The power value of the vent cap hole diameter, (m), the pool water subcooling temperature, (K), and the pressure of the discharging water, (Pa), in the empirical equation Equation (1) can be explained by Equation (17). In the assumptions used in the analysis, the FO duration is basically dominated by bubble motion in the pool water, and there is no consideration of the nozzle length which is one of the steam generating conditions. This means that the location where flashing occurs, at the nozzle of in the pool, does not affect the FO duration. The FO duration would be affected by the nozzle length if the duration is affected by the steam generating conditions. As shown in Equation (1), nozzle length. lV, had little effect on the frequency (at a power value of 0.04) in the experiment. This means that the location where flashing occurs and steam is generated (inside the nozzle or in the pool) does not affect the frequency of the FO.
The assumptions used in the analysis are sufficient to explain the fact that the frequency of the FO, is dominated by condensation controlled bubble motion in the pool water, but is not affected by the nozzle length which is one of the steam generating conditions. The phenomenon of FO involves the overlapping phenomena of saturated water flashing and steam condensation in the pool water. The phenomenon might be so complicated, nevertheless the simple bubble model has explained well the effects of the experimental settings on the frequency of FO. This shows that FO is a phenomenon which is basically dominated by bubble motion in the pool water.
Conclusion
We did an experimental study on high-pressure saturated water discharging into pool water to clarify the behavior when high-pressure saturated water occasionally contacts low-pressure, low-temperature on cold water in ship's heat plant or a LICA in an advanced reactor. The results are summarized as follows:
1) Flashing oscillation (FO) occurred when high-pressure saturated water was discharged into pool water under specified experimental settings. The occurrence of flashing oscillated between a point very close to the vent hole and a point some distance away. We named the flashing very close to the vent hole "Phase A", and the flashing at some distance from the vent hole "Phase B". The pressure in the vent tube oscillated periodically and synchronized with the pressure of the pool water. The pressure in the vent and of the pool water varied according to the FO and peaked when pressure oscillation appeared in the region of Phase B.
2) The oscillation of the pressure and flashing location might have been caused by a balancing action among the supply of saturated water, flashing in the control volume, and steam condensation on the steam-water interface. Flashing close to the vent is initiated (Phase A). The steam bubble generated by the flashing shrinks due to steam condensation on the interface. 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 starts again and the next cycle starts;
3) A test using two vent tubes, the length of which differed by 1.0 m and 2.0 m from the original one (1.4 m from the saturated water tank to the pool water tank), showed that the physical behavior of the FO was consistent under different vent tube length conditions. As, the FO phenomena was influenced weakly by the length of the vent tube, the frequency of the FO depended on the control volume, consisting of steam bubbles in the pool water and a part of the outlet space of the vent tube.
4) A linear analysis was conducted using a spherical flashing bubble model and three basic equations: the continuity equation of flashing and condensation, the momentum equation for water-steam interface motion and the equations of state for steam and water. The period of the flashing oscillation in the experiments can be explained by theoretical analysis. The phenomena of FO involves the overlapping phenomena of the saturated water flashing and steam condensation in the pool water. The phenomena might be so complicated, nevertheless the simple bubble model has explained well the effects of the experimental settings on the frequency of FO. This shows that FO is a phenomenon which is basically dominated by bubble motion in the pool water.
5) When the saturated water pressure and vent hole diameter increases, 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 (during which flashing occurs some distance from vent hole) diminishes. Phase A (during which flashing occurs very close to the vent hole) appears, and FO stops when the saturated water pressure and vent hole diameter are large, and the pool water subcooling is low. With a short nozzle, bubbles cannot develop and Phase B occurs. Therefore FO with an alternating flashing location in the pool might stop when a long nozzle is used. The effects of the experiment settings on the behavior of the flashing in the pool are represented in the maps obtained through this study.
6) Bailey's correlation for the flow rate of discharging saturated water into gas field can predict well the present result. And subcooling of pool water little affects on the flow rate. These facts indicate that the discharging flow rate into pool water is little affected by the outlet condition except ambient pressure.
We have many unstable or oscillatory thermo hydraulic gas-liquid two-phase behaviors in energy plants. To improve the efficiency, safety, and reliability of the plants, the mechanism behind the phenomena should be clarified.
Acknowledgements
Authors are grateful to Dr Yamaji of SRI, Japan, Drs. T. Hoshi and N. Ishida of JAERI for their help to conduct experiments.
