The convection heat transfer of CO2 at supercritical pressures in a 0.0992 mm diameter vertical tube at relatively high Reynolds numbers (Rein = 6500), various heat fluxes and flow directions are investigated experimentally and numerically. The effects of buoyancy and flow acceleration resulting from the dramatic property variations are studied. The Results show that the local wall temperature varied non-linearly for both upward and downward flow when the heat flux was high. The difference in the local wall temperature between upward and downward flow is very small when the other test conditions are held the same, which indicates that for supercritical CO2 flowing in a micro tube as employed in this study, the buoyancy effect on the convection heat transfer is insignificant and the flow acceleration induced by the axial density variation with temperature is the main factor leading to the abnormal local wall temperature distribution at high heat fluxes. The predicted temperatures using the LB low Reynolds number turbulence model correspond well with the measured data. To further study the influence of flow acceleration on the convection heat transfer, air is also used as the working fluid to numerically investigate the fluid flow and heat transfer in the vertical micro tube. The results show that the effect of compressibility on the fluid flow and heat transfer of air in the vertical micro tube is significant but that the influence of thermal flow acceleration on convection heat transfer of air in a vertical micro tube is insignificant.

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