Andreas Cramer

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Investigation of the flow driven by an alternating magnetic field

The flow induced by a single-phase alternating magnetic field is studied numerically and in a phy... more The flow induced by a single-phase alternating magnetic field is studied numerically and in a physical model. It is shown that the flow structure depends drastically on the frequency of the field, owing to a changeover of the rotor of the Lorentz force in the corners from a single local maximum to a pair of local maxima, one in close vicinity of another, with different signs of vorticity.

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The sensitivity of the flow driven by a travelling magnetic field to axial alignment

The present experimental study is concerned with the sensitivity of the flow driven by a travelli... more The present experimental study is concerned with the sensitivity of the flow driven by a travelling magnetic field (TMF) to axial alignment. Referring to the center axis of the TMF generating coil system the fluid volume was stepwise dealigned. Because the flow induced in a TMF is, basically, of a torus type, vertical velocity components are representative for the motion in the meridional plane. To acquire velocity profiles the Ultrasound Doppler velocimetry (UDV) was used which allows gathering the whole profile along the ultrasonic beam. Several transducers were mounted at the bottom of the fluid covering vessel and connected to the multiplexer channels of the UDV device. Eutectic GaInSn was used as working fluid. Analysing mean velocity profiles and the spatiotemporal properties of the flow, the study shows, that already for small deviations from coaxial conditions the flow topology changes.

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Thermoelectromagnetic stirring in metallurgy

Thermoelectromagnetic convection in cubic containers was studied experimentally. Two opposing sid... more Thermoelectromagnetic convection in cubic containers was studied experimentally. Two opposing side walls were cooled and heated, respectively, to produce an uniform temperature gradient. Inhomogeneous magnetic field distributions B were achieved either with a permanent magnet or with specifically shaped pole shoes of an electromagnetic system. Ultrasonic Doppler velocimetry demonstrated that even a moderate temperature gradient may drive distinct convection. Two different flow regimes were investigated with the permanent magnet. Located at an isothermal wall, it produced a single vortex spreading the whole container while the flow was relatively stable. Moving the magnet to the center altered the flow structure. Four vortices developed and the velocity fluctuations were intensified. The more generic case realised with the electromagnet provided a gradient of B only in one direction. Since the field strength and the area of impact on the melt were larger, developed turbulent regimes were accomplished.

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Magnetic Field Control of the Melt Extraction Process

Direct extraction out of a crucible is a rapid solidification process for the production of advan... more Direct extraction out of a crucible is a rapid solidification process for the production of advanced materials such as intermetallic compound fibres. The main industrial applications require these fibres to have a maximum diameter significantly below 100 μm. Today the melt extraction technology does not fulfil this requirement without additional control mechanisms. The main limitations of the process are due to turbulence within the melt and a wavy motion on the surface. In the patent literature many examples can be found which try to overcome these problems by applying mechanical parts, usually made of heat resistant ceramics. But none of them works reliably.

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Sound velocity measurements in fluids using echo signals from scattering particles

A novel approach for measuring the speed of sound in fluids with scattering particles is presente... more A novel approach for measuring the speed of sound in fluids with scattering particles is presented. Potential fields of application for sound velocity measurements in fluids are process control, environmental measurement technology and medicine, where sound velocity can be used as an indicator of temperature, concentration or mass density. Similar to the pulsed Doppler application, the method also works non-invasively and uses the echo signals from scattering particles suspended in the fluid. The basic idea is that the ultrasonic time of flight to the focus position z depends on the speed of sound c in a well-defined way. The time of flight to the focus can be extracted from the echo signals, because the stray echo is strongest for the scattering particles being located in the sound focus and can thus be used to determine the speed of sound. Results are shown for different homogeneous fluids with sound velocities between 1116 m/s (ethanol, 50 °C) and 2740 m/s (eutectic GaInSn). Meas...

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Flow measurements in a model of the Czochralski crystal growth process

An experimental study of the buoyancy-induced flow in a model of a Czochralski crystal growth sys... more An experimental study of the buoyancy-induced flow in a model of a Czochralski crystal growth system was conducted. Ultrasonic velocimetry was used to measure fluid velocities. To have similar thermal boundary conditions as in an industrial growth facility, a double walled glass crucible flown through by a heating fluid was chosen to hold the fluid. Similarity of the heat transfer conditions was achieved by selecting GaInSn as liquid metal under investigation. Due to the double-walled crucible, measurements through the container wall are difficult if ever possible. Since the availability of short ultrasonic transducers it is practicable to have the sensor immersed into the fluid. Measurements of the radial velocity component shortly below the melt surface across the entire diameter of the crucible at various azimuthal angles reveal the complex flow structure of natural convection in a Czochralski crucible. As it is not to be expected to grow high quality mono-crystalline crystals fr...

