Special heat transfer characteristics of supercritical CO₂ flowing in a vertically-upward tube with low mass flux

Due to limited studies on heat transfer of supercritical CO₂ (S-CO₂) at low mass flux, an experimental study was conducted on the High-TaP-SCO2 test loop to study heat transfer of S-CO₂ in a 16-mm diameter tube with low mass flux. Fundamental data were collected at a pressure range of 7.5–10.5 MPa, mass flux of 50–500 kg/m² s and heat flux of 5–100 kW/m². The effects of heat flux, mass flux, and pressure on heat transfer at low mass flux were analyzed. Results showed that, with increasing heat flux, a completely different heat transfer phenomenon was observed at lower mass flux (G < 300 kg/m² s) compared to a normal mass flux (G ≥ 300 kg/m² s). Heat transfer at lower mass flux was not deteriorated but rather enhanced with a rising heat transfer coefficient, which is about 2.6 times higher than single-phase convective heat transfer. With mass flux decreasing from 400 to 100 kg/m² s, heat transfer was enhanced about 5 times and turned from deterioration to enhancement. The mechanism of this heat transfer transition was further discussed. Results suggested that the special heat transfer enhancement at lower mass flux was mainly induced by the combined effects of strong buoyancy and high c_p fluid. Based on the present dataset, existing Nusselt correlations for supercritical fluids were reevaluated, and these correlations failed in capturing the heat transfer enhancement occurring at low mass flux. Finally, a modified correlation for the heat transfer of S-CO₂ at low mass flux was proposed within ±20% error and is available for the design of relevant devices with S-CO₂.