Wall temperature measurement and heat transfer correlation of turbulent supercritical carbon dioxide flow in vertical circular/non-circular tubes

Experimental data are presented which describe heat transfer characteristics of turbulent supercritical carbon dioxide flow in vertical tubes with circular, triangular, and square cross-sections. Experiments are conducted at a constant pressure of 8 MPa under various conditions such as inlet bulk temperatures ranging from 15 to 32 °C, imposed heat fluxes from 3 to 180 kW/m², and mass velocities from 209 to 1230 kg/m² s. The corresponding Reynolds and Grashof numbers are in the range of 3 × 10⁴ to 1.4 × 10⁵, and 5 × 10⁹ to 4 × 10¹¹, respectively. The test section is composed of an entrance region of 0.6 m long and a heating region of 1.2 m long. Wall temperatures are measured by thermocouples installed at the outer surface of the heating region. In order to identify the effect of the cross-sectional shape on the supercritical heat transfer, wall temperature distributions in the streamwise direction are compared at the same heat flux and mass velocity conditions. Based on the wall temperature data, an improved heat transfer correlation, which can be applicable to both forced convection and mixed convection regimes, is proposed, and compared with previous ones.