Two-phase damping and interface surface area in tubes with internal flow

Two-phase flow is common in the nuclear industry. It is a potential source of vibration in piping systems. In this paper, two-phase damping in the bubbly flow regime is related to the interface surface area between phases and, therefore, to flow configuration. Two sets of experiments were performed with a vertical tube clamped at both ends. First, gas bubbles of controlled geometry were simulated with glass spheres let to settle in stagnant water. Second, air was injected in stagnant alcohol to generate a uniform and measurable bubble flow. In both cases, the two-phase damping ratio is correlated to the number of bubbles (or spheres). Two-phase damping is directly related to the interface surface area, based on a spherical bubble model. Further experiments were carried out on tubes with internal two-phase air-water flows. A strong dependence of two-phase damping on flow configuration in bubbly flow regime is observed. A series of photographs attests to the fact that two-phase damping increases for a larger number of bubbles, and for smaller bubbles. It is highest immediately prior to the transition from bubbly flow to slug or chum flow regimes. Beyond the transition, damping decreases. An analytical model is proposed to predict two-phase flow damping in bubbly flow, based on a spherical bubble model. The results also reveal that the transition between bubbly flow and slug/churn flow depends on tube diameter. Consequently, the tube diameter also has an effect on two-phase damping. The above results could lead to some modifications of existing flow regime maps for small diameter tubes.