Experimental investigation in the local heat transfer of supercritical carbon dioxide in the uniformly heated horizontal miniature tubes

This study experimentally investigated the local heat transfer characteristics of supercritical carbon dioxide, which was uniformly heated in horizontal circular smooth tubes with inner diameters of 1.0 mm, 0.75 mm and 0.5 mm. The experimental results illustrated heat transfer enhancement with an increase in fluid temperature before the pseudocritical point and a decline in heat transfer just after the pseudocritical point. Subsequently, the heat transfer coefficient increased again when the fluid temperature was sufficiently higher than the pseudocritical value due to the enhancement of turbulence resulting from the decrease in viscosity. In addition, the parametric effects of outlet pressure, heat flux, mass flux, inlet temperature, and tube diameter on local heat transfer were investigated. The system demonstrated the optimal heat flux with the highest thermal performance when the outlet fluid condition was close to the corresponding pseudocritical point, at which both the specific heat and Prandtl number attained peak values. With other parameters fixed, the system with a higher mass flux, lower inlet temperature, or smaller diameter exhibited higher heat transfer performance. A new empirical correlation developed on the basis of the experimental data can reasonably predict the local Nusselt number of supercritical carbon dioxide along the flow path in uniformly heated horizontal tubes.