Abstract
A numerical study is presented that deals with the flow in the mold of a continuous slab caster under the influence of a DC magnetic field (electromagnetic brakes (EMBrs)). The arrangement and geometry investigated here is based on a series of previous experimental studies carried out at the mini-LIMMCAST facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The magnetic field models a ruler-type EMBr and is installed in the region of the ports of the submerged entry nozzle (SEN). The current article considers magnet field strengths up to 441 mT, corresponding to a Hartmann number of about 600, and takes the electrical conductivity of the solidified shell into account. The numerical model of the turbulent flow under the applied magnetic field is implemented using the open-source CFD package OpenFOAM®. Our numerical results reveal that a growing magnitude of the applied magnetic field may cause a reversal of the flow direction at the meniscus surface, which is related the formation of a “multiroll” flow pattern in the mold. This phenomenon can be explained as a classical magnetohydrodynamics (MHD) effect: (1) the closure of the induced electric current results not primarily in a braking Lorentz force inside the jet but in an acceleration in regions of previously weak velocities, which initiates the formation of an opposite vortex (OV) close to the mean jet; (2) this vortex develops in size at the expense of the main vortex until it reaches the meniscus surface, where it becomes clearly visible. We also show that an acceleration of the meniscus flow must be expected when the applied magnetic field is smaller than a critical value. This acceleration is due to the transfer of kinetic energy from smaller turbulent structures into the mean flow. A further increase in the EMBr intensity leads to the expected damping of the mean flow and, consequently, to a reduction in the size of the upper roll. These investigations show that the Lorentz force cannot be reduced to a simple damping effect; depending on the field strength, its action is found to be topologically complex.
Highlights
ENSURING the quality of continuous cast (CC)products is becoming increasingly important in view of growing production rates
Our parametric study based on wide and highly resolved magnetic field variations addresses the following main questions: (1) How does the flow structure change with the growing magnetic field strength? (2) What is the origin of initial meniscus acceleration and its later deceleration? (3) Which conditions and mechanisms are responsible for the formation of the opposite meniscus flow?
The wall-adapting local eddy-viscosity (WALE) turbulence model is used in the present work to simulate sSGS: It is robust for complex geometries with strong mesh refinements, and it is capable of predicting the formation of coherent structures that can exist under the influence of the applied magnetic field.[9,24,27]
Summary
Products is becoming increasingly important in view of growing production rates. Uncontrolled fluid flow in the continuous casting mold is suspected of being responsible for various casting defects. Turbulent jet flow is an important phenomenon during the continuous casting process as the mold flow is mainly driven by the submerged jet emanating from the submerged entry nozzle (SEN) It influences the free surface stability, promotes superheat transport to the solidified shell as well as to the slag band, or poses the risk of introducing impurities and inclusions into the bulk of the slab. Cho and Thomas[18] classified the influence of the applied magnetic field on the formation of different casting defects and suggested corresponding guidance for practical use of EMBrs. Complementary to the studies mentioned here, the authors of this work have recently presented a very detailed numerical study of the induced electric current distribution during the EMBr process, focusing on the interaction with the turbulent flow and considering the effects of the presence of the solid shell, which is very important during real solidification in the CC process.[19]. Our parametric study based on wide and highly resolved magnetic field variations addresses the following main questions: (1) How does the flow structure change with the growing magnetic field strength? (2) What is the origin of initial meniscus acceleration and its later deceleration? (3) Which conditions and mechanisms are responsible for the formation of the opposite meniscus flow?
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