Lorentz Force Flowmeter for Liquid Aluminum: Laboratory Experiments and Plant Tests
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This article aims to demonstrate that molten metal flow at a high temperature can be measured effectively in a contactless manner by using external direct current magnetic fields. The device applied in the present work is termed Lorentz force flowmeter (LFF) and is based on exposing the flow to a magnet system and measuring the drag force acting on it. Two series of measurements are reported. In the first series, we perform a model experiment in the laboratory using the eutectic alloy GaInSn, which is liquid at room temperature. The second series of measurements is devoted to two plant tests on flow measurement of a liquid aluminum alloy. In both tests, the force acting on the magnet system is measured that is equal to the Lorentz force acting on the flow. To generalize our results, we also derive the scaling law that relates the force acting on a localized magnet system to the flow rate of a fluid with arbitrary electrical conductivity. This law shows that LFF, if properly designed, has a wide range of potential applications in ferrous and nonferrous metallurgy.
KeywordsLiquid Metal Mass Flow Rate Lorentz Force Magnet System Volumetric Flow Rate
This present work is devoted to demonstrating the applicability of a recently developed electromagnetic noncontact flow measurement device—the Lorentz force flowmeter (LFF)—to flow measurement in the production of secondary aluminum.
A considerable part of the world’s aluminum demand is satisfied through secondary aluminum production.[1, 2] During the production process, aluminum scrap is molten in special rotary furnaces fired by heavy oil to support a sufficiently high temperature. The burner system generates temperatures higher than 1973 K (1700 °C) inside the furnace. The capacity of the furnaces shows a typical range of 8 to 14 tons. After tapping the furnace, the primary melt flows under the action of gravity through an open channel into converters that are located below ground level. In these converters, the final composition of the alloy is prepared by adding alloying elements including Si, Cu, Fe, Mn, and Cr among others. Finally, after tapping of the converter, the final melt flows again through an open channel either to the casting machine, where it will solidify, or into a preheated crucible, which is used to deliver the liquid aluminum to the customer.[1, 2, 3]
To monitor and control the production process, it would be desirable to measure the mean velocity of the liquid aluminum continuously to deduce the mass-flux and volume-flux. Until now, however, no melt-flow measurement system is commercially available. In general, the production process is controlled only by weighing the scrap and either the solid blocks or the containers holding the final liquid melt (i.e., by taking data only at the beginning and the end of the process). During the production, melting salt (NaCl + KCl) is added to avoid heat waste and to absorb the burn-off resulting from scrap contamination with plastics, oil, and dirt. So, after tapping, it is not known how much aluminum remains in the rotary furnace and/or how much aluminum is in the converter. Therefore, important information during the production process is not available. For instance, the scrap performance cannot be evaluated exactly and the exact amount of additions into the converter cannot be calculated.
The object of the present study is the development and demonstration of the feasibility of a noncontact electromagnetic system to measure a liquid metal flow. Our approach embodies the Lorentz force velocimetry[4, 5, 6, 7, 8, 9, 10] technique, which is physically based on measuring the force acting on an external magnet system that interacts with the flow. This force is exactly equal to the braking Lorentz force induced in a flowing conducting fluid by an external magnetic field. In what follows, a measurement system based on the Lorentz force velocimetry principle will be called LFF.
As will be detailed in the following, the LFF possesses several advantages that make it attractive to measure flows in hot and aggressive melts. First, the method is entirely contactless. Second, the relation between the measured force and the desired volumetric flow rate is often linear. Third, the slope of this linear function—the calibration coefficient—is independent of the viscosity and density of the liquid metal and depends only on the magnetic field magnitude, the electrical conductivity of the liquid metal, and the geometry of the channel in which the liquid metal flows. Therefore, a calibration curve obtained for one liquid metal can be used directly for other liquid metals provided that their electrical conductivities are known. In aluminum production, for instance, numerous standard alloys exist that have well-defined conductivities for which the calibration factors thus can be tabulated.
In what follows, namely in Section II, we give a brief review of the principles on which a LFF is based. In Section III, we explain how the measured force signal of an LFF can be converted into the desired volume flux. In Section IV, which is the main body of the present work, we report the results of velocity measurements both in a laboratory, room-temperature liquid metal flow using a small-scale LFF and in the plant of a producer of secondary aluminum using a full-scale LFF prototype for industrial flows. In Section V we summarize our conclusions and indicate future research directions.
