Enhancement of sCO2 Heat Transfer Near the Critical Point Using a Twisted Elliptical Mini Tube

In this study, comprehensive CFD simulations are carried out to investigate the heat transfer characteristics of supercritical carbon dioxide (sCO2) heating inside a horizontal twisted elliptical mini tube near the critical point. The effects of various parameters, such as mass flux, heat flux, and average bulk enthalpy, on heat transfer are considered. The current computational fluid dynamics (CFD) simulations successfully captured the phenomenon of heat transfer deterioration at elevated heat flux to mass flux ratios. Throughout this work, the outcomes are explained in connection with the drastic changes in sCO2 properties near the pseudo-critical point. It was observed that the mixing induced by the tube twist significantly reduced the effects of buoyancy, increasing the uniformity of the fluid properties and, therefore, enhancing the thermal performance. The obtained results were compared to those for a circular tube of the same hydraulic diameter under the same operating conditions to highlight the advantages of utilizing twisted elliptical tubes. It was found that the twisted elliptical tube can enhance heat transfer by up to 35% compared to the circular one.