Search

In this work an optimization of a Rankine cycle, nested into a long-range intercity bus for the recovery of exhaust gas exergy is carried out. In order to optimize WHR-systems, it is of great importance to carry out a holistic optimization of the system in combination with all its subsystems and components. The applied working fluid is cyclopentane and the expander design is of scroll type. The main contributors to efficiency changes are the expansion machine and the heat exchangers. As can be seen in the literature, the process optimization is often carried out by assuming a constant efficiency of the expansion machine. If this is done, the relationship between the thermodynamic points of state applied to the in- and outlet of the expander and its efficiency is neglected. Thus, the full optimization potential of the process remains unused and leads to a non-realistic behaviour of the expander. In this paper a holistic approach to optimize the expansion machine and the thermodynamic process states is presented. The optimization is carried out by using accurate thermophysical properties of the working fluid. Part of this optimization is a novel approach to enable the scalability of the expander. The basis is a semiempirical model well-known in the literature. This model allows the prediction of the expander performance in a wide operating range. To enable the optimization of the sizing of the expander, the model parameters are scaled based on the geometry and selected characteristic numbers. A stationary optimization is carried out. Thanks to its reasonable computational effort of the modelling approaches, an application in transient optimizations is feasible in future works

Ziel dieser Arbeit war es, möglichst viele Aspekte der elektrochemischen Hochgeschwindigkeitsabscheidung von Zink aufzuklären, insbesondere den Reaktionsmechanismus. Dazu dienten rotierende Zylinderelektroden-Experimente und eine Versuchsmatrix (Variation von pH-Wert, Konzentration, Leitsalz, Rotationsgeschwindigkeit,…) wurde aufgestellt. Es wurde ein weiter Konzentrationsbereich von ZnSO4 untersucht und Modellsysteme mit einer Konzentration von 0.61 M definiert. Diese experimentellen Ergebnisse dienten der Einführung einer Strategie zur Aufklärung des Reaktionsmechanismus über die Modellierung. Durch die systematische Vorgehensweise in der gewählten Strategie erhielt man einen vollständiger Satz an elektrochemischen Parametern. Die Modellierung basiert auf einem „Multi Ion Continuum Approach“, der auch den Einfluss homogener Reaktionen berücksichtigt: „Multi Ion Transport and Reaction Model“ (MITReM). In der Interpretation der Hochgeschwindigkeitsabscheidung mit Hilfe von Experimenten und Simulationen lag der Schwerpunkt auf der auftretenden Alkalisierung in Elektrodennähe und der Gasblasenrührung bei stark negativen Potentialen durch die Wasserstoffproduktion. Die erhaltenen experimentellen Ergebnissen konnten mittels Modellierung mit verifiziert werden und es wurde eine Übereinstimmung der erstellten Theorie (pH-abhängiger Reaktionsmechanismus) mit den gemessenen Strom-Spannungskurven, Effizienzmessungen oder spektroskopischen Messungen gefunden., The aim of this work is the onvestigation of high-speed zinc deposition from aqueous solutions. For this purpose, a well considered experimental matrix was established in order to explore the characteristic features of the electrochemical system. As can be assumed, these characteristic of the system depend on the potential, the electrolyte composition (ZnSO4 concentration, supporting electrolyte concentration, pH value), the electrolyte convection due to the rotation speed of the cylinder electrode, the temperature, etc. The strategy was to start the investigation with simple systems and to proceed to more complex model systems of interest. This approach includes the investigation of all possible electrochemical reaction mechanisms. These experiments were performed using a Rotating Cylinder Electrode (RCE). In order to obtain the maximum amount of information, particularly on the occuring reaction mechanisms, simulations have been performed using a suitable mathematical model. This model is based on a multi-ion continuum approach. The employed simulation tool MITReM (Multi-Ion Transport and Reaction Model) even considers homogeneous reactions in addition to the electrode reactions. Through the applied simulation approach not only assumptions are possible, but a proof of the exact behaviour of the system can be made. The results obtained from modelling are in good agreement with experimental findings, therefore a good description by the choosen electrochemical parameters has been achieved. Besides the electrochemical parameters and the investigation of the mechanism, the main emphasis in the interpretation of high-speed zinc deposition by means of experiments and simulation lies in the observed alkalisation close to the surface of the zinc electrode, the gas-bubble stirring at negative potentials caused by hydrogen production, and the change of the surface structure during depostition.

Roland Scharf Graz, Univ., Dipl.-Arb., 2009

[no abstract]