Damping and fluid-elastic instability of a tube bundle submitted to two-phase flow

Heat exchangers from nuclear steam generators are submitted to two-phase flow (steamwater). The U-bend region of nuclear steam generators is particulary vulnerable to vibration due to two-phase cross-flow. Fluidelastic coupling between the vibrating structure and the flowing fluid may lead to fluidelastic instability. It is admitted that fluidelastic instability is initiated by a loss of damping. Fluidelastic instability may be formulated in terms of a dimensionless flow velocity and a dimensionless mass-damping parameter (in which the damping is the one in still flow). A classical method of damping calculation suggests that damping in two-phase flow is due to a specific two-phase mechanism in addition to classical viscous damping. Yet, some difficulties may arise, for example in the definition of the viscosity of the two-phase mixture. In that sense, we propose to consider the fluid damping as a global entity. Experimental data in air-water from the literature and from our mock-up are analysed. A new reduction formula is defined. This approach is then extended to take into account the effect of two-phase flow velocity. Comparison of the evolution of damping with flow velocity between two-phase flow and single-phase flow is made. From this study, a new formulation for fluidelastic instability in two-phase flow can be proposed. It is formulated in terms of dimensionless flow velocity and void fraction. (author)