Numerical investigation on turbulent mixing of liquid lead-bismuth in wire-wrapped rod bundles

Accurate prediction of subchannel temperatures in lead–bismuth eutectic reactors requires distinguishing directed crossflow from stochastic turbulent mixing. A Reynolds stress model, assessed against experimental temperatures and pressure-drop correlations, was applied to a 19-rod wire-wrapped bundle. The complete Reynolds stress tensor was projected onto the local normal direction of each gap, and quantitative statistics were evaluated downstream of the first wire-wrap pitch. For the 328 mm-pitch bundle, gap-normal velocity fluctuations exhibit approximately periodic variations downstream of the entrance region. The ratios of the pitch-averaged gap-normal fluctuating velocity to the pitch-averaged crossflow velocity are approximately 0.50 for Gap 1, 0.40 for Gap 3, and 0.12 for Gaps 5 and 7, with only minor variation across the tested mass flow rates. The CFD-derived internal mixing parameters are 8.48%-10.43% higher than the Cheng-Todreas predictions, whereas the peripheral sweeping parameters are 18.31%-18.38% lower. The proposed method provides a quantitative link between gap-resolved CFD results and subchannel mixing closures.