Numerical study on fluid flow and heat transfer characteristics of supercritical CO2 in horizontal tube under various non-uniform heating conditions

Supercritical CO₂ (sCO₂) is deemed the potential working medium for thermodynamic cycles in the next generation of power machinery. In this paper, the flow and heat transfer characteristics of highly buoyant turbulent sCO₂ in horizontal tubes are numerical studied under the non-uniform heating conditions. The tube with a typical internal diameter of 10 mm is selected with mass flux equaling to 300 kg/(m²·s), effective heat fluxes ranging from 20 to 60 kW/m², and pressure equaling to 8 MPa. The results confirmed that the heat transfer of sCO₂ inside the horizontal tube in the circumferential or axial non-uniform heating conditions is greatly deteriorated by 13 %-68 % compared to the uniform heating. The overall heat transfer performance of semi-circumferential axial non-uniform heating is about 2.5 % higher than that of bottom semi-circumferential uniform heating, while the axial temperature difference of the bottom surface exceeds 150 K. Screening three representative heating positions, the bottom heating has the best temperature uniformity. The corresponding turbulent streamlines featured by unique helical structures can substantially improve the circumferential and radial heat transfer over 10 %, enhancing the heat transfer performance of the smooth tube close to that of rifled tubes. Based on the numerical data, viable correlations were developed for calculating the axial local Nusselt number of forced convection heat transfer of sCO₂ in horizontal tubes considering the effects of cross-sectional vorticity, secondary flow intensity, and buoyancy-natural convection, and the prediction values are in good agreement with experimental data.