Page 81 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

56 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

FFT works well with high data rate only. If the data rate is low or if the interarrival
times are quite different from each other, the contribution of the so-called “white
noise” to the spectrum becomes more important (Adrian and Yao, 1986) as the longer
the “hold” phase, the more the resampled signal is biased with respect to the real one.
The alternative solution is to first calculate the discretized correlation function of the
velocity by means of the “slotting” technique, and then to apply the Fourier transform
to compute the spectrum (Mayo, 1974; Tummers and Passchier, 2001).

Light reflection
Experimenters performing LDA experiments in rod bundles have to deal quite often
with the reflection of light from the rod walls close to the elliptical measurement vol-
ume. Reflection is interpreted by the postprocessing software as a zero-velocity signal
besides to the real fluid velocity; this artificially decreases the average values and it
alters also the root mean square. If the real velocity is not close to zero and the rod wall
does not touch the probe volume, a filter on the velocity can be applied to leave out the
reflection contribution of the near-zero-velocity samples and solve/minimize the
problem.

Light distortion
When the laser enters the rod bundle, light refraction occurs as predicted by the law of
Snell. It can be minimized either by adopting materials for the rod bundle with the
same refractive index of water or by employing a working fluid with same refractive
index of the other components (see Section 3.1.2.4.4 for further details). Either way,
the refraction also affects the θ angle between the two beams as the laser passes from
air to a different medium. This implies a change in the size of the LDA probe volume,
as reported by Chang et al. (2014). The probe length is evaluated by

d 0
l p ¼ ,
sinθ=2
where d 0 is the laser beam diameter at the focal point as defined by Guenther (1990)
and θ is the beam angle. The value for θ is usually given for the air side and depends on
the refractive index of the medium: as the medium becomes denser, the refraction
increases and the beam angle decreases. It follows that the probe volume into the fluid
becomes longer. This fact has to be considered in the design of the experimental facil-
ity because the elongated probe volume has to fit anyhow the tight regions of the setup
such as the rod gaps, where possible.

3.1.2.4.2 Particle image velocimetry
PIV is a nonintrusive optical technique for flow measurements that allows to detect the
velocity field of seeding particles on a plane or volume illuminated by a laser sheet.
The PIV system usually consists of a double-pulsed laser source emitting the light col-
limated in the optics (spherical lens, cylindrical lens, or their combination) and then
focused at the measurement region. The light of the two laser pulses is scattered by the