Experimental Study on Fluid-Elastic Instability of the Heat-Exchange Tube with Unequal Frequencies

The steam generator is the center of the primary and secondary circuits of the pressurized water reactor(PWR)type of nuclear power plant. It plays an important role in transferring the heat of the primary circuit coolant to the secondary circuit to generate high-pressure steam that drives the rotation of the steam turbine unit to produce electricity. The heat-exchange tubes inside the steam generator which form an inverted U-shaped structure are easily affected by fluid-elastic instability and thus produce a large amount of vibration，which renders the tubes susceptible to wear. This could eventually lead to the failure of the steam generator，resulting in economic losses. The typical use of anti-vibration strips to support the elbow section of the internal heat-exchange tubes produces a difference in the natural frequency of tubes between the in-plane and out-of-plane directions of the clamping surface. As the anti-vibration strips relax and fail，the natural frequency in the in-plane direction of the elbow section will be lower than that in the transverse direction. In this paper，the water tunnel experiment method is used to study the fluid-elastic instability of a single flexible tube，with a pitch diameter ratio of 1.33 and a triangular arrangement. The natural frequency ratios of the in-plane direction and the out-of-plane direction are 50%，60%，70%，and 80%，respectively. The results of the experiment show that with a low flow velocity，the equal-frequency tube exhibits a small overall vibration. With an increase in the flow velocity，the phenomenon of fluid-elastic instability appears in both the streamwise and transverse directions. Compared with the flexible equal-frequency tube，the unequal-frequency tube produces a longer locking zone and a larger vibration amplitude in the process of vortex shedding excitation. The phenomenon of fluid-elastic instability occurs in the transverse direction only. Within the scope of the experimental conditions，whenever the vortex-induced vibration phenomenon occurs，the flow velocity is positively correlated to the natural frequency in the streamwise direction. In the transverse direction，the critical velocity of the fluid-elastic instability of the heat-exchange tube is controlled by the natural frequency. © 2023 Tianjin University. All rights reserved.