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A series of experiments have been performed to investigate thermal stratification induced by steam-air mixture vertical injection, in the range of steam mass flux 3–30 kg/m2s, air mass fraction 0–5% and submergence depth 100–300 mm. Experimental results show that: thermal stratification occurs at the region where none of chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume is strong enough. For pure steam experiments, thermal stratification occurs when the steam mass flux above 15 kg/m2s. The pool is mixed well firstly, then thermally stratified, and then re-mixed in sequence, as the water temperature increases. A little air can make thermal stratification and re-mixing occur ahead of time and more air can completely remove thermal stratification. Gas bypass induced by shallow submergence depth makes thermal stratification occur earlier and more severe. The submergence depth has little effect on thermal stratification in chugging regime. • Shallow submergence depth is 1.25 D -3.75 D. • Gas bypass is observed when submergence depth is 1.25 D. • A little air can make thermal stratification occur ahead of time and more severer. [ABSTRACT FROM AUTHOR]

Highlights • Dynamic pressure signals and jet images are recorded and processed. • The effect of air mass fraction and water temperature on pressure oscillation interfacial fluctuation height are analysed. • The validity of previous correlation for steam dominant frequency is evaluated. • A modified function for dominant frequency of mixture gas jets is developed. Abstract Present paper aims to experimentally investigate pressure oscillation induced by steam-air mixture gas sonic jets in subcooled water. Dynamic pressure signals and jet images are recorded and analyzed. Experimental results show that: the pressure oscillation dominant frequency decreases with the rise of water temperature, inlet pressure and air mass fraction. Air mass fraction has a complex effect on pressure oscillation intensity. When water temperature is less than 55 °C, as the air mass fraction increases, the pressure oscillation intensity increases first, and then decreases slightly, and then increases quickly. When water temperature is more than 55 °C, as the air mass fraction increases, the pressure oscillation intensity decreases first, and then increases quickly. The interfacial fluctuation height and pressure oscillation intensity dependency to the air mass fraction is the same near the nozzle exit. In addition, the validity of previous analysis model for steam sonic jet pressure oscillation dominant frequency is evaluated. A modified function considering the effect of air mass fraction is developed. The predicted results of steam-air mixture gas sonic jet pressure oscillation dominant frequency agree well with the experimental results. More than 98% of the data points are inside the error band of ±20%. [ABSTRACT FROM AUTHOR]

• Bubble resonance with 35 Hz is observed, which could intensify pressure oscillation intensity. • Air is present in dispersive small bubbles or a toroid after condensation. • The onset of chugging is given by correlations. • A new frequency correlation modified with probability is developed. An experimental study is conducted on the condensation behavior, onset of chugging, fluid oscillation, and pressure oscillation when a steam–air mixture is injected vertically into subcooled water at a low mass flux. Air is present in the form of dispersive small bubbles after condensation at a low water temperature and in the shape of a toroid at a high water temperature. Bubble resonance is first observed experimentally; a large bubble is almost stationary below the pipe outlet and air is sustainably released by small bubbles from the upper surface. The upper air mass fraction of chugging is obtained and compared with a transient conduction–diffusion model and momentum balance model. The dependence of different parameters on the amplitude, frequency, and probability of fluid oscillation is analyzed. A new frequency correlation modified with probability is developed and shows good agreement with the experimental data. During bubble resonance, the wave shape of pressure oscillation is harmonic with the frequency at about 35 Hz, and the intensity is larger than that under pure steam conditions. When bubble resonance does not occur, the presence of air can reduce the pressure oscillation intensity. [ABSTRACT FROM AUTHOR]

• Four typical interfacial shapes are observed. • The spreading rate of interfacial radius is in the range of 0.12–0.27. • The interfacial fluctuation height is proportional to the axial position with spreading coefficient 0.03 ± 0.008. • The maximum velocity and acceleration of interface fluctuation are estimated to be around 6 m/s and 20,000 m/s2, respectively. Interfacial characteristics of steam-air mixture sonic jets are recorded and processed using image processing technique. Interfacial shape, radius, fluctuation height, fluctuation velocity and fluctuation acceleration are analyzed. Four typical interfacial shapes including expansion-contraction, double expansion-contraction, expansion-contraction-divergence and expansion-divergence are observed and a three-dimensional interfacial shape map is developed. Gas dynamic effect is notable in the axial region of the near field. The interfacial radius increases linearly with the axial distance in the far field. The spreading rate increases with the rise of air mass fraction and its value is in the range of 0.12–0.27. The interfacial fluctuation height is proportional to the axial distance both in the near field and far field. The spreading coefficient is in the range of 0.03 ± 0.008. In general, both interfacial fluctuation velocity and acceleration increase with the axial distance. The growth rate decreases rapidly in the near field and then increases slightly again in the far field. The maximum velocity and acceleration of interface fluctuation are estimated to be around 6 m/s and 20,000 m/s2, respectively. [ABSTRACT FROM AUTHOR]