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
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 (a), CIFT-reconstructed flow without (b) and with (c) 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 2 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(c) 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.
Abbildung 3: Fluxgate sensor 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.
Application example: model mould with electromagnetic brake
|Figure 4: Visualization of the setup with DC electromagnet (a) and reconstructed flow (b).|
At the continuous casting model it could be shown that under the influence of an applied magnetic field for flow control with a strength of 300 mT flow-induced magnetic field changes in the order of 100 nT can be measured precisely, from which in turn the flow in the mould can be reconstructed. Figure 4 shows the corresponding setup and the mould flow, which, under certain boundary conditions, shows regular flow oscillations when the brake is switched on.
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 5: Experiment inside a closed cylinder (a) and reconstructed flow (b).|
This could be demonstrated at the following experiment, shown is Figure 5. 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 5(b) shows the reconstructed flow for an experiment where the fluid was first pumped downward and then upward.
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