Direct numerical simulation of the turbulent flow and heat transfer of supercritical CO_2 in a semicircular pipe

摘要

The heat transfer characteristics of supercritical carbon dioxide (SCO_2) in micro-channels is a major concern in the thermal-hydraulic design of printed circuit heat exchangers (PCHEs). In the present study, direct numerical simulation was utilized to study the heat transfer of SCO_2 in a semicircular pipe under mixed and forced convection. The inlet pressure and temperature were set to 8 MPa and 301.15 K to study the effects of thermophysical property variations on heat transfer. The Reynolds number at the pipe inlet was 4000, which is comparable to that of a PCHE channel. Emphasis is placed on revealing the effects of heat flux, secondary flow, thermal acceleration, corner effect, and geometric orientation on heat transfer. It was found that the temperature at the top wall was always higher than that at the bottom wall owing to the effects of buoyancy. A high-temperature region was identified near the pipe corner (known as the corner effect), which is caused by the combined effects of reduced velocity and small local hydraulic diameter. Secondary flows with two large vortices were observed in the channel cross-section. The secondary flow intensity was strengthened by the increase in heat flux. At the same time, the vortex centers bent toward the bottom at high heat fluxes. The orientation of the semicircular pipe has a large effect on the cross-sectional vector distribution and circumferential temperature gradient, which should be considered in the design of PCHEs.