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Unsteady Pressure and Velocity Measurements in 5x5 Rods Bundle Using Grids With and Without Mixing Vanes

Abstract : The vibration of rods is of prime importance to estimate their wearing due to the frictions with grids' elements. To predict this wearing, the forces exerted on the rods need to be identified and quantified. Pressure fluctuations are explored using an instrumented rod centred within a 5x5 rods bundle. Two configurations of spacer-grids are used: with and without mixing vanes respectively noted WMV and NMV. The position of the measurement point is rotated over 360° and its distance from the grids is varied from 0.5D$_h$ to 20D$_h$. The results show the non-homogeneity of the azimuthal distribution of the pressure fluctuations in the near wake of the spacer-grids for both configurations. The return to a homogeneous azimuthal distribution of the pressure fluctuations is faster without mixing vanes than with mixing vanes. The intensities of the pressure fluctuations are (in average) higher for WMV than for NMV: higher by 13% and 136% respectively at distances 0.5D$_h$ and 20D$_h$ from the grids. Power law approximations show decreases of $\langle$$P'_{rms}$$\rangle$$_\theta$ like $(y/D_h)$$^{-0.46}$ and $(y/D_h)$$^{-0.68}$. respectively for the configurations WMV and NMV. The profiles of mean velocity and velocity fluctuations are explored around the instrumented rod using LDV. Results show oscillation of the profiles and a damping with the distance from the grids. Power law approximations show decreases of $\langle$$V'_{rms}$$\rangle$$_{Psq}/V_{flow}$ ⁄ like $(y/D_h)$$^{-0.38}$ and $(y/D_h)$$^{-0.46}$. respectively for WMV and NMV. Between 2D$_h$ and 15D$_h$, the dimensionless ratio of "pressure fluctuations to velocity fluctuations" $\langle$$P'_{rms}$$\rangle$$_\theta$/$1\over 2$$\rho$ $\langle$$V'_{rms}$$^2$$\rangle$$_{Psq}$ is found to increase by 70% and 100% respectively for the configurations NMV and WMV. This increase demonstrated by the combination of pressure and velocity measurements further reveals the complexity of the turbulent flow in the wake of spacer-grids.
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F. Moreno, S. Bantiche, F. Bazin, T. Lohez, D. Picard, et al.. Unsteady Pressure and Velocity Measurements in 5x5 Rods Bundle Using Grids With and Without Mixing Vanes. NUTHOS-12 - 12th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety, Apr 2018, Qingdao, China. ⟨cea-02338866⟩

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