Christian Karcher

Electromagnetic flow measurements in liquid metals using time-of-flight Lorentz force velocimetry

Time-of-flight Lorentz force velocimetry is a non-invasive electromagnetic measurement technique that can be used to determine both the flow rate and/or the local velocities in electrically conducting fluids like liquid metals. Using this technique, two identical so-called Lorentz force flow meters—each consisting of a permanent magnet system and an attached digital force sensor—are arranged in a row and separated by a defined distance. Each flow meter measures the Lorentz force that is generated within the melt when the electrically conducting liquid metal passes the magnetic field. This time-of-flight technique can be exploited for the flow measurement by purely cross-correlating the two force signals. Hence, the measurement becomes independent of any fluid properties and magnetic field parameters. We present results of two model experiments that demonstrate that time-of-flight Lorentz force velocimetry is feasible for non-contact measurement of both global flow rates and local surface velocity in turbulent liquid metal flow. In these experiments, we use the low-melting eutectic alloy Ga68In20Sn12 as a test melt. Moreover, to support these experimental findings, we present results of numerical simulations using the commercial codes FLUENT and MAXWELL. The numerical predictions are in good agreement with the experimental findings.

Local Lorentz force flowmeter at a continuous caster model using a new generation multicomponent force and torque sensor

Lorentz force velocimetry is a non-invasive velocity measurement technique for electrical conductive liquids like molten steel. In this technique, the metal flow interacts with a static magnetic field generating eddy currents which, in turn, produce flow-braking Lorentz forces within the fluid. These forces are proportional to the electrical conductivity and to the velocity of the melt. Due to Newtons third law, a counter force of the same magnitude acts on the source of the applied static magnetic field which is in our case a permanent magnet. In this paper we will present a new multicomponent sensor for the local Lorentz force flowmeter (L2F2) which is able to measure simultaneously all three components of the force as well as all three components of the torque. Therefore, this new sensor is capable of accessing all three velocity components at the same time in the region near the wall. In order to demonstrate the potential of this new sensor, it is used to identify the 3-dimensional velocity field near the wide face of the mold of a continuous caster model available at the Helmholtz-Zentrum Dresden-Rossendorf. As model melt, the eutectic alloy GaInSn is used.

Low-Prandtl-Number Marangoni Convection Driven by Localized Heating on the Free Surface: Results of Three-Dimensional Direct Simulations

Marangoni convection in a square container with localized heating on the free liquid surface is simulated numerically using a pseudospectral Fourier-Chebyshev method. The extensive three-dimensional computations are performed for a low Prandtl number typical of liquid metals. Upon increasing the Marangoni number, the initially steady flow becomes oscillatory and eventually chaotic. The transitions reduce the spatial symmetries of the flow. Suitably defined integral velocity and temperature show scaling on the Marangoni number. The scaling exponents are compared with predictions from theoretical models for the heat transport in flow geometries relevant for the industrial process of electron beam evaporation.

Contactless flow measurement in liquid metal using electromagnetic time-of-flight method

Measuring flow rates of liquid metal flows is of utmost importance in industrial applications such as metal casting, in order to ensure process efficiency and product quality. A non-contact method for flow rate control is described here. The method is known as time-of-flight Lorentz force velocimetry (LFV) and determines flow rate through measurement of Lorentz force that act on magnet systems that are placed close to the flow. In this method, a vortex generator is used to generate an eddy in the flow, with two magnet systems separated by a known distance placed downstream of the vortex generator. Each of the magnet systems has a force sensor attached to them which detects the passing of the eddy through its magnetic field as a significant perturbation in the force signal. The flow rate is estimated from the time span between the perturbations in the two force signals. In this paper, time-of-flight LFV technique is demonstrated experimentally for the case of liquid metal flow in a closed rectangular duct loop that is driven by an electromagnetic pump. A liquid metal alloy of gallium (Ga), indium (In) and tin (Sn)—GaInSn—is used as the working fluid. In contrast to prior works, for the first time, three-dimensional strain gauge force sensors were used for measuring Lorentz force to investigate the effect of flow disturbances in different directions for flow measurements by the time-of-flight LFV method. A prototype time-of-flight LFV flowmeter is developed, the operation of which in laboratory conditions is characterised by different experiments.

