Magnetocaloric materials with first-order magnetic (FOM) phase transitions are of interest as low-cost working materials in magnetic cycles. Hysteresis is a property associated with first order transitions, and is undesirable as it can reduce performance. Devices using FOMs in active magnetic refrigeration have shown performance comparable to more expensive second-order materials, so some degree of hysteresis appears to be acceptable; however, the amount of hysteresis that may be tolerated is still an unanswered question. Among the FOM, the family of MnP-based is one of the promising materials for magnetic heat pump applications near room temperature. The present study describes the experimental investigation of a single-layer MnFeP1-xSix active magnetic regenerator (AMR), under different test conditions and following a protocol of heating and cooling processes. The results for the FOM are compared with a Gd AMR that is experimentally tested following the same protocol, with the objective to study the irreversibilities associated with FOM. The experimental tests are performed in a PM I test apparatus at a fixed displaced volume of 5.09 cm3 and a fixed operating frequency of 1 Hz. The results indicated a significant impact of the hysteresis on the heating and cooling temperature span for FOM regenerator. For certain operating conditions, multiple points of equilibrium (MPE) exist for a fixed hot rejection temperature. It is shown that the existence of MPEs can affect the performance of an AMR significantly for certain operating conditions. The present work advances our understanding since the combined hysteresis and MPE are two significant features which can impact layered AMR performance using MnFeP1-xAsx FOM by systematic experimental testing. With this objective, three multilayer MnFeP1-xAsx FOM regenerator beds are experimentally characterized under a range of applied loads and rejection temperatures. Thermal performance and the impacts of MPE are evaluated via heating and cooling experiments where the rejection (hot side) temperature is varied in a range from 283 K to 300 K. With fixed operating conditions, we find multiple points of equilibrium for steady-state spans as a function of warm rejection temperature. The results indicate a significant impact of MPE on the heating and cooling temperature span for multilayer MnFeP1-xAsx FOM regenerator. Unlike single material FOM tests where MPEs tend to disappear as load is increased (or span reduced), with the layered AMRs, MPEs can be significantly even with small temperature span conditions. A third experimental study examines the performance of MnFeP1-xAsx multilayer active magnetic regenerators. Five different matrices are tested: (i) one with three layers; (ii) one with six layers; and (iii) three, eight layer regenerators where the layer thickness is varied. The tests are performed using a dual regenerator bespoke test apparatus based on nested Halbach permanent magnets (PM II test apparatus). Operating variables include displaced volume (3.8 - 12.65 cm3), operating frequency (0.5 - 0.8 Hz) and hot-side rejection temperature (293-313 K).The results are mainly reported in terms of zero net load temperature span as a function of rejection temperature; a few tests with non-zero applied load are also presented. A maximum temperature span of 32 K is found for an 8-layer regenerator, which is similar to a previous work performed with gadolinium in the same experimental apparatus. A 1D active magnetic regenerator model accounting for thermal and magnetic hysteresis is developed and compared to experimental data for both a Gd-based and MnFeP1-xSix based AMR. Magnetic and thermal hysteresis are quantified using measured data for magnetization and specific heat under isothermal and isofield warming and cooling processes. Hysteresis effects are then incorporated in the model as irreversible work and reduced adiabatic temperature change. Model results are compared to measured temperature spans for regenerators operating with different thermal loads. Simulated results for temperature span as a function of cooling power and rejection temperature show good agreement with experimental data. The irreversible work due to hysteresis is found to have a small impact on predicted spans, indicating that useful cooling power is well predicted using cyclic measurements of adiabatic temperature change. / Graduate

No description available.

This thesis deals with regenerative heat exchangers and their operational issues. The theoretical part explains rotary and fixed bed regenerators. Further it includes the most common issues we encounter in operation. Practical part presents a design of rotary regenerator which cools flue gas from wood chip boiler. Calculation contains stechiometry of flue gas and air, design of rotor, calculation of outlet temperatures, strength calculation, dynamic calculation and design of heat exchanger peripherals.

