Experimental study on heat transfer to supercritical CO2 flowing in vertical upward tube at medium mass flux

An experimental study was performed on heat transfer characteristics of supercritical pressure CO₂ (SC-CO2) flowing at medium mass flux conditions in a vertically-upward tube of 16 mm inner diameter at the Heat Transfer and Flow test loop of Supercritical CO2 (HTF-SCO2) in Xi’an Jiaotong University. Experimental parameters included the pressure ranging from 7.5 to10.5 MPa, the mass flux of 400-600 kg/m²s, and the heat flux of 20-100 kW/m². Based on the experimental data, effects of mass flux, heat flux and operation pressure on heat transfer characteristics of SC-CO₂ were thoroughly discussed. With the decrease of mass flux and increase of heat flux, heat transfer characteristics of SC-CO₂ becomes worse and worse. The wall temperature rises to high levels with the occurrence of a wall temperature peak and the wall temperature peak also rises remarkably with the decrease in mass flux and increase in heat flux. Especially, effect of pressures on the heat transfer of SC-CO2 was found to be quite different from that previously reported in literature. When the heat flux is low (such as 30 kW/m²), the HTD was diminished with the increase in pressures, but when the heat flux is up to 50 kW/m², the HTD is surprisingly intensified by the increase of pressure. The buoyancy effect was considered to explain this distinct influence of pressure on the heat transfer of SC-CO₂ by employed a non-dimensional parameter Bu. With the increase of pressure, buoyancy effect was diminished owing to the decrease of density difference between fluids near the wall and the center. When heat flux was lower, the Bu was located between 5×10^-6 and 10^-4, where buoyancy effect impaired heat transfer, so the heat transfer coefficient increased by rising pressure. But when heat flux was larger, the Bu was above 10^-4, where buoyancy effect began to enhance heat transfer, as a result, the heat transfer coefficient was reduced by weakened buoyancy effect because of the increase of pressure (CSPE).