Heat transfer deterioration in vertical sCO2 cooling in 3 mm tube
Andreas Wahl, Rainer Mertz, Eckart Laurien, Jörg Starflinger
Abstract
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 is used to evaluate the heat transfer deterioration. The effects of the CO2 mass flux of 141−354kg/m²s and bulk fluid temperatures of 20−50°C with a constant pressure of 80bar on the heat transfer were examined. 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 (Tpc) disappears. The deteriorated heat transfer in the downwards flow showed significant lower wall temperatures compared to the upwards flow. The separate evaluation of the liquid-like and gas-like region indicates that the deterioration may vary based on the fluid properties. The results were regressed within +-15% in the forced convection region and +-40% in the mixed convection region. The comparison with the literature data of vertical cooling in 6 mm showed different behaviour which indicates an influence of the tube diameter.