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For boiling water reactors and small reactors, an effective way to reduce containment pressure is to discharge gas from the containment into the suppression pool. Based on the above background, a series of experiments were conducted to investigate the pressure oscillations frequency of steam-air gas discharged into a supercooled water pool. The air mass fraction was 0.0-0.4, and the hole pitch was 1.5d-3d. The air compressor and electric boiler generated air and steam, respectively. The steam and air were mixed and discharged into the supercooled water pool. A high-frequency dynamic pressure sensor was used at the nozzle outlet to record the pressure oscillation signal, and a high-speed camera captures jet plume images. The fast Fourier transform (FFT) was used to process the pressure signal, and the peak value of the frequency domain signal is the dominant frequency of the oscillation. The image observation reveals that in the case of a double-hole jet, the interference of the two gas plumes causes the gas plumes to converge or even agglomerate to form a combined jet. The dominant frequency of the double-hole jet is lower than that of the single-hole jet because of the increase in the dimension of the double-hole jet plume, which leads to a longer condensation time. Therefore, increasing the number of holes will result in a decrease in the dominant frequency of the oscillation, which may be closer to the equipment's natural frequency and lead to resonance damage. For temperature effects, the condensation time of the plume is shorter as the subcooling increases, so the oscillation frequency increases with subcooling. For the effect of hole pitch, the dominant frequency increases with hole pitch. The water temperature between the two gas plumes decreases as the hole pitch increases, and the gas plumes condense faster and take less time to condense. Based on the research conclusions of the dominant frequency characteristics of the steam jet condensation, an empirical equation for predicting the dominant frequency was proposed, and the predicted deviation is within the range of -15%-20%. The oscillation dominant frequency of the steam-air mixture jet increases with the subcooling degree and the hole pitch, which is consistent with the conclusion of the steam jet. When steam-air mixture gas jets, the dominant frequency is inversely proportional to the non-condensable gas mass fraction. The reason is that during the steam-air mixture gas jets, the concentration of non-condensable gas increases after steam condensation between the gas-water interfaces, which inhibits the mass transfer process, resulting in a longer condensation time. As the fraction of non-condensable gases increases, the mass transfer inhibition becomes more pronounced.

The transport and retention of aerosols in the containment gap are closely related to the gas flow in the cracks. In order to evaluate the aerosol retention efficiency in the cracks after an accident by the existing aerosol penetration coefficient relationships, taking the capillary as the representative crack, a methodology for pressure difference driven gas flow calculation was proposed based on the one-dimensional compressible adiabatic flow equation, and the calculation program was compiled based on FORTRAN language. The input parameters of the program were capillary upstream pressure and temperature, downstream pressure, capillary length, inner diameter and roughness. When calculating the resistance, the classical laminar resistance relationship and turbulent Colebrook relationship were adopted, and the form resistance coefficients at the inlet and outlet of the capillary were 0.5 and 1.0 respectively. The program can calculate the subsonic speed, critical speed and choking state of air flow. The one-dimensional calculation method was validated by the capillary flow characteristic experiment carried out by Kagoshima University in Japan. The capillary length is 12 cm, the inner diameter is 397 μm and the roughness was 0.02%. The downstream pressure of the test was 100 kPa and the upstream pressure was 150-700 kPa. In addition, in order to obtain the detailed flow information in the capillary, the computational fluid dynamics (CFD) software ANSYS Fluent 19.2 was also used to model and calculate the experimental device. The structured grid scheme was adopted in the CFD modeling, and the number of grids was about 1.7 million. The numerical results show that the flow change trend is consistent, and most of the deviation between the gas flow and the experimental value is less than 10%, which proves the accuracy and rationality of the one-dimensional calculation method, and the efficiency of one-dimensional calculation method is better than that of CFD method. On this basis, the influences of pressure difference and inner diameter on gas flow were analyzed by one-dimensional calculation method. The results show that the leakage flow of small inner diameter capillary increases quadratic with the increase of pressure difference, and when the inner diameter is greater than 100 μm, the growth trend of mass flow transits to linearization if the pressure difference exceeds a certain value. When the pressure difference makes the flow in the transition zone between laminar flow and turbulence, the leakage flow through the capillary tube will appear a platform, affected by the change trend of resistance coefficient. The larger the inner diameter, the earlier the platform appears. The results of this paper can provide a technical basis for the retention evaluation of aerosols in the cracks. In the follow-up, the numerical methodology will be extended to the analysis of pressure-driven gas flow with multi-components through capillary tubes.

The void fraction and interfacial area concentration are important phase interfacial parameters in two-phase flow. Accurate acquisition of void fraction and interfacial area concentration under churn flow and annular flow in a narrow rectangular channel is the key to construct and improve the two-fluid model. In this paper, a visualization experiment was conducted to study the flow characteristics of gas-liquid two-phase flow in a narrow rectangular channel with a cross-section of 65 mm×2 mm. The range of gas-phase superficial velocity is 1.9 m/s and liquid-phase superficial velocity is 0.1-1.5 m/s. Flow pattern includes churn flow and annular flow. Based on the idea of gas-phase splitting calculation, an analytical calculation method to obtain the void fraction and interfacial area concentration under the churn flow and annular flow was proposed using high-speed camera. And the calculation method for the local parameters in the direction of the wide edge of the narrow rectangular channel was also proposed. For the two-phase flow video under each experimental condition, picture extraction and data processing were performed at 0.1 s time interval, and the duration of data processing is 60 s for each condition. The relative deviation between the calculated void fraction of the empirical formular and experiment value in the narrow channel is within 10%. The correlation coefficient K between interfacial area concentration and void fraction was proposed. As the void fraction increases, the value of K gradually converges to 1 from greater than 1. This result is consistent with the theoretical analysis and verifies the rationality of interfacial area concentration calculation method. To verify the accuracy of the local parameter calculation method, the experimental images were divided into 10 equal parts along the broad side of the narrow rectangular channel. The relative deviation between the average values of the local void fraction and interfacial area concentration and the results calculated without partitioning is within 5%. The calculation method can be helpful for the study of phase interfacial parameters in narrow rectangular channels under complex flow patterns.