Über Zeitkonstanten von Kondensatoren

Zusammenfassung Bei hinreichend langer Trocknung und Bernsteinisolation lassen sich mit Luftkondensatoren Zeitkonstanten erreichen, die in der Größenordnung 10⁷ s bzw. 10³ s/pF liegen. Die physikalischen Ursachen der Entladung werden näher untersucht. Praktisch erfolgt die Entladung nur über die Bernsteinisolation; die Leitfähigkeit der Luft ist zu vernachlässigen. Maßgebend für die Entladung ist das Verhältnis Volumen zur Kapazität. Hohe Zeitkonstanten lassen sich erreichen, wenn das Verhältnis Volumen zur Kapazität klein ist. Kondensatoren mit sehr hohen Zeitkonstanten finden ihre Anwendung, z. B. wenn ein aufgeladener Kondensator als Vergleichsspannungsquelle dienen soll, bei der elektrischen Kurzzeitmèssung als Zeittransformator oder zur Messung sehr hoher Widerstände von der Größenordnung 10¹⁰ bis 10¹⁵ nach dem Kondensatorentladungsverfahren. Über eine weitere Anwendung wird demnächst berichtet werden.Die vorliegende Untersuchung wurde von Herrn Prof. Dr.-Ing. E. h. Dr.-Ing. W. Rogowski angeregt, dem ich für seine vielseitige Unterstützung zu großem Dank verpflichtet bin. Der Deutschen Forschungsgemeinschaft danke ich für die Mittel zur Durchführung der Arbeit.     

Optimisation models to enhance resilience in evacuation planning

We argue that the concepts of resilience in engineering science and robustness in mathematical optimisation are strongly related. Using evacuation planning as an example application, we demonstrate optimisation techniques to improve solution resilience. These include a direct modelling of the uncertainty for stochastic or robust optimisation as well as taking multiple objective functions into account.     

Integrated synthesis and extraction of short‐chain fatty acid esters by supercritical carbon dioxide

We developed an efficient, integrated reaction‐extraction process for the production of short‐chain fatty acid ethyl esters (FAEE) from milk fat, using carbon dioxide as the only processing solvent. FAEE were synthesized using a short‐chain fatty acid selective lipase. The expansion of the liquid mixture of reactants by dense carbon dioxide enhanced the apparent lipase selectivity. In situ extraction of FAEE by a continuous flow of supercritical carbon dioxide proved to increase the lipase production rate. When the integrated process was operated with alternated periods of synthesis and product removal, the overall selectivity for short‐chain FAEE increased as well, as a result of the combination of the selectivities of lipase and extraction solvent. A two‐fold increase of the lipase productivity was achieved at these conditions, compared to a single batch reaction. The developed process enables the synthesis and isolation of high‐value fatty acid derivatives from a natural source such as milk fat. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Prof. Dr. Dr. h.c. Karl Kratzl Emeritus

Ohne Zusammenfassung     

Maximizing efficiency in the transition to a coal-based economy

Energy is the lynchpin of modern society. Since the early 1970s, growing dependence on foreign energy sources, oil in particular, has constrained US independence in foreign policy, and at times, inhibited economic stability and growth. Addressing oil dependence is politically and economically complex. Proposed solutions are multifaceted with various objectives such as energy efficiency and resource substitution. One solution is the partial transition from an oil- to coal-based economy. A number of facts support this solution including vast coal reserves in the US and the relative price stability of coal. However, several roadblocks exist. These include uncertain recoverable reserves and the immaturity of “clean” coal technologies. This paper provides a first order analysis of the most efficient use of coal assuming the transition from oil to coal is desirable. Scenario analysis indicates two possible transition pathways: (1) bring the transportation sector onto the electric grid and (2) use coal-to-liquid fuels to directly power vehicles. The feasibility of each pathway is examined based on economic and environmental factors, among which are energy availability, affordability and efficiency, and environmental sustainability. Results indicate that partial transition of the transportation sector onto the electric grid offers the more viable solution for coal-based reduction of the US oil dependence.     

MELODY: A completely decentralized adaptive file system for handling real-time tasks in unpredictable environments

We describe the major features of the completely decentralized adaptive file system MELODY which was designed for realizing anintegrated system design for a distributed real-time system working in a hazardous and unpredictable environment. MELODY's adaptivity mechanisms are based on novel services rendered by the distributed operating system DRAGON SLAYER. The file system, in order to both meet real-time constraints and provide for high availability, allows for replicating, relocating, or deleting file copies. Such copies may also bepublic orprivate. At every site aLocal Task Scheduler tries to schedule the arriving critical tasks, based on the availability of resources at this site such that deadline failures are minimized. Depending on the deadline failure history, status changes as well as file replication, deletion, or relocation are analyzed and managed by the cooperatingLocal File Assigners. In order to analyze MELODY's real-time performance we report on simulation experiments in which its capability of minimizing deadline failures of time-critical tasks was compared to other file system models: an idealbest-case model, abaseline model with no file replication, a file system allowingonly for replication ofprivate copies, and a model which allows forreplication and relocation of public copies only. While the best-case is unrealistic for a distributed implementation, the other models embody only part of MELODY's mechanisms yet have the benefit of a considerably smaller communication overhead. We report on the distributed simulation results which unambiguously show MELODY's superior performance, in addition to the built-in sensitivity to changes in the environment. A DRAGON SLAYER/MELODY prototype has been completed in our labs in order to serve as a distributed real-time testbed in our future work with MELODY.This work was partially supported by IBM Endicott (research Agreement No. 6073-86) by the State of Michigan (IMR-87-146751), and by General Dynamics Land Systems (#DEY-605089).     

Model-based test case generation and prioritization: a systematic literature review

Software and Systems Modeling - Model-based test case generation (MB-TCG) and prioritization (MB-TCP) utilize models that represent the system under test (SUT) for test generation and...     

Hardness Loss of Plastic Mold Steels: Phenomenon in Injection Molding

The standard application-oriented test for abrasive wear of polymers on plastic mold steels is the platelet wear tester method in which two steel specimens form a thin wear slit. The glass fiber-reinforced polymer melt is injected through the wear slit, and the surface of both specimens is abraded. In the injection molding process, similar conditions often occur in thin-walled parts or film gates. Drastic loss of the hardness of a powder metallurgical steel (PM steel) is discovered after approx. 200 injection cycles while performing platelet wear tests. For experimentally analyzing the reason for the loss of steel hardness during injection molding, a new platelet wear testing apparatus is developed, which can be used to measure the increase in temperature inside the steel specimen while testing. The first results show and support the hypothesis that a temperature increase above the annealing temperature occurs inside the steel due to viscous dissipation at the steel surface. In cooperation with the company voestalpine BÖHLER Edelstahl & Co KG, tests on the PM steel in a dilatometer are performed to simulate the cycle-by-cycle heat pulses occurring in the injection molding process. With the results of those tests, it is possible to state a hypothesis for the reason of this hardness loss.     

Carbon Segregation in CoCrFeMnNi High-Entropy Alloy Driven by High-Pressure Torsion at Room and Cryogenic Temperatures

Herein, a CoCrFeMnNi high-entropy alloy with reduced Cr content and with the addition of 2 at% C interstitial is processed via high-pressure torsion (HPT) under 6.5 GPa by three turns at room and cryogenic temperatures. The microstructure is investigated by transmission electron microscopy (TEM) and atom probe tomography (APT). The results indicate that C atoms segregate at the boundaries of the nanograins in the sample processed at room temperature, while the sample processed at cryogenic temperature does not show any notable segregations of carbon.