Richard Nauber

Dual-plane ultrasound flow measurements in liquid metals

An ultrasound measurement system for dual-plane, two-component flow velocity measurements especially in opaque liquids is presented. Present-day techniques for measuring local flow structures in opaque liquids disclose considerable drawbacks concerning line-wise measurement of single ultrasound probes. For studying time-varying flow patterns, conventional ultrasound techniques are either limited by time-consuming mechanical traversing or by the sequential operation of single probes. The measurement system presented within this paper employs four transducer arrays with a total of 100 single elements which allows for flow mapping without mechanical traversing. A high frame rate of several 10 Hz has been achieved due to an efficient parallelization scheme using time-division multiplexing realized by a microcontroller-based electronic switching matrix. The functionality and capability of the measurement system are demonstrated on a liquid metal flow at room temperature inside a cube driven by a rotating magnetic field (RMF). For the first time, the primary and the secondary flow have been studied in detail and simultaneously using a configuration with two crossed measurement planes. The experimental data confirm predictions made by numeric simulation. After a sudden switching on of the RMF, inertial oscillations of the secondary flow were observed by means of a time-resolved measurement with a frame rate of 3.4 Hz. The experiments demonstrate that the presented measurement system is able to investigate complex and transient flow structures in opaque liquids. Due to its ability to study the temporal evolution of local flow structures, the measurement system could provide considerable progress for fluid dynamics research, in particular for applications in the food industry or liquid metal technologies.

Phased Array Ultrasound System for Planar Flow Mapping in Liquid Metals

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.

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.

Time reversal ultrasound focusing through multimode waveguides

Abstract Ultrasound imaging in harsh environments, such as the continuous steel casting process, benefits from a spatial separation of sensors and measuring volume to avoid damaging e.g. because of high temperatures. This can be achieved through acoustical multimode waveguides. To focus ultrasound in the measuring volume despite the complex sound propagation, we propose using the time reversal technique. We present numerical simulations and experiments using the phased array ultrasound Doppler velocimeter to focus through a water filled waveguide with a 64 element array. A resolution in the millimetre range is achieved for a 68 mm long waveguide.

Instabilities and spin-up behaviour of a rotating magnetic field driven flow in a rectangular cavity

This study presents numerical simulations and experiments considering the flow of an electrically conducting fluid inside a cube driven by a rotating magnetic field (RMF). The investigations are focused on the spin-up, where a liquid metal (GaInSn) is suddenly exposed to an azimuthal body force generated by the RMF and the subsequent flow development. The numerical simulations rely on a semi-analytical expression for the induced electromagnetic force density in an electrically conducting medium inside a cuboid container with insulating walls. Velocity distributions in two perpendicular planes are measured using a novel dual-plane, two-component ultrasound array Doppler velocimeter with continuous data streaming, enabling long term measurements for investigating transient flows. This approach allows identifying the main emerging flow modes during the transition from stable to unstable flow regimes with exponentially growing velocity oscillations using the Proper Orthogonal Decomposition method. Characteris...

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.

Modulares Ultraschall-Array-Doppler-Velozimeter zur Messung von komplexen Strömungen in Flüssigmetallen

Zusammenfassung Bei verschiedenen industriellen Verfahren der Metallurgie und der Halbleiterkristallzucht werden Magnetfelder eingesetzt, um Flüssigkeitsströmungen in elektrisch leitfähigen Schmelzen zu beeinflussen. Die Überwachung der Strömungsprozesse erfolgt häufig mit Ultraschall. Zeit- und Ortsauflösung der gegenwärtig kommerziell verfügbaren Ultraschallmesssysteme reichen jedoch nicht aus, um beispielsweise komplexe Wirbelstrukturen oder Turbulenzen aufzulösen. Dieser Beitrag stellt mit dem modularen Ultraschall-Array-Doppler-Velozimeter ein neuartiges Messsystem vor, das es ermöglicht, solche Strömungen mehrdimensional und mehrkomponentig zu erfassen. Durch den Einsatz von vier Ultraschallwandlerarrays mit je 25 Piezoelementen und einer parallelisierten Ansteuerung der Elemente im Zeitmultiplex werden Bildwiederholraten von 28 Hz bei einer Ortsauflösung von ca. 3 mm erreicht. Damit ergeben sich neue Möglichkeiten zu einem weiterführenden Verständnis von komplexen Strömungsprozessen in der Magnetohydrodynamik. Abstract

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.