Time and frequency domain analyses of fluid force fluctuations in a normal triangular tube array in forced vibrations

Abstract

Vibrations of tube bundles in heat exchange equipment, such as nuclear steam generators and tube-and-shell type heat exchangers, are often caused by unsteady fluid forces. The study of flow-induced vibration excitation forces on tubes is necessary for understanding the fluid-elastic instability phenomenon. In this paper, a forced vibration method is introduced to investigate unsteady motion-dependent fluid forces in a normal triangular tube array with the pitch ratio of 1.32 that is subjected to water crossflow. Fluid force behaviors of a single flexible central tube and its lift response spectra are investigated in various forced vibration modes. In a seven-tube kernel unit, coupling effects of surrounding tubes on the central tube are also discussed. Results show that the lift force on a flexible vibrating tube mainly depends on the vibration amplitude of tube itself in the lift direction, and is less affected by the degree of freedom, the rotation direction and the main vibration direction of tube vibration. The frequency spectrum of lift responses is dominated by two main frequencies: the vortex alternating frequency and the tube natural frequency. Surrounding flexible tubes increase higher order harmonics of the periodic lift force fluctuations of the central tube. Compared with rear-column tubes, vibrations of the front-column tubes have more significant effects on lift force responses of the tube kernel, which facts causes strong coupling to promote the fluid-elastic instability of tubes.