Application of Ultrasound Doppler Velocimetry to flows of hot metallic melts

During the last decades the Ultrasound Doppler Velocimetry (UDV) became a very powerful tool to measure the velocity structure 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 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 [1] and gallium [2]. Now, first successful measurements in liquid sodium at 150°C are published [3]. We will present mean profiles of a sodium flow in a rectangular duct exposed to an external, transverse magnetic field. To demonstrate the capability of UDV the transformation of a well-known turbulent, piston-like profile to a M-shaped velocity profile with growing magnetic field strength was observed. The significance of artefacts such as caused by the existence of reflecting interfaces in the measuring domain will be discussed.
An integrated ultrasonic sensor with acoustic wave guide has been developed to overcome the limitation of ultrasonic transducers to temperatures lower than 200°C. This sensor can presently be applied at maximum temperatures up to 400°C, but an extension up to 800°C can reasonably be expected. In this presentation we show some experimental results obtained in PbBi bubbly flows at temperatures up to 350°C. Ar bubbles were injected through a single orifice in a cylindrical container filled with stagnant PbBi. Velocity profiles were measured in the bubble plume. Mean values of the liquid as well as the bubble velocity were extracted from the data and will be presented as function of the gas flow rate.
Velocity profiles of a single-phase PbBi flow in a tube were successfully obtained at the KALLA laboratory of FZK. By means of the integrated sensor stable velocity signals could be received over a period of about 72 hours at temperatures between 180°C and 350°C.

[1] Y. Takeda: Measurement of velocity profile of mercury flow by ultrasound Doppler shift method. Nucl. Techn. 79 (1987), 120-124
[2] D. Brito, H.-C. Nataf, P. Cardin, J. Aubert, J.-P. Masson: Ultrasonic Doppler velocimetry in liquid gallium. Exp. Fluids 31 (2001) 6, 653-663
[3] S. Eckert, G.Gerbeth: Velocity measurements in liquid sodium by means of Ultrasound Doppler Velocimetry. accepted for publication in Exp. Fluids (2002)