In the present experimental study, the flow behavior within a semi-closed two-phase natural circulation loop was examined with an emphasis placed on the role of the expansion-tank line resistance. The loop is composed of the heater, riser, condenser and the down-comer sections and the expansion tank. Water was used as the working fluid. Heat flux, inlet subcooling and inlet-, exit- and expansion-tank- restrictions were taken as the controlling parameters. The circulation modes were examined by measuring the circulation rate(heater-inlet velocity), liquid velocity through the expansion-tank line, void fraction at the riser, heater inlet and outlet temperatures and the loop pressure.
Six circulation modes of loopwise natural circulation were identified. They are the single-phase(liquid) circulation, periodic circulation(A), two-phase continuous circulation and periodic circulations(B), (C) and (D). Among these modes, single-phase and two-phase continuous circulations exhibit no significant oscillations and are considered to be the stable operation modes. The single-phase natural circulation is the unique feature observed with the semi-closed loop. Periodic circulation (A) is characterized by the regular occurrence of the large two-phase flow carry-over with the single-phase(liquid) circulation mode between them. Hence, the large amplitude oscillations are induced. Periodic circulation (B) is featured by the flow oscillations with the continuous boiling inside the heater section, and is considered to be the density wave oscillation. Periodic circulation(C) also occurs with the continuous boiling, but with its period being much shorter than that of periodic circulation(B), and is considered to be the manometric oscillation. On the other hand, in the mode of periodic circulation(D), the continuous boiling is still maintained but with the longer period and the larger amplitude of oscillations than those in periodic circulation(B). Periodic circulation(D) is considered to be the pressure drop oscillation.
The circulation rate increases and then decreases with increasing of the heat flux. Parametric study shows that the increases of the flow restrictions at the heater inlet and at the expansion-tank line and the decreases of the exit-restriction and the inlet subcooling broaden the range of the two-phase continuous circulation mode. Unlike the analytical prediction, the excursive instability was not identified with the semi-closed loop because the liquid inventory can not be changed easily due to the flow restriction at the expansion-tank line.
