Effect of pressure on heat transfer between supercritical CO2 in tube and pulverized coal combustion flue gas

To further improve the thermal efficiency of coal-fired boilers, lower the environmental impact, and minimize the size of system components, the potential utilization of supercritical CO2 (S-CO2) as the working fluid has drawn considerable attention recently. In this study, the thermal characteristics between supercritical CO2(S-CO2) in a vertically upward tube and the flue gas in the combustion chamber were investigated numerically using the standard k-ε model, the P-1 radiation model, and the conjugate heat transfer model. The effects of the S-CO2 pressure on the surface heat flux, the wall temperature, the bulk temperature, the bulk velocity, the surface heat transfer coefficient, and the Nusselt number were examined. The distributions of the flue gas temperature and S-CO2 temperature were further elaborated. The results show that the supercritical pressure has a significant effect on the S-CO2 tube wall temperature, the wall temperature is higher under a low S-CO2 pressure. The wall heat flux is dominated by connection in the entrance region of the tube but is contributed by both convection and radiation at larger axial distances. The heat transfer coefficient increases with increasing the supercritical pressure, and the supercritical pressure has slight impact on the tube wall temperature. The present numerical study helps gain improved understanding of the thermal performance and optimize the design of boilers using S-CO2 as the working fluid.