Influence of step discontinuity in porous plate temperature in free convection with transpiration

A simple finite difference scheme is used to solve the second order, simultaneous, non-linear partial differential equations with a nonhomogeneous boundary condition, to study the influence of a step discontinuity in the surface temperature on heat transfer from a vertical porous plate in free convection with uniform transpiration. The numerical procedure is described in detail and results are presented forPr =0.72 in the blowing parameter range, - 1.5 < ©₁ < 1.5, for non-dimensional temperature range of 0.25 to 2.0.When the downstream section is cooler than the upstream section, an equilibrium point is defined, which moves towards the discontinuity on suction and away from it on blowing. The readjustment of the boundary layer is slow, causing the recovery length to be considerable.

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Einfluß einer stufenweisen Temperaturänderung in einer porösen Platte mit freier Konvektion und Verdunstung

Zusammenfassung Es wurde ein einfaches Finite-Differenzen-Verfahren verwendet, um ein System gekoppelter, nicht-linearer, partieller Differentialgleichungen 2. Ordnung mit nicht-homogenen Randbedingungen zu lösen und den Einfluß einer stufenförmigen Diskontinuität in der Oberflächentemperatur einer vertikalen, porösen Platte auf den Wärmeübergang bei freier Konvektion mit gleichmäßiger Verdunstung zu studieren. Das numerische Verfahren wird im Detail beschrieben und Ergebnisse werden fürPr=0,72, sowie für einen Anblasparameter zwischen –1,5 und +1,5 und für eine dimensionslose Temperatur von 0,25 bis 2, 0 mitgeteilt.Wenn der stromabwärts gelegene Abschnitt kälter ist als der Bereich stromaufwärts, kann ein Gleichgewichtspunkt definiert werden, der sich bei Saugzug in Richtung der Diskontinuität bewegt und von ihr weg beim Anblasen. Das Wiederausbilden der Grenzschicht erfolgt langsam und verursacht eine erhebliche Einstell-Länge.

     

Acceleration-Performance Optimization for Motors With Large Air Gaps

This paper presents an optimization-procedure yielding for minimal acceleration times for different speed ranges using the example of a magnetically levitated slice motor with a large air gap. The optimization is based on a set of analytical equations together with selected 3-D finite element method simulations with the aim to optimize both the stator geometry and the number of drive turns. It is shown that the use of 3-D instead of 2-D simulation tools is obligatory for motors with large air gaps for achieving sufficient simulation accuracy. The relevant equations for the optimization are derived, and the accuracy of the proposed method is verified by measurements on a prototype system.     

One‐dimensional finite element grids based on a localized truncation error analysis

With the exponential increase in computing power, modelers of coastal and oceanic regions are capable of simulating larger domains with increased resolution. Typically, these models use graded meshes wherein the size of the elements can vary by orders of magnitude. However, with notably few exceptions, the graded meshes are generated using criteria that neither optimize placement of the node points nor properly incorporate the physics, as represented by discrete equations, underlying tidal flow and circulation to the mesh generation process. Consequently, the user of the model must heuristically adjust such meshes based on knowledge of local flow and topographical features—a rough and time consuming proposition at best. Herein, a localized truncation error analysis (LTEA) is proposed as a means to efficiently generate meshes that incorporate estimates of flow variables and their derivatives. In a one‐dimensional (1D) setting, three different LTEA‐based finite element grid generation methodologies are examined and compared with two common algorithms: the wavelength to Δx ratio criterion and the topographical length scale criterion. Errors are compared on a per node basis. It is shown that solutions based on LTEA meshes are, in general, more accurate (both locally and globally) and more efficient. In addition, the study shows that the first four terms of the ordered truncation error series are in direct competition and, subsequently, that the leading order term of the truncation error series is not necessarily the dominant term. Analyses and results from this 1D study lay the groundwork for developing an efficient mesh generating algorithm suitable for two‐dimensional (2D) models. Copyright © 2000 John Wiley & Sons, Ltd.     

Fission of237Np+n through a double humped fission barrier

The subthreshold fission cross section of neutron induced fission of²³⁷Np has been measured between 20 and 60 eV with the CBNM time-of-flight spectrometer. The resonances in this energy range have been analysed with a single level shape fitting computer program. The result is interpreted in the frame of Strutinsky's theory of nuclear deformation.     

Polymorphic modifications of barium metagermanate

Polymorphic transformations of BaGeO₃ were investigated by DTA and X-ray diffraction methods and the data obtained were compared with those previously published. BaGeO₃ undergoes a reversible phase transformation at 1100° from a low-temperature hexagonal polymorph to a medium-temperature polymorph having an unknown crystal structure, which undergoes a further reversible transformation at 1200° to a high-temperature orthorhombic polymorph found to be stable up to the melting temperature of 1287°. On cooling from the melt, the high-temperature form can be retained metastably at room temperature, but transforms to the low-temperature form at 900°.

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Zusammenfassung Polymorphe Umwandlungen von BaGeO₃ wurden unter Anwendung der DTA- und Röntgendiffraktionsmethoden untersucht und die erhaltenen Angaben mit den früher veröffentlichten verglichen. Bei 1100° vollzieht sich eine reversible Phasentransformation der BaGeO₃ von der hexagonalen Struktur bei niedrigen Temperaturen in eine Verbindung unbekannter Kristallstruktur bei mittleren Temperaturen, welche wiederum bei 1200° eine weitere reversible Umwandlung in eine orthorhombische Struktur erfuhr und sich bis zur Schmelztemperatur von 1287° als stabil erwies. Beim Abkühlen der Schmelze kann die Form bei hohen Temperaturen bei Zimmertemperatur in metastabilem Zustand erhalten bleiben, wird jedoch bei 900° in die Form bei niedrigen Temperaturen umgewandelt.

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Résumé On a étudié les transformations polymorphiques de BaGeO₃ par ATD et diffraction des rayons X et comparé les résultats avec ceux obtenus antérieurement. A 1100° C BaGeO₃ subit une transformation de phase réversible de la forme hexagonale stable à basse température à la forme stable à moyenne température, de structure cristalline inconnue, qui se transforme réversiblement à 1200° C en une phase orthorhombique qui reste stable jusqu'à 1287° C. Au refroidissement, la forme stable à haute température peut être maintenue à l'état métastable à la température ambiante, mais elle se transforme à 900° C en la forme stable à basse température.

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BaGeO₃ . BaGeO₃ 1100° - - 1200° , 1287°. - , - 900°.

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This investigation was supported by the Boris Kidri Foundation for Scientific Research     

Improving mains current quality for three-phase three-switch buck-type PWM rectifiers

Modulation schemes for three-phase three-switch buck-type pulsewidth modulation rectifiers where the switching state of one bridge leg is clamped within a /spl pi//3-wide interval of the mains period guarantee minimum switching losses as well as minimum input filter capacitor voltage ripple and minimum dc current ripple. However, as shown in this paper by a detailed analysis of the time behavior of the input filter capacitor voltages within a pulse period such modulation schemes are characterized by the occurrence of sliding intersections of the filter capacitor voltages which causes input current distortion. An advanced modulation scheme is proposed which prevents the input current distortion and allows it to maintain the optimum performance of conventional modulation schemes. The theoretical considerations are finally verified by measurements on a 5-kW hardware prototype.