Thomas Wondrak

Contactless inductive flow tomography for a model of continuous steel casting

The contactless inductive flow tomography (CIFT) aims at reconstructing the velocity field in electrically conducting melts from externally measured induced magnetic fields. One of its possible applications is the velocity reconstruction in the continuous casting process. In this paper, we apply this method to the flow field in a small model (containing approximately 1.4 l of the eutectic alloy GaInSn) of a mould for thin slab casting. It is shown that the flow structure, in general, and the jet position and intensity, in particular, can be reliably determined from magnetic field data using only a modest number (in the order of 5) of sensors.

Numerical simulation of the Tayler instability in liquid metals

The electrical current through an incompressible, viscous and resistive liquid conductor produces an azimuthal magnetic field that becomes unstable when the corresponding Hartmann number exceeds a critical value of the order of 20. This Tayler instability (TI), which is not only discussed as a key ingredient of a nonlinear stellar dynamo model (Tayler-Spruit dynamo), but also as a limiting factor for the maximum size of large liquid metal batteries, was recently observed experimentally in a column of a liquid metal (Seilmayer et al 2012 Phys. Rev. Lett. 108 244501). On the basis of an integro-differential equation approach, we have developed a fully three-dimensional numerical code, and have utilized it for the simulation of the Tayler instability at typical viscosities and resistivities of liquid metals. The resulting growth rates are in good agreement with the experimental data. We illustrate the capabilities of the code for the detailed simulation of liquid metal battery problems in realistic geometries.

Use of electromagnetic induction tomography for monitoring liquid metal/gas flow regimes on a model of an industrial steel caster

Monitoring of the steel flow through the submerged entry nozzle (SEN) during continuous casting presents a challenge for the instrumentation system because of the high temperature environment and the limited access to the nozzle in between the tundish and the mould. Electromagnetic inductance tomography (EMT) presents an attractive tool to visualize the steel flow profile within the SEN. In this paper, we investigate various flow regimes over a range of stopper positions and gas volume flow rates on a model of a submerged entry nozzle. A scaled (approximately 10:1) experimental rig consisting of a tundish, stopper rod, nozzle and mould was used. Argon gas was injected through the centre of the stopper rod and the behaviour of the two-phase GaInSn/argon flow was studied. The experiments were performed with GaInSn as an analogue for liquid steel, because it has similar conductive properties as molten steel and allows measurements at room temperature. The electromagnetic system used in our experiments to monitor the behaviour of the two-phase GaInSn/argon flow consisted of an array of eight equally spaced induction coils arranged around the object, a data acquisition system and a host computer. The present system operates with a sinusoidal excitation waveform with a frequency of 40 kHz and the system has a capture rate of 40 frames per second. The results show the ability of the system to distinguish the different flow regimes and to detect the individual bubbles. Sample tomographic images given in the paper clearly illustrate the different flow regimes.

Some methodological improvements of the contactless inductive flow tomography

The goal of contactless inductive flow tomography (CIFT) is the velocity reconstruction in electrically conducting melts which are used in many metallurgical and crystal growth applications. In this paper, we discuss some recent methodological improvements of this method, in particular the automatic search for an optimum regularisation parameter and the amended treatment of the boundary integrals.

Local Lorentz force flowmeter at a continuous caster model using a new generation multicomponent force and torque sensor

Lorentz force velocimetry is a non-invasive velocity measurement technique for electrical conductive liquids like molten steel. In this technique, the metal flow interacts with a static magnetic field generating eddy currents which, in turn, produce flow-braking Lorentz forces within the fluid. These forces are proportional to the electrical conductivity and to the velocity of the melt. Due to Newtons third law, a counter force of the same magnitude acts on the source of the applied static magnetic field which is in our case a permanent magnet. In this paper we will present a new multicomponent sensor for the local Lorentz force flowmeter (L2F2) which is able to measure simultaneously all three components of the force as well as all three components of the torque. Therefore, this new sensor is capable of accessing all three velocity components at the same time in the region near the wall. In order to demonstrate the potential of this new sensor, it is used to identify the 3-dimensional velocity field near the wide face of the mold of a continuous caster model available at the Helmholtz-Zentrum Dresden-Rossendorf. As model melt, the eutectic alloy GaInSn is used.

Contactless Inductive Flow Tomography: Brief History and Recent Developments in Its Application to Continuous Casting

The contactless inductive flow tomography (CIFT) aims at reconstructing the velocity field of electrically conducting fluids, with special focus on applications in metallurgy and crystal growth technologies. The method relies on the induction of secondary magnetic fields if the moving fluid is exposed to a primary magnetic field. The theoretical foundation of the method is delineated, and some early experiments on the reconstruction of the three-dimensional flow in a cylinder are sketched. Then, the recent efforts to apply CIFT to various model problems in connection with the continuous casting of steel are summarized.

Contactless Inductive Bubble Detection in a Liquid Metal Flow

The detection of bubbles in liquid metals is important for many technical applications. The opaqueness and the high temperature of liquid metals set high demands on the measurement system. The high electrical conductivity of the liquid metal can be exploited for contactless methods based on electromagnetic induction. We will present a measurement system which consists of one excitation coil and a pickup coil system on the opposite sides of the pipe. With this sensor we were able to detect bubbles in a sodium flow inside a stainless steel pipe and bubbles in a column filled with a liquid Gallium alloy.

Enhancing robustness and applicability of contactless inductive flow tomography

Measuring the flow velocity in hot, chemically aggressive and opaque melts is a challenging task even for todays measurement techniques. The contactless inductive flow tomography (CIFT) could provide a solution by applying magnetic fields to an electrically conducting melt and measuring the small flow-induced magnetic perturbances outside of the container. In this paper we will demonstrate how the robustness of CIFT can be enhanced by means of excitation with time-harmonic magnetic fields, making it more insensitive to the ubiquitous changes of the environmental magnetic field. Further we will show how the problem of an electrically conducting container can be treated, which is necessary, e.g., for industrial application in continuous casting.

Flow Visualization by Means of Contactless Inductive Flow Tomography in the Presence of a Magnetic Brake

Abstract In continuous casting DC magnetic fields perpendicular to the wide faces of the mold are used to control the flow in the mold. Especially in this case, even a rough knowledge of the flow structure in the mold would be highly desirable. The contactless inductive flow tomography (CIFT) allows to reconstruct the dominating two-dimensional flow structure in a slab casting mold by applying one external magnetic field and by measuring the flow-induced magnetic fields outside the mold. For a physical model of a mold with a cross section of 140 mm×35 mm we present preliminary measurements of the flow field in the mold in the presence of a magnetic brake. In addition, we show first reconstructions of the flow field in a mold with the cross section of 400 mm×100 mm demonstrating the upward scalability of CIFT.

Spatial and temporal resolution of a local Lorentz force flowmeter

Electromagnetic measurement techniques are very promising for accessing the flow properties of liquid melts. We extend one of the recently developed techniques, Lorentz force velocimetry, to the measurement of spatial flow structures close to the wall of the confining container. The sensor we use is called local Lorentz force flowmeter (L2F2). It comprises a small permanent magnet which is attached to a force measurement system. We demonstrate that it is possible to reconstruct the complex flow in the vicinity of the wall of a confined vessel using the L2F2. Additionally, we show with the help of a solid body experiment that the L2F2 responds to temporal changes in the flow in the order of 1 Hz.