In dieser Arbeit wird eine Methode zur Leistungsoptimierung der Kolbenpfade von Stirling-Motoren entwickelt, die auf der Theorie der optimalen Steuerung beruht. Für die effiziente praktische Umsetzbarkeit ist dabei ein geringer numerischer Aufwand des eingesetzten thermodynamischen Modells entscheidend. In detaillierten Modellen von Stirling-Motoren resultiert ein Großteil des numerischen Aufwandes aus der Beschreibung des Regenerators, einem gasdurchströmten Kurzzeit-Wärmespeicher. Im ersten Teil der Arbeit wird der Fokus deshalb auf die Entwicklung eines effizienten Regeneratormodells gelegt. Hierbei wird ein neuartiger Ansatz gewählt, der sich aus der Perspektive der Endoreversiblen Thermodynamik ergibt: Der Regenerator wird als endoreversibles Teilsystem betrachtet, welches an zwei Kontaktpunkten durch irreversible Interaktionen mit den benachbarten Teilsystemen Gasteilchen, Entropie und Energie austauscht. Innere Irreversibilitäten des Regenerators werden als Entropiequellterme in die Modellierung einbezogen. Im zweiten Teil der Arbeit wird dann ein iterativer Optimierungsalgorithmus erarbeitet, der die Leistung von Stirling-Motoren unter periodischen Randbedingungen für eine vorgegebene Periodendauer maximieren kann. Der Algorithmus startet mit vorgegeben initialen Kolbenpfaden, die im Laufe der Iterationen graduell verschoben und so den optimalen Pfaden angenähert werden. Um diese graduelle Verschiebung zu bestimmen, muss in jedem Iterationsschritt neben dem Differentialgleichungssystem, das die Thermodynamik des Stirling-Motors beschreibt, ein konjugiertes Differentialgleichungssystem gelöst werden. Der erarbeitete Algorithmus nutzt dabei die Existenz eines Grenzzyklus des konjugierten Differentialgleichungssystems unter Zeitumkehr zu dessen Lösung für periodische Randbedingungen aus. Unter Verwendung des endoreversiblen Regeneratormodells wird mit diesem iterativen Optimierungsalgorithmus die Theorie der optimalen Steuerung erstmals für die Kolbenpfadoptimierung eines beispielhaften Stirling-Motors in α-Konfiguration eingesetzt. / In this thesis a method for power optimization of the piston paths of Stirling engines is developed, which is based on Optimal Control Theory. For the efficient practical feasibility of this task, low numerical effort of the utilized thermodynamic model is crucial. In detailed models of Stirling engines, a large part of the numerical effort results from the description of the regenerator, which is a short-time heat storage. Therefore, in the first part of this thesis the focus is on the development of an efficient regenerator model. Here, a novel ansatz is chosen which arises from the perspective of Endoreversible Thermodynamics: The regenerator is described as an endoreversible subsystem that has two contact points, at which it exchanges particles, entropy, and energy with the adjacent subsystems through irreversible interactions. Internal irreversibilities of the regenerator are included in the model as entropy source terms. In the second part of the thesis an iterative optimization algorithm is worked out, which can maximize the power output of Stirling engines under periodic boundary conditions for given cycle time. The algorithm starts with predefined initial piston paths, which are gradually shifted over the course of the iterations and thus approach the optimal paths. To determine this gradual shift, in every iteration not only the system of differential equations describing the thermodynamics of the Stirling engine needs to be solved, but also a conjugate system of differential equations. The algorithm here exploits the existence of a limit cycle of the conjugate system under time reversal to solve it for periodic boundary conditions. By means of the endoreversible regenerator model, with this iterative optimization algorithm Optimal Control Theory is applied for the first time to optimize the piston paths of an exemplary Stirling engine in α-configuration.

Energy conversion devices using solid-state magnetocaloric materials have the potential to reduce energy consumption and mitigate environmental pollutants. To overcome the limited magnetic entropy change of magnetocaloric materials, magnetic refrigeration devices typically use the active magnetic regenerator (AMR) cycle. AMR devices have demonstrated promising performance, however costs must be reduced for broad market penetration. Although the magnet cost is of greatest importance for commercialization, literature has decoupled magnet design from AMR optimization. And while multilayered regenerators can improve performance without increasing cost, a number of questions remain unanswered as a result of the prohibitive parameter space. This dissertation explores methods of improving AMR performance and decreasing cost both at the subsystem level, namely the magnetocaloric regenerator, fluid flow system and magnetic field source, and the device level by coupling the regenerator and magnet design problems in a cost optimization framework. To improve AMR performance, multilayered regenerators with second-order magnetocaloric materials are experimentally and numerically investigated, yielding insight on how individual layers behave and interact over a wide range of regenerator compositions and operating parameters. An efficient AMR modeling approach is presented where individual layers are treated as cascaded AMR elements, and simulations are in excellent agreement with experiments. Insights from the computationally efficient model are used to inform device modifications, and a no-load temperature span of 40 K is measured in close proximity to the simulated optimum; one of the highest in literature. To simultaneously decrease AMR costs, a permanent magnet optimization framework is explored that is conducive to nonlinear objectives and constraints. This is used to investigate the optimal design of permanent magnet structures with reduced rare-earth permanent magnet materials. The regenerator and magnet design problems are then coupled in a permanent magnet topology optimization to minimize the combined capital and operating costs of an AMR. The optimal magnetic field waveform and the optimal means of producing this waveform are simultaneously obtained. The lifetime ownership costs of the optimized AMR device are shown to be in the realm of existing entry-level cooling devices. The presented cost optimization framework is of interest to both scientists and engineers, and demonstrates the importance of fast AMR models in identifying system designs, regenerator compositions and operating regimes that increase AMR performance and decrease cost. / Graduate

