Contact

Dr. Thomas Wondrak

Head Inductive measurement techniques
t.wondrakAthzdr.de
Phone: +49 351 260 2489

Dr. Max Sieger

Staff member
Inductive measurement techniques
m.siegerAthzdr.de
Phone: +49 351 260 3594

Contactless Inductive Flow Tomography (CIFT)

The contactless inductive flow tomography (CIFT), developed at HZDR, is able to visualise the three-dimensional flow profile of fluids with high electrical conductivity. It employs the principle of the induction of electrical currents due to the movement of a conducting material through a magnetic field. Because of its contactless operating principle, it is well-suited for hot and chemically agressive melts, which are, for example, encountered in continuous steel casting or semiconductor chrystal growth.

Working principle for two-dimensional flows

Foto: CIFT Blender ©Copyright: Dr.-Ing. Matthias Ratajczak
Figure 1: Principle of CIFT

The working principle of CIFT can be illustrated using a manly two-dimensional flow profile inside a continuous steel casting mould. Figure 1 shows a sketch of a mould with a submerged entry nozzle with outlets faced sidewards. From these outlets, the liquid metal flows inside the mould. When a static magnetic field is applied in the vertical direction, an electric current is generated inside the melt because of its relative motion in comparison with the magnetic field. This electric current, in turn, generates a magnetic field that can be measured outside the mould. This magnetic field carries information about the flow structure in the melt. From the laws of Ohm and Biot-Savart, a linear model of this induction process can be derived. By inverting this model and applying appropriate regularization techniques, the flow in the mould is reconstructed from the magnetic field measurements outside the mould.

Application example: two-phase flow inside a model mould

Figure 2: Schematic sketch of the measurement
Figure 3: CIFT-reconstructed flow without (left) and with (right) gas injection.

Figure 2 displays a realization of this method in a model experiment, where the static magnetic field is generated by a rectangular coil around the mould. The superposition of the applied magnetic field and the flow induced magnetic field is measured with 7 sensors along both narrow faces of the mould. In this setup a time resolution of 1 Hertz is achieved. The videos in Figure 3 show two different flow regimes in the mould that were provoked by two different amounts of gas which was injected into the submerged entry nozzle. In the video shown in Figure 3(right) an undesired change from the so-called double-roll flow to a single-roll flow can be observed.

Challenges for the measurement

The challenges of this measurement technique are the numeric solution of the linear inverse problem, as well as the measurement of the very small flow-induced magnetic fields. The strength of the induced magnetic fields is typically two to five orders of magnitude smaller than the applied magnetic field. For example, an applied magnetic field of about 1 mT creates a flow-induced magnetic field of approximately 100 nT. In comparison, earth's magnetic field is about 50,000 nT. Thus it appears that smallest magnetic disturbances arising e.g. from switching of electric devices, arc-welding activities or solar activity, superimpose the measurement signal and must be filtered out. This can be achieved by excitation with an AC magnetic field of a constant frequency in the order of 1 Hz. Then, like in lock-in amplifiers, only the signal with the excitation frequency is very precisely extracted and noise signals in other frequency ranges are filtered out.

Figure 4: Fluxgate sensors and induction coils

Besides Fluxgate sensors, air-core coils are applied for robust magnetic field measurements. These have the advantage in comparison with Fluxgate probes that they have no intrinsic upper range limit. However, they require a large number of turns in order to detect the tiny flow-induced magnetic field. Currently, induction coils sensors with a size of 29 mm and 340,000 turns are used. With the help of an analog-digital-converter having a dynamic range of 100 dB, the small flow-induced magnetic fields can be measured. Figure 4 exemplarily shows a Fluxgate sensor and different induction coils.

CIFT as an online process control tool

The developement of the fast reconstruction algorithm was a milestone for the CIFT method: The precalculation of the inverse matrizes for different Tikhonov regularisation parameters λ allowed the solution of the inverse problem on off-the-shelf GPUs in less than a second. This enabled to use CIFT as an online process control tool.

