Thermal effects on high-pressure transcritical wall-bounded turbulence

The sustainment of turbulence under high-pressure transcritical conditions is examined using direct numerical simulations of channel flow subject to various combinations of fluid pressure and wall temperatures. The working fluid is carbon dioxide at supercritical reduced pressures P/Pc∈{1.5,2,5} (the subscript c denotes the critical value) driven through a channel bounded by differentially heated isothermal walls at reduced temperatures Tcw/Tc∈{0.8,0.9,0.95} (cold wall) and Thw/Tc∈{1.1,1.2,1.4} (hot wall). The resulting friction Reynolds numbers fall within the ranges Reτcw∈[56,111] at the cold wall and Reτhw∈[118,167] at the hot wall. The computational domain has dimensions (Lx,Ly,Lz)=(2π,2,2π/3)δ in the streamwise, wall-normal, and spanwise directions, corresponding to a minimal flow unit (δ is the channel half-height). The results show that setting the temperature of the hot wall slightly above the pseudo-boiling point, specifically in the range Thw/Tpb∈[1.01,1.18], maximizes the Nusselt number associated with both the hot and cold walls, while the bulk and friction Reynolds numbers peak in the narrower range Thw/Tpb∈[1.01,1.10]. Furthermore, both turbulent activity and heat transfer across the channel are found to be highest in the early supercritical regime, with pressures just above critical. These findings highlight the importance of tuning the thermophysical parameters precisely if turbulence is to be sustained and heat transfer enhanced in high-pressure transcritical fluid flows.