Ce travail a été effectué dans le cadre d’études de la miniaturisation d’un cryo-réfrigérateur de type tube à gaz pulsé (TGP) et particulièrement pour mieux comprendre l’écoulement et le transfert de chaleur dans un régénérateur, l’élément clé du TGP.Nous présentons les études numérique et expérimentale du facteur de frottement et du nombre de Nusselt pour les écoulements stationnaires et continus à nombre de Reynolds modéré O(1 − 100) au sein d’un régénérateur micro-fabriqué. L’influence de la porosité et de la géométrie est étudiée. La micro-structure précisément contrôlée représente des canaux incurvés de largeur de 10, 20 et 40 μm et de profondeur de 100 à 300 μm qui forment un réseau de colonnes ayant des profiles de losanges ou sinusoïdaux. Les micro-canaux sont gravés sur un substrat de silicium par la technologie DRIE. Une technologie d’implantation de thermomètres à l’intérieur de la micro-structure de régénérateur a été développée et mise en œuvre. Les performances des micro-régénérateurs ont été étudiées selon deux approches : la première se base sur le rapport des pertes de charges dans l’écoulement et de l’efficacité du transfert thermique (NPH/NTU) ; la deuxième, sur le coefficient de transfert de chaleur globale proposé par Bejan. L’étude numérique de ces deux critères montre tout le potentiel des micro-structures proposées. / This research is done in the framework of miniaturisation of pulse tube cryocoolers studies and especially to gain a better understanding of the mass flow and heat transfert in the regenerator, which is a crucial component of these type of cryocoolers.In this work we present a numerical and experimental study of the Darcy-Weisbach friction factor and Nusselt number for a continuous and steady flow at moderate Reynolds number O(1−100) in a micro-machined regenerators. The influence of porosity from 40 to 80 % and of the geometry parameters are studied. Well-controlled microstructures represent convoluted channels of 10, 20 or 40 μm width and 100 or 300 μm depth generated by rhombic- or sinusoidal-shaped columns.The channels are etched in Silicon wafers using DRIE MEMS technology. The thermometers are integrated inside the regenerator’s micro-structure to measure the temperature evolution. The efficiency of the regenerators is estimated using two different approaches : the first, as a ratio of pressure drop losses and heat transfer efficiency (NPH/NTU) ; the second, as a volumetric heat transfer density coefficient proposed by Bejan. The numerical study of the efficiency shows theinterest of proposed micro-structures.

Stirling cryocoolers are required for a wide variety of applications, especially in military equipment, due to their small size, low weight, long lifetime and high reliability considering their efficiency. Thus, it is important to be able to investigate the operating performance of these coolers in the design stage. This study focuses on developing a computer program for simulating a Stirling cryocooler according to the second order analysis. The main consideration is to simulate thermodynamic, fluid dynamic and heat transfer behavior of Stirling cryocoolers. This goal is achieved by following the route of Urieli (1984), which was focused on Stirling cycle engines. In this research, a simulation for performance prediction of a Stirling cryocooler is performed. In addition to that, the effects of system parameters are investigated. This attempt helps to understand the real behavior of Stirling cryocoolers using porous regenerator material. Results implied that first order analysis methods give optimistic predictions where second order method provides more realistic data compared to first order methods. In addition to that, it is shown that regenerator porosity has positive effect on heat transfer characteristics while affecting flow friction negatively. As a conclusion, this study provides a clear understanding of loss mechanisms in a cryocooler. Performed numerical analysis can be used as a tool for investigation of effects of system parameters on overall performance.