Foto: CIFT Controler - model ©Copyright: Ivan Glavinic

Figure 5: Scheme of a CIFT-based process control loop

Figure 5 depicts the scheme of a CIFT-based process control loop, which controls the strength of an electromagnetic brake (EMBr) in dependence of the reconstructed velocity field inside the mould. Strong hysteresis effects of the EMBr due to ferromagnetic parts are compensated by an advanced mathematical model (cf.  Glavinic et al., 2022).

This control loop was succesfully applied in a model laboratory experiment:

Foto: CIFT Controler: Online Prozesskontrolle - Testfall verstopfter Auslass, Vektorplots ©Copyright: Ivan Glavinic

Figure 6: Results of a laboratory experiment with incorporated CIFT-based process control loop

The common problem of nozzle clogging in steel casting is simulated by an artificial blockage of the right-sided outlet port of the submerged entry nozzle (cf. Fig. 6 middle and right), which diminishes the flow velocities and shifts the jet impignement point (marked by blue arrow) in the right side of the mould. The CIFT-based control loop reliably detects the difference in height between left and right impignement point and initiates a shut-down of the EMBr, whereby the height difference is mitigated.

Three-dimensional reconstruction

When a three-dimensional flow has to be reconstructed, it is not sufficient to apply only one sngle magnetic field. In this case at least one additional magnetic field must be applied in a different direction. An approach with one horizontal and one vertical field yields good results. For one reconstruction, at first the horizontal magnetic field is applied and the resulting flow-induced magnetic field is measured. Thereafter the vertical magnetic field is applied and the flow-induced magnetic field is measured again. From both measurements the three-dimensional flow can be reconstructed.

Figure 7: Experiment inside a closed cylinder
Figure 8: Reconstructed flow inside an experiment inside a closed cylinder

This could be demonstrated at the following experiment, shown is Figure 7. In an acrylic glass cylinder the flow is driven by a propeller in the centre of the container. The guiding blades at the top of the container allow for the creation of two different flow structures: When pumping upwards, the guiding blades dampen the rotation of the fluid and a dominating poloidal flow is created, whereas pumping downward creates a toroidal flow in addition to the poloidal flow. Figure 8 shows the reconstructed flow for an experiment where the fluid was first pumped downward and then upward.

Application example: liquid metal convection

Thermally driven movements in liquid metals are relevant processes inside stars and planets and likewise present in technical applications, e.g., semiconductor crystal growth. A deep understanding of the non-inear dynamics of such hydrodynamic systems can be achieved with so-called Rayleigh-Bénard convection (RBC) model experiments. Liquid metal RBC experiment are substantially different from experiments with water with respect to fluid density and thermal conductivity and the opaqueness of liquid metals sets high demands to the measurement techniques used. CIFT, for the first time, enables a complete view of the global 3D flow field in dependence of time even in large RBC geometries. Figure 9 depicts an experimental RBC set-up at HZDR. The central liquid metal-filled cylinder with a height of 640 mm and a diameter of 320 mm is surrounded by the CIFT excitation coils, 42 fluxgate probes (inset) and further sensors. This enables to reconstruct the global flow field in the whole cylinder as representatively shown in Figure 10.

Foto: CIFT-Rayleigh-Bénard: experimenteller Aufbau ©Copyright: Dr. Max Sieger

Figure 9: CIFT-Rayleigh-Bénard experimental set-up

Foto: CIFT-RB: result ©Copyright: Dr. Max Sieger

Figure 10: flow reconstruction of a liquid metal Rayleigh-Bénard convection experiment (height of the cylinder: 640 mm)

Literature

Patents

  • F. Stefani u. a., „Verfahren und Anordnung zur kontaktlosen Bestimmung von räumlichen Geschwindigkeitsverteilungen in nicht-kugelförmigen elektrisch leitfähigen Flüssigkeiten“, DE 100 26 052 B4, 17. März 2005.
  • F. Stefani u. a., „Verfahren und Anordnung zur kontaktlosen Bestimmung von Geschwindigkeitsverteilungen eines flüssigen Metalls in einer Stranggießkokille“, DE 10 2008 055 034 A1, 01. Juli 2010.