Development of Lorentz Force Flow Meter Test Setup and its Evaluation with a Moving Brass Plate
T. V. Shyam1, S. K. Apraj1, Nirupam Das1, S. K. Sinha1
1
Reactor Engineering Division, Bhabha Atomic Research Centre, Mumbai, India [email protected]
ABSTRACT
Lorentz Force flow meter has good prospects for its application in High temperature reactor applications where molten metal alloy based coolants are used. One of the salient features of this technique, is that, it can be employed as non intrusive technique which allows the sensing element to be easily maintainable.
The Lorentz force velocimetry is a non-contact technique for velocity measurement in electrically conducting fluids. It is based on exposing the fluid to a magnetic field and measuring the drag force acting upon the magnetic field lines. The force on a fixed magnet system is measured directly. The measured signal is a linear function of the flow velocity. The paper describes the development of a test setup for Lorentz force flow meter and which can be tested with a moving metal plate at room temperature conditions instead of using molten metal which necessitates the use of expensive high temperature experimental loop setup. The system comprises of a horse shoe shaped magnet system fabricated by means of a rectangular Neodymium Iron Boron (Nd-Fe-B) rare earth permanent magnet with vanadium permendur legs. The air gap in the magnet system provides the space for allowing the passage of the moving metal medium whose velocity or flow has to be determined. The breaking force experienced by the magnet system is determined by means of ‘C’ shaped member which houses a set of strain gauges and as well as supports the magnet system to over hang from a top mount plate. The moving metal medium is simulated by driving a metal plate through the air gap of the magnet system with the help of a linear actuator system.
Experiments were carried over by moving a brass plate at varying speeds and strain gauge readings were acquired, analyzed and correlated with speed of the moving metal.
Keywords: Lorentz force velocimetry , Vanadium Permendur, Strain gauge
Introduction:
Molten metal alloys like Lead Bismuth (Lb-Bi) are proposed as suitable candidates for future High temperature reactors considering their physical properties exhibiting lower melting point and higher boiling point. At the same time they show highly corrosive behavior, which hinders to large extent, the employment of intrusive type instrumentation for measurement of process parameters like flow.
Lorentz force velocimetry (LFV) is a noncontact technique for velocity measurement in electrically conducting fluids. It is based on exposing the fluid to a magnetic field and measuring the drag force acting upon the magnetic field lines. The force on a fixed magnet system is measured directly. The measured signal is a linear function of the flow velocity.
Lorentz force velocimetry (LFV) technique is based on measuring the drag force on magnetic field lines which cross the flowing conducting medium. This noncontact technique is appropriate
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for high-temperature applications because it is free from the inevitable electrode corrosion predicament that has hindered Faraday’s classical method for Electromagnetic Flow meter. By contrast, measurement of flow of liquid metals at high temperatures cannot be carried out using conventional inductive flow meters since electrodes, which are essential to apply Faraday’s principle, cannot be inserted in the flow.
When an electrically conducting fluid travels across magnetic field lines, formed by a permanent magnet as in Figure 1, the induced eddy currents guide to a Lorentz force which brakes the flow. The Lorentz force density is approximately
where σ is the electrical conductivity of the fluid, v is its velocity and B is the magnitude of the magnetic field.
Figure 1. Principle of Lorentz flow velocimetry
By virtue of Newton’s law, an reverse force acts upon the magnetic-field-generating system and drags it along the flow direction as if the magnetic field lines were indiscernible obstacles. A Lorentz force flow meter is a device which determines the flow rate from a measurement of this force. Lorentz force flow meters can be fabricated by designing as static flow meters where the magnet system is at rest and one measures the force acting on it. The force acting on a Lorentz force flow meter depends both on the velocity distribution and on the shape of the magnet system.
The force is proportional to the velocity and conductivity of the fluid, and its measurement is the basis of LFV. With the advent of powerful rare earth permanent magnets and design tools for permanent magnet systems that a practical realization of this principle has now become possible.
