Porous Media Thermoacoustic Stacks: Measurements and Models

The present research analyzes random porous thermoacoustic stack systems analytically, experimentally, and numerically with a primary objective to develop a comprehensive analytical porous media modeling for random porous (such as Reticulated Vitreous Carbon (RVC) foams) environment. Mathematical models are developed for flow, thermal, and energy fields within the random porous medium stack. The Darcy and Brinkman-Forchheimer-extended Darcy models are used for modeling the momentum equation and local thermal equilibrium assumption between the porous matrix and trapped fluid in the void space for energy equation. The expressions of temperature, energy flux density, and acoustic work absorbed or produced by a thermoacoustic device are compared with existing literature and observed good agreements. After obtaining the flow and thermal fields’ information, the present study examines the entropy generation distribution within the stack. One important item revealed in this study is that entropy generation inside the porous medium completely follows the trend of the imaginary part of Rott’s first function profile. Another major contribution of this research is to identify the location of maximum entropy generation which is identical to the location of maximum thermoacoustic heat and work transport. The expression of Nusselt number for steady flow cannot be used in oscillatory random porous medium because of the phase difference between the temperature gradient at the wall and the temperature difference between the wall and the space averaged temperature. The present research experimentally examines novel stack configuration by considering “alternating conducting and insulating materials” as stack in thermoacoustic devices. The objective of considering such stack arrangement is to reduce the conduction heat transfer loss from the hot end of the stack to the cold end, thereby increasing the performance of the stack. Eight different heterogeneous stack arrangements are studied in this research. The performance of the heterogeneous stack arrangement is compared with the typical homogeneous stacks. This research shows that heterogeneous stacks can be used in thermoacoustic devices particularly in small (millimeter) scale thermoacoustic devices. Numerically the present study investigates the influence of working fluid, geometric, and operating conditions on stack performance by solving the full Navier-Stokes, mass, energy equation, and equation of state

Récupération des déchets thermiques dans les usines d’aluminium à travers les machines Stirling

Abstract: Among all industrial sectors, the aluminum-production industry is one of the most energy intensive: most up-to-date processes require a quantity of electrical energy ranging between 11 and 15 MWh per ton of produced aluminum. Most of this energy is needed to sustain the electrolysis process that produces aluminum, namely, the Hall-Héroult process; however, a detailed analysis of a conventional aluminum production line demonstrates that almost half of the aforementioned energy is lost under the form of heat. The aluminum companies fund private research projects to find solutions that increase the energy efficiency of their smelters. This is accomplished by analyzing the thermal losses and the possibility of converting them into useful power. For instance, heat wasted in the aluminum plant can be the source for space or district heating. Another solution consists in introducing a heat engine that uses the thermal waste to generate electrical power. In both cases, a useful output (i.e. thermal or electrical power) is produced by means of energy that would be otherwise wasted. There is a thermal waste that has been widely analyzed in the scientific literature, namely the release of exhaust gases at high temperature from the aluminum plant to the environment. Several solutions to recover this form of energy loss have been proposed (for instance, the introduction of an organic Rankine engine); however, these technologies have not been implemented because of their high capital costs. This is the reason why other forms of thermal waste must be investigated. It is well known that there is a considerable temperature difference between the walls of the electrolytic cell (in which aluminum is produced) and the surrounding air; more precisely, the wall surface has a temperature between 200 C and 400 C, while the air and the surrounding walls are at ambient temperature. Therefore, heat transfer phenomena arise, such as natural convection and thermal radiation. Nowadays, technologies able to recover these forms of heat (as sidewall heat exchangers) are not sufficiently advanced; hence, further research in this field is needed. The main objective of this thesis is to study the recovery and the conversion of the electrolysis cell thermal wastes occurring at the pot sidewall. After a brief introduction, the literature review is presented. The review highlights the sources of thermal waste in the Hall-Héroult process; then the (few) technologies able to harvest them without affecting the safety of the smelter are listed. Therefore, it is proposed to recover the cell-sidewall heat wastes by thermal radiation (and, to a lesser extent, by conduction) and to convert them into useful power by means of the Stirling engine technology. The recovery of the thermal losses and their conversion are the main topics of two scientific papers presented in this thesis. Finally, in the last chapters of this document, this research project and future works are discussed.Parmi tous les secteurs industriels, celui de la production d’aluminium est l’un des plus importants consommateurs d’énergie : les processus les plus modernes nécessitent une quantité d’énergie électrique comprise entre 11 et 15 MW h par tonne d’aluminium produite. La plus grande partie de cette énergie est nécessaire pour réaliser le processus d’électrolyse qui produit de l’aluminium (c’est-à-dire, le procédé Hall-Héroult). La moitié de cette énergie est perdue sous forme de déchets thermiques. Tous les producteurs d’aluminium financent des projets de recherche privés pour trouver des solutions qui améliorent l’efficacité énergétique de leurs usines. Ceci est réalisé par l’analyse des sources de pertes thermiques et par la suite par la valorisation de ces pertes. Par exemple, la chaleur gaspillée dans l’usine d’aluminium peut être utilisée aux fins de chauffage urbain ou pour produire de la puissance électrique. Dans les deux cas, un effet utile (c’est-à-dire, l’énergie thermique ou électrique) est produit à travers une source de chaleur qui serait autrement perdue. Le déchet thermique qui a été largement analysé dans la littérature scientifique est la libération des gaz d’échappement à haute température depuis l’usine d’aluminium vers l’environnement. Plusieurs solutions pour récupérer cette forme d’énergie thermique ont été proposées. Mais ces solutions n’ont pas été appliquées en raison de leurs coûts d’investissement élevés. Ceci est la raison pour laquelle d’autres formes de récupération et de conversion des rejets thermiques doivent être étudiées. Il est bien connu qu’une grande différence de température se produit entre les parois de la cuve électrolytique (dans lesquelles l’aluminium est produit) et l’air ambiant ; plus précisément, la surface de paroi a une température comprise entre 200 ◦ C et 400 ◦ C, tandis que l’air est à la température ambiante. Par conséquent, des phénomènes de transfert de chaleur tels que la convection naturelle et le rayonnement thermique apparaissent. Aujourd’hui, les technologies en mesure de récupérer et de convertir cette forme de chaleur (comme par exemple, les échangeurs de chaleur installés sur la paroi latérale) ne sont pas suffisamment au point, donc davantage de recherche dans ce domaine est nécessaire. L’objectif de cette thèse est d’étudier la récupération des déchets thermiques et leur conversion en puissance utile. Après une brève introduction, une revue de la littérature existante est réalisée afin de mettre en évidence les sources de gaspillage thermique dans le procédé de Hall-Héroult et les technologies disponibles pour les récupérer et les valoriser en respectant les critères de sécurité de l’usine. Il est donc proposé de récupérer les déchets thermiques de la paroi de la cuve par rayonnement (et, dans une moindre mesure, par conduction) et de les convertir en puissance utile à travers des machines Stirling. La récupération des déchets et leur conversion ont été traitées par l’auteur dans deux articles scientifiques présentés dans cette thèse. Les derniers chapitres de la thèse sont dédiés à la discussion de ce projet de recherche et aux travaux futurs

