Page 84 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Rod bundle and pool-type experiments in water serving liquid metal reactors 59

displacement between two frames can be determined at most with 1 pixel accuracy
(also referred to as pixel-locking or peak-locking effect) and the image is then
under-sampled. If the particle image extends to more than 1 pixel, the position of
the particle can be interpolated relying also on the light intensity from the neighboring
pixels in order to achieve the better subpixel accuracy. In order to have the particle
images always in focus, the laser sheet thickness at the measurement region has to
be thinner than the focal depth of the camera. The thinnest part of the laser sheet
(“beam waist,” w 0 in Fig. 3.1.3B) is, in principle, the most suitable region where
to carry out the measurements. However, properties, such as the sheet thickness
and the beam energy, present sharp spatial gradients in and near the beam waist, which
is not good for the measurements. The solution is to illuminate the measurement
region with a portion of the laser beam, which is close to the waist but far enough
not to have such gradients. PIV measurements in rod bundle facilities can be affected
by the reflection of light on the walls; the results are that the imaged particle cannot be
distinguished anymore from the background light. The subtraction of a background to
all the recorded pictures is a possible workaround during the data preprocessing phase.
Another solution to limit the background noise is to paint black the rod walls.
A coating layer of Rhodamine fluorescent die can also be applied on the surface as
described by Jones (2016); the light reflected from the wall would then result in a dif-
ferent wavelength and could be filtered out.

3.1.2.4.3 Laser-induced fluorescence

The planar laser-induced fluorescence (LIF) thermometry technique allows the mea-
surement of scalar quantities in a 2D plane. The realization of the technique requires
the use of a fluorescent dye and relies on the dependency of the fluorescence intensity
on the dye temperature and concentration. If the dye is mixed homogeneously with the
fluid, the technique allows the measurement of the temperature field in the plane.
The fluorescent signal should be converted to temperature by means of a previous cal-
ibration performed in the same test section. LIF requires a light source, which can illu-
minate the desired test section. For this purpose a laser source is used and the plane
illumination is obtained by shaping the light beam into a sheet through appropriate
optics (Fig. 3.1.4A). The wavelength of the laser source should fall in the absorption
spectra of the dyes molecule. A camera provided with a proper filter records the
fluorescent intensity emitted. The technique is based on the well-assessed dependence
of the probability of fluorescence emission to the temperature of the fluorescent dye/
fluid solution. The dye molecule is first excited with an incident photon and absorbs
the photon energy. An example of absorption and emission spectrum can be found in
Fig. 3.1.4B. The absorbed energy will then be partly or totally dissipated in vibrational
and collisional transfer with the other molecules. The remaining energy will be
re-emitted in the form of a photon of higher wavelength (and lower energy) with
respect to the incident one. The higher the temperature of the dye solution, the higher
the kinetic energy of the molecules, the higher the probability of total dissipation
of the energy absorbed. As a consequence, a lower number of photons will be
re-emitted with increasing fluid temperature, decreasing the fluorescent signal