Numerical Investigation on Heat Transfer Enhancement of Supercritical CO₂ Flowing in Heated Vertically Upward Tubes

Heat transfer enhancement (HTE) of supercritical CO₂ flowing in heated vertical tube with an inner diameter of 6.32 mm was investigated numerically in present paper. The studies were performed for HTE cases with the pressure being 8.12 MPa, the mass flux being 400 and 1000 kg/m²s, and the heat flux being 30 and 50 kW/m². Four turbulence models, including the RNG k-ε, the SST k-ω and two low-Reynolds number models (LB and LS), were evaluated with the experimental data collected from literatures. The SST k-ω model was shown to be the best among the four models, and then was used for the simulation in this study. The effect of five factors, including buoyancy, thermal acceleration, thermal conductivity, specific heat and viscosity of the fluid, on HTE were respectively analyzed according to the numerical results. It was shown that the buoyancy had a little negative influence on HTE and was negligible for the heat transfer. The thermal acceleration effect was detrimental to the HTE by accelerating the fluid near the wall and at the same time reducing the turbulence kinetic energy in the core of the flow. The rapid decrease of thermal conductivity of the fluid at the pseudo-critical region was also bad for HTE. Variation of the specific heat of the fluid had strong positive effect on the HTE. When most of the buffer layer was occupied by fluid with the large specific heat, the heat absorbing capability of the fluid was increased and more heat was carried away efficiently. Moreover, decrease in viscosity of the fluid with increasing temperature also significantly promoted the HTE because of the increase in flow turbulence and the reduction in thermal-conduction resistance. After that, the weight of above five factors effect on HTE was compared quantitatively.