Proceedings of the First International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Kruger Park, 8-10 April 2002.This lecture is a principle-based review of a growing body of fundamental work stimulated by multiple opportunities to optimize geometric form (shape, structure, configuration, rhythm, topology, architecture, geography) in systems for heat and fluid flow. Currents flow against resistances, and by generating entropy (irreversibility) they force the system global performance to levels lower than the theoretical limit. The system design is destined to remain imperfect because of constraints (finite sizes, costs, times). Improvements can be achieved by properly balancing the resistances, i.e., by spreading the imperfections through the system. Optimal spreading means to endow the system with geometric form. The system construction springs out of the constrained maximization of global performance. This 'constructal' design principle is reviewed by highlighting applications from heat transfer engineering. Several examples illustrate the optimized internal structure of convection cooled packages of electronics. The origin of optimal geometric features lies in the global effort to use every volume element to the maximum, i.e., to pack the element not only with the most heat generating components, but also with the most flow, in such a way that every fluid packet is effectively engaged in cooling. In flows that connect a point to a volume or an area, the resulting structure is a tree with high conductivity branches and low-conductivity interstices.tm201

High-frequency operation and miniaturization aspects of pulse-tube cryocoolers

Cryocoolers are small refrigerators capable of achieving useful refrigeration below 120 K. Recent developments in the field of high Tc superconductors spawned a wide range of applications such as terahertz sensors, SQUIDS, low noise amplifiers, filters for microwave applications and many more. These devices are typically, nondissipating and require a cryocooler delivering refrigeration power of about 10 mW operating at 80 K. The existing commercial closed loop cryocoolers are huge, less reliable and expensive. Several research groups have been investigating development of cryocoolers using microsystems technologies for on-chip cryocooling. Gas cycles which, can be broadly divided into recuperative (steady flow) and regenerative (oscillating flow) cycles are the only current means of reaching cryogenic temperature in a single stage. The aim of this thesis is to investigate miniaturization of regenerative cycles. Pulse-tube cryocoolers, a variation of the Stirling cycle (regenerative type), are a fairly recent development in cryocooler technology. The principal advantage of a pulse-tube refrigerator is that it has no cold moving parts in the refrigerator

Sustainable energy for a resilient future: proceedings of the 14th International Conference on Sustainable Energy Technologies

Volume I, 898 pages, ISBN 9780853583134 Energy Technologies & Renewables Session 1: Biofuels & Biomass Session 5: Building Energy Systems Session 9: Low-carbon/ Low-energy Technologies Session 13: Biomass Systems Session 16: Solar Energy Session 17: Biomass & Biofuels Session 20: Solar Energy Session 21: Solar Energy Session 22: Solar Energy Session 25: Building Energy Technologies Session 26: Solar Energy Session 29: Low-carbon/ Low-energy Technologies Session 32: Heat Pumps Session 33: Low-carbon/ Low-energy Technologies Session 36: Low-carbon/ Low-energy Technologies Poster Session A Poster Session B Poster Session C Poster Session E Volume II, 644 pages, ISBN 9780853583141 Energy Storage & Conversion Session 2: Heating and Cooling Systems Session 6: Heating and Cooling Systems Session 10: Ventilation and Air Conditioning Session 14: Smart and Responsive Buildings Session 18: Phase Change Materials Session 23: Smart and Responsive Buildings Session 30: Heating and Cooling System Session 34: Carbon Sequestration Poster Session A Poster Session C Poster Session D Policies & Management Session 4: Environmental Issues and the Public Session 8: Energy and Environment Security Session 12: Energy and Environment Policies Poster Session A Poster Session D Volume III, 642 pages, ISBN 9780853583158 Sustainable Cities & Environment Session 3: Sustainable and Resilient Cities Session 7: Energy Demand and Use Optimization Session 11: Energy Efficiency in Buildings Session 15: Green and Sustainable Buildings Session 19: Green Buildings and Materials Session 24: Energy Efficiency in Buildings Session 27: Energy Efficiency in Buildings Session 28: Energy Efficiency in Buildings Session 31: Energy Efficiency in Buildings Session 35: Energy Efficiency in Buildings Poster Session A Poster Session D Poster Session