Conversion of a scroll compressor to an expander for organic Rankine Cycle: modeling and analysis / UOIT

The subject of this thesis is searching in unstructured peer-to-peer systems. Such systems have been used for a variety of different applications, including file-sharing, content distribution and video streaming. These applications have been very popular; they contribute to a large percentage of today's Internet traffic and their users typically number in the millions. By searching, we refer to the process of locating content stored by peers. Searching in unstructured peer-to-peer systems poses a challenge because of high churn: both the topology and the content stored by peers can change quickly as peers arrive and depart, while the network formed under this churn process can be arbitrary at any point in time. As a result, a search mechanism must operate without any a priori assumptions on this dynamic topology. Ideally, a search mechanism should be scalable: as, typically, peers have limited bandwidth, the traffic generated by queries should not grow significantly as the peer population increases. Moreover, a search mechanism should also be reliable: if certain content is in the system, searching should locate it with reasonable guarantees. These two goals can be conflicting, as generating more queries increases a mechanism's reliability but decreases its scalability. Hence, a fundamental question regarding searching in unstructured systems is whether a mechanism can exhibit both properties, despite the network's dynamic and arbitrary nature. In this thesis, we show this is indeed the case, by proposing a novel mechanism that is both scalable and reliable. This is shown under a mathematical model that captures the evolution of both network and content in an unstructured system, but is also verified through simulations. To the best of our knowledge, this is the first provably scalable and reliable search mechanism for unstructured peer-to-peer systems. In addition to the above problem, we also consider a hybrid peer-to-peer system, in which the peer-to-peer network co-exists with a central server. The purpose of this hybrid architecture is to reduce the server's traffic by delegating part of it to its clients ---\emph{i.e.}, the peers: a peer wishing to retrieve certain content first propagates a query over the peer-to-peer network, and downloads the content from the server only if the query fails. This hybrid architecture can be used to partially decentralize a content distribution server, a search engine, an online encyclopedia, etc. The trade-off between scalability and reliability translates, in the hybrid case, to a trade-off between the peer and the server traffic loads. We propose a search mechanism under which both loads remain bounded as the peer population grows. This is surprising, and has an important implication: one can construct hybrid peer-to-peer systems that can handle traffic generated by a large (unbounded) peer population, even when both the server and peer bandwidth capacities are limited. Again, this is proved under a model capturing the hybrid system's dynamic nature and verified through simulations. To the best of our knowledge, our work is the first to show that hybrid systems with such properties exist.

We explore the relationships between the computational problem of recognizing expander graphs, and the problem of efficiently approximating proof length in the well-known system of \emph{resolution}. This program builds upon known connections between graph expansion and resolution lower bounds. A proof system $P$ is \emph{(quasi-)automatizable} if there is a search algorithm which finds a $P$-proof of a given formula $f$ in time (quasi)polynomial in the length of a shortest $P$-proof of $f$. It is open whether resolution is (quasi-)automatizable. We prove several conditional non-automatizability results for resolution modulo new conjectures concerning the complexity of identifying bipartite expander graphs. Our reductions use a natural family of formulas and exploit the well-known relationships between expansion and length of resolution proofs. Our hardness assumptions are unsupported; we survey known results as progress towards establishing their plausibility. The major contribution is a conditional hardness result for the quasi-automatizability of resolution.

We explore the relationships between the computational problem of recognizing expander graphs, and the problem of efficiently approximating proof length in the well-known system of \emph{resolution}. This program builds upon known connections between graph expansion and resolution lower bounds. A proof system $P$ is \emph{(quasi-)automatizable} if there is a search algorithm which finds a $P$-proof of a given formula $f$ in time (quasi)polynomial in the length of a shortest $P$-proof of $f$. It is open whether resolution is (quasi-)automatizable. We prove several conditional non-automatizability results for resolution modulo new conjectures concerning the complexity of identifying bipartite expander graphs. Our reductions use a natural family of formulas and exploit the well-known relationships between expansion and length of resolution proofs. Our hardness assumptions are unsupported; we survey known results as progress towards establishing their plausibility. The major contribution is a conditional hardness result for the quasi-automatizability of resolution.

