Publikationen

Non-contact electromagnetic control and shaping of liquid metal free surfaces is crucial in a number of high-temperature metallurgical processes like levitation melting and electromagnetic sealing, among others. Other examples are the electromagnetic bending or stabilization of liquid metal jets that frequently occur in casting or fusion applications. Within this context, we experimentally study the influence of strong axial magnetic fields on the dynamics of falling metal droplets and liquid metal jets. GaInSn in eutectic composition is used as test melt being liquid at room temperature. In the experiments, we use a cryogen-free superconducting magnet (CFM) providing steady homogeneous fields of up to 5 T and allowing a tilt angle between the falling melt and the magnet axis. We vary the magnetic flux density, the tilt angle, the liquid metal flow rate, and the diameter and material of the nozzle (electrically conducting/insulating). Hence, the experiments cover a parameter range of Hartmann numbers Ha, Reynolds numbers Re, and Weber numbers We within 0 < Ha < 440, 340 < Re < 4500, and 0.09 < We < 12.1. As major results we find that under the influence of the strong magnetic field, droplet rotation ceases and the droplets are stretched in the field direction. Moreover, we observe that the jet breakup into droplets (spheroidization) is suppressed, and in the case of electrically conducting nozzles and tilt, the jets are bent towards the field axis.

We report a method to detect and to measure the size and velocity of elongated bubbles or drops in a dispersed two-phase flow. The difference of the magnetic susceptibilities between two phases causes a force on the interface between both phases when it is exposed to an external magnetic field. The force is measured with a state-of-the-art electromagnetic compensation balance. While the front and the back of the bubble pass the magnetic field, two peaks in the force signal appear, which can be used to calculate the velocity and geometry parameters of the bubble. We achieve a substantial advantage over other bubble detection techniques because this technique is contactless, non-invasive, independent of the electrical conductivity and can be applied to opaque or aggressive fluids. The measurements are performed in an inclined channel with air bubbles and paraffin oil drops in water. The bubble length is in the range of 0.1-0.25 m and the bubble velocity lies between 0.02-0.22 m s-1. Furthermore we show that it is possible to apply this measurement principle for nondestructive testing (NDT) of diamagnetic and paramagnetic materials like metal, plastics or glass, provided that defects are in the range of 10-2 m. This technique opens up new possibilities in industrial applications to measure two-phase flow parameters and in material testing.

Local Lorentz force velocimetry is a local velocity measurement technique for liquid metals. Due to the interaction between an electrically conducting liquid and an applied magnetic field, eddy currents and flow-braking Lorentz forces are induced in the fluid. Due to Newtons third law, a force of the same magnitude acts on the source of the applied magnetic field, which is a permanent magnet in our case. The magnet is attached to a gauge that has been especially developed to record all three force and three torque components acting on the magnet. This new-generation local Lorentz force flowmeter (L2F2) has already been tested in a test stand for continuous casting with a 15 mm cubic magnet providing an insight into the three-dimensional velocity distribution of the model melt GaInSn near the wide face of the mold. For better understanding of these results, especially regarding torque sensing, we propose dry experiments which consist in replacing a flowing liquid by a moving solid. Here, as the velocity field is fixed and steady, we are able to decrease considerably the variability and the noise of the measurements providing an accurate calibration of the system. In this paper, we present a numerical study of this dry calibration using a rotating disk made of aluminum and two different magnet systems that can be shifted along the rotation axis as well as in the radial direction.

A study of turbulent Taylor-Couette flow between two cylinders in the presence of a uniform axial magnetic field is presented. The flow is driven by the rotating inner cylinder, and the outer cylinder is set to be fixed. Applying fully-3D numerical simulations in the approximation of low magnetic Reynolds number, the influence of the magnetic field on the turbulence intensity and structure is investigated with a variation of the Hartmann number. In the first part, periodic boundary conditions in the axial direction are taken into account that means no Hartmann layer formation. However, due to the high aspect ratio, the flow behavior can present a simplified version close to the real flow. In the second part, this study is compared with a study, where the Hartmann end-walls perpendicular to the magnetic field are introduced so that the flow is confined by the bottom and upper boundaries. In particular, the effects of the Hartmann walls and periodic condition on the process of generation and dissipation of turbulence under the magnetic field influence are compared.

Lorentz force velocimetry (LFV) is a contactless flow measurement technique for electrically conducting liquids. LFV is based on measuring the flow-induced force acting on an externally arranged permanent magnet (or a magnet system) and being proportional to the velocity (or mass flux) of the flow. This force is equal in magnitude to the braking Lorentz forces induced in the moving conductor. In case of flow rate measurement, the magnetic field lines of the used permanent magnet (or magnet system) penetrate the entire crosssection of the flow. In contrast, in local Lorentz force velocimetry (LLFV), tiny permanent magnets are used of which the penetration depth of its field lines is much smaller than the dimension of the flow. The present study aims to extend LLFV to liquid metal two-phase flow measurement. Such flows are of interest in high-temperature metallurgic processes, such as continuous casting of steel, where injection of argon bubbles into the melt is applied to prevent clogging, to mix the melt and to remove inclusions. In a first test, we investigate the transient response of Lorentz force to a simple arrangement of bubble/particle injected into liquid melt GaInSn at rest. The results show that the recorded Lorentz forces vary significantly and their maximum values are different for each rising bubble/particle. The shapes of Lorentz force signals depend on local liquid flow structures and non-conducting volume effects, both of which are dominated by bubble/particle positions.