Andreas Cramer

Velocity Measurement Techniques for Liquid Metal Flows

Analysis and control of fluid flows, often subsidiary to industrial design issues,require measurements of the flow field. For classical transparent fluids such aswater or gas a variety of well-developed techniques (laser Doppler and parti-cle image velocimetry, Schlieren optics, interferometric techniques, etc.) havebeen established. In contrast, the situation regardingopaque liquids still lacksalmost any commercial availability. Metallic and semiconductor melts oftenpose additional problems of high temperature and chemical aggressiveness,rendering any reliable determination of the flow field a challenging task. Thisreview intends to summarise different approaches suitable for velocity mea-surements in liquid metal flows and to discuss perspectives, particularly inview of some recent developments (ultrasound, magnetic tomography).Focus-ing mainly on local velocity measurements, it is subsequently distinguishedbetween invasive and non-invasive methods, leaving entirely aside the acqui-sition of temperature, pressure, and concentration, for which [1] may serve asa comprehensive reference.

Measurement of the sound velocity in fluids using the echo signals from scattering particles.

With conventional methods the sound velocity c in fluids can be determined using the back wall echo. This paper proposes a novel technique, in which the signals reflected by scattering particles suspended in a fluid are analysed instead. The basic idea is that the particles generate the strongest echo signal when being located in the sound field maximum. Therefore the position of the echo signal maximum is a measure for the propagation time to the sound field maximum. Provided that calibration data or sound field simulations for the ultrasonic transducer are available, this propagation time suffices to determine both sound velocity and the location of the sound field maximum. The feasibility of the new approach is demonstrated by different kinds of experiments: (i) Measurements of the sound velocity c in four fluids covering the wide range between 1116 and 2740m/s. The results show good agreement with values published elsewhere. (ii) Using the dependence of the sound velocity on temperature, it is possible to vary c over the comparatively small range between 1431 and 1555m/s with increments of less than 10m/s. The measured statistical variation of 1.4m/s corresponds to a relative uncertainty not worse than 0.1%. (iii) The focus position, i.e. the distance of the maximum of the sound field from the transducer, was varied by time-shifted superposition of the receive signals belonging to the different elements of an annular array. The results indicate that the novel method is even capable of measuring profiles of the sound velocity along the ultrasonic beam non-invasively.