The subject of this thesis is searching in unstructured peer-to-peer systems. Such systems have been used for a variety of different applications, including file-sharing, content distribution and video streaming. These applications have been very popular; they contribute to a large percentage of today's Internet traffic and their users typically number in the millions. By searching, we refer to the process of locating content stored by peers. Searching in unstructured peer-to-peer systems poses a challenge because of high churn: both the topology and the content stored by peers can change quickly as peers arrive and depart, while the network formed under this churn process can be arbitrary at any point in time. As a result, a search mechanism must operate without any a priori assumptions on this dynamic topology. Ideally, a search mechanism should be scalable: as, typically, peers have limited bandwidth, the traffic generated by queries should not grow significantly as the peer population increases. Moreover, a search mechanism should also be reliable: if certain content is in the system, searching should locate it with reasonable guarantees. These two goals can be conflicting, as generating more queries increases a mechanism's reliability but decreases its scalability. Hence, a fundamental question regarding searching in unstructured systems is whether a mechanism can exhibit both properties, despite the network's dynamic and arbitrary nature. In this thesis, we show this is indeed the case, by proposing a novel mechanism that is both scalable and reliable. This is shown under a mathematical model that captures the evolution of both network and content in an unstructured system, but is also verified through simulations. To the best of our knowledge, this is the first provably scalable and reliable search mechanism for unstructured peer-to-peer systems. In addition to the above problem, we also consider a hybrid peer-to-peer system, in which the peer-to-peer network co-exists with a central server. The purpose of this hybrid architecture is to reduce the server's traffic by delegating part of it to its clients ---\emph{i.e.}, the peers: a peer wishing to retrieve certain content first propagates a query over the peer-to-peer network, and downloads the content from the server only if the query fails. This hybrid architecture can be used to partially decentralize a content distribution server, a search engine, an online encyclopedia, etc. The trade-off between scalability and reliability translates, in the hybrid case, to a trade-off between the peer and the server traffic loads. We propose a search mechanism under which both loads remain bounded as the peer population grows. This is surprising, and has an important implication: one can construct hybrid peer-to-peer systems that can handle traffic generated by a large (unbounded) peer population, even when both the server and peer bandwidth capacities are limited. Again, this is proved under a model capturing the hybrid system's dynamic nature and verified through simulations. To the best of our knowledge, our work is the first to show that hybrid systems with such properties exist.

Une turbine diphasique est utilisée pour remplacer le détendeur classique des systèmes de réfrigération. Dans la turbine, une tuyère transforme l'enthalpie de l'écoulement en énergie cinétique. Remplacer la détente isenthalpique classique par une détente isentropique augmente le coefficient de performance d'un système de réfrigération d'une valeur qui peut aller jusqu'à 20 %, pour la même consommation électrique du compresseur. Pendant cette transformation, la pression statique de l'écoulement diminue et un changement de phase se produit au col de la tuyère. La détente d'un écoulement diphasique est étudiée analytiquement et simulée avec le logiciel de CFD Fluent 13. Un modèle d'évaporation adapté à la détente diphasique est ajouté à l'algorithme de dynamique de fluide dans Fluent 13, afin de créer et simuler le changement de phase à l'intérieur de la tuyère. Le modèle d'évaporation est basé sur la théorie de la nucléation et sur des résultats expérimentaux. Le modèle calcule le taux de nucléation hétérogène initiée par les cavités de la paroi intérieur de la tuyère, et le taux de nucléation hétérogène initiée par la présence de bulles de vapeur au cœur de l'écoulement. Outre les tuyères, la roue de la turbine est aussi conçue. La roue récupère l'énergie cinétique d'un écoulement diphasique et la transforme en un couple de moment. La géométrie des différents éléments de la roue est définie afin d'assurer l'intégrité mécanique et un bon rendement énergétique. La conception de la roue prend en considération le comportement de l'écoulement en contact avec les paliers de la roue, la perte de puissance due aux frottements aérauliques, ainsi que de nombreux autres facteurs. Fluent 13 est utilisé pour simuler l'écoulement diphasique à l'intérieur des augets de la roue, ainsi que pour calculer les pertes dues aux frottements aérauliques. Le rendement calculé de la turbine est vérifié avec des tests d'une turbine diphasique dans un groupe refroidisseur d'eau ayant une puissance de réfrigération de 700 kW. / A two-phase turbine used to replace the usual expander in a refrigeration system needs a nozzle/expander to transform the flow enthalpy into kinetic energy. Replacing the isenthalpic expansion by the isentropic one, increases the coefficient of performance of refrigeration system up to 20% for the same compressor input power. During this transformation, the static pressure of the flow decreases and a phase change occurs at the nozzle throat. The expanding and flashing flow in the convergent and divergent parts of a nozzle is analytically studied and then simulated using the CFD software Fluent 13. A separate evaporation model is added to the fluid dynamics algorithms in Fluent 13 in order to create the phase change inside the nozzle. The evaporation model is based on the classical theory of nucleation and on experimental results: it calculates the heterogeneous nucleation initiated by the wall cavities and the heterogeneous nucleation initiated by the presence of vapor bubbles in the flow. The turbine is composed of the designed nozzles and a rotor that recovers the kinetic energy of the two-phase flow and transforms it into torque. The geometry of the rotor components is designed to provide mechanical integrity and high efficiency. The design takes into consideration the flow behavior inside the rotor, the power loss due to drag force, and many other factors. Fluent 13 is used to study the flow inside the rotor buckets and to estimate the power loss due to drag force. The turbine calculated efficiency is verified by testing a two-phase turbine in a water chiller having a refrigeration capacity of 700 kW.

No description available.

