TWO-PHASE CROSS FLOW STRUCTURES IN BUBBLY AND CHURN FLOWS IN ROD BUNDLE GEOMETRIES

摘要

Two-phase cross-flow induced vibrations in the inverted U tube bundles of a nuclear steam generator can cause serious tube failures. To avoid such vibration problems demands a better understanding of two phase cross flow structures in the rod bundle geometries subjected to two-phase bubbly and churn flows. Particularly, churn flow may be closely related to fluidelastic vibrations. However, vibration excitation and damping mechanisms due to the two-phase cross flow are unfortunately much more complex and less understood than those in a single-phase flow. We therefore, studied experimentally and numerically the two-phase flow behaviors in cross flow tube bundles. In experiments, the phase distributions of air-water two-phase flow in the horizontal and the inclined rod bundles with in-line and staggered arrays in a vertical rectangular duct (200 mm square x 1090 mm in height) were mainly analyzed. The results exhibit non-homogeneous phase distribution both in bubbly and churn flows depending on the angle of inclination of rod arrays. A numerical simulation of bubble trajectories was carried out in Lagrangian scheme, using a single-phase k-e model. The results predicted well the global structures of the two-phase flow obtained by experiments. However, bubble behaviors observed by a high speed video were not well simulated in their details due to the lack of information about the forces acting on the bubbles. Some discussions have been presented on this problem as well.