Numerical study on the heat transfer enhancement of supercritical CO2 in vertical ribbed tubes

Highlights: • Rib configuration can reduce the influence of buoyancy force on heat transfer. • The transverse vortices induced by ribs are the main reason of local heat transfer improvement. • Increase in rib height and decrease in rib pitch can lead to further heat transfer enhancement. • The field synergy principle can be used as an indicator of heat transfer enhancement. It is a promising treatment strategy to use rib turbulators within the tube for suppressing heat transfer deterioration at supercritical pressure, which can significantly increase the heat transfer coefficient and reduce the wall temperature. The present study aims to numerically investigate the heat transfer enhancement of supercritical CO₂ flowing in heated vertical tube with rib configurations. By the analysis of the buoyancy effects and the influence of vortex structures on the flow field, the mechanism of heat transfer enhancement is preliminarily recognized. Results show that, buoyancy effects on flow and convective heat transfer are significantly weakened by the rib-induced vortices. Furthermore, rib-induced secondary flows and enhanced turbulent kinetic energy in the near-wall region also lead to heat transfer improvement. By increasing rib height or decreasing rib pitch can further improve convective heat transfer in ribbed tube. In addition, field synergy principle is proved to successfully predict the degree of heat transfer deterioration or enhancement due to the variation of volume-averaged field synergy angle along the tube is opposite to that of the local Nusselt number. The conclusions presented here can help more effective engineering designs for supercritical heat transfer applications.