Norman Thieme

Ultrasound Flow Mapping for the Investigation of Crystal Growth

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.

Modular ultrasound velocimeter for adaptive flow mapping in liquid metals

Understanding the complex interaction of conductive fluids and time-varying magnetic fields is the main goal of research in magnetohydrodynamics (MHD). Customized magnetic fields can be used to optimize flows in technical and industrial processes involving liquid metals. For example the performance of batteries with replaceable liquid electrolytes, such as zinc slurry energy storage systems, can be improved by magnetically influencing the flow of the electrolyte. However, necessary experimental studies are often limited by the performance of flow instrumentation for opaque liquids. We present a modular research platform for flow mapping in liquid metals, the phased array ultrasound Doppler velocimeter (PAUDV). It is based on the pulsed-wave ultrasound Doppler principle in combination with the phased array technique to provide an electrically steerable sound field, enabling novel applications in MHD research. The ability to dynamically focus the ultrasound beam allows to resolve smaller flow structures in planar measurements compared to fixed-beam sensors. The PAUDV can be applied to flows in narrow channels and two velocity components can be measured with only one acoustical access via the cross beam technique. Fast electrical traversing of the measurement volume allows to obtain and visualize turbulence statistics. A two-point correlation function can be retrieved by interleaving velocity measurements at two focal points of varying distance. The PAUDV consists of a modular electronics unit with up to eight beamformer cards, capable of driving a total of 256 channels. Each channel can be individually configured regarding the excitation pattern (three-level quantization, 64 samples) and the delay (1.6 ns resolution). Data acquisition and processing is implemented on multiple FPGAs, control and data visualization are performed on a PC. The capabilities of the modular research platform PAUDV are demonstrated on measurements in the alloy gallium-indium-tin at room temperature. Time resolved planar velocity vector maps are shown for a flow in a cubic vessel under the influence of a transient rotation magnetic field.

Modular research platform for adaptive flow mapping in liquid metals

Experimental studies in the field of magnetohydrodynamics (MHD) involving complex 3d flows are often limited by the performance of flow instrumentation for opaque liquids. We present a modular research platform for flow mapping in liquid metals, the phased array ultrasound Doppler velocimeter (PAUDV). It is based on the pulsed-wave ultrasound Doppler principle in combination with the phased array technique to provide an electrically steerable sound field, enabling novel applications in MHD research. The ability to dynamically focus and steer the ultrasound beam allows to resolve smaller flow structures compared to fixed-beam sensors. Two velocity components can be measured with only one acoustical access via the cross beam technique. Fast electrical traversing of the measurement volume allows to obtain and visualize turbulence statistics. A two-point correlation function can be retrieved by interleaving velocity measurements at two focal points of varying distance quasi-simultaneously.

Ultrasound flow mapping for 3D turbulent liquid metal flows

Conductive fluids, e.g. metallic melts, can be driven by magnetic fields, which is a branch of magnetohydrodynamics (MHD). MHD can be used for driving a melt flow during the crystal growth of photovoltaic silicon in order to improve the mass and heat transfer in the melt for better structural and electrical properties of the silicon crystals. However, the optimal application of MHD requires a good understanding of the flow, which is generally complex and unsteady during crystal growth. Substantial knowledge about the flow is usually gained through numerical simulations and MHD model experiments at room temperature. For model experiments, a comprehensive flow mapping of complex and unsteady flow phenomena is required.

Ultrasound flow mapping for the investigation of crystal growth

The production of high quality solar cells requires a deep understanding of the solidification process. Especially when time-dependent magnetic fields are used to improve the material and heat transfer in the melt, the resulting flow structures are complex and unsteady. Hence, numerical simulations are used to gain an insight into the melt flow. For the calibration of the numerical simulations, model experiments using liquid metals at room temperature are used. The melt flow is strongly influenced by the melt height, that constantly decreases during a solidification process. Hence, measuring the position and the shape of the solidification front is required for an understanding of the melt flow. Furthermore a comprehensive flow mapping of complex and unsteady flow phenomena is necessary. Commercial flow instrumentation systems usually utilize only one or a few single element probes that are operated strictly in sequential multiplex. This leads to low frame rates and limits their application to quasi-static flow fields.

Modular ultrasound platform for flow mapping in magnetohydrodynamics

Abstract Flow control based on time-dependent magnetic fields is used in various industrial processes involving liquid metals, i. e. during the solidification process of silicon to improve the quality and efficiency of wafers. In order to investigate the interactions between spatiotemporal-varying magnetic fields and conductive fluids numerical simulations are performed. The numerical models are verified by model experiments with opaque low-melting alloys. A suitable measurement technique for flow mapping in such model experiments is ultrasound Doppler velocimetry. In contrast to conventional systems employing transducers with fixed sound field our approach is to use a phased array with the ability to focus and steer its acoustic field. We present the Phased Array Ultrasound Doppler Velocimeter (PAUDV) for flow mapping in magnetohydrodynamics. With its custom host software for control and signal processing experiments that are flexible regarding the experimental setup can be defined.