Abstract
Precise velocity measurement in a fuel rod bundle is required to improve the thermal hydraulic characteristics of Pressurerized Water Reactor (PWR) spacer grids. However, the use of conventional laser Doppler velocimetry (LDV) is restricted by the existence of invisible region in fuel rod bundles and insufficient spatial resolution for the narrow gaps in rod bundles. The new LDV was therefore developed to overcome these problems. The LDV is miniaturized with fiber optics embedded in a fuel cladding and can be inserted in an arbitrary grid cell instead of a fuel rod. The rod-embedded fiber LDV (rod LDV) is applied for cross-flow and axial-flow measurements at rod gaps and sub-channels to investigate turbulent velocity field in a rod bundle with mixing spacer grids. Distributions of mean velocity and turbulent intensity around the center rod of a 5×5 rod bundle are obtained. The measurements of this study are compared with particle image velocimetry (PIV) measurement for the same geometry and flow condition. Similarity of both measurements is confirmed qualitatively and some of data are in good agreement quantitatively.
