Numerical simulation of vitrification processes: glass homogeneity by gas bubbling study
Résumé
A direct induction process using the cold crucible technology has been developed by the French Alternative Energies and Atomic Energy Commission and AREVA-NC for 25 years. In this process, walls of the crucible are cooled by internal water circulation which protects them from the corrosion and allow higher temperature of the molten glass. Moreover, a mechanical stirrer and air bubbling system promote a good homogeneity of the melt. In results, the lifetime of the crucible as the incorporation rate of the nuclear waste is increased.The 3D numerical simulation of glass elaboration by induction in cold crucible needs a coupled approach of the different phenomena induction, thermal and hydrodynamic. Indeed, those three phenomena are strongly coupled because of the temperature dependence of the glass properties.This presentation describes how the air bubbling in the glass is modelled in our simulation and enlightens its important role in the homogenisation of the glass during its elaboration. Air bubbling in liquids as viscous as the glass (viscosity up to 10 Pa.s) is not common and specific models have to be built. Two different models are used. The first one is a semi-empirical based model and the second is a two phase model (VOF). In order to model air bubbling in the molten glass, specific experiments have been performed based on the principle of hydraulic similarity. They consist in replacing the glass by silicon oil of equivalent viscosity in a tank of the same size. Non-dimensional Reynolds number as Galilee numbers are well respected between the two configurations. Only the Morton number differs, but it reflects tension surface effect which has been proven to have a weak effect on the formation of air bubbles in a very viscous liquid. This experiment facility allows us to acquire data as bubbles size, bubbles velocity and their behaviour. These data are used to build specific correlations for the semi-empirical model and also to validate the two phase model.The importance of air bubbling is quantified in simulations of the cold crucible process in a case with thermal convection forces only compared with a case with the air bubbling.Thanks to this numerical simulation new capability, the internal design of the crucible can be optimized and the comprehension of glass elaboration can be improved.
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