On the Impact of Aspect Ratio and Other Geometric Effects on the Stability of Rectangular Thermosiphons

Abstract

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

Direct Numerical Simulation of High Prandtl Number Fluid Flow in the Downcomer of an Advanced Reactor

“…11b. To better understand the physics behind this phenomenon, POD analyses 7,24 have been implemented for a series of cases with and without budget peaking. The POD is chosen because it is a very effective mathematical method to identify the dominant flow structure, which in turn can reveal the difference in dynamic behavior between cases with and without peaking.…”

“…5) oriented toward use in GPU architectures that has demonstrated its capability to perform high-fidelity large eddy simulation (LES) and DNS in both simple and complex geometries. [6][7][8] The downcomer can be simplified as flow between two heated parallel plates due to the high aspect ratio of most configurations. The working fluid is FLiBe, which is the interest of our industrial partner KP.…”