließt. Anhand eines realen Anwendungsfalls werden die Möglichkeiten und Grenzen der aktiven Nutzung der Speicherfähigkeit von Fernwärmenetzen aufgezeigt. Insbesondere werden monetäre Gewinnpotentiale bei Anwendung der aktiv gesteuerten Netzspeicherung in durch KWK-Anlagen versorgten Fernwärmenetzen bei gleichzeitigem Stromhandel auf dem Spotmarkt abgeschätzt.

Modern aircraft, military and commercial, rely extensively on hydraulic systems. However, there is great interest in the avionics community to replace hydraulic systems with electric systems. There are physical challenges to replacing hydraulic actuators with electromechanical actuators (EMAs), especially for flight control surface actuation. These include dynamic heat generation and power management. Simulation is seen as a powerful tool in making the transition to all-electric aircraft by predicting the dynamic heat generated and the power flow in the EMA. Chapter 2 of this dissertation describes the nonlinear, lumped-element, integrated modeling of a permanent magnet (PM) motor used in an EMA. This model is capable of representing transient dynamics of an EMA, mechanically, electrically, and thermally. Inductance is a primary parameter that links the electrical and mechanical domains and, therefore, is of critical importance to the modeling of the whole EMA. In the dynamic mode of operation of an EMA, the inductances are quite nonlinear. Chapter 3 details the careful analysis of the inductances from finite element software and the mathematical modeling of these inductances for use in the overall EMA model. Chapter 4 covers the design and verification of a nonlinear, transient simulation model of a two-step synchronous generator with three-phase rectifiers. Simulation results are shown