Experimental Study on Turbulent Convection Heat Transfer of Supercritical CO2 in Cooled Inclined Miniature Tubes

This study explores experimentally turbulent convection heat transfer characteristics of supercritical carbon dioxide (CO2) in cooled miniature tubes (Dh = 1 mm) with various inclined angles (theta = 0, 30, 60 and 90 degrees). The influence of tube inclined angles, CO2 mass flow rates and wall heat fluxes on wall temperature and heat transfer coefficient were analyzed. Results related to buoyancy effect and thereby affected heat transfer were discussed. It was found that the wall temperature and the heat transfer coefficient markedly increase with the increased inclined angle, indicating the gradual weakness of mixed convection flow with the increased inclined angle. This effect was strengthened as mass flow rate decreased. In addition, the liquid-like region witnesses more significant influence of buoyancy effect on the heat transfer coefficient. Increasing mass flow rate could promote the heat transfer intensification, accompanied by the flow shifting from mixed convention flow to forced convection flow. The heat transfer coefficient is independent of the wall heat flux in liquid-like region, and slightly increased with the augment of wall heat flux in gas-like region. Given that the previously existing heat transfer correlations showed large deviations in predicting the present experimental data, a set of new correlations were proposed for cooling heat transfer prediction of supercritical CO2 in inclined miniature tubes and yield excellent predictive capability, evidenced by overall mean obsolete error of 11.13%, 7.72%, 12.30% and 14.24% for the inclined angle theta = 0 degrees, 30 degrees, 60 degrees and 90 degrees respectively.