Electromagnetic control of oscillating flows in a cavity

Abstract

In continuous steel casting, liquid steel flows turbulently through a submerged nozzle into a thin, vertical mould. In the mould the liquid steel is cooled, such that it solidifies and plate steel is formed. On top of the liquid steel in the mould, a slag layer is present and due to the turbulent behavior of the flow, particles and droplets from the slag layer can get entrained into the bulk flow. This leads to inclusions in the final product, which is unwanted. The flow in the mould needs to be stabilized, such that entrainment effects no longer play a role. For this, electromagnets are generally installed next to the mould. The flow of the electrically conductive liquid steel through the magnetic field induces an electrical current, from which an induced Lorentz force emerges, which acts as a so-called electromagnetic brake. This dissertation presents an experimental study on flow dynamics, heat transfer, and electromagnetic interaction in a thin slab continuous casting mould. To mimic the continuous casting process, a glass model of the mould was fabricated and (salt) water was used as modeling fluid, such that particle image velocimetry measurements could be performed. The flow of both single and bifurcated jets is studied. The jets issuing into the thin cavity and the induced flow in the cavity exhibit a self-sustained oscillating behavior with a frequency that grows linearly with the jet velocity. It was found that the self-sustained oscillations exist due to an imbalance between the inertial forces in the recirculation zones alongside the jets and the pressure force due to a low pressure zone in these recirculation zones. The low pressure zone in the center of the recirculation zones can exist due to the (semi) two dimensionality of the flow. When a thicker cavity is employed, the self-sustained jet oscillations vanish due to a less structured, and more three dimensional, flow pattern. Next, the influence of the self sustained jet oscillations on heat transfer at the wall is studied for flow from a bifurcated nozzle. A constant high inlet flow temperature is applied, in combination with cooling of one of the broad walls. Measurements of the temperature at the cooled wall are performed using thermochromic liquid crystals (TLC’s) attached to the cooled wall. The self-sustained jet oscillations show an imprint on the TLC’s. At the point where the shear layers of the jet reach the wall a hot spot is formed, and in the center of the recirculation zone alongside the jet a cold spot is found. The cold spot moves with the jet oscillation, leading to a non-uniform and time-dependent temperature distribution at the cooled wall. Measurements of the temperature drop of the liquid over the cavity have been performed and the average heat transfer coefficient h was found to scale with Re^0.8. Subsequently, the self-sustained jet oscillations are influenced by means of an applied electromagnetic force. This is done by applying an electrical current through a saline solution across the width of the cavity, in conjunction with a permanent magnetic field perpendicular to the electrical current. The combination of the electrical and magnetic field with a liquid with high electrical conductivity (as compared to tap water) leads to a permanent and local Lorentz force. This Lorentz force can be applied such that the jet oscillations are either suppressed or enhanced. In the oscillation suppressing configuration, the flow due to the Lorentz force prohibits the recirculation zones from forming, and so no low pressure zones emerge alongside the jet. Above a critical forcing strength, this suppresses the self-sustained jet oscillations completely. In the opposite, oscillation enhancing, configuration, the flow due to the Lorentz force increases the formation of the recirculation zones and hence the oscillation frequency increases. We finalize this thesis by discussing how the experimental results from this work can be used in the design and optimization of actual steel casters, and in the validation of numerical models to be used for that same purpose. Shortcomings of the experimental methods are discussed, as well as the appropriate scaling of physical parameters.