Numerical investigation on conjugate cooling heat transfer to supercritical CO2 in vertical double-pipe heat exchangers

Conjugate cooling heat transfer to supercritical CO2 in a vertical double-pipe heat exchanger was numerically investigated in the present study. With the aim to better understand the conjugate cooling mechanism of supercritical fluid, detailed information on heat transfer behavior is provided. The results demonstrate that the numerical results predicted by the Abe, Kondoh, and Nagano (AKN) model show the best agreement with the experimental data. After validation, the influences of cooling water Re and temperature at the shell side, supercritical fluid Re at the tube side, flow direction, and pipe diameter on conjugate cooling heat transfer were investigated based on velocity fields. We conclude that cool water Re and temperature at the shell side have a significant effect on the cooling phenomenon at the tube side. Reduction in heat transfer could be avoided by either an increase in Re-CO2 or a decrease in d(i). In addition, variations in density and c(p) are the most significant factors to determine the occurrence of abnormal heat transfer phenomena. In comparison with the heating process of supercritical CO2, the sharply increased viscosity noted would hinder the distortion of the flow field to ameliorate heat transfer deterioration during the cooling process.