Numerical analysis on flow and heat transfer characteristics of supercritical pressure CO<sub>2</sub> in inclined tube

Abstract

In this study, numerical simulation of flow and heat transfer of S-CO2 (supercritical CO2) in the inclined upward and downward smooth tubes with an inner diameter of 10 mm and a heating length of 2000 mm at an inclination angle of 45° were conducted. The SST k - ω low Reynolds number turbulence model was used, and the accuracy of the calculation model was verified by the experimental data of S-CO2 flowing upward in the vertical smooth tube. The variation characteristics of the axial and circumferential inner wall temperature T w,i and the convective heat transfer coefficient h were analyzed. The S-CO2 would undergo the phase change from a liquid-like to the vapor-like at the pseudocritical temperature T pc by assuming supercritical pseudo-boiling. Based on this assumption, the reasons for a difference in the wall temperature distribution at the top generatrix during S-CO2 flowing in different directions were studied. By the detailed analysis of the velocity distribution, thermophysical properties distribution, including specific heat capacity at constant pressure cp and thermal conductivity λ , and the turbulent kinetic energy k distribution in the cross-sections of the inclined circular tube, the heat transfer mechanism related to the wall temperature difference at upward and downward was studied. The results showed that the thickness of vapor-like film, axial velocity u , turbulent kinetic energy k , and the thickness of viscosity sub-layer δ y +=5 were the main factors affecting the difference in the wall temperature distribution at the top generatrix in different flow directions.