A supercritical carbon dioxide (s-CO2) Brayton power cycle was extensively applied in several energy systems in this study. This paper presents an experimental investigation of the heat transfer characteristics of supercritical CO2 in a vertical, small circular tube with an inner diameter of 5 mm at a low mass flux but high heat fluxes. Experiments were performed for inlet temperatures ranging from 5 to 40 degrees C, pressures from 7.5 to 10.5 MPa, mass flux from 50 to 200 kg/m(2)s, and heat flux from 38 to 234 kW/m(2). Results reveal marked differences in heat transfer characteristics under different levels of heat flux at low mass fluxes in the small channel. Buoyancy plays a substantial role in heat transfer at low mass flux, especially under conditions with high heat fluxes. Based on data from this study and experimental results in small channels described in the literature, a new heat transfer model was developed using a deep learning method with the TensorFlow tool, which captures 90% of experimental data within a +/- 20% error band. (C) 2019 Elsevier Ltd. All rights reserved.