Experimental studies were performed on the two-phase natural circulation phenomena with the heater power input, loopwise friction and the liquid temperature taken as parameters. Initial single-phase natural circulation was followed by the steady cyclic behavior of two-phase natural circulation with regular oscillatory periods. Circulation rate, represented by the liquid velocity at the heater inlet, increases with the increase of the heater power input and/or the liquid temperature and with the decrease of the loopwise friction. On the other hand, oscillation periods decrease with the increase of the heater power input and/or the liquid temperature and with the decrease of the loopwise friction ; that is, the oscillation periods are inversely proportional to the circulation velocity and related to the residence time of a fluid particle around the loop.
