Flat-bar supports have frequently been used to control transverse vibration of tubes in the U-bend region of recirculating steam generators. Recently, it was discovered that the frictional effects of these flat bars are not always sufficient to prevent streamwise or in-plane (IP) fluidelastic instability (FEI). Due to the highly nonlinear nature of the tube-support interaction, the parameters that control the vibration in the IP direction (support clearance and preloads), have not been well characterized. In order to improve the characterization and modelling of tube-support interaction effects, CNL has developed experimental facilities that allow better control of tube and support positions and has improved its numerical models.

This paper investigates the parameters controlling IP FEI through experimental measurements and numerical modelling. Experiments were conducted on a straight tube bundle in a parallel triangular configuration with a pitch ratio of 1.5. The tubes were fixed at one end while, at the other end, flat bars were accurately positioned with respect to the tubes to control their vibration in the transverse direction. The effects of the support clearance and preload were investigated. Support clearance was varied by changing the thickness of the flat bars while relative position and preload were controlled by the precise lateral position of the flat bars. Tests were conducted in air-water flow for various void fractions. For each combination of support, preload and void fraction, the flow velocity was varied and the vibration of the free ends of all the tubes was measured using a high speed camera. FEI behaviour and thresholds were determined based on the vibration amplitude.

Numerical simulations were also conducted to use the test data for validating the numerical model. FEI was modelled using the quasi-steady approach with inter-tube coupling. The tube-support contact normal force was modelled using a spring-damper system while the friction was modelled using two friction models: the velocity-limited friction model and force balance friction model. The results demonstrate that the model can reproduce the experimental results with acceptable accuracy.