Browsing by Author "Hofmann, Otto R."
Now showing 1 - 3 of 3
Results Per Page
Sort Options
- ItemBoussinesq 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.
- ItemElectromagnetic 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. Α 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. Α 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. Α first computer calculation shows that the Lorentz force will become the second driving force besides buoyancy near electrodes, provided electrode currents are about 800 Α or higher. Α 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.
- ItemImportance of the Lorentz force in electrically heated glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Hofmann, Otto R.; Philipp, GerdIn 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.