Thermodynamic performance analysis of supercritical pressure CO2 in tubes

Abstract

Thermodynamic characteristics of supercritical pressure CO2 were numerically investigated in tubes under cooling and heating conditions. The effects of mass and heat fluxes, pressure, cross section shape and buoyancy were studied, and the determinant of the peak heat transfer coefficient was discussed. The results indicated that smaller ratio of heat flux to mass flux q/G and pressure are beneficial to the heat transfer and could lead to smaller fluid friction and entropy generation. Among three tubes with different cross sections, the circular one has the best bulk heat transfer performance in terms of both first and second laws of thermodynamics. Under heating conditions, the heat transfer deterioration may occur at the entrance when the ratio of q/G is relatively large. Replacing the circular tube with a semicircular one could alleviate local deteriorations efficiently. The local heat transfer coefficient has a lot to do with the near-wall effective thermal conductivity instead of the static thermal conductivity. The heat transfer augmentation caused by buoyancy and the deterioration mitigation in the semicircular tube could be attributed to improved near-wall effective thermal conductivity as well.