The key question for a successful implementation of LFF is how the unknown flow rate of a liquid metal can be computed from the measured force. This question can be answered either by performing a numerical simulation of the full magnetohydrodynamic problem or by calibration of the measuring system. In the present work, we are concerned exclusively with the second method.
Lorentz Force Measurement
An industrial channel for liquid aluminum transportation on the external wall surface has a real temperature that does not exceed 323 K (50 °C). This is a consequence of the large thickness of 60 to 80 mm of the walls made of concrete, of their comparatively low thermal conductivity of about 0.5 W/m K, and of natural air convection around the channel. Taking this into account, we are led to the conclusions that the magnetic induction deviation does not exceed 1 to 1.5 pct. Nonetheless, to monitor a possible temperature variation in the industrial tests, we install a thermocouple on the surface of magnet system.
To determine c for our laboratory LFF described in Section IV–A, we measure the Lorentz force F directly for different volumetric flow rates q that are set up in a room-temperature, liquid-metal test channel. We choose the magnetic field induction B 0 equal to its value on the flow axis. In Section IV–B, for the calibration of our industrial LFF, we use one of two tests at aluminum production in a plant in which the total mass of discharged aluminum is known. In this case, we also measure Lorentz force and use the cumulative weight of aluminum in the end of the process to compute its flow rate through the channel.
Volumetric Flow Rate and Cumulative Volume
Mass Flow Rate and Cumulative Mass
Laboratory and industrial measurements
Physical Properties of Working Fluids
Ga68 pctIn20 pctSn12 pct T L = 293 K (20 °C)
AlSi12 T L = 1053 K (780 °C)
Mass density ρ (kg/m3)
Kinematic viscosity ν (m/s)
3.40 × 10−7
4.21 × 10−7
Electrical conductivity σ (Ohm m)−1
3.31 × 106
3.01 × 106
Governing Parameters for Experimental Studies of LFF in Laboratory and Industry (Maximum Values)
Lorentz force F (N)
Magnetic field induction B 0 (mT)
Reynolds number Re
0.185 × 105
2.38 × 105
Interaction parameter N
Volumetric flow rate q (m3/s)
0.5 × 10−3
10.55 × 10−3
Mass flow rate m (kg/s)
Accumulated mass M (kg)
Calibration factor of LFV c (m)
1.605 × 10−2
4.57 × 10−2
The experiment is carried out with a laboratory LFF and mentioned working fluid Ga68 pctIn20 pctSn12 pct with the properties given in Table I. The aim of this experiment is to determine a procedure of calibration factor c for a well-defined flow for which the flow rate can be measured and monitored accurately.
The distributions of the magnetic field along the channel height and in the direction of flow are shown in Figures 2(a) and (b). In vertical direction z within the height of channel, the nonhomogeneity of the field does not exceed 2.5 pct, whereas nonhomogeneity of field in flow direction x is strong. High nonhomogeneity along the flow is favorable to the production of intensive electric currents j as shown in Figure 1(b) and, consequently, to the generation of a large Lorentz force and the same force acting on magnet system.
The mechanical part of the LFF is a vertical metallic rod 500 mm in length linked with the magnetic system in its upper end and fixed in its middle point on a horizontal rotational axis. The point in which the force acts on the magnet system is at a distance of 305 mm from the rotational axis. The other end of the rod is linked with a balance weight for a stable equilibrium of the system at the initial instant of time of measurement. An additional horizontal rod of 175 mm in length is connected to the main vertical rod on the axis, and the other end is attached to the scales. The balance weight is chosen so that the imbalance of the mechanical system in the absence of measured force does not exceed 10−2 N. This provides a high accuracy of force measurement because in a range of forces up to 10 N acting on the magnet system a deformation of scales sensor is less than 100 μm (i.e., imbalance of mechanical system at this deformation remains practically previous).
A Vivès probe (5; Figure 3(a)) is installed in the middle point of the channel cross section at a distance of 410 mm from the cross-section inlet. A signal from the Vivès probe is fed into a multivoltmeter 2700 DMM (Keithley Instruments, Inc., Cleveland, OH) (6) and then to a computer for mean velocity calculation.
