Experimental study on heat transfer of supercritical CO2 flowing in a mini tube under heating conditions

Supercritical carbon dioxide (S-CO2) power cycles hold great potential applications in various energy utilization systems, in which the heat transfer of S-CO2 is one of the most important issues. However, due to the complex thermophysical properties, it is still insufficient to predict heat transfer coefficient of S-CO2, especially under the heating conditions with high ratios of heat flux to mass flux (q/G). In this study, the heat transfer of S-CO2 is experimentally investigated in a mini tube (d = 2 mm) in the range of p = 7.6-8.4 MPa, q = 100-200 kW/m(2) and G = 400-700 kg/m(2)s, in which the ratio of q/G is within 250-500 J/kg. The parameter influences on heat transfer coefficient of S-CO2 are analyzed. It is found that the heat transfer coefficient decreases with increasing heat flux and decreasing mass flux, but is little affected by pressure. Under high q/G conditions, buoyancy effect is significant and heat transfer is deteriorated in the whole test section. Available heat transfer correlations for S-CO2 are compared with the experimental data; however, the prediction errors of these correlations are relatively high, especially in pseudo-critical regions. As a result, a new heat transfer correlation is developed for S-CO2 under high q/G conditions, in which buoyancy effect and variations in thermophysical properties are both taken into account. (C) 2020 Published by Elsevier Ltd.