  Single-phase natural circulation thermosiphon loops have been attracting
increased interest as they represent the prototype of passive safety systems.
However, the stability properties of thermosiphon loops, which can affect and
compromise their functionality, are still actively investigated. Traditionally,
the stability analysis of thermosiphon loops has been simplified to
one-dimensional (1D) calculations, on the argument that the flow would be
mono-dimensional when the diameter of the pipe D is orders of magnitude smaller
than the length of the loop Lt. However, at lower Lt/D ratios, rectangular
thermosiphon loops show that the flow presents 3D effect, which also has been
confirmed by stability analyses in toroidal loops. In this paper, we performed
a series of high-fidelity simulations using the spectral element code Nek5000
to investigate the stability behavior of the flow in rectangular thermosiphon
loops. A wide range of Lt/D ratio from 10 to 200 has been considered and the
results show many different outcomes compared to previous 1D analytical
calculations or stability theory. Moreover, we analyzed the flow in rectangular
thermosiphon loops using Proper Orthogonal Decomposition (POD) and we observed
that the cases without flow reversal are characterized by swirl modes typical
of bent pipes and high-frequency oscillation of the related time coefficients
obtained by Galerkin projection. However, the swirl mode was not observed in
cases with flow reversals, these cases are characterized by symmetric flow
field at 2nd POD mode and the similarity of low-frequency oscillation in the
projection of POD modes.