The signal of electronic scales (7) measuring a force on the magnet system is fed into the computer. The LFF prototype (8) is installed at a distance of 70 mm downstream from the Vivès probe (5). To examine the flow shape influence on readings of the LFF, we use a controlling gate (9), which is an obstacle partly blocking the cross section.
The form and dimensions of the magnet system are in agreement with a geometry of the production channel (Figures 5(a) and (b)). Each magnet pole 3 consists of 16 NdFeB magnets with sizes of 30 × 30 × 100 mm. The poles are linked to each other by an iron yoke 4. Each pole area facing the lateral channel wall is a × b = 100 × 260 mm (where a coincides with aluminum flow direction). The thickness of the poles is 60 mm. The distance between the low edges of the poles is 300 mm, whereas the upper edges are 550 mm apart (Figure 5(a)). The magnetic induction on the magnet surface is 450 mT. The magnetic field B at a height of 50 mm from the channel bottom in the middle plane is 13.5 mT. The magnet system is linked to a counterweight (5) by a steel rod (6) that can turn around the axis (7). The force F is measured by commercial scales (8), and a digital signal from scales is fed to the computer (9) to compute the volumetric flow rate, mass flow rate, and accumulated volume and mass of aluminum.
The magnetic field in this LFF at first glance is represented as nonhomogeneous along the height for such a magnet system design. However, a special measurement of magnetic induction inside the channel directly before the industrial tests demonstrates a magnetic field homogeneous enough along the channel height of 0…125 mm with a deviation of 1.13 pct. A height of flow can vary between 65 mm and 80 mm. Such homogeneity is a result of the affinity of the horizontal part of the iron yoke to the lower edges of magnet poles as shown in Figure 5(a), which weakens this area transversal component of the magnetic field that provides equalization of the field along the vertical. In regard to the field distributions in two other directions, they are also identical for both industrial tests. Thus, for both tests used, factor c reflects magnetic field nonhomogeneity in flow direction. In the industrial flow meter with a large distance between poles, the magnetic field is 30 times less than in the laboratory case. However, the much bigger flow rate in industrial tests takes place; therefore, this point provides a reliable measurement of Lorentz force in industry (Table II).
The positions of the main channel and the installed LFF from test to test do not vary. We emphasize that the channel and LFF are separated from each other with a gap of 10 mm between the magnet poles and the channel wall outer surface. The LFF is installed on a concrete foundation. Because of this, a channel position relative to LFF is rigorously fixed.
The weight of magnet system is 40 kg. The counterweight, also about 40 kg, provides equilibrium of the mechanical system. In the same manner as the laboratory experiment, we properly select a mass of counterweight to realize an initial unbalance of the mechanical system equal to 10−2 N.
As shown in Figure 5(b) the eddy currents j result in the vicinity of the edges of the magnetic field. Because the channel walls are nonconducting, the electric currents flow wholly inside the fluid. We emphasize that the aluminum oxide film on the upper surface of flow is motionless and plays a similar role as a nonconducting wall (i.e., eddy currents are fully closed in the liquid aluminum as sketched in Figure 5(b)). These currents are considerable as they are closed in the fluid in which the magnetic field is absent and an electromotive force is not generated. Therefore, the confined magnetic field provides a measurement of the Lorentz force by LFF at even a small induction of magnetic field.
The operating temperature of liquid aluminum in both tests is approximately 1053 K (780 °C). Despite this, the temperature of the external surface of the channel walls near the LFF prototype does not exceed 323 K (50 °C); it makes cooling the magnet system unnecessary. To take into account the variation of the magnetic field induction in consequence of the magnet temperature changes (Eqs.  and ), we place a thermocouple on the magnet surface for a temperature correction of the processing data.
Two tests are executed. Test 1 is used as a base to define factor c (Eq. ), using the known alloy mass received in this production cycle. It should be noted that this factor keeps a constant value if the magnetic field along the flow height changes within 1.13 pct (see previous sections). In the case of growth of the flow rate, an increase in the top level of flow adds a force according to Eq. .
We note that, in another case of nonhomogeneous magnetic field along the height, lifting of the flow level reaches an area of weaker magnetic field and gives a smaller increase in the Lorentz force (i.e., the factor c seems underestimated). Therefore, we give a special attention to the vertical distribution of the magnetic field, the precise position of LFF, and inspection of the upper levels of the aluminum flows in the operating channel at a distance of the LFF installation (3.5 m from the channel inlet) in the beginning and during both tests. It is important because, each time after aluminum transportation, the position of the channel outlet, which influences the height level of flow in the channel, is technically compelled to be disassembled for cleaning and reconditioning between the two transportation processes.
