Page 96 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

70 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Frequency
Maximum displacement
Divergence Flutter
(buckling)
region
Stable
regime
Other
Divergence Other Turbulence-
(buckling) dynamic induced Restablizing dynamic
Stable regime region regions vibrations zone regions

Flow velocity Flow velocity
Fig. 3.1.9 Frequency and maximum amplitude of a flexible rod in a cylindrical channel as a
function of flow velocity.
(Adopted from Ridder, J.D., Doar e, O., Degroote, J., Tichelen, K.V., Schuurmans, P.,
Vierendeels, J., 2015. Simulating the fluid forces and fluid-elastic instabilities of a clamped-
clamped cylinder in turbulent axial flow. J. Fluids Struct. 55, 139–154.)

The behavior of rods in such kind of flows is rather complex. It is interesting to
summarize the description provided by Ridder et al. (2015) (based on experiments
by Modarres-Sadeghi et al. (2008)), who describe the motion of a single, suspended
rod in a cylindrical channel. At very low velocities, the rod stays in the center of the
channel. When the velocity is increased, turbulence will cause the rod to vibrate with
small amplitudes. When the velocity is increased again the amplitude will increase
and, at a certain specific velocity, the rod will buckle to one side with a large ampli-
tude. When the velocity is increased even more, the cylinder repositions itself at the
center of the channel. When reaching a certain velocity, the rod starts to flutter around
the central position. The aforementioned process is shown in Fig. 3.1.9.
Obviously, the behavior would be even more complex in a bundle geometry. The
flow pulsations described in Section 3.1.3.3, for example, are of influence as well as
described by, for example, De Ridder et al. (2016). Flow-induced vibrations by axial
flow in rod bundles are subject to a range of experimental techniques in water-based
facilities. The displacement of the rod needs to be recorded, which can be accom-
plished by a range of techniques. Pauw et al. (2013) apply a significant number of
different techniques to determine the deformation and vibration of a nuclear fuel
pin, being strain gauges (Basile et al., 1968), accelerometers (Takano et al., 2016),
a laser Doppler vibrometer (LDV) (Choi et al., 2004), and a grid method
(Badulescu et al., 2009). The nonintrusive laser speckle photography-based tech-
niques can be used as well to measure in-plane translation, rotation, or vibration of
a surface (Keprt and Bartonek, 1999). Fabert et al. (2014) applied the method to fuel
cladding and were able to measure amplitudes in the micrometer range.

3.1.3.5 Effect of wire wraps

Wire wraps are primarily meant as a spacer to prevent the fuel rods in the tight fuel
bundle from touching each other by swelling and buckling. If rods do touch each other,
local deterioration of heat transfer (and thus hotspots) may occur due to a local