The study of heat exchangers with both the hot and cold fluids driven by buoyancy forces is an area of considerable interest due to their inherent passivity and non-existence of moving parts. The present work aims to study such heat exchange devices employing Coupled Natural Circulation Loop (CNCL) systems. A CNCL is a simplified version of the multi-loop heat exchange systems utilised in the nuclear domain under emergency and in energy-intensive processes. A CNCL comprises of two thermally linked fluid filled closed loop conduits in a gravitational field with heat source and sink on either of the loops, and operates completely on the buoyancy forces generated at the heating and cooling sections enabling heat transfer at the location of thermal coupling.

The thesis presents a Fourier series based 1-D transient modelling of the CNCL system. The developed model has been extensively verified against the 3-D CFD data and the experimental data from literature, for transient and steady state cases. Because of the many inherent limitations of the transient 3-D CFD study, the developed 1-D model has been used to study the stable and chaotic transient behaviour and develop the stability maps of the CNCL, employing non-dimensional numbers. Different non-dimensional numbers which characterise the transient behaviour of the CNCL system have been identified, which include the thermal coupling sensitivity coefficient, the flow resistance coefficient, in addition to the Grashof number, the Fourier number and the Stanton number. The CNCL system behaviour is dependent on the orientation of the system (vertical or horizontal) and the heater cooler configuration employed. The introduction of conjugate effects in the CNCL leads to additional non-dimensional terms. If the magnitude of the thermal coupling sensitivity coefficient is close to unity, the thermal coupling between the loops of the CNCL is strong. The work identifies that the CNCL systems have multiple steady-states and provides an approach to determine the stability of the steady-states using the developed 1-D model. The horizontal CNCL exhibits counterflow and parallel flow configurations at the common heat exchanger with the stability domain of counterflow being greater than that of parallel flow. An increase in the flow resistance coefficient and the Fourier number leads to an increase in the domain of stable operation of the CNCL system. The thermal coupling at the common heat exchanger section of the CNCL system leads to a decrease in the domain of stability compared to the NCL system.