| With the development of current aerospace technology, cryogenic propellants are increasingly used in the missile industry. Geysering has been an important problem in long cryogenic propellant feeding lines connecting the launch vehicle propellant tank and the rocket engines. The phenomenon termed geysering can be described as the unstable expulsion of a boiling liquid and its vapor from a tube. When the feed-line is heated along the tube wall, the bubbles are formed, start to rise and eventually coalesce into a larger slug bubble called a Taylor bubble, which fills the cross section of the line. As the Taylor bubble rises, it expels the liquid from the line into the tank ahead of it; cold liquid at the bottom of the tank then rushes into the empty line propelled not only by gravity, but also by the low pressure ahead of it created by condensation of the vapor in the line. This column of liquid impacts a closed valve or other obstructions at the bottom of the line with a sufficiently high velocity creating a potentially destructive water hammer with surge pressure. So it's necessary to study the bubbles generation, coalescence, rise as well as Taylor bubbles formation and development in tubes. Based on previous studies, through experiments and theoretical analysis, aiming at the cryogenic liquid with small density difference between liquid phase and vapor phase, small latent heat of evaporation, and other unique characteristics of thermodynamics, heat transfer and boiling dynamics, overcoming the experimental difficulty of cryogenic two phase flow, it deeply studies the phenomenon of vapor-liquid two-phase flow in the cryogenic vertical transfer pipelines. In this paper, bubbles production, coalescence, Taylor bubble formation mechanism, the moving velocity and length distribution of Taylor bubble and liquid slug and the dynamic flow characteristics in pipeline have been experimentally studied as following aspects:(1) Visualization experiments and the experimental apparatus have been set up in order to investigate the mean initial position of Taylor bubbles, the moving velocities and the length distributions of Taylor bubbles and liquid slugs, the log-normal shape is fitted to the measured distributions of Taylor bubbles and liquid slugs. Through analyzing the pressure signal of two-phase flow in vertical cryogenic tubes, the influence of the characteristics of bubbly flow and slug flow, the law of flow pattern transition and pipe dimension on change of the flow pattern and the dynamic flow characteristics of two-phase flow are studied.(2) The results show as follows: 1) The mean initial position of Taylor bubbles is affected by the inclination angle from the vertical of the tube. With the inclination angle increasing, the mean initial position of Taylor bubbles increases first, and then decreases. And when the inclination angle is 30°, the mean initial position of Taylor bubbles reaches the maximum; 2) With the increase of inclination angle, the mean lengths of Taylor bubbles and liquid slugs both increase first, and then decrease. And when the inclination angle is 30°, the mean length of Taylor bubbles and liquid slugs reach the maximum. The mean lengths of Taylor bubbles and liquid slugs would increase with the increasing of the position along the tube. When at the same inclination angle and the position with the same x/D, the mean length of Taylor bubbles in the tube with small diameter is greater than that of Taylor bubbles in the tube with large diameter. And the reverse is true in the liquid slugs; 3) With the increase of inclination angle, the moving velocities of Taylor bubbles and liquid slugs both increase first, and then decrease. And when the inclination angle is 30°, the rising velocity of Taylor bubbles reaches the maximum. And the rising velocity of liquid slugs can reach the maximum when the inclination angle is 20o or 30o.(3) The influence of system pressure on the pressure fluctuation in vertical upward gryogenic two-phase flow is studied. Qualitative results show that the pressure fluctuation of cryogenic vapor-liquid two phase flow in vertical tube is similar to that of room-temperature vapor-liquid two phase flow, which is affected by some factors such as inclination angle, heat flux and system pressure. And the flow pattern transition is basically the same; 5) When the system pressure was 0 kPa, the trend of change on PSD of pressure signal was almost the same for all experimental working conditions. And with the increase of the position of measure points, the high PSD frequency gradually moved rightward. And the curve had the trend that changed from single peak to double peaks. When system pressure was not equal to 0 kPa, the fluctuation of pressure signals was inhibited. And with the increase of system pressure, inhibition on the fluctuation of pressure signal would strengthen. Hence on the cryogenic vapor-liquid two-phase systems, it's an effective measure to increase the system pressure in order to suppress the pressure fluctuations under the premise of the system, which can prevent geysering phenomenon occurrence in cryogenic liquid transfer pipelines.(4) The mechanisms of bubble collision and coalescence were theoretically analyzed and a MUSIG(MUlti-SIze-Group) model was proposed to describing this process. Then three-dimensional numerical simulation was performed using the newly proposed model. Comparison of the numerical results against the experimental data illustrates that the model can give accurate predictions on the locations of Onset of Bubble Coalescence (OBC) and Forming of Slug Bubble (FSB), as well as the local bubble size distributions. The results show as follows: 1) The MUSIG model could effectively predict the process that the dispersed bubbles coalesce into Taylor bubbles in the tubes; 2) Through increasing the inclination angle, the coalescence rate of bubbles was decreased and the formation of Taylor bubbles was delayed at higher cross section mean void fraction; 3) This model could predict the composing condition of bubbles dimension, and it could help to analysis the probability of formation of Taylor bubbles and geysering phenomenon in order to take measures to inhibit geysering.The formation mechanism of Taylor bubbles and the dynamic flow characteristics of two-phase flow in vertical upward tubes are experimentally and theoretically researched. The researched results are reasonable and helpful for further work on the formation and development law of Taylor bubbles, and dynamic characteristics in cryogenic tubes. And it could provide theory support to reveal the mechanism of geysering phenomenon.
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