In dieser Arbeit sollte im Tierversuch die lokale Gewebeverträglichkeit der neuen, mit PVVV 80/20 beschichteten, hoch hydrophilen Expander (osmed GmbH, Ilmenau, Deutschland) getestet werden und allgemeine Indikationen für die Anwendung von Hydrogelexpandern erstellt werden. Material und Methoden: Für den Versuch standen 24 Kaninchen als Versuchstiere zur Verfügung, welche in vier Gruppen unterteilt wurden, wobei die Gruppe 1 als Kontrollgruppe (Expander mit Silikonhülle) diente. Die Gruppen 2 - 4 wurden mit den neu entwickelten Hydrogelexpandern („Cylinder M4 x 12“) beschickt. Die Implantation der Expander erfolgte bei allen Versuchstieren in die linke Kniefalte. Nach einem zeitlich vorbestimmten Explantationsplan (nach 10, 21 und 90 Tagen) wurden die Expander entnommen, gewogen und das, den Expander direkt umgebene Gewebe entnommen, histologisch aufbereitet und untersucht. Zusätzlich wurden Proben aus der rechten, nicht implantierten Kniefalte, als Referenzprobe entnommen. Desweiteren wurde die Anwendbarkeit von Hydrogelexpandern anhand von zwei Fallbeispielen vorgestellt und untersucht. Ergebnisse: Histologisch konnte bei beiden Expandertypen eine lokale Fremdkörperreaktion (Entzündungszellinfiltration, Granulationsgewebe), im Sinne einer Kapselbildung, nachgewiesen werden. Die Alterationen des direkt angrenzenden Gewebes der beschichteten Expander sind insgesamt nur geringgradig stärker ausgeprägt als in den Proben mit Verwendung des Implantats mit Silikonhülle. Somit ist die Gewebeverträglichkeit der neu entwickelten Expander mit den silikonumhüllten Expandern vergleichbar. Diese Arbeit konnten weiterhin zeigen, dass die „Cylinder M4 x 12“-Expander mit der PVVV 80/20-Beschichtung ein deutlich höheres Quellverhalten aufweisen als die herkömmlichen, mit Silikonhülle umgebenen, Expander. Die in den Fallbeispielen verwendeten Hydrogelexpander, zeigten in beiden Fällen das gewünschte kosmetische Endergebnis. Schlussfolgerung: Für beide Expandertypen lässt sich eine gute lokale Verträglichkeit ableiten. Damit bieten die getesteten Expander „Cylinder M4 x 12“ mit der PVVV 80/20-Beschichtung die Möglichkeit, die Silikonhülle in ausgewählten Indikationen zu ersetzen und neue Einsatzmöglichkeiten zu erschließen. Zudem bieten die beschichteten Expander den Vorteil, dass es durch ihre minimale Größe nur einer kleinen Inzision für die Implantation bedarf und sie damit besonders für den Einsatz in der minimal invasiven Chirurgie geeignet erscheinen. Einsatzmöglichkeiten der Hydrogelexpander in der Veterinärmedizin sind unter medizinischen und ästhetischen Gesichtspunkten zu betrachten. Im Vordergrund der medizinischen Indikationen stehen vor allem Wunden mit erheblichem Gewebeverlust (z. Bsp. nach Traumata, Verbrennungen u.a.), bei denen eine plastische Operation keine spannungsarme Adaptation der Wundränder gewährleisten kann. Auch die kosmetische Indikation gewinnt immer mehr an Bedeutung. Möglich ist der Einsatz des Expanders zum Beispiel als Hodenimplantat oder als Dauerimplantat nach Enukleatio bulbi und zur Narbenkorrektur. Ihr klinischer Einsatz bedarf jedoch weiterer experimenteller – vor allem – in-vivo-Untersuchungen, um ihre Einsatzmöglichkeiten beim Tier - und in Zukunft auch beim Menschen – zu evaluieren.

In this thesis, an experimental research is conducted on scroll-based Organic Rankine Cycle (ORC) focusing on the expansion process. An important feature of the ORC is the ability to utilize low or moderate temperature heat sources derived from renewable energy such as concentrated solar radiation, biomass/biofuels combustion streams, geothermal heat and waste heat recovery. The ORC is more appropriate than steam Rankine cycle to generate power from low capacity heat sources (5-500 kW thermal). For example, expansion of superheated steam from 280oC/1000 kPa to a pressure corresponding to 35oC saturation requires a volume ratio as high as 86, whereas for the same operating conditions toluene shows an expansion ratio of 6 which can be achieved in a single stage turbine or expander. The objective of this work is to experimentally study the performance of a selected refrigeration scroll compressor operating in reverse as expander in an ORC. To this purpose, three experimental systems are designed, built and used for conducting a comprehensive experimental programme aimed at determining the features of the expansion process. In preliminary tests the working fluid utilized is dry air while the main experiments are done with the organic fluid R134a. Experimental data of the scroll expander are collected under different operating conditions. Power generation in various conditions is analyzed in order to determine the optimum performance parameters for the scroll expander. In addition, thermodynamic analysis of the system is conducted through energy and exergy efficiencies to study the system performance. Based on the experimental measurements, the optimum parameters for an ORC cycle operating with the Bitzer-based expander-generator unit are determined. The cycle energy and exergy efficiencies are found 5% and 30% respectively from a heat source of 120oC. / UOIT

No description available.