Experimental investigation on the heat transfer characteristics of supercritical CO2 at various mass flow rates in heated vertical-flow tube

Highlights: • Experimental study is conducted on the heat transfer of SCO₂ heated in vertical tube. • Heat transfer characteristics at the various mass flow rates have marked difference. • The mechanisms of deterioration are discussed at various mass flow rates. • A correlation is developed based on dimensionless parameters and experimental data. -- Abstract: In this paper, experimental study is performed to investigate the characteristics of abnormal heat transfer of supercritical carbon dioxide (SCO₂) at various ranges of mass flow rate in heated vertical-flow tube. The experimental results indicate that the heat transfer characteristics at the various mass flow rates have marked difference. Heat transfer deterioration (HTD) is not observed at the low-mass flow rate (G = 80–120 kg/(m²·s)) even with a higher q/G. While the obvious HTD is detected in moderate (G = 120–180 kg/(m²·s)) and high (G > 180 kg/(m²·s)) mass flow rate. HTD occurs firstly near the outlet of test section at moderate mass flow rate (where the bulk fluid temperature exceeds the pseudo-critical temperature), and it moves toward the upstream of the test section as the mass flow rate increases. The laminarization of the low-density fluid layer in the heated surface is an important mechanism of HTD. Based on the wall-to-bulk temperature differences generating the modification of properties gradient and shear stress, the impacts of properties gradient, buoyancy effect and flow acceleration as well as the laminarization of the low-density fluid layer on heat transfer are considered in dimensionless form. Consequently, a new correlation is developed based on the heat transfer data more than 2800 which sets from 10 independent experiments.