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Crystal growth processes can profit from an electromagnetically driven melt flow since controlling them allows optimizing the mass and heat transfers in the melt and, thereby, improves the structural and electrical properties of the grown crystals. This process optimization requires a precise understanding of magnetohydrodynamics (MHD) phenomena in crystal growth. Studying time-dependent MHD demands for a high temporal resolution combined with a long measurement duration to analyze the transitional flow behavior. Furthermore, a spatially resolved measurement of the global flow structure is desired to capture the complex 3-D flow structures. We present an ultrasound array Doppler velocimeter (UADV) for time-resolved flow imaging in MHD model experiments with low-melting metals. Flow imaging at frame rates of several Hertz is achieved by using a combined spatial and temporal multiplexing scheme. Long-running measurements are enabled by a field-programmable gate array (FPGA)-based signal processing that reduces the measurement data rate by a factor of 5. A reconstruction of the 3-D flow structure in cylindrical containers with a rotation-symmetric flow is proposed. The UADV is demonstrated at an MHD experiment with a melt flow in a cylindrical container driven by a traveling magnetic field. The transition from a laminar to a time-dependent flow is studied, revealing an oscillating flow. The demonstration of the 3-D reconstruction gives comprehensive insight into the global flow structure. Hence, the UADV method is shown to be a valuable tool for measuring complex, time-dependent melt flows, which can contribute to a better understanding of the flow phenomena during crystal growth. [ABSTRACT FROM AUTHOR]

Controllable magnetic fields can be used to optimize flows in technical and industrial processes involving liquid metals in order to improve quality and yield. However, experimental studies in magnetohydrodynamics often involve complex, turbulent flows and require planar, two-component (2c) velocity measurements through only one acoustical access. We present the phased array ultrasound Doppler velocimeter as a modular research platform for flow mapping in liquid metals. It combines the pulse wave Doppler method with the phased array technique to adaptively focus the ultrasound beam. This makes it possible to resolve smaller flow structures in planar measurements compared with fixed-beam sensors and enables 2c flow mapping with only one acoustical access via the cross beam technique. From simultaneously measured 2-D velocity fields, quantities for turbulence characterization can be derived. The capabilities of this measurement system are demonstrated through measurements in the alloy gallium-indium–tin at room temperature. The 2-D, 2c velocity measurements of a flow in a cubic vessel driven by a rotating magnetic field (RMF) with a spatial resolution of up to 2.2 mm are presented. The measurement results are in good agreement with a semianalytical simulation. As a highlight, two-point correlation functions of the velocity field for different magnitudes of the RMF are presented. [ABSTRACT FROM AUTHOR]

A high energy conversion and cost efficiency are keys for the transition to renewable energy sources, e.g., solar cells. The efficiency of multicrystalline solar cells can be improved by enhancing the understanding of its crystallization process, especially the directional solidification. In this paper, a novel measurement system for the characterization of flow phenomena and solidification processes in low-temperature model experiments on the basis of ultrasound (US) Doppler velocimetry is described. It captures turbulent flow phenomena in two planes with a frame rate of 3.5 Hz and tracks the shape of the solid-liquid interface during multihour experiments. Time-resolved flow mapping is performed using four linear US arrays with a total of 168 transducer elements. Long duration measurements are enabled through an online, field-programmable gate array (FPGA)-based signal processing. Nine single US transducers allow for in situ tracking of a solid–liquid interface. Results of flow and solidification experiments in the model experiment are presented and compared with numerical simulation. The potential of the developed US system for measuring turbulent flows and for tracking the solidification front during a directional crystallization process is demonstrated. The results of the model experiments are in good agreement with numerical calculations and can be used for the validation of numerical models, especially the selection of the turbulence model. [ABSTRACT FROM PUBLISHER]