Convective heat transfer at high subcritical pressures in tubes with and without flow obstacles

Heat transfer measurements to CO2-cooled tubes with and without flow obstacles at high subcritical pressures were obtained at the University of Ottawa's supercritical test facility. Tests were performed for vertical upward flow in a directly heated 8 mm ID tube for a pressure range from 6.6 to 7.2 MPa, a mass flux range from 115 to 1000 kg m−2 s−1, and a wall heat flux range from 1 to 122 kW m−2. Cases with spacing between obstacles equal to 250, 500 and 1000 mm were investigated. The results are presented in plots of wall temperature and heat transfer coefficient vs. bulk fluid enthalpy. The effects of flow conditions and obstacle geometry and spacing on high subcritical heat transfer have been discussed. Obstacles were found to suppress nucleate boiling, increase the CHF and lower the maximum post-CHF temperature. The maximum increases in CHF and in post-dryout heat transfer coefficient were observed for the obstacles having a spacing of 250 mm. Comparisons of the experimental data with prediction methods for single-phase heat transfer, CHF and post-dryout heat transfer are also presented.