Measurements of fluid fluctuations around an oscillating nuclear fuel assembly

In this paper, dynamic measurements of fluid velocity in the by-passes of a test-section representing a nuclear fuel assembly are presented. The test-section was designed to identify stiffness, damping and mass coefficients of a fuel assembly under axial flow, and previous studies have shown that the by-passes have an influence on the identified coefficients. The results presented in this paper show that the motion of the fuel assembly induces fluctuations in the axial fluid velocity in the by-passes. These fluctuations depend on the excitation frequency and position. A delay has been observed between the fuel assembly displacement and the fluid velocity fluctuations. The delay decreases when the axial velocity increases which means that it is a convection driven phenomenon.