This paper describes the phenomena of flow behaviors in a closed natural circulation loop. Three flow regimes were identified according to the intensity of the heat flux: The first one is the single phase regime where the mass flow rate is proportional to the one-third power of the heat flux. The second one is the two phase circulation regime where the mass flow rate increases with the heat flux, which is followed by another two phase circulation regime where the mass flow rate decrease with increasing the heat flux. In the two-phase circulation regimes, the explosive vaporization subsequent to boiling suppression repeated regularly with the low heat flux. When the heat flux was increased the circulation with relatively regular pumping could be attained. Further increase of the heat flux yields the dryout condition. Effects of the inlet- and exit-restrictions on the flow behaviors were also examined.