L’objectif de ce travail est de développer un modèle multi-physique et multi-échelle de Régénérateur Magnétique Actif en vue d’optimiser le fonctionnement d’un système de réfrigération magnétique. Le modèle numérique développé lors de cette thèse est un modèle multi-physique et multi-échelle qui prend en compte trois phénomènes distincts (le magnétisme, la fluidique et le transfert de chaleur), chacun à une échelle différente (micro-échelle, mini-échelle et macro-échelle). Une étude expérimentale a été menée afin de déterminer les propriétés thermophysiques des matériaux magnétocaloriques et d’intégrer les résultats de ces mesures dans le modèle numérique. Le modèle a été validé par une comparaison avec des données expérimentales et les résultats obtenus ont montré une bonne corrélation entre les résultats du modèle et les mesures. Enfin, le modèle a été exploité par une analyse de sensibilité des paramètres en vue d’étudier le fonctionnement ainsi que les performances du système. Ce modèle permettra d’identifier une stratégie de conception optimale d’un Régénérateur Magnétique Actif afin de concevoir des systèmes de réfrigération magnétique performants. / The objective of this work is the developing of a multi-physics and multi-scale numerical model of an Active Magnetic Regenerator in order to optimize the operation of a magnetic refrigeration system. The numerical model developed in this thesis is a multi-physics and multi-scale model that takes into account simultaneously three distinct phenomena (magnetism, fluid flow and heat transfer), each on a different scale (micro-scale, mini-scale scale and macro-scale). An experimental study was conducted to determine the thermophysical properties of magnetocaloric materials and integrate the results of these measurements in the numerical model. The model has been validated by comparison with experimental data and the results showed a good correlation between the model results and measurements. Finally, the model was exploited by an analysis of parameter sensitivity allowing studying the operation and performance of the system. This model will identify an optimal design strategy of an Active Magnetic Regenerator in order to design high-performance magnetic refrigeration systems.

yes / Heavy petroleum industries, including the Fluid Catalytic Cracking (FCC) unit, are among some of the biggest contributors to global greenhouse gas (GHG) emissions. The FCC unit’s regenerator is where these emissions originate mostly, meaning the operation of FCC regenerators has come under scrutiny in recent years due to the global mitigation efforts against climate change, affecting both current operations and the future of the FCC unit. As a result, it is more important than ever to develop models that are accurate and reliable at predicting emissions of various greenhouse gases to keep up with new reporting guidelines that will help optimise the unit for increased coke conversion and lower operating costs. Part 1 of this paper was dedicated to reviewing the riser section of the FCC unit. Part 2 reviews traditional modelling methodologies used in modelling and simulating the FCC regenerator. Hydrodynamics and kinetics of the regenerator are discussed in terms of experimental data and modelling. Modelling of constitutive parts that are important to the FCC unit, such as gas–solid cyclones and catalyst transport lines, are also considered. This review then identifies areas where the current generation of models of the regenerator can be improved for the future. Parts 1 and 2 are such that a comprehensive review of the literature on modelling the FCC unit is presented, showing the guidance and framework followed in building models for the unit.

Les pulses tubes sont des cryoréfrigérateurs similaires aux machines Stirling mais sans pièce mobile à froid, ce qui diminue les vibrations et augmente leur fiabilité. Toutes ces caractéristiques sont les bienvenues pour les applications spatiales, un domaine où le poids et la taille de la machine sont critiques. C'est dans ce cadre qui s'insère la recherche sur la miniaturisation des pulses tubes ; pour le réussir, on peut diminuer le volume déplacé pendant un cycle de la machine, en augmentant simultanément la fréquence d'opération. Pour savoir quels sont les limitations à l'augmentation de fréquence, des simulations sur le comportement du régénérateur et l'inertance à très hautes fréquences ont été faits; une étude expérimentale sur les pertes thermiques dans le tube a été aussi élaborée; et, finalement, des prototypes de pulse tube ont été dimensionnés et caractérisés, l'un desquels satisfait les spécifications initialement données : 0.25 W à 120 K avec 20 W puissance mécanique, à 100 Hz. / Pulse Tubes are a kind of cryocoolers similar to Stirling refrigerators, apart from the cold mobile element, absent in the first, which lessens their exported vibrations and increases their reliability. Spatial applications seek these characteristics for the instruments embarked, along with small weight and size. These needs stimulate the research on pulse tube miniaturization; to achieve this reduction, one can decrease the swept volume per cycle, whilst increasing the frequency of operation. To understand the barriers to carry out very high frequency operation, simulations on the behavior of the regenerator and inertances were made, as well as an experimental study on the thermal losses of the expansion tube. Finally, several very high frequency pulse tube prototypes were built and benchmarked, one of which fulfills the requirements we had initially set : 0.25 W at 120 K, with 20 W of input power at 100 Hz.