Radiative heating of supercritical carbon dioxide flowing through tubes

Brayton power cycles using high temperature, high pressure supercritical carbon dioxide (s-CO₂) as the working fluid have been increasingly considered as attractive candidates for solar-thermal power plants. Several configurations of heat exchangers and solar receivers are under investigation, with predicted tube temperatures ∼1000 K. The inclusion of radiation modeling to capture the effect of absorption bands of s-CO₂ and the radiative heat transfer among the equipment surfaces makes the computation costly and time consuming, and is often neglected on the basis of convection being the dominant transport mechanism. In this work, a numerical study has been performed to characterize the heat transfer in simultaneously developing laminar flow of s-CO₂ through a circular pipe. The combined effects of convection and radiation are presented by varying the Reynolds number, pipe diameter, length to diameter ratio, wall emissivity and the total wall heat flux. It is shown that neglecting the effects of radiative heat transfer, and in particular the participation of s-CO₂ in thermal transport can lead to large errors in predicting wall temperature, and by extension, the component lifetime. The error in wall temperature also leads to erroneous predictions on losses to the environment. The calculations indicate that there is a range of flow conditions over which the design process needs to incorporate radiation modeling.