Local velocity measurements in high temperature liquid metals by means of mechano-optical probes

Model experiments are an important tool to understand the details of the flow structure and the transport properties of
flows occurring in real-scale metallurgical facilities as well as to validate the multitude of numerical codes for flow
simulation. Generally, water experiments are often performed to characterise the flow. Nevertheless, due to the large
differences of material properties like density, heat conductivity, surface tension or electrical conductivity the use of liquid
metals has clearly to be preferred if heat transfer phenomena, two-phase flows or the influence of electromagnetic fields
on the flow should be investigated. The application of suitable alloys with low melting points, for instance PbBi (Tm = 125
°C) or InGaSn (Tm = 5...10 °C), makes such kind of experiments very flexible and offers the ability to measure the
essential flow quantities like velocity, pressure or void fraction.

In this lecture we want to discuss a novel sensor which has been developed by the Rossendorf group to measure the local
velocities in opaque liquid flows. The measuring principle is based on the separation of a direct mechanical interaction
between flow and sensor tip and the optical acquisition and processing of the signal. In principle, this fact allows the
extension of the range of applicability to higher temperatures. Furthermore, the insensitivity of the system to electrical
noise and external magnetic fields can be considered as an important advantage. Until now, the sensor has been tested in
metallic melts up to temperatures of about 350 °C . In principle, an extension of the range of application up to
temperatures of about 1100 °C should be possible by the utilisation of quartz glass as material for the sensitive sensor tips.
The first sensors have been manufactured and tested with low temperature melts.

We present measurements of the local velocity obtained in an eutectic InGaSn melt driven by a rotating magnetic field.
The interest is focussed on geometry and parameters relevant for crystal growth technologies and mixing processes in
metallurgical applications, respectively. Measured profiles of the azimuthal velocity have been obtained at different
frequencies and field amplitudes.