A small permanent magnet with dipole moment m is situated at a distance L above a semi- infinite fluid moving with consistent velocity v parallel to its free surface. The magnetic field of the dipole which is referred as the primary field is of the order
at the surface of the fluid. By virtue of the fluid’s motion, eddy currents with amplitude
are induced which are horizontal and concentrated below the surface as shown in Figure 2(a).
The eddy currents interact with the primary field to produce the Lorentz force which tends to sever the flow. The eddy currents surround themselves with a magnetic field b, which is denoted as secondary field and which is depicted in Figure 2(b). This magnetic field extends to the location of the dipole, and its magnitude there is
The magnetic dipole experiences a force of the order
when subjected to a magnetic field gradient b/L . This provides the estimate for the force acting on the magnet.
Figure 2(a). Generation of eddy currents in flow medium
Figure2(b). Generation Lorentz force by interaction of secondary field and magnet
dipole
Experimental Setup
The schematic of Lorenz force flow meter is shown in the Figure 3. It consists of .Horse shoe shaped magnet consisting NdFeB permanent magnet and magnetic flux guiding limbs made of vanadium permendur. The distribution of magnetic flux density of the horse shoe shaped
magnet obtained through electromagnetic simulation is shown in Figure 5. The flowing metallic medium is simulated by a flat plate which is moved through the Lorentz force flow meter by a linear actuator. The drag force is coupled to the static magnet system is measured by means of strain measurement system. The experimental trials were carried out by slewing the actuator at variable speeds. The strain measurement were carried out with rosette strain gauges placed on both sides of the C shaped bridge used for supporting the magnet system from the top plate. The photograph of the Lorenz Force Flow meter test setup is shown in Figure 4
Figure 3. Schematic of Lorenz Force Flow meter
.
Figure 4. Lorenz Force Flow meter test setup
Figure 5. Magnetic flux density distribution in horse shoe shaped magnet
The experiments were carried out with brass plate. The speeds of the actuator were varied from 50mm/s to 350mm/s in steps of 50mm/s. The outputs from the various channels of strain
measurement system are shown in Figure 6a, 6b and 6c. The signals acquired for multiple runs and were offset corrected and averaged.
Figure 6a. The output of Channel 0 of the strain measurement system
Figure 6b. The output of Channel 1 of the strain measurement system
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Figure 6c. The output of Channel 2of the strain measurement system Discussion of Results
It can be seen that there is distinct relation between the amplitude of the peak value of the signal with the velocity of the moving plate. The output of the channel 1 of strain measurement system has shown better sensitivity. The experiments were carried out with moving brass plate for simulating the molten metal medium. The literature shows the resistivity of the brass in the range 0.6 x 10-7 to 0.9 x 10-7 Ohm-m. For accounting the effect of the material of the duct which carries molten metal , was compensated by attaching SS plates to the magnet pole faces. The results show a transient nature of the signals, which are due to the entry and exit of the moving plate in and out of the Lorentz force flow sensor system. In case of continuous motion of the conductive medium, it is expected to have static value for the output from the strain measurement system. Further to use the system in high temperature regime, improvisation have to be made by incorporating cooling ducts in the vandium permendur limbs of the magnet system and thermal insulation to be provided for heat Loss. The limbs of the magnet can also be extended if required, to attain lower ambient temperature zone for the permanent magnet.
Conclusion
The Lorentz force flow meter are non invasive type of flow meter for molten metal flow and are apt for high temperature applications like in High Temperature Reactors . They have prominent features like ease of maintainability due to no contact with the high temperature flow medium and further permanent magnet can be maintained at lower temperature than that of the flow medium by use of high permeability ferromagnetic long shunt legs.. They have shown it is possible for application in flow of molten lead based alloys.
References
[1] P A Davidson, An introduction to Magnetohydrodynamics (Cambridge University Press, Cambridge, 2001)
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