Natural Circulation in Thermal-Hydraulic Systems

Natural circulation - the self-sustained, pump-free movement of fluid driven solely by buoyancy forces - is one of the most important and elegant phenomena in engineering thermophysics. It underpins the passive safety philosophy of modern nuclear reactors, powers the domestic solar water heater, drives geothermal energy systems, and has quietly enabled reliable industrial heat exchange for more than two centuries. Despite this ubiquity, the thermal-hydraulic behaviour of natural circulation loops remains an active area of research, particularly in the context of nuclear reactor safety analysis, where accurate prediction of passive cooling performance is a licensing requirement.