Application of the Ultrasonic Doppler Method in liquid metal flows: examples and perspectives

The Ultrasound Doppler Velocimetry (UDV) is a non-intrusive technique to measure velocities of liquid flows. Because of the ability to work in opaque fluids and to deliver complete velocity profiles in real time it becomes very attractive for liquid metal applications. But, in case of hot metallic melts the user is confronted with a number of specific problems: First of all the application of the ultrasonic transducers is usually restricted to maximum temperatures of about 150°C. The transmission of a sufficient amount of ultrasonic energy from the transducer to the fluid has to be guaranteed. Here, the acoustic coupling and the wetting conditions have to be considered as important issues. Moreover, the flow has to be seeded with reflecting particles to obtain Doppler signals from the fluid.
The feasibility of velocity profile measurements by UDV has already been demonstrated for low temperature liquid metals as mercury and gallium. Complications arise for UDV applications in liquid metal flows at high temperatures. Recently, an integrated ultrasonic sensor with acoustic wave guide has been developed. The feasibility of this sensor concept has been demonstrated in diverse experiments using miscellaneous metallic melts until temperatures of about 700°C.
In this presentation we show various applications of UDV in liquid metal flows to demonstrate the capabilities and current restrictions of this technique. For instance, we consider single- and multi-transducer arrangements for flow mapping or present velocity measurements obtained during the solidification of a metallic melt. Besides the determination of velocity profiles in the liquid phase the UDV data allow for an assessment of the current position of the solidification front too. Specific problems arising in the context of UDV measurements in liquid metal experiments will be discussed.