Effect of array flexibility on transverse fluidelastic instability in parallel triangular tube bundles under single- and two-phase cross-flows

Fluidelastic instability (FEI) represents a significant risk in steam generators due to high-velocity two-phase water-steam cross-flows acting on tube bundles. While FEI in square tube bundles has been extensively studied, triangular tube arrays remain an interesting subject. This paper presents new experimental insights into transverse FEI in a parallel triangular tube bundle. The experiments were conducted at CEA-Saclay (France) using a 50-tube bundle ( P / D = 1.44, D = 30 mm) subjected to cross-flows of air, water, and air-water with homogeneous void fractions ranging from 25% to 90%. Six configurations with one to seven flexible tubes within a rigid tube bundle were tested. A key feature of this study is the detailed modal analysis of the bundle under natural turbulent excitation at various flow velocities and void fractions, which allows the identification of modes prone to transverse FEI as well as their complexity. The influence of array flexibility on transverse FEI was investigated. Under single-phase water flow conditions, fluidelastic coupling induces collective in-phase motion of the tubes. In single-phase air flow, the unstable mode is characterized by predominantly out-of-phase motion of the tubes in the central column, and instability occurs only when at least two tubes in the same column are allowed to move. In two-phase flows, the transition to instability is more gradual and dependent on the flexible layout of the array, with mode shapes showing out-of-phase motion patterns of the tube clusters as the void fraction increases. For each void fraction, the deviation in critical velocity between the different configurations remains less than 20%. Finally, a stability map comparing the present and literature data shows Connors’ constants of about 5 for water and two-phase flows, and >10 for air flow.