Application of the ultrasound Doppler method for flow measurements in LBE alloys

Design and optimisation of the thermal hydraulics of liquid metal reactor systems is strongly based on numerical simulations of the related fluid flow and heat & mass transfer processes. Whereas these numerical simulations are essentially based on local flow phenomena (small-scale vortices, turbulence or sub-grid scale modeling), experimental results are often limited to integral flow rates or local related data like temperature or pressure. Local velocity measurements would be highly desirable but are mostly lacking due to the very limited possibilities for velocity measurements in liquid metals.
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. In addition, it can principally operate through the channel wall though a direct contact to the melt reduces ultrasonic losses. However, 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.