Natural circulation is the most important heat removal mechanism for passive protection systems in a lot of industrial applications, such as nuclear power plants, solar energy systems, reboilers and cooling of electronic systems. The aim of the present work is to investigate the flow and heat transfer characteristics of parallel loops, connected in the lower heated sections, in single-phase natural circulation. The test facility was composed by 2 vertical circuits connected in parallel; each of

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... them was rectangular in geometry, the aspect ratio (defined as the height to width ratio) was 1.63, with circular copper tube of 4 mm (I.D.). An upper cold heat exchanger provided the heat sink, while the heat source at the bottom was a power supply system. Several calibrated thermocouples (T-type) placed in the fluid along the vertical tubes allowed the evaluation of the hot and cold legs average fluid temperature differences. Tests were carried out imposing 3 different heat sink temperatures (10, 20, 30°C); for each of these temperatures the power supply at the lower heater was increased from 20 to 90 W. The fluid investigated was distillate water. The experimental results have been analysed in terms of thermal performance of the single or connected loops. Collected data have also been compared with Vijaiyan's correlation.