Electromagnetic detection of nonconducting inclusions in a liquid metal flow

The paper describes a first experimental test of a new electromagnetically based bubble detection method in an electrically conducting fluid. A reliable and contin-uous bubble detection method is important for a lot of liquid metal flows, in particular, flows of reactive liquid metals like sodium or lithium. Moreover, the method is suitable for the detection of any inhomogeneity having different electrical conductivity compared to the bulk flow.
The experimental approach for the detection of inhomogeneities was developed at RWTH Aachen for an early slag detection in steel production. The method is based on the fact that a primary electromagnetic field is changed by the motion of an electrically conducting fluid. This change is very sensitive to any inhomogeneities in the flow which have a different electrical conductivity compared to that of the fluid. The signals of suitably arranged secondary coils are evaluated in order to separate the influence of the inhomogeneity. In the experiments performed at the sodium facility of RCR we were able to detect single bubbles which were of diameters in the range 0.5 - 2 mm.
Compared to usual, mainly acoustically based detection methods the following advantages of the electromagnetic approach are obvious:

• No time delay between bubble occurence in the coil region and the signal.
• High sensitivity up to single bubble detection.
• Contactless method. No need to install sensors at hot walls, even a thermal isolation between the coils and the channel is possible.
• Every flowing bubble is detected whereas acoustic methods are able to detect acoustically active bubbles only.

The results of our experiment show convincingly the efficiency of such an electromagnetic detection method.