Heat transfer under buoyancy-acceleration competition in strongly heated helium-xenon flow through a horizontal tightly packed triangular rod bundle

Helium-xenon (He-Xe) mixtures are promising working fluids for compact gas-cooled reactors. However, their heat transfer behavior in horizontal tight rod bundles under strong heating is unclear. In these channels, strong heating induces both buoyancy and flow acceleration. This study experimentally investigated a 40 g/mol He-Xe mixture in a horizontal tightly packed rod bundle ( P / D = 1.15). We conducted 63 steady-state runs. Local Reynolds numbers (Re) ranged from 422 to 3403, and Richardson numbers (Ri) from 0.01 to 0.90. At low Reynolds numbers, existing correlations predict average Nusselt number (Nu) values below the experimental results. To account for the effects of strong heating, an empirical competition-form Nu correlation was developed using Ri and the acceleration parameter (Kv) as correction terms for buoyancy and flow acceleration, respectively. Relative to the baseline forced-convection fit, the correlation reduced the mean absolute percentage error from 5.61% to 3.67% and the maximum absolute percentage error from 38.35% to 25.61%. Overall, the results indicate that buoyancy-induced heat-transfer enhancement was the predominant effect, whereas acceleration-induced weakening became evident only under specific high-Kv conditions. Local results were evaluated at three discrete axial measurement points. Between the first and last points, Re and Nu decreased by 34.1% and 42.0%, respectively. The buoyancy parameters decayed rapidly downstream, whereas the acceleration effect persisted. These trends indicate a stronger relative buoyancy influence near the inlet. Farther downstream, forced convection and flow acceleration become increasingly important.