STABILIZATION EFFECT OF SHOCK NON-LINEARITY ON THE DYNAMICS OF A STEAM GENERATOR TUBE

Abstract : In Pressurized Water Reactors of nuclear power plants, steam generators act as heat exchangers between primary and secondary coolant fluids. They consist of a bundle of U-tubes in which flows the primary coolant fluid. Several support plates guide these tubes vertically. Secondary coolant fluid flows along the U-tubes and passes through the space between tubes and plates. This space, initially of a foliate shape, is filled with sludge deposits. Consequently, fluid flow is accelerated and the tubes are more excited. Moreover, the mechanical bonding between tube and plate is changed. The combination of these two phenomena can lead to dynamic instabilities and tube cracks. This paper focuses on the effects of shocks on tube instability. As we concentrate on a mechanical point of view, the study of the nonlinear dynamics is made in the following specific conditions: the tube is in air with no flowing fluid; an instability is generated by injecting a force proportional to the velocity of a point of the leg of the tube. The tube is modeled as an Euler-Bernoulli beam. The contact between tube and plate is computed assuming circular obstacles and a contact force that is linear with a gap. A reduced model is generated to represent the bandwidth of interest and the effect of contact forces. Time evolution is then computed using a nonlinear Newmark scheme. Numerical simulations show the effect of shock nonlinearities on the dynamics. Cases of unstable unconstrained tubes leading to bounded stable dynamics when shocks occur are analyzed. The instability in the unconstrained condition does not imply that the bilateral contact condition is unstable. Areas of the contact stiffness/feedback gain plane are shown to lead to stable modes of the bilateral model. The work then presents results on an experimental bench that includes the curved part of a tube and one Support Plate. The instability is generated by feeding back to a shaker the amplified signal of a velocity measurement performed with an integrated accelerometer. For great enough feedback gains, the vibration amplitudes grow until shocks occur. The permanent regime eventually found is analyzed. A good similarity between test and analysis is found.
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Proceedings of COMPDYN 2011, May 2011, France. paper 262, 2011
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Thibaud Thenint, Etienne Balmès, Mathieu Corus. STABILIZATION EFFECT OF SHOCK NON-LINEARITY ON THE DYNAMICS OF A STEAM GENERATOR TUBE. Proceedings of COMPDYN 2011, May 2011, France. paper 262, 2011. 〈hal-00607080〉

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