Flow measurement in metal melts using a non-contact surface velocity sensor based on time-of-flight Lorentz force velocimetry

Abstract

This paper attempts to present a novel development of an electromagnetic non-contact surface velocity measurement technique in electrically conducting liquids which could be applied in high-temperature metallurgical processes involving metal melts. The technique is based on Lorentz force velocimetry, i.e. on measuring the force which is generated by the interactions of the melt flow and an externally applied magnetic field that is spanned by permanent magnet system. The electromagnetically induced force pushes the magnet system into the direction of the flow and can be measured using a force sensor that is attached to the magnet system. As the measured force linearly depends on melt velocity, a non-contact evaluation of the velocity can be achieved. However, the recorded force also depends on the electrical conductivity of the melt. In application this material property is a priori unknown as it strongly depends on both temperature and composition of the melt. Hence, calibration of such a measuring device becomes a cumbersome task. Our development aims to circumvent this deficit by applying a time-of-flight technique. According to this principle we design and test a prototype of sensor for measuring free-surface velocity. In the model experiments we use both solid bodies and the liquid metal GaInSn as test liquids. During continuous casting of steel, control of free-surface melt flow in the mold is crucial for final steel quality. However, flow in the mold cannot be directly observed or measured using optical methods or submerging sensors due to the presence of an opaque layer of slag or casting powder on top and due to the chemical aggressiveness of liquid metals at high temperatures (1). Information of flow patterns in the mold may be obtained from the velocity at the free surface (meniscus). Various methods to measure velocities in high-temperature metal melts have been developed and reported. For instance, a similar measurement sensor to the present one is the Mass Flow Control sensor (2) which is also based on electromagnetic principles and the time-of-flight technique. The drawbacks of this sensor are twofold. First, both the rate and the quality of signals are limited as the application of this sensor is restricted to steady flow situations. Second, this sensorshould be embeddedin the mold wall. A much simpler design