Lorentzkraft — Anemometrie für die berührungslose Durchflussmessung von Metallschmelzen

Zusammenfassung In der Metallurgiebranche fehlen derzeit geeignete Verfahren zur präzisen Erfassung, Regelung und Dosierung der zwischen den einzelnen Produktionsstufen übertragenen Mengen an Metallschmelze. Durch den Einsatz des patentierten Verfahrens der Lorentzkraft-Anemometrie, bei dem der direkte Kontakt zur heißen Metallschmelze nicht erforderlich ist, lässt sich diese Aufgabe lösen und somit ein nachhaltiger Beitrag zu zukünftig energie- und kostenoptimierter Produktion leisten. Im vorliegenden Artikel werden das Prinzip des Verfahrens erläutert und die wissenschaftlich-technischen Wege zur Entwicklung, Prüfung und Kalibierung von entsprechenden Lorentzkraft-Anemometern vorgestellt. Desweiteren werden Beispiele aktueller Anwendungen der Lorentzkraft-Anemometrie in der Praxis diskutiert. Abstract

Electromagnetic interaction between a rising spherical particle in a conducting liquid and a localized magnetic field

Lorentz force velocimetry (LFV) is a non-contact electromagnetic flow measurement technique for electrically conductive liquids. It is based on measuring the flow-induced force acting on an external permanent magnet. Motivated by extending LFV to liquid metal two-phase flow measurement, in a first test we consider the free rising of a non-conductive spherical particle in a thin tube of liquid metal (GaInSn) initially at rest. Here the measured force is due to the displacement flow induced by the rising particle. In this paper, numerical results are presented for three different analytical solutions of flows around a moving sphere under a localized magnetic field. This simplification is made since the hydrodynamic flow is difficult to measure or to compute. The Lorentz forces are compared to experiments. The aim of the present work is to check if our simple numerical model can provide Lorentz forces comparable to the experiments. The results show that the peak values of the Lorentz force from the analytical velocity fields provide us an upper limit to the measurement results. In the case of viscous flow around a moving sphere we recover the typical time-scale of Lorentz force signals.

Control of heat transfer in engine coolers by Lorentz forces

In engine coolers of off-highway vehicles convective heat transfer at the coolant side is a limiting factor of both efficiency and performance density of the cooler. Here, due to design restrictions, backwater areas and stagnation regions appear that are caused by flow deflections and cross-sectional expansions. As appropriate coolants, mixtures of water and glysantine are commonly used. Such coolants are characterized by their electrical conductivity of some S/m. This gives rise to control coolant flow and therefore convective heat transfer by means of Lorentz forces. These body forces are generated within the weakly conducting fluid by the interactions of an electrical current density and a localized magnetic field both of which being externally superimposed. In application this may be achieved by inserting electrodes in the cooler wall and a corresponding arrangement of permanent magnets. In this paper we perform numerical simulations of such magnetohydrodynamic flow in three model geometries that are frequently apparent in engine cooling applications: Carnot-Borda diffusor, 90° bend, and 180° bend. The simulations are carried out using the software package ANSYS Fluent. The present study demonstrates that, depending on the electromagnetic interaction parameter and the specific geometric arrangement of electrodes and magnetic field, Lorentz forces are suitable to break up eddy waters and separation zones and are thus significantly increasing convective heat transfer in these areas. Furthermore, the results show that due to the action of the Lorentz forces the hydraulic pressure losses can be reduced.

Computational MHD Part III — Application to Electromagnetic Control of Convective Flows

Convective flow in a liquid metal heated locally at its upper surface and affected by an applied time-dependent magnetic field is investigated. The system under consideration serves as a physical model for the industrial process of electron beam evaporation of liquid metals. In this process, the strong energy input induces strong temperature gradients along the free surface and in the interior of the melt. Thus, the liquid metal is subject to both thermocapillary and natural convection. The vigorous convective motion within the melt leads to highly unwelcome heat losses through the walls of the crucible. The strong convective heat transfer limits the temperature rise in the hot spot and, therefore, the thermodynamic efficiency of the evaporation process. The present paper aims to demonstrate that the melt-flow can be effectively controlled by using external magnetic fields in order to considerably reduce the convective heat losses. As examples, we employ numerical simulations based on the finite element method to study the effects of both a traveling magnetic field and a rotating magnetic field.