Ultrasound flow mapping for 3D turbulent liquid metal flows

Flows in metallic melts can be driven by magnetic fields, which is a branch of magnetohydrodynamics (MHD). Crystal growth processes can profit from magnetically driven flows, since it can optimize the mass and heat transfer in the melt and thereby improve the structural and electrical properties of the grown crystals. For a better understanding of MHD phenomena in crystal growth, model experiments with low-melting metals were developed to investigate the melt flow with ultrasound Doppler velocimetry. To study complex, time-dependent MHD flows, there is a need for temporary and spatially resolved 2D flow measurements. We present an Ultrasound Array Doppler Velocimeter (UADV) for time-resolved flow mapping in MHD model experiments with low-melting metals, e.g. the alloy gallium-indium-tin (GaInSn, melting point 10.5°C). The UADV uses a line-scanning approach with linear arrays of large US elements (pitch <; λ). The measurement of turbulent melt flows demands for a 2D flow mapping at high frame rates of several Hertz, which is achieved by employing a spatial and time multiplexing scheme. The investigation of time-dependent flows requires long-running measurements. Therefore, an FPGA-based signal processing reduces the measurement data by a factor of 10. The UADV is demonstrated at an MHD experiment with a melt flow driven by a traveling magnetic field. The transition from a laminar to a turbulent flow is shown.