Ultrasound Doppler measurements in liquid metal two-phase flows

Because of the ability to work in opaque fluids and to deliver complete velocity profiles in real time the Ultrasound Doppler Velocimetry (UDV) can be considered as an attractive technique to obtain velocity data from liquid metal flows. This paper presents several examples for UDV measurements in liquid metal two-phase flows. The measuring technique shows the capability to detect both the bubble and the liquid velocity simultaneously. The probability density function of the recorded velocity data was examined to the signals arising from the liquid and the gaseous phase. Multiple reflections of the ultrasonic beam occurring between the gas bubbles at higher gas flow rates may complicate a precise discrimination between both phase velocities. An iterative threshold method was applied to obtain accurate velocity profiles by identifying and excluding artifacts in the signal. Nevertheless, the signal quality deteriorates significantly with increasing gas flow rate.
Bubble-driven flows play a prominent role in many industrial facilities and processes. In metallurgical applications gas bubbles are injected into furnaces, ladles or similar melt containing transfer vessels in order to homogenize the melt and their physical and chemical properties. On the other hand, uncontrolled entrainment of cover gas into the liquid metal in the casting process causes a worsening of the mechanical properties of the casting products. In the field of nuclear engineering, gas entrainment into the coolant is one of the essential safety issues in the design of innovative liquid metal-cooled fast reactors. Model experiments using low-melting metal alloys can be considered as an effective tool to gain a better understanding of the complex character of liquid metal two-phase flows. In this paper, we present exemplary measurements performed at various experimental configurations concerning the behaviour of single bubbles and bubble plumes under the influence of AC and DC magnetic fields and a visualization of gas entrainment at the free surface of a liquid metal.