Investigation on steam contact condensation injected vertically at low mass flux: Part I pure steam experiment
Introduction
The pressure suppression containment is widely applied in traditional boiling water reactors (BWRs), due to two merits: reducing the containment design pressure or volume and good fission product retention effectiveness. It also has the potential to be used in pressurized water reactors (PWRs) because of reducing the required number of passive containment cooling system (PCCS) heat exchangers [1]. At a postulated loss of coolant accident (LOCA), a large volume of steam associated with the reactor pressure vessel (RPV) blowdown is vented to drywell (DW), and then vented to suppression pool (SP) where it condensates to a small volume of water. However, unfavorable fluid and pressure oscillations occur under specific conditions, which threaten the structural integrity of containment [2], [3].
The most severe pressure oscillation was observed in chugging regime which occurs at a low mass flux [4], [5], [6], [7], [8], [9]. Liang and Griffith [10] developed chugging criterion through transient conduction model and concluded the smaller diameter pipe, the larger upper threshold of chugging. With et al. [6] reviewed previous experimental data and assumed that chugging boundary increases linearly with the pipe diameter. This assumption was found in contradiction with the PPOOLEX MIX experiments [11]. Recently, Zhao and Hibiki [5] reviewed the condensation regimes in detail. Considerable bias was found among the existing analytical model, empirical correlations and empirical maps.
Aya et al. [4], [12] experimentally investigated fluid and pressure oscillations in vent pipes and DW in a wide parameters. Thermocouples in the pipe measured water temperature and steam temperature periodically (back flow of water), which indicates chugging is taking place. The steam mass flux at the upper threshold of chugging occurrence is almost on a downward straight line against the water temperature. Later, Aya and Nariai [13] analyzed boundaries between various condensation regimes through linear stability theory. Recently, Laine et al. [14], [15] performed twelve experiments to obtain verification data of the Aya and Nariai model for prediction of fluid oscillation in the pipe. Villanueva et al. [16] proposed that Froude number (Fr) can be used as a scaling criterion for non-dimensional amplitude and frequency of the fluid oscillation. The non-dimensional amplitude had maximum at Fr ≈ 2.8 and the non-dimensional frequency had minimum at Fr ≈ 6, based on the data of Aya et al. and Laine et al. The Aya and Nariai analytical theory failed to capture the dependence of amplitude and frequency on water temperature. Later, Gallego-Marcos et al. [11] revised scaling approach and developed correlations for non-dimensional amplitude and frequency based on Gaussian distribution. In addition, Youn et al. [17] investigated characteristics of pressure oscillation in the water. The pressure pulse frequency was little affected by the water temperature, but increased with increasing steam mass flux. Gregu et al. [18] measured pressure spikes of around 1.2 MPa in the pipe because of the condensation-induced water hammer (CIWH).
Summaries of available experimental investigations on pure steam contact condensation at low mass flux are shown in Table 1. Three experimental methods (temperature measurement, dynamic pressure measurement and visual observation) were used in the references. From the literature review mentioned above, there is no enough experimental data about upper threshold of chugging, fluid oscillation and pressure oscillation, especially at high water temperature. Therefore, ten experiments have been performed to investigate steam contact condensation injected vertically at low mass flux, using the three methods.
Section snippets
Experimental facility
Schematic diagram of experimental facility is shown in Fig. 1(a), which consists of a water tank, gas supply system and measuring system. The upper open water pool has dimensions (length × width × height) 2400 mm × 1200 mm × 2400 mm and two transparent sidewalls for optical measurements. Two screw air compressors are applied to produce dry air with maximum mass flow rate of 1000 kg/h and maximum operating pressure of 0.8 MPa. An air storage tank with volume of 3 m3 is used to keep air flow rate
Condensation behavior
Chugging is a condensation regime characterized by periodic bubble formation, bubble collapse, water flow back and ejection, as shown in Fig. 4. When the subcooled water stays in the nozzle, steam condensation occurs on the steam-water interface. Once condensation rate is less than steam flow rate, steam will speed up water. Then, steam is carried over and steam bubble forms in the nozzle outlet. When the steam bubble reaches its maximum size, bubble surface becomes rough. Later, steam bubble
Conclusion
Ten pure steam experiments have been carried out to investigate steam contact condensation in subcooled water, in the range of steam mass flux 5–80 kg/m2 s and water temperature 19–96 °C. Steam condensation behave, fluid oscillation and pressure oscillation are recorded and analyzed. Main conclusions can be drawn as:
- (1)
In chugging regime, the bubble shape transitions from cone to sphere as the water temperature increases. Typical Rayleigh-Taylor instability on the bubble interface and vortex ring
Conflict of interest statement
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgement
The financial supports of National Nature Science Foundation of China (Nos. 11605032, 11775060 and KY11500170030) and the Open Fund Program of the Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education of China (ARES-2017-02) are gratefully acknowledged.
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