Flashing close to the vent is initiated (Phase A). The steam bubble generated by the flashing shrinks due to steam condensation on the interface. Previous studies on steam condensation in pool water showed that pressure peak occurred when the steam bubble condensed and the ambient water moved to the bubble center quickly in the CO and chugging regions [6, 7]. The internal pressure of the attached bubble increases by the momentum of the ambient water and the difference between the pressure inside the nozzle (vent hole) and outside the nozzle becomes small. Then the saturated water discharges without flashing close to the vent hole and flashes at some distance from the vent hole (Phase B). The pressure outside the nozzle decreases. Flashing close to the vent hole start again and the next cycle starts.
Figure 6 shows the result of the FFT analysis on the pressure traces in the vent tube and pool water. The peak intensity of the frequency component (or the dominant frequency) of 193-195 Hz agreed with the frequency of the FO. Figure 7 shows the relation between the frequency of the FO and the experimental settings. The dominant frequency or frequency of the FO increased as the pool water subcooling increased, and as the discharging water pressure and vent hole diameter decreased. We have applied a curve fit to collapse the data using the method of least squares. The frequency is best arrived at using the following equation:
The solid lines in Fig. 7 represent Equation (1).
Figure 8 shows phenomena maps for flashing and condensation phenomena in the pool. The phenomena are divided into the following patterns:
1) Flashing (F): Flashing close to the vent hole (Phase A) continues without FO. It has a stable steam-water interface and little pressure oscillation;
2) Flashing oscillantion (FO): The range of the flashing oscillates between a point close to vent hole (Phase A) and some distance from the vent hole (Phase B). The pressures in the vent and the pool water vary according to the FO and peaks when pressure oscillation appears in the region of Phase B;
3) Transition region (TR); The transition region between patterns (1) and (2) or the transition region between pattern (1) and discharging water without flashing.
As mentioned, FO is caused by a balancing action between the flashing in the control volume and steam condensation on the steam-water interface. When the saturated water pressure and vent hole diameter increase, and pool water subcooling decreases, the amount of flashing in the pool water increases and the location where the flashing occurs moves closer to the outlet vent. This means that Phase B is diminished, Phase A appears, and FO stops when the saturated water pressure and vent hole diameter are large, and pool water subcooling is low. Some experimental studies on flashing into gas field, using an orifice and a nozzle, have shown that vapor bubbles cannot develop in a short nozzle and that flashing (or vaporization) of the discharging water takes place at some distance from the nozzle [1, 2].
