Numerical insights into the transient behaviour of single-phase Natural Circulation Loops for nuclear passive cooling applications

Natural Circulation Loops (NCLs), where fluid is driven through a closed circuit solely by thermal imbalance, offer potential for use in passive cooling systems within nuclear power plants. Transient CFD simulations of two-dimensional NCLs of different aspect ratios have been performed using the Unsteady Reynolds-averaged Navier–Stokes framework for a range of Rayleigh numbers (8×109<Ram<8×1013), at three Prandtl numbers (Pr=0.01,0.71,7.1) and for two different loop aspect ratios in order to provide insight into the transient and stability behaviour of such systems. Results predict that NCL systems exhibit complex and rich dynamic behaviour, with strong sensitivity to both the imposed Rayleigh number and Prandtl number. Initial thermal transients led to the establishment of unstable and oscillatory flow behaviour which, in several cases, led to flow reversals. At the high and moderate Prandtl number, cases at the lower Ram=8×109 reached a statistically steady-state whilst flows at the higher Ram=8×1013 demonstrated more oscillatory, but relatively regular, behaviour, including persistent flow reversals. A significant reduction in Prandtl number to Pr=0.01 led to much more stable behaviour, with cases at all Ram producing shorter initial transients and settling quickly to a statistically steady-state. This is primarily as a consequence of the increased dominance of heat transfer by conduction and subsequent reduction in overall temperature variations. For cases that produced statistically steady-state behaviour, output Reynolds numbers demonstrated good agreement with existing experimental trends and established correlations. These results suggest that utilizing fluids with low Prandtl numbers may be an effective means of improving the stability of NCLs for a given heat input without compromising the output Reynolds number.