  Energy efficiency in process industry and passive safety systems in nuclear
power plants necessitate the use of buoyancy driven heat exchangers. The
current study presents a 1-D numerical model of the buoyancy driven fluid
system in a Coupled Natural Circulation Loop (CNCL) comprising of water as the
working fluid. In this study water at different temperatures is considered as
the operating fluid in each of the loops. The study employs a 1-D model derived
using a semi-analytical Fourier series. A validation study was carried out
using the relevant literature to verify the mathematical model. A detailed
parametric study on individual NCLs and their coupled system was conducted by
varying the cross-sectional area keeping the other parameters constant to gain
insights into the effect of thermal coupling on the long-term transient
dynamics of each of the individual loops of the CNCL system, for stable,
neutrally stable, and unstable conditions. The dynamics were analysed using the
difference between transient buoyancy and viscous forces, and it was found that
the overall heat transfer coefficient influences the coupling behaviour and the
dynamics of the component loops in a CNCL. At lower values of the overall heat
transfer coefficient, the component loops in the CNCL nearly retain their
independent behaviour, i.e., the component loops hardly influence each other.
It was also found that the temperature dependent fluid properties influence the
stability of the CNCL system in some cases.
