Experimental and numerical investigation of LBE flow and heat transfer in a grid-spaced hexagonal 7-rod bundle with cosine axial power distribution

To support thermal-hydraulic design and safety assessment of lead-cooled fast reactor fuel assemblies, this study investigates the flow and heat transfer characteristics of lead-bismuth eutectic (LBE) in a grid-spaced hexagonal 7-rod bundle under a cosine axial power distribution. Experiments were performed over a Reynolds number range of 2.4 × 10 4 to 5.5 × 10 4 using heater rods with an axial cosine power profile. The measured Nusselt numbers are significantly lower than those predicted by classical correlations developed for uniform heating. The validated CFD analysis suggests that this discrepancy is mainly associated with the continuous restructuring of the thermal boundary layer under cosine heat flux and the inherent confinement effects of the bundle geometry. For the present low-blockage, vane-free grid, CFD predicts a flow–thermal decoupling behavior, characterized by pronounced flow disturbances accompanied by comparatively limited local heat-transfer enhancement. Using the experimental measurements together with the validated CFD results, the heat-transfer phase lag caused by the thermal entrance effect was quantified, and a configuration-specific empirical correlation for the axial Nu distribution under cosine heating was developed for the present bundle configuration and investigated operating range. Furthermore, the validated CFD results showed that, driven by fluid thermal inertia, the peak cladding temperature was located at approximately z / L = 0.70 within the heated section, downstream of the power peak at z / L = 0.50, and remained insensitive to variations in heat load within the investigated range. These findings offer significant insights into boundary layer restructuring in low-Pr fluids and provide a valuable reference for the thermal-hydraulic design of LFR fuel assemblies.