Bubble detection in liquid metals

Weakly- and non-conducting inclusions, e.g. gas bubbles, distort magnetic fields in highly conducting liquid metals. The magnetic signature of the distortion can be measured and gives information on the size, shape, position and movement of the bubble.

Working principle

Usually, properties of liquid metals such as their high temperature, chemically aggressive nature and opaqueness, place high demands on measurement techniques and exclude invasive methods. However, the high electrical conductivity of metallic melts enables the use of inductive techniques.

The rise of a gas bubble in a rectangular liquid metal column is schematically depicted: The magnetic field of an excitation coil (black) permeates through the liquid metal and is detected by a pair of receiver coils (red/blue). Both halfs of the receiver coil pair are wound in opposite directions and connected in series, which lifts the overall induced voltage for a fluid at rest in the absence of a gas bubble. The rise of gas bubble distorts the distribution of induced current within the liquid metal and introduces an imbalance in the receiver coil pair. The temporal evolution typically defines an S-shaped curve, whose structure and amplitude contain information on bubble size, position and velocity.

The measurements are particularly challenging, because the high sampling frequency (on the order of several hundred Hertz) necessary to trace fast-moving gas bubbles restricts the measurement to a thin boundary layer within the liquid metal due to high-frequency shielding (skin effect).

In the case of a moving liquid metal, the fluid flow induces an additional voltage in the sensor. As the flow velocity typically is considerably different from the bubble's upward velocity, both effects can be disentangled.

Detection of non-conducting inclusions in GaInSn

A laboratory experiment consisting of one excitation coil and four pairs of receiver coils around a rectangular liquid metal column was designed:

Non-conducting spheres of different diameters were moved inside the liquid metal column in a controlled manner. The induced voltages inside the receiver coil pairs carry a "fingerprint" of the size and position of the spheres. The experimental data show close agreement with numerical simulations (cf. Fig. 2).

Detection of gas bubbles in liquid sodium

The rise of gas bubbles in a liquid sodium-filled cylinder (NAFEX) was investigated within the framework of the european HORIZON-EURATOM-project ESFR-SIMPLE. An inductive sensor was specifically designed to fit HZDRs ultrafast electron beam X-Ray computed tomographer ROFEX.

The rise of Argon gas bubbles could be detected by the contactless inductive bubble detection (CIBD) technique as well as by ultrafast X-ray tomography:

Fig. 4 depicts the measured signal of an representative event for one CIBD receiver coil pair. The start and end times of the S-shaped signal along with the known geometrical dimensions of the sensor enable calculation of the rising velocity of the bubble. The amplitude of the signal provides information about the bubble size and position, that can be directly verified by the X-Ray data.

• Bieberle, M.; Gundrum, T.; Räbiger, D.; Bieberle, A.; Eckert, S.
  3-D shape and velocity measurement of argon gas bubbles rising in liquid sodium by means of ultrafast X-ray CT imaging
  Flow Measurement and Instrumentation 95(2024), 102503
• Gundrum, T.; Büttner, P.; Dekdouk, B.; Peyton, A.; Wondrak, T.; Galindo, V.; Eckert, S.
  Contactless Inductive Bubble Detection in a Liquid Metal Flow
  Sensors 16(2016)1, 63