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Biogenic amorphous silica as main driver for plant available water in soils

Scientific Reports

More frequent and longer drought periods are predicted threatening agricultural yield. The capaci... more More frequent and longer drought periods are predicted threatening agricultural yield. The capacity of soils to hold water is a highly important factor controlling drought stress intensity for plants. Biogenic amorphous silica (bASi) pools in soils are in the range of 0–6% and are suggested to help plants to resist drought. In agricultural soils, bASi pools declined to values of ~1% or lower) due to yearly crop harvest, decreasing water holding capacity of the soils. Here, we assessed the contribution of bASi to water holding capacity (WHC) of soil. Consequently, ASi was mixed at different rates (0, 1, 5 or 15%) with different soils. Afterwards, the retention curve of the soils was determined via Hyprop method. Here we show that bASi increases the soil water holding capacity substantially, by forming silica gels with a water content at saturation higher than 700%. An increase of bASi by 1% or 5% (weight) increased the water content at any water potential and plant available water in...

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Experimental investigation on the electromagnetically controlled buoyancy-induced flow in a model of a Czochralski puller

International Journal of Applied Electromagnetics and Mechanics

ABSTRACT A great deal of work with respect to both physical and numerical modelling of Czochralsk... more ABSTRACT A great deal of work with respect to both physical and numerical modelling of Czochralski crystal growth has been conducted in the generic Rayleigh-Bénard system. In order to come closer to the conditions in a real Czochralski puller, specific effects such as the influence of a rounded crucible bottom, deviations of the thermal boundary conditions from the generic case, crucible and/or crystal rotation, and the influence of magnetic fields are often studied separately. Within this paper we present a model experiment focusing on investigations of the impact of magnetic fields on the flow in a Czochralski puller. To achieve similar thermal boundary conditions as in an industrial growth facility, a double-walled rounded bottom glass crucible was chosen to hold the fluid. Similarity of the heat transfer conditions was guaranteed by selecting the ternary alloy GaInSn as the model fluid. Measurements of the fluid flow have been conducted by means of the ultrasound Doppler velocimetry. The results reveal the complex flow structure of natural convection in a Czochralski crucible. Because the growth of high quality mono-crystalline crystals is impeded by such a non-axisymmetric flow, rotating magnetic fields (RMF) are often proposed to render the flow more axisymmetric. To study the effect of an RMF on the natural convection in a Czochralski system the experimental apparatus was mounted inside the home-made MULTIpurpose MAGnetic field facility (MULTIMAG). In the present contribution the three-dimensional convective patterns as well as the resulting temperature fluctuations will be discussed both for the pure buoyant case and for the application of an RMF.

Macroscopic thermomagnetic convection: a more generic case and optimization

Magnetohydrodynamics

ABSTRACT

Model experiments on macroscopic thermoelectromagnetic convection

Magnetohydrodynamics

The interaction between a thermoelectric current and an imposed magnetic field may produce thermo... more The interaction between a thermoelectric current and an imposed magnetic field may produce thermoelectromagnetic convection (TEMC). In the present paper, an experimental study on TEMC in a generic configuration is reported. While the necessary temperature gradient grad T in a square box was accomplished by heating and cooling of two opposing side walls, respectively, utilising a massive nickel plate for the bottom of the electrically conducting container established a material discontinuity with respect to the liquid metal layer. Primarily, such a jump in the related Seebeck coefficient non-parallel to grad T is a prerequisite for the existence of a thermoelectric current. The second condition for TEMC, which is a non-vanishing curl of the Lorentz force, was fulfilled using a permanent magnet producing an inhomogeneous magnetic field. Ultrasonic Doppler velocimetry was used to quantify the TEMC flow field. The measurements demonstrate that even a moderate temperature difference can produce a distinct convection. Locating the magnet, the direction of magnetization of which was parallel to grad T , close to either side wall produced a single vortex spreading throughout the entire box. Moving the magnet to the centre led to a modified distribution of the magnetic field, which, in turn, altered the flow pattern. A convective pattern consisting of four vortices developed and the velocity fluctuations were intensified. The numerical results for the distribution of the magnetic field in the presence of the ferromagnetic bottom support the experimental findings.

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A physical model for electromagnetic control of local temperature gradients in a Czochralski system

Magnetohydrodynamics

The shape of the solidification front and the related mono-crystalline growth in the Czochralski ... more The shape of the solidification front and the related mono-crystalline growth in the Czochralski crystal growth process is thought of being strongly influenced by the ratio of the horizontal and the vertical temperature gradient r * = ∆T h /∆Tv at the triple point liquid-solid-atmosphere, which ratio desirably should be in the order of unity. A liquid metal model experiment was therefore built that allows studying r * under the influence of magnetic fields. The cylindrical liquid metal column was homogeneously heated from below, whereas on top the heat was extracted in a centrical region covering only one third of the surface in order to simulate the growing crystal. Without flow control, r * ≈ 3 is far removed from unity. It was then possible to reach the target value r * = 1 for any temperature difference between the bottom and the top at a moderate field strength while applying a rotating magnetic field.

