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.