Browsing by Author "Hofmann, Otto R."

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    Boussinesq approximation to compute the temperature and velocity distributions in glass melts
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Hofmann, Otto R.
    The objective of the paper is to discuss the basis of glass melt modelling. For the governing equations to compute the Joule heat, temperatures and velocities in glass melts, conditions and criteria have been derived which enable the use of simplified equations and material properties. The computed examples base on the properties of the typical mass glasses: float glass, green container glass, fibre glass (E-glass) and Pyrex-type borosilicate glass. The computations show that in future it will be necessary to get more knowledge about the cubic and temperature-dependent expansion coefficient of glass melts. The per volume element constant viscosity, thermal and electric conductivity cause inaccuracies of more than 5 % in numerical modelling. The proposal is made to name a similarity number of direct Joule heat production Staněk number.
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    Electromagnetic force in electric glass melting
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Hofmann, Otto R.
    Electromagnetic effects occur in electrically heated glass melts. Sometimes this fact causes disadvantages but it also offers the chance to influence the glass flow beneficially. A Lorentz force can be generated by a strong external magnetic field called "Fremdfeld" (foreign field). The Lorentz force in the "Eigenfeld" (eigenfield) that is caused by the magnetic field around the current density in the glass can be neglected. A specific Lorentz force in the "Eigenfeld of the electrode" occurs in electric glass melting using rod electrodes and results from the magnetic field around these electrodes. The numeric JENA-HLX code was employed to calculate the current density distribution for complex voltages and the temperature-dependent electric conductivity. The magnetic field was built up according to the Biot-Savart law. A first computer calculation shows that the Lorentz force will become the second driving force besides buoyancy near electrodes, provided electrode currents are about 800 A or higher. A second numeric trial dealt with a side-wall, a bottom and a top electrode in R-S-T connection. Here the most significant effect occurred at side-wall electrodes, where horizontal velocities increased. The third test was carried out to learn more about the Lorentz force in an electrically heated crucible. Here the most interesting effects were to observe when a Fremdfeld was applied to the electrically heated fluid. In modelling external horizontal and vertical magnetic fields, the resulting fluid flow depends on the mutual orientation and the phase shift between current density and magnetic field. For instance, the forced glass melt rotation around the electrodes is reversed if this phase shift changes from 0° to 180°. To sum up, the Lorentz force offers various opportunities to control the glass flow.
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    Importance of the Lorentz force in electrically heated glass melts
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Hofmann, Otto R.; Philipp, Gerd
    In all computations of the flow velocities and temperature distributions in electric glass melts the electromagnetic Lorentz force has been neglected. This force is caused by the interaction between electric current density and the magnetic flux density in the region around electrodes. For some examples of real glass melting furnaces the necessity of including the Lorentz force into flow modelling was demonstrated. Some problems, e.g. the thermal stability of the batch layer in furnaces with top electrodes and the residence-time distribution of tanks with horizontal electrodes, cannot be explained and interpreted realistically if this second driving force besides the buoyancy is neglected. Moreover, the method of physical modelling of the electrically heated glass melts remains doubtful as long as the similarity criteria also for the Lorentz force cannot be fulfilled.
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    Zur Berechnung der in der Elektroglasschmelze direkt freigesetzten Jouleschen Wärme
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1981) Hofmann, Otto R.; Hilbig, Gerhard
    Für die Projektierung bzw. Nachrechnung von direkt elektrisch beheizten Glasschmelzen ist die Kenntnis der Elektrodenströme, der Potential-, Stromdichte- und Leistungsdichteverteilungen im Glasbad erforderlich. Ihre Berechnung erfolgt durch die Lösung der elektrischen Potentialgleichung, wobei allgemein von einer lokal verteilten elektrischen Leitfähigkeit auszugehen ist. Nur für konstante Leitfähigkeit ergeben sich analytische Berechnungsmöglichkeiten für die oben genannten interessierenden Größen. Mit diesem Beitrag wird versucht, eine zusammenfassende Darstellung der Berechnungsmöglichkeiten bei der Modellierung der elektrischen Verhältnisse in Elektroschmelzen zu geben, wobei die Erörterung spezieller Fragen (z. B. zur Genauigkeit der analytisch errechneten Werte bei Anwendung des Spiegelungsverfahrens oder zur Lage der Spannungszeiger bei galvanisch getrennt einspeisenden Spannungen) noch aussteht. On the calculation of the Joule effect heat liberated in electric glassmelting For predicting or confirming the behavior of direct electric melting furnaces knowledge of the electrode current, potential distribution, current density and energy denshity distributions in the melt are all necessary. These parameters can be calculated by solving the electrical potential equation with a locally varying electrical conductivity. Only when conductivity is constant are analytical solutions possible. This paper summarizes the possibilities of obtaining the information required by modelhng the behavior of the electric furnace although some particular questions (such as the accuracy of analytical values obtained by the mirror Image method or the position of the voltage indicator) still remain. Calcul de la chaleur libérée dans la fönte de verre par effet Joule Pour la conception et le calcul des fontes de verre chauffees electriquement, il faut connaître les courants d'électrodes et la distribution du potentiel, des densités de courant et des puissances volumiques dans le bain de verre. Ce calcul s'effectue par la résolution d'une équation du potentiel électrique en partant en général d'une distribution locale des conductivités électriques. Il est possible de trouver des expressions analytiques des grandeurs citées ci-dessus seulement si la conductivité reste constante. On a tenté de donner une vue d'ensemble sur les differentes possibilités de calcul dans la représentation par un modèle des grandeurs électriques dans les fontes mais il reste encore à discuter certains points particuliers tels que la précision des valeurs calculées par simulation et la position des phases pour des alimentations séparées.