Ultrasound Doppler Velocimetry
For many years the Ultrasound Doppler Velocimetry (UDV) has been applied at HZDR for measuring velocity fields in liquid metal flows. This echographic method originates from the medical imaging for measuring the blood flow in the human body. Since mid of 1980s it has been deployed for the application field of research and engineering beyond the medical branch, especially at opaque fluids or systems without optical access, since such fluid flows cannot be gathered by established optical flow measurement techniques as PIV (Particle Image Velocimetry) und LDA (Laser Doppler Anemometry). This technique is suited as well for measuring flow velocities in metal melts as demonstrated for the first time in a moving mercury melt at room temperature . For the application in further liquid metals as e.g. Gallium-Indium-Tin (Ga68In20Sn12) [2-5], also at room temperature, or sodium, aluminum as well as various tin and lead alloys with temperatures far over 100°C [6-8] the HZDR has achieved successful advances in the development.
In literature the principle of UDV is known among different names such as Ultrasound Doppler Velocity Profiling (UVP), Acoustic Doppler Velocity Profiler (ADVP) as well as PW Doppler (Pulse Wave Doppler) in the medical branch.
The ultrasound Doppler velocimetry is based on the pulse-echo method and requires acoustic inhomogeneities in the fluid to measure. These may be of natural origin as the case for many metal melts (these are presumably oxides of pure metal or alloy) or artificial scattering particles has to be added. In the simplest implementation an ultrasonic measuring system deploys a single ultrasound transducer (normally based on the piezoelectric effect) serving as emitter as well as receiver of acoustic waves. It emits a short ultrasonic pulse consisting of a few harmonic wave trains in a frequency range of 1 to 10 MHz, which propagates in direction of the normal of the transmission plane (the acoustic axis) as longitudinal wave with sound velocity of the media. At this process a part of the acoustic energy is scattered at the inhomogeneities (particles) and reverberated to the transducer (see figure 1). The transducer receives an echo signal composed of the scattering echos of all particles in the propagation zone of the pulse with the transit time of the scattering echos relative to the pulse emission in the received signal reflecting the axial position of the respective scattering particles. After a specific time the pulse emission is repeated. By means of differences in transit time of the scattering echos between consecutive pulse emissions, resulting from a finite shift of particle position, the movement of the scattering particles and consequently the flow velocity in axial direction can be determined. Consequently, this method provides a profile of the axial velocity component in propagation direction. In the practical application usually 8 to 100 pulse emissions are required for measuring a single velocity profile to compensate the commonly very low signal-to-noise ratio. The spatial resolution of the measuring method in axial as well as in lateral direction is defined by wave length, size of the transmission plane and pulse length. The lateral resolution deteriorates with increasing measuring depth due to the divergence of the ultrasonic pulse, whereas the extent of the measuring volume in axial direction may be considered as constant.
The acquisition of linear velocity profiles in real time and the most widely non-invasive measurement make the ultrasound Doppler velocimetry to a very important and beneficial measuring technique of research in the field of magnetohydrodynamics.
|Figure 1: Operation principle of the ultrasound Doppler velocimetry|
Multidimensional flow field measurements
In order to realize a multidimensional imaging flow measurement in a simple implementation several transducers may be arranged laterally to each other. Commercial UDV measuring devices as applied by HZDR enable such an operation with up to 10 transducers. However, due to the sequential excitation of the transducer channels the hereby achievable temporal resolution is strongly limited. Furthermore, the spatial resolution of the flow structures is distinctly confined by the low number of available measuring lines.
In cooperation with the TU Dresden a novel measurement principle  on the basis of transducer arrays was developed fitted to the particular requirements of measurements in liquid metal flows. The application of specific excitation methods as a simultaneous acquisition on several measuring lines and an interlaced pulsing strategy allows the acquisition of a flow field by a large number of measuring lines in the same time the conventional technique of commercial devices requires for measuring a single profile. Figure 2 shows the implementation of a corresponding measurement system comprising of two transducer arrays each with 24 measuring lines in an orthogonal arrangement providing a direct measurement of both velocity components in a measuring plane of extent 67 x 67 mm² (with 24 x 24 vectors). Thereby, frame rates up to 100 Hz may be obtained.
|Figure 2: Array arrangement for measuring the vector field in the plane of the transducer arrays|
|Figure 3: Application example of the array measurement system : time-dependent flow field (as contour plot) of an electro-vortex flow excited by an electrical current flow across two parallel electrodes. At the time t=0 the electrical current is powered up. Lorentz forces generated below the electrodes by this current induce the formation of downward directed jet streams recirculating in the bulk of the cylindrical vessel. The flow field was measured with one transducer array installed at the vessel bottom gathering the vertical flow component (with blue representing the downward directed flow and red the upward directed flow).|
Measurements at hot melts
Ultrasound measurements at hot melts (meaning T>150°C) necessitate extended transducer technologies to meet the requirements of high temperatures and chemical reactivity of such melts. For that two technological approaches are available: High temperature transducer and combined transducers with integrated acoustic wave guide.
High temperature transducers are principally constructed as conventional transducers except using piezo materials resisting temperatures up to 230°C. For reasons of system safety, the protection of the transducer as well as the complexity of wetting the probe a direct contact of the transducer surface with the melt is difficult. An alternative exists in the coupling through an interior wall (acoustic transmission plate e.g. made of stainless steel). For this purpose at HZDR specific probe sockets for ultrasound transducers (see figure 4a) were developed ensuring an optimal acoustic transmission path and a sufficient melt wetting, compensating effects of thermal expansion as well as enabling an easy maintenance.
By means of this technology measurements at various hot metal melts as sodium, tin-bismuth (SN60Bi40) and bismuth-lead (Bi55,5Pb44,5) may be performed. Such measurements were carried out in sodium  at the in-house facility NATAN, in SnBi at the LIMMCAST facility of HZDR (see figure 4) and in BiPb at facilities of collaboration partners in Latvia and Belgium .
To measure flow velocities of metal melts with temperatures above 230°C acoustic wave guides (see figure 5) are required. These guides consist of an adequate thin coiled metal foil in order to prevent the propagation of unwanted wave modes inside the guide. Over the length of the guide of typically 200 to 500 mm a sufficiently large temperature decrease is obtained to protect the piezo transducer at the end surface of the wave guide from overheating. In the current state of development this technique can be employed reliably in melts up to about 700°C. Beside measurements in Na, SnBi and BiPb combined transducers with acoustic wave guides were for example applied successfully for measuring flow velocities in a bronze melt (CuSn) at 620°C  and salt melt (of KaNO3) at 550°C. Drawbacks of the usage of wave guide probes compared to conventional transducers in the lower temperature range are a reduced signal-to-noise ratio a limitation of measurement depth.
|Figure 4: Measurement of the velocity profile of a tube flow at LIMMCAST facility (SnBi at 200°C) by applying high temperature transducers (a) schematic illustration of the mounted probe socket in cross-sectional view, (b) measured velocity profiles for various flow rates, (c) probe socket and partially installed high temperature transducer and (d) probe socket and transducer mounted at the horizontal test section of LIMMCAST facility|
|Figure 5: Acoustic wave guide probe|
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 S. Franke, D. Räbiger, V. Galindo, Y. Zhang, S. Eckert: Investigations of electrically driven liquid metal flows using an ultrasound Doppler flow mapping system, Flow Measurement and Instrumentation 48 (2016), 64-73.