Flow induced instabilities of fuel rods bundles subjected to a plane jet cross flow: experimental and numerical analysis

摘要

Experimental and analytical programs on PWR fuel rods subjected to local transverse jets are undertaken at French atomic energy commission (CEA/DEC/SECA/LHC) in order to study the rod behaviour with respect to baffle jetting. Baffle jetting is correctly simulated by a plane jet cross flow. When increasing the cross-flow rate, instabilities are characterised in particular by a sudden increase in vibrational amplitude. The fluid-elastic instabilities are now well understood. However. in some test configurations, resonance amplitudes have been observed for cross flow rates lower than those corresponding to fluid-elastic instability (early instabilities). The aim of the paper is to present and analyse such resonance amplitudes. The conditions in which these early instabilities have been observed are reported in connection with previous studies. Then we present experimental and numerical analyses which have been performed in the case of a plane jet flowing on one side of a comer rigid rod. The hypothesis of the coincidence between a periodic vortex shedding and a rod eigenfrequency is particularly studied. Experimental investigations concern visualisations, wall pressure fluctuations and velocimetry. The results lead us to conclude in the presence of vortex and vortex shedding inside and on the edge of the bundle. However, periodicity of this shedding is not present in the frequency range of interest. Numerical analysis, performed by unsteady Reynolds Averaged Navier-Stokes numerical simulations leads to the same conclusion. Experimental and numerical investigations both suggest other causes of rod instability, in particular the jet instability.