A new set of direct numerical simulations (DNS) of spanwise-rotating square duct flow has been conducted at a global friction Reynolds number Re tau = 180 and global friction rotation numbers Ro tau of up to 10. In this study, a systematic comparison was performed between the constant heat flux (CHF) and constant temperature difference (CTD) thermal boundary conditions to clarify the differences in turbulent heat transfer mechanisms under rotation-induced secondary flow. The results show marked differences in the basic temperature statistics between the two boundary conditions. Under CHF, a peak in the outer region appears at high rotation numbers, whereas under CTD, a double-peak structure already emerges at low rotation numbers. Budget analysis reveals that, under different thermal boundary conditions, temperature redistribution by secondary flow generates large local mean temperature gradients, which in turn enhance turbulent heat flux and temperature variance production and lead to local peaks in the temperature statistics. On the unstable side, the characteristic scales of the temperature field in the near-wall region are insensitive to the thermal boundary condition, whereas stronger large-scale components are observed in the outer region under the CTD condition. At the same rotation number, the turbulent Prandtl number is higher under the CTD condition, and the overall heat transfer enhancement is more pronounced. These results deepen the understanding of turbulent heat transfer mechanisms and provide a useful reference for turbulence modeling.