To convert the measured forces into volumetric flow rates or mass flow rates, it is necessary to know the calibration constant c for the LFF used in the plant tests. An accurate determination of this quantity would require an independent possibility to measure the instantaneous volumetric flow rate in the plant test. Because such a possibility does not exist, we had to determine the calibration constant in an indirect manner as is described next.
Strictly speaking, the calibration Eq.  is only valid if the cross section of the flow is constant; in which case, the flow rate q is related to the mean velocity v and the cross section A by q = vA, where A is a constant. In the open channel flow typical of secondary aluminum production, however, the cross section changes because of level fluctuations of the liquid metal. Hence, the calibration constant weakly depends on the level of the liquid metal and thereby on q. In what follows, we assume that this dependence is so weak that c can be regarded as a constant. We then can determine c from the cumulative mass or volume.
To show the role of natural oscillations of LFF and channel in pulsating signal, we estimate their frequency. The magnet system with an arm length of L = 0.35 m and a mass of M = 80 kg possesses an inertia moment I = 10.17 kg m2 that leads to a natural frequency of oscillations f′ 0/mag = 1.17 Hz. These oscillations do not coincide with measured oscillations (Figure 6). Estimations of inertia moment for the channel of M = 1250 kg in mass and L = 10 m in length give a value of inertia moment of I = 5218 kg m2. These parameters provide the natural frequency of oscillations f′ 0/ch = 0.55 Hz. This last frequency coincides in value with the legitimated frequency but is related to the channel oscillations in the vertical direction. In this connection, it should be noted that the LFF measures streamwise velocity oscillations and the mean velocity in this direction (i.e., does not measure oscillations in vertical direction).
In addition to the raw data shown in Figure 6 by curves 1 and 2, Figures 7 and 8 show the instantaneous mass flow rates m(t) (labeled 1) and cumulative mass M (labeled 2) calculated according to Eqs.  and . For the calculation, we use the density for alloy AlSi12 given in Table I. The curve 2 of the cumulative mass grows with time. The final value M (Δt = 1200 seconds) provides the total amount of aluminum that is passed through the channel. Curve 2 has the highest slope in the range between t 1 = 400 seconds and t 2 = 650 seconds in which the flow rate is maximum. In this test, the accumulated mass M adds up to 8350 kg.
Figure 8 demonstrates the dependences for test 2 as the same for test 1 in Figure 7. This process distinctive by its duration is also represented in the terms of mass flow rate (curve 1) and cumulative mass (curve 2) as functions of time. Curve 1 faithfully reproduces the behavior of curve 2 for the flow rate shown in Figure 6. In the construction of curves in Figure 6 and Figure 8, we use Eqs. , [8,] and  as well as calibration factor c (Eq. ). We emphasize that q here at a smaller process duration is threefold higher than q for test 1. The accumulated mass reaches 8930 kg.
It should be emphasized that the calibration factor c, whose dimension is meter, approximately seems to be half the length of the magnet system poles in the flow direction both for the laboratory and for the industrial LFF systems.
Error Estimations of Industrial Measurements
We have described a noncontact measurement method for the velocity of electrically conducting fluids and demonstrated its feasibility for measuring flow rates. Laboratory experiment and industrial tests showed that the LFF is a reliable tool for the measurement of flow characteristics of aggressive molten metal even for a small magnitude of the magnetohydrodynamics interaction parameter.
Our measurements in the laboratory show that a linear dependence between the Lorentz force and the flow rate holds for a closed channel. Two industrial tests also demonstrate the feasibility of measuring the flow characteristics in open channels.
The results serve as a basis for the development of new LFF systems with improved performance. Systems currently under construction have parallel magnet poles and have a higher magnetic field. This is not only important for the measurements in aluminum but also for molten metals with a lower electrical conductivity such as steel, lead, and tin. Other applications besides metallurgy such as semiconductor crystal growth and glass manufacturing are forthcoming.
We are grateful to the Bundesministerium für Bildung und Forschung (BMBF) for partial financial support in the framework of the ForMaT program under grant number 03FO2202. Moreover, H. Schreiber and D. van Kalken are gratefully acknowledged for useful discussions.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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