In the frame of EU-project sCO₂-flex the design of a 25 MWe supercritical CO₂ (sCO₂) Brayton cycle will be designed. The system will be optimized to meet flexibility requirements, while reducing environmental impact and focusing on cost efficiency. In the context of a sCO₂ Brayton cycle, the gas cooler is a key component to achieve a high overall efficiency. Close to the critical point, due to varying properties, heat transfer and pressure drop of carbon dioxide (CO₂) are difficult to predict. In case of vertical flow, acceleration and buoyancy effects induced by strong density gradients can cause a significant deterioration of the heat transfer.

In this publication, the cooling heat transfer coefficient (htc) is investigated in a 3 mm diameter tube with vertical flow orientation. Commonly used calculation methods of the heat transfer coefficient are presented. Although developed for heating of sCO₂, the mixed convection criterion of Jackson and Hall [7] is used to evaluate the heat transfer deterioration. The effects of the CO₂ mass flux of 141 − 354 kg/m²s and bulk fluid temperatures of 20 − 50 °C with a constant pressure of 80 bar on the heat transfer were examined. The transition between forced and mixed convection can be explained by the htc -values. The upwards flow shows a steady decrease in the htc with the reduction of the mass flux. However, the downwards flow shows significant effects of buoyancy. At low mass flux the distinct peak in the htc at the pseudocritical temperature (T_pc) disappears.