
Elliptic and Parabolic Problems
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Haim Brezis has made significant contributions in the fields of partial differential equations and functional analysis, and this volume collects contributions by his former students and collaborators in honor of his 60th anniversary at a conference in Gaeta. It presents new developments in the theory of partial differential equations with emphasis on elliptic and parabolic problems.
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A. Bermudez, R. Leira, M.C. Müniz and F. Pena
Abstract.
We present a numerical model describing the thermal behavior of a silicon purification process which takes place into a so-called casting ladle. We consider, simultaneously, the phase change in the silicon and a nonlinear non-local boundary condition arising from the Stefan-Boltzmann radiation condition at the enclosure surfaces within the ladle. We also propose a numerical approximation using a finite element method. An iterative algorithm and numerical results are presented.
1. Introduction
In many engineering applications involving high-temperature processes numerical simulation provides an insight into the radiative analysis of these complex systems and it promotes improvements of several process optimization (see [3, 5]). The motivation of this work is to compute the numerical solution of the problem addressed in [6] applied to a silicon purification process - see [1]. We consider, simultaneously, the phase change in the silicon and the non-local boundary condition arising from the Stefan-Boltzmann radiation condition at the enclosure surfaces within the ladle.
The outline of this paper is as follows. In Section 2 the physical problem is introduced. In Section 3 using the axisymmetry of the domain, we formulate the mathematical problem in a two-dimensional domain by means of cylindrical coordinates. Section 4 is devoted to introduce space and time discretization of the aforementioned problem and to present an iterative algorithm. Finally, in Section 5, several numerical results are shown.
2. The physical problem
Metallurgical grade silicon (MG-Si) is obtained from a silicon oxide in electrical submerged arc furnaces. A technique of MG-Si puri.cation is to melt it and to induce its directional solidification. This method of removing impurities is based on the fact that most impurities tend to remain in a molten region rather than re-solidify.
This purification process is taking place into a casting ladle which consists of a finite axisymmetric cylinder containing a cylindrical enclosure. After the casting ladle being electrically heated, its lid is open and molten silicon is poured into its inner cavity keeping a gap between the top of the silicon and the upper part of the inner ladle surface where several heating elements are located. The objective is now to push upwards the metal impurities by means of inducing its one-directional solidification switching on the heating elements and then keeping molten the top of the silicon ingot.
In doing so, the solid silicon grows gradually upwards into the liquid and the metallic impurities are segregated into the melt region during solidification, thus, at the end of the process most of impurities are concentrated at the top of the silicon ingot. Radiation heat transfer is considered at the inner cavity and materials of the enclosure are assumed to be opaque (see [4]), therefore radiation may be treated as a surface phenomenon.
Moreover, we assume both that the walls of the cylindrical enclosure behave as black surfaces and that the medium within the enclosure is radiatively nonparticipating so that it has no effect on the radiation transfer between inner surfaces.
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