Page 181 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Measurement techniques for liquid metal based nuclear coolants 153

Stopper
Tundish rod
level

Tundish

z (mm)
Meniscus level
335 1 8
2 9
300
3 10
4 11
250 X 5 12 X
6 13
200
7 14
S S
150
100
50
0
–0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5
0 70 140 x (mm)
v (m/s)
Fig. 3.5.6 Schematic sketch of the mold (A) of the mini-LIMMCAST facility equipped with 1
excitation coil and 14 magnetic field sensors and a reconstruction of the velocity field in the
mold for one instant in time (B).

perturbation of one or more externally applied magnetic fields. From these measure-
ments, the flow structure is reconstructed solving a linear inverse problem with an
appropriate regularization technique. Besides the appropriate mathematical modeling
of the inverse problem, the challenge of this technique is the reliable measurement of
the tiny flow-induced magnetic field that is about three to five orders of magnitude
smaller than the applied magnetic field. Therefore, a very stable current source for
generating the excitation magnetic field and magnetic field sensors with a high
dynamic range and a very linear response are needed. Using fluxgate probes and a
static excitation magnetic field, it could be shown that the flow in the mold of a con-
tinuous caster could be reconstructed with good agreement with accompanying UDV
measurements (Wondrak et al., 2010, 2011). Fig. 3.5.6 shows a schematic sketch and
the reconstructed velocity field in the mold of a model of a continuous caster. Success-
ful measurements of the time-dependent behavior of the flow in a modified Rayleigh-
B enard setup showed the applicability of CIFT for very low velocities of about
20mm/s (Wondrak et al., 2018). By using an AC excitation magnetic field with rather
low frequency in the order of 1Hz, the flow-induced magnetic field can be separated
from environmental noise suppressing undesired signals, for example, originating
from moving ferromagnetic materials or from switching of electric currents in the