Page 339 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

308 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Fig. 6.2.2.15 Argand diagram
in restabilizing regime.
Adapted from De Ridder, J.,
Doar e, O., Degroote, J., Van
Tichelen, K., Schuurmans, P.,
Vierendeels, J., 2015.
Simulating the fluid forces and
fluid-elastic instabilities of a
clamped-clamped beam in
turbulent axial flow. J. Fluids
Struct. 55, 139–154.

6.2.2.4 Conclusions

First, this chapter describes the occurrence of vortex streets caused by axial flow in
bundles of tubes and the vibrations they trigger. Simulations with rigid tubes are able
to capture the vortex streets, which originate from the difference in axial flow speed in
the gap between tubes and the subchannel center. The instability becomes stronger as
the P/D ratio decreased, although even lower P/D ratios might have an opposite
behavior. Fluid–structure interaction simulations predict the vibration of a flexible
tube mounted in this bundle. These simulations show that tubes at a corner of the bun-
dle experience stronger oscillations in one specific direction.
Second, the dynamics of a flexible tube in axial flow have been studied using
coupled CFD and CSM calculations. To verify the validity of the calculations, a spe-
cific case for which experimental data are available is simulated. The dynamics of the
system were also computed by fluid-structure interaction simulations. The stable,
divergence, restabilization, and flutter regimes were obtained by increasing the flow
velocity. In the divergence regime, it was shown that small misalignment errors
with the mean flow might be a significant contribution to the maximal displacement.
Misalignment can also cause skipping the restabilization regime between divergence
and flutter.

Acknowledgment

This work was performed in the framework of the Horizon 2020 SESAME project and the Hori-
zon 2020 MYRTE project. It has received funding from the European Commission Euratom
Research and Training Program on Nuclear Energy under grant agreement No. 654935
(SESAME). It has received funding from the European Commission Euratom Research and
Training Program on Nuclear Energy under grant agreement No. 662186 (MYRTE). The
authors gratefully acknowledge the funding by the Research Foundation-Flanders (FWO),