Keywords: continuous casting, interfaces, turbulence modelling.

This paper concerns the development and validation (using an oil/water system) of a finite volume computer model of the continuous casting process for steel flat products. The emphasis is on hydrodynamic aspects and in particular the dynamic behaviour of the metal-slag interface. The study is of interest to the industry, since instability and wave action encourage the entrainment of inclusions into the melt affecting product quality. Conventional volume-of-fluid (VOF) type simulations for this type of problem are very costly in three dimensions. For this reason, the interface between oil and water is tracked using a new implicit algorithm, called the Counter Diffusion Method. Standard RANS turbulence models such as k-e were found to too diffusive for this application leading to rapid damping of the interface. To overcome this deficiency, a time-filtered version of the k-epsilon model, was used; this was coupled with appropriate density stratification terms to account for kinetic to potential energy conversion at the interface.

In normal operation argon gas is injected into the mould to prevent clogging of the submerged entry nozzle (SEN) delivering the metal from the tundish. This was simulated experimentally by injecting air into the oil-water system. The presence of air alters the hydrodynamic behaviour of the jets as it adds buoyancy and also affects the interface, causing mixing of the oil and water layers. The presence of gas bubbles was represented in the model using a Lagrangian tracking scheme, which updates the position of typical tracks for a range of bubble diameters to compute the average gas holdup in the mould. This information is then used to update the metal-gas mixture density in each computational cell providing feedback to the continuum momentum equations. The bubble tracks are affected by fluid turbulence through a stochastic term in the interface drag computation.

With these modifications the model produced a realistic behaviour of interface dynamics in both amplitude and frequency. It was shown that as in the experiment, shear due to high jet velocities produced fragmentation of the interface and for low oil layer thickness led to uncovering of the water in some areas. As in the experiment, the model predicts a significant effect of the bubbles on the behaviour of the layers, promoting mixing. Further work aims to refine the physics of bubble behaviour by introducing a delay crossing parameter to account for interfacial tension effects such as film draining.

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