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Physical modelling of temperature fluctuations in a high aspect ratio model of the Czochralski crystal growth

Journal of Crystal Growth

Applications of Ac and DC Magnetic Fields in Metallurgical and Crystal Growth Processes

Materials Processing in Magnetic Fields, 2005

ABSTRACT Nowadays, magnetic fields are widely utilized in metallurgical and crystal growth applic... more ABSTRACT Nowadays, magnetic fields are widely utilized in metallurgical and crystal growth applications on an industrial scale. This paper focuses on laboratory studies using liquid metals with a melting point up to 300 °C to model the realistic processes. Based on four examples it is shown that this physical modeling has a great potential in both, revealing an insight into the basic phenomena involved, and the optimization of the technological details. Direct melt extraction of fibers and the aluminum investment casting have been chosen representatively for the damping of flow by a DC magnetic field. Stabilization, i.e. the retarding of instabilities, is demonstrated with electromagnetic levitation. We are considering the floating-zone technique as an example from the large field of crystal growth.

Method and System for the Contactless Determination of Spatial Speed Distributions in Electrically Conductive Fluids

Thermoelectromagnetic convection - An alternative stirring technique in metallurgy

ABSTRACT

Experimental Investigation of Rotating Magnetic Field Driven Flow by Highly Sensitive Potential Probe Measurements

ABSTRACT

Fluid Flow Measurements in Electrically Driven Vortical Flows

This class of flows is characterized by the feeding of strong electrical currents into a volume o... more This class of flows is characterized by the feeding of strong electrical currents into a volume of liquid metal. A prominent example of industrial interest is the electrode-welding where the current is supplied via a locally restricted area, leading to high current densities. The interaction of these currents with their own magnetic field drives flows which exhibit phenomena such as formation of jets and the pinch-effect. Further increasing the current may lead to disruption of the melt column accompanied by a discharge. This can be used for a liquid metal current limiter with self-healing properties. Local measurements of the flow structures are of obvious interest for a better understanding of these phenomena. The paper will give results from a systematic study of the flow structure performed by means of our mechano-optical velocity probe. The measured flow structure of the evolving jet was found to be in a reasonable agreement with our numerical simulations.

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Effect of the Superimposed DC Magnetic Field on a Melt Flow in an Induction Furnace Experimental Model

The unsteady flow field has been studied experimentally in a cold metal model of an existing 10 k... more The unsteady flow field has been studied experimentally in a cold metal model of an existing 10 kHz induction furnace for the intermetallic fiber extraction in presence of the DC magnetic field. It has been shown that a vertical DC magnetic field normal to a free liquid metal surface efficiently decreases turbulence in the melt volume and suppresses surface deformations. Whereas the turbulence and free surface deformations were not suppressed by the DC field parallel to the free surface.

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Velocity Measurement Techniques for Liquid Metal Flows

Fluid Mechanics And Its Applications, 2007

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Measurement of the sound velocity in fluids using the echo signals from scattering particles

Ultrasonics, 2012

With conventional methods the sound velocity c in fluids can be determined using the back wall ec... more With conventional methods the sound velocity c in fluids can be determined using the back wall echo. This paper proposes a novel technique, in which the signals reflected by scattering particles suspended in a fluid are analysed instead. The basic idea is that the particles generate the strongest echo signal when being located in the sound field maximum. Therefore the position of the echo signal maximum is a measure for the propagation time to the sound field maximum. Provided that calibration data or sound field simulations for the ultrasonic transducer are available, this propagation time suffices to determine both sound velocity and the location of the sound field maximum. The feasibility of the new approach is demonstrated by different kinds of experiments: (i) Measurements of the sound velocity c in four fluids covering the wide range between 1116 and 2740 m/s. The results show good agreement with values published elsewhere. (ii) Using the dependence of the sound velocity on temperature, it is possible to vary c over the comparatively small range between 1431 and 1555 m/s with increments of less than 10 m/s. The measured statistical variation of 1.4 m/s corresponds to a relative uncertainty not worse than 0.1%. (iii) The focus position, i.e. the distance of the maximum of the sound field from the transducer, was varied by time-shifted superposition of the receive signals belonging to the different elements of an annular array. The results indicate that the novel method is even capable of measuring profiles of the sound velocity along the ultrasonic beam non-invasively.

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